In this radio's past someone replaced the 7F8 with a 7AF7 and did a little re-wiring to make it work. I remember mentioning this here, but I don't remember when. Anyway, AM Broadcast band is working. When I got it, there was a 500 ohm resistor connecting the both cathodes together. This was incorrect. I added the correct 2.2K ohm resistor to the mixer cathode, and disconnected the 500 ohm resistor. So the BC is fine.
The resistor on the SW front end is missing. But I don't see how it would make any difference. It's in parallel with a coil and trimmer capacitor. I get continuity on the SW oscillator coil. Maybe this tube wont work at SW frequencies? It seems that it should. So I'm not going to re-wire for the 7F8. SW does pick up clicks and things but no actual broadcasts.
I haven't tried to put another radio radio near the oscillator coil to see if it's oscillating on the SW band. But I think that it's a good next step, not finding anything else wrong.
What should I look at next?
Thanks,
Dave
: In this radio's past someone replaced the 7F8 with a 7AF7 and did a little re-wiring to make it work. I remember mentioning this here, but I don't remember when. Anyway, AM Broadcast band is working. When I got it, there was a 500 ohm resistor connecting the both cathodes together. This was incorrect. I added the correct 2.2K ohm resistor to the mixer cathode, and disconnected the 500 ohm resistor. So the BC is fine.
: The resistor on the SW front end is missing. But I don't see how it would make any difference. It's in parallel with a coil and trimmer capacitor. I get continuity on the SW oscillator coil. Maybe this tube wont work at SW frequencies? It seems that it should. So I'm not going to re-wire for the 7F8. SW does pick up clicks and things but no actual broadcasts.
: I haven't tried to put another radio radio near the oscillator coil to see if it's oscillating on the SW band. But I think that it's a good next step, not finding anything else wrong.
: What should I look at next?
:
:Thanks,
:
:Dave
If it were mine, I would put it back the way it was designed, then go from there. A resistor across a coil is for dampening; if it was made that way it was necessary for proper operation.
OK, I will add the resistor. The 7AF7 may be identical to the 7F8 except for Pin-out. So it might work. I'll let you know what happened when the 10K ohm resistor is added.
Thanks,
Dave:
:Hello All,
:: In this radio's past someone replaced the 7F8 with a 7AF7 and did a little re-wiring to make it work. I remember mentioning this here, but I don't remember when. Anyway, AM Broadcast band is working. When I got it, there was a 500 ohm resistor connecting the both cathodes together. This was incorrect. I added the correct 2.2K ohm resistor to the mixer cathode, and disconnected the 500 ohm resistor. So the BC is fine.
:: The resistor on the SW front end is missing. But I don't see how it would make any difference. It's in parallel with a coil and trimmer capacitor. I get continuity on the SW oscillator coil. Maybe this tube wont work at SW frequencies? It seems that it should. So I'm not going to re-wire for the 7F8. SW does pick up clicks and things but no actual broadcasts.
:: I haven't tried to put another radio radio near the oscillator coil to see if it's oscillating on the SW band. But I think that it's a good next step, not finding anything else wrong.
:: What should I look at next?
::
::Thanks,
::
::Dave
:
:
:If it were mine, I would put it back the way it was designed, then go from there. A resistor across a coil is for dampening; if it was made that way it was necessary for proper operation.
I just checked the tube data. A 7AF7 is the same as a 7N7 which is also the same as a 6SN7 or 12AX7 dual triode. It has an amplification factor of 16. A 7A8 has an amplification factor of 48. So it's 3 times as strong in amplifying than is a 7AF7.
7A8 is the better tube, it seems. If I cannot get SW working, I'll assume that the 7AF7 cannot oscillate fast enough for SW frequencies.
Thanks,
Dave
:Johnnysan,
: OK, I will add the resistor. The 7AF7 may be identical to the 7F8 except for Pin-out. So it might work. I'll let you know what happened when the 10K ohm resistor is added.
:
:Thanks,
:
:Dave:
::Hello All,
::: In this radio's past someone replaced the 7F8 with a 7AF7 and did a little re-wiring to make it work. I remember mentioning this here, but I don't remember when. Anyway, AM Broadcast band is working. When I got it, there was a 500 ohm resistor connecting the both cathodes together. This was incorrect. I added the correct 2.2K ohm resistor to the mixer cathode, and disconnected the 500 ohm resistor. So the BC is fine.
::: The resistor on the SW front end is missing. But I don't see how it would make any difference. It's in parallel with a coil and trimmer capacitor. I get continuity on the SW oscillator coil. Maybe this tube wont work at SW frequencies? It seems that it should. So I'm not going to re-wire for the 7F8. SW does pick up clicks and things but no actual broadcasts.
::: I haven't tried to put another radio radio near the oscillator coil to see if it's oscillating on the SW band. But I think that it's a good next step, not finding anything else wrong.
::: What should I look at next?
:::
:::Thanks,
:::
:::Dave
::
::
::If it were mine, I would put it back the way it was designed, then go from there. A resistor across a coil is for dampening; if it was made that way it was necessary for proper operation.
May be kinda like the saga of replacing the 1L6 with a 1R5, would work at the lower frequencies but struggled at the higher frequencies.
Radiodoc
***********
:Johnnysan,
: I just checked the tube data. A 7AF7 is the same as a 7N7 which is also the same as a 6SN7 or 12AX7 dual triode. It has an amplification factor of 16. A 7A8 has an amplification factor of 48. So it's 3 times as strong in amplifying than is a 7AF7.
: 7A8 is the better tube, it seems. If I cannot get SW working, I'll assume that the 7AF7 cannot oscillate fast enough for SW frequencies.
:
:Thanks,
:
:Dave
::Johnnysan,
:: OK, I will add the resistor. The 7AF7 may be identical to the 7F8 except for Pin-out. So it might work. I'll let you know what happened when the 10K ohm resistor is added.
::
::Thanks,
::
::Dave:
:::Hello All,
:::: In this radio's past someone replaced the 7F8 with a 7AF7 and did a little re-wiring to make it work. I remember mentioning this here, but I don't remember when. Anyway, AM Broadcast band is working. When I got it, there was a 500 ohm resistor connecting the both cathodes together. This was incorrect. I added the correct 2.2K ohm resistor to the mixer cathode, and disconnected the 500 ohm resistor. So the BC is fine.
:::: The resistor on the SW front end is missing. But I don't see how it would make any difference. It's in parallel with a coil and trimmer capacitor. I get continuity on the SW oscillator coil. Maybe this tube wont work at SW frequencies? It seems that it should. So I'm not going to re-wire for the 7F8. SW does pick up clicks and things but no actual broadcasts.
:::: I haven't tried to put another radio radio near the oscillator coil to see if it's oscillating on the SW band. But I think that it's a good next step, not finding anything else wrong.
:::: What should I look at next?
::::
::::Thanks,
::::
::::Dave
:::
:::
:::If it were mine, I would put it back the way it was designed, then go from there. A resistor across a coil is for dampening; if it was made that way it was necessary for proper operation.
Looking at the specs of the two tubes, there is quite a difference if you were wanting a unit for some serious RF work.
Both of the units being dual triodes within a common envelope, the envelope profile of the 7F8 being a short and
squatty unit and the 7AF7 unit being of standard full envelope length.
The 7AF7 tube being of the "common amplifier" category, whereas the 7F8's specs exemplifies all of a full 300 Mhz
freq rating range. It also having the desired lower interelectrode capacitances all across the board.
The 7AF7 rates a 2100 umho transconductance figure while the 7F8 sports a 3300 figure.
Then there is the gain aspect of the 7AF7 at a mere 16....with the 7A8 being at a 48 level.
Now I don't know about your band of shortwave that you are referring to , but the lower ones nearest the BC band would
be much less discriminating, but on up the spectrum and particularly if for the FM band, the choice would be simple.
Back in the earlier big tubes sockets days and their leads and internal wiring.......the direct leaded configuration of the
locktals certainly solved that "baggage" problem in scaling up the frequency capabilities of circuitry incorporating them.
Referencing to 7F8 tube data is HERE
Referencing to 7AF7 tube data is HERE
73's de Edd
:Dave,
:
:May be kinda like the saga of replacing the 1L6 with a 1R5, would work at the lower frequencies but struggled at the higher frequencies.
:
:Radiodoc
:***********
:
::Johnnysan,
:: I just checked the tube data. A 7AF7 is the same as a 7N7 which is also the same as a 6SN7 or 12AX7 dual triode. It has an amplification factor of 16. A 7A8 has an amplification factor of 48. So it's 3 times as strong in amplifying than is a 7AF7.
:: 7A8 is the better tube, it seems. If I cannot get SW working, I'll assume that the 7AF7 cannot oscillate fast enough for SW frequencies.
::
::Thanks,
::
::Dave
:::Johnnysan,
::: OK, I will add the resistor. The 7AF7 may be identical to the 7F8 except for Pin-out. So it might work. I'll let you know what happened when the 10K ohm resistor is added.
:::
:::Thanks,
:::
:::Dave:
::::Hello All,
::::: In this radio's past someone replaced the 7F8 with a 7AF7 and did a little re-wiring to make it work. I remember mentioning this here, but I don't remember when. Anyway, AM Broadcast band is working. When I got it, there was a 500 ohm resistor connecting the both cathodes together. This was incorrect. I added the correct 2.2K ohm resistor to the mixer cathode, and disconnected the 500 ohm resistor. So the BC is fine.
::::: The resistor on the SW front end is missing. But I don't see how it would make any difference. It's in parallel with a coil and trimmer capacitor. I get continuity on the SW oscillator coil. Maybe this tube wont work at SW frequencies? It seems that it should. So I'm not going to re-wire for the 7F8. SW does pick up clicks and things but no actual broadcasts.
::::: I haven't tried to put another radio radio near the oscillator coil to see if it's oscillating on the SW band. But I think that it's a good next step, not finding anything else wrong.
::::: What should I look at next?
:::::
:::::Thanks,
:::::
:::::Dave
::::
::::
::::If it were mine, I would put it back the way it was designed, then go from there. A resistor across a coil is for dampening; if it was made that way it was necessary for proper operation.
I found the problem. Someone replaced the 150pf capacitor with a .0047 mf capacitor. 4700pf is a bit high. With the .0047 disconnected I receive short wave just fine. But I probably need the 150 pf capacitor because I only get 1400 and above on the BC band.
I'll find a tiny capacitor and try it out.
This radio has no FM. So on to the capacitor re-replacement.
Thanks,
Dave
:
:
:
:
:
:
:Looking at the specs of the two tubes, there is quite a difference if you were wanting a unit for some serious RF work.
:Both of the units being dual triodes within a common envelope, the envelope profile of the 7F8 being a short and
:squatty unit and the 7AF7 unit being of standard full envelope length.
:The 7AF7 tube being of the "common amplifier" category, whereas the 7F8's specs exemplifies all of a full 300 Mhz
:freq rating range. It also having the desired lower interelectrode capacitances all across the board.
:The 7AF7 rates a 2100 umho transconductance figure while the 7F8 sports a 3300 figure.
:Then there is the gain aspect of the 7AF7 at a mere 16....with the 7A8 being at a 48 level.
:Now I don't know about your band of shortwave that you are referring to , but the lower ones nearest the BC band would
:be much less discriminating, but on up the spectrum and particularly if for the FM band, the choice would be simple.
:
:Back in the earlier big tubes sockets days and their leads and internal wiring.......the direct leaded configuration of the
:locktals certainly solved that "baggage" problem in scaling up the frequency capabilities of circuitry incorporating them.
:
:
:Referencing to 7F8 tube data is HERE
:
:
:Referencing to 7AF7 tube data is HERE
:
:
:
:
:73's de Edd
:
:
:
:
:
:
:
:
:
:
:
::Dave,
::
::May be kinda like the saga of replacing the 1L6 with a 1R5, would work at the lower frequencies but struggled at the higher frequencies.
::
::Radiodoc
::***********
::
:::Johnnysan,
::: I just checked the tube data. A 7AF7 is the same as a 7N7 which is also the same as a 6SN7 or 12AX7 dual triode. It has an amplification factor of 16. A 7A8 has an amplification factor of 48. So it's 3 times as strong in amplifying than is a 7AF7.
::: 7A8 is the better tube, it seems. If I cannot get SW working, I'll assume that the 7AF7 cannot oscillate fast enough for SW frequencies.
:::
:::Thanks,
:::
:::Dave
::::Johnnysan,
:::: OK, I will add the resistor. The 7AF7 may be identical to the 7F8 except for Pin-out. So it might work. I'll let you know what happened when the 10K ohm resistor is added.
::::
::::Thanks,
::::
::::Dave:
:::::Hello All,
:::::: In this radio's past someone replaced the 7F8 with a 7AF7 and did a little re-wiring to make it work. I remember mentioning this here, but I don't remember when. Anyway, AM Broadcast band is working. When I got it, there was a 500 ohm resistor connecting the both cathodes together. This was incorrect. I added the correct 2.2K ohm resistor to the mixer cathode, and disconnected the 500 ohm resistor. So the BC is fine.
:::::: The resistor on the SW front end is missing. But I don't see how it would make any difference. It's in parallel with a coil and trimmer capacitor. I get continuity on the SW oscillator coil. Maybe this tube wont work at SW frequencies? It seems that it should. So I'm not going to re-wire for the 7F8. SW does pick up clicks and things but no actual broadcasts.
:::::: I haven't tried to put another radio radio near the oscillator coil to see if it's oscillating on the SW band. But I think that it's a good next step, not finding anything else wrong.
:::::: What should I look at next?
::::::
::::::Thanks,
::::::
::::::Dave
:::::
:::::
:::::If it were mine, I would put it back the way it was designed, then go from there. A resistor across a coil is for dampening; if it was made that way it was necessary for proper operation.
I seem to have misplaced my teeny tiny value capacitors. I almost never need that size and RS doesn't carry that size anymore. So I guess (as silly as it sounds) I'll have to mail-order a 150pf capacitor. Maybe if I search my place, after several hours I'll find it. This is very disappointing.
When I do find it, I'll let everyone know how the radio is working.
Thanks,
Dave
:Edd,
: I found the problem. Someone replaced the 150pf capacitor with a .0047 mf capacitor. 4700pf is a bit high. With the .0047 disconnected I receive short wave just fine. But I probably need the 150 pf capacitor because I only get 1400 and above on the BC band.
: I'll find a tiny capacitor and try it out.
: This radio has no FM. So on to the capacitor re-replacement.
:
:Thanks,
:
:Dave
::
::
::
::
::
::
::Looking at the specs of the two tubes, there is quite a difference if you were wanting a unit for some serious RF work.
::Both of the units being dual triodes within a common envelope, the envelope profile of the 7F8 being a short and
::squatty unit and the 7AF7 unit being of standard full envelope length.
::The 7AF7 tube being of the "common amplifier" category, whereas the 7F8's specs exemplifies all of a full 300 Mhz
::freq rating range. It also having the desired lower interelectrode capacitances all across the board.
::The 7AF7 rates a 2100 umho transconductance figure while the 7F8 sports a 3300 figure.
::Then there is the gain aspect of the 7AF7 at a mere 16....with the 7A8 being at a 48 level.
::Now I don't know about your band of shortwave that you are referring to , but the lower ones nearest the BC band would
::be much less discriminating, but on up the spectrum and particularly if for the FM band, the choice would be simple.
::
::Back in the earlier big tubes sockets days and their leads and internal wiring.......the direct leaded configuration of the
::locktals certainly solved that "baggage" problem in scaling up the frequency capabilities of circuitry incorporating them.
::
::
::Referencing to 7F8 tube data is HERE
::
::
::Referencing to 7AF7 tube data is HERE
::
::
::
::
::73's de Edd
::
::
::
::
::
::
::
::
::
::
::
:::Dave,
:::
:::May be kinda like the saga of replacing the 1L6 with a 1R5, would work at the lower frequencies but struggled at the higher frequencies.
:::
:::Radiodoc
:::***********
:::
::::Johnnysan,
:::: I just checked the tube data. A 7AF7 is the same as a 7N7 which is also the same as a 6SN7 or 12AX7 dual triode. It has an amplification factor of 16. A 7A8 has an amplification factor of 48. So it's 3 times as strong in amplifying than is a 7AF7.
:::: 7A8 is the better tube, it seems. If I cannot get SW working, I'll assume that the 7AF7 cannot oscillate fast enough for SW frequencies.
::::
::::Thanks,
::::
::::Dave
:::::Johnnysan,
::::: OK, I will add the resistor. The 7AF7 may be identical to the 7F8 except for Pin-out. So it might work. I'll let you know what happened when the 10K ohm resistor is added.
:::::
:::::Thanks,
:::::
:::::Dave:
::::::Hello All,
::::::: In this radio's past someone replaced the 7F8 with a 7AF7 and did a little re-wiring to make it work. I remember mentioning this here, but I don't remember when. Anyway, AM Broadcast band is working. When I got it, there was a 500 ohm resistor connecting the both cathodes together. This was incorrect. I added the correct 2.2K ohm resistor to the mixer cathode, and disconnected the 500 ohm resistor. So the BC is fine.
::::::: The resistor on the SW front end is missing. But I don't see how it would make any difference. It's in parallel with a coil and trimmer capacitor. I get continuity on the SW oscillator coil. Maybe this tube wont work at SW frequencies? It seems that it should. So I'm not going to re-wire for the 7F8. SW does pick up clicks and things but no actual broadcasts.
::::::: I haven't tried to put another radio radio near the oscillator coil to see if it's oscillating on the SW band. But I think that it's a good next step, not finding anything else wrong.
::::::: What should I look at next?
:::::::
:::::::Thanks,
:::::::
:::::::Dave
::::::
::::::
::::::If it were mine, I would put it back the way it was designed, then go from there. A resistor across a coil is for dampening; if it was made that way it was necessary for proper operation.
Yep....local osc to mixer injection capacitor.....C5 150 pf.....between the LO/MIX cathodes circuitry...that MUCH higher value would tend to overcouple.....load down...and swamp down things a bit.
Addenda:...have several new El Menco silver micas of that value ...can mail if needed...RSVP.
73's de Edd
:Edd,
: I seem to have misplaced my teeny tiny value capacitors. I almost never need that size and RS doesn't carry that size anymore. So I guess (as silly as it sounds) I'll have to mail-order a 150pf capacitor. Maybe if I search my place, after several hours I'll find it. This is very disappointing.
: When I do find it, I'll let everyone know how the radio is working.
:
:Thanks,
:
:Dave
::Edd,
:: I found the problem. Someone replaced the 150pf capacitor with a .0047 mf capacitor. 4700pf is a bit high. With the .0047 disconnected I receive short wave just fine. But I probably need the 150 pf capacitor because I only get 1400 and above on the BC band.
:: I'll find a tiny capacitor and try it out.
:: This radio has no FM. So on to the capacitor re-replacement.
::
::Thanks,
::
::Dave
:::
:::
:::
:::
:::
:::
:::Looking at the specs of the two tubes, there is quite a difference if you were wanting a unit for some serious RF work.
:::Both of the units being dual triodes within a common envelope, the envelope profile of the 7F8 being a short and
:::squatty unit and the 7AF7 unit being of standard full envelope length.
:::The 7AF7 tube being of the "common amplifier" category, whereas the 7F8's specs exemplifies all of a full 300 Mhz
:::freq rating range. It also having the desired lower interelectrode capacitances all across the board.
:::The 7AF7 rates a 2100 umho transconductance figure while the 7F8 sports a 3300 figure.
:::Then there is the gain aspect of the 7AF7 at a mere 16....with the 7A8 being at a 48 level.
:::Now I don't know about your band of shortwave that you are referring to , but the lower ones nearest the BC band would
:::be much less discriminating, but on up the spectrum and particularly if for the FM band, the choice would be simple.
:::
:::Back in the earlier big tubes sockets days and their leads and internal wiring.......the direct leaded configuration of the
:::locktals certainly solved that "baggage" problem in scaling up the frequency capabilities of circuitry incorporating them.
:::
:::
:::Referencing to 7F8 tube data is HERE
:::
:::
:::Referencing to 7AF7 tube data is HERE
:::
:::
:::
:::
:::73's de Edd
:::
:::
:::
:::
:::
:::
:::
:::
:::
:::
:::
::::Dave,
::::
::::May be kinda like the saga of replacing the 1L6 with a 1R5, would work at the lower frequencies but struggled at the higher frequencies.
::::
::::Radiodoc
::::***********
::::
:::::Johnnysan,
::::: I just checked the tube data. A 7AF7 is the same as a 7N7 which is also the same as a 6SN7 or 12AX7 dual triode. It has an amplification factor of 16. A 7A8 has an amplification factor of 48. So it's 3 times as strong in amplifying than is a 7AF7.
::::: 7A8 is the better tube, it seems. If I cannot get SW working, I'll assume that the 7AF7 cannot oscillate fast enough for SW frequencies.
:::::
:::::Thanks,
:::::
:::::Dave
::::::Johnnysan,
:::::: OK, I will add the resistor. The 7AF7 may be identical to the 7F8 except for Pin-out. So it might work. I'll let you know what happened when the 10K ohm resistor is added.
::::::
::::::Thanks,
::::::
::::::Dave:
:::::::Hello All,
:::::::: In this radio's past someone replaced the 7F8 with a 7AF7 and did a little re-wiring to make it work. I remember mentioning this here, but I don't remember when. Anyway, AM Broadcast band is working. When I got it, there was a 500 ohm resistor connecting the both cathodes together. This was incorrect. I added the correct 2.2K ohm resistor to the mixer cathode, and disconnected the 500 ohm resistor. So the BC is fine.
:::::::: The resistor on the SW front end is missing. But I don't see how it would make any difference. It's in parallel with a coil and trimmer capacitor. I get continuity on the SW oscillator coil. Maybe this tube wont work at SW frequencies? It seems that it should. So I'm not going to re-wire for the 7F8. SW does pick up clicks and things but no actual broadcasts.
:::::::: I haven't tried to put another radio radio near the oscillator coil to see if it's oscillating on the SW band. But I think that it's a good next step, not finding anything else wrong.
:::::::: What should I look at next?
::::::::
::::::::Thanks,
::::::::
::::::::Dave
:::::::
:::::::
:::::::If it were mine, I would put it back the way it was designed, then go from there. A resistor across a coil is for dampening; if it was made that way it was necessary for proper operation.
I found some low value capacitors. I tried 180 pf, no SW great BC, 150 pf, no SW geat BC. 10pf gives nice strong SW but nice weak BC. So it must be somewhere between 10 and 68 because I tried that too. I'll try 30pf and 47pf. These are teeny tiny but I've seen values like this in 5 tube table radios. I remember once I was working on one where someone substituted a 47mf electrolytic capacitor for a 47pf mica capacitor. That didn't work well either.
Dave
:
:
:
:
:Yep....local osc to mixer injection capacitor.....C5 150 pf.....between the LO/MIX cathodes circuitry...that MUCH higher value would tend to overcouple.....load down...and swamp down things a bit.
:
:
:Addenda:...have several new El Menco silver micas of that value ...can mail if needed...RSVP.
:
:
:
:
:73's de Edd
:
:
:
:
:
:
:
:
:
::Edd,
:: I seem to have misplaced my teeny tiny value capacitors. I almost never need that size and RS doesn't carry that size anymore. So I guess (as silly as it sounds) I'll have to mail-order a 150pf capacitor. Maybe if I search my place, after several hours I'll find it. This is very disappointing.
:: When I do find it, I'll let everyone know how the radio is working.
::
::Thanks,
::
::Dave
:::Edd,
::: I found the problem. Someone replaced the 150pf capacitor with a .0047 mf capacitor. 4700pf is a bit high. With the .0047 disconnected I receive short wave just fine. But I probably need the 150 pf capacitor because I only get 1400 and above on the BC band.
::: I'll find a tiny capacitor and try it out.
::: This radio has no FM. So on to the capacitor re-replacement.
:::
:::Thanks,
:::
:::Dave
::::
::::
::::
::::
::::
::::
::::Looking at the specs of the two tubes, there is quite a difference if you were wanting a unit for some serious RF work.
::::Both of the units being dual triodes within a common envelope, the envelope profile of the 7F8 being a short and
::::squatty unit and the 7AF7 unit being of standard full envelope length.
::::The 7AF7 tube being of the "common amplifier" category, whereas the 7F8's specs exemplifies all of a full 300 Mhz
::::freq rating range. It also having the desired lower interelectrode capacitances all across the board.
::::The 7AF7 rates a 2100 umho transconductance figure while the 7F8 sports a 3300 figure.
::::Then there is the gain aspect of the 7AF7 at a mere 16....with the 7A8 being at a 48 level.
::::Now I don't know about your band of shortwave that you are referring to , but the lower ones nearest the BC band would
::::be much less discriminating, but on up the spectrum and particularly if for the FM band, the choice would be simple.
::::
::::Back in the earlier big tubes sockets days and their leads and internal wiring.......the direct leaded configuration of the
::::locktals certainly solved that "baggage" problem in scaling up the frequency capabilities of circuitry incorporating them.
::::
::::
::::Referencing to 7F8 tube data is HERE
::::
::::
::::Referencing to 7AF7 tube data is HERE
::::
::::
::::
::::
::::73's de Edd
::::
::::
::::
::::
::::
::::
::::
::::
::::
::::
::::
:::::Dave,
:::::
:::::May be kinda like the saga of replacing the 1L6 with a 1R5, would work at the lower frequencies but struggled at the higher frequencies.
:::::
:::::Radiodoc
:::::***********
:::::
::::::Johnnysan,
:::::: I just checked the tube data. A 7AF7 is the same as a 7N7 which is also the same as a 6SN7 or 12AX7 dual triode. It has an amplification factor of 16. A 7A8 has an amplification factor of 48. So it's 3 times as strong in amplifying than is a 7AF7.
:::::: 7A8 is the better tube, it seems. If I cannot get SW working, I'll assume that the 7AF7 cannot oscillate fast enough for SW frequencies.
::::::
::::::Thanks,
::::::
::::::Dave
:::::::Johnnysan,
::::::: OK, I will add the resistor. The 7AF7 may be identical to the 7F8 except for Pin-out. So it might work. I'll let you know what happened when the 10K ohm resistor is added.
:::::::
:::::::Thanks,
:::::::
:::::::Dave:
::::::::Hello All,
::::::::: In this radio's past someone replaced the 7F8 with a 7AF7 and did a little re-wiring to make it work. I remember mentioning this here, but I don't remember when. Anyway, AM Broadcast band is working. When I got it, there was a 500 ohm resistor connecting the both cathodes together. This was incorrect. I added the correct 2.2K ohm resistor to the mixer cathode, and disconnected the 500 ohm resistor. So the BC is fine.
::::::::: The resistor on the SW front end is missing. But I don't see how it would make any difference. It's in parallel with a coil and trimmer capacitor. I get continuity on the SW oscillator coil. Maybe this tube wont work at SW frequencies? It seems that it should. So I'm not going to re-wire for the 7F8. SW does pick up clicks and things but no actual broadcasts.
::::::::: I haven't tried to put another radio radio near the oscillator coil to see if it's oscillating on the SW band. But I think that it's a good next step, not finding anything else wrong.
::::::::: What should I look at next?
:::::::::
:::::::::Thanks,
:::::::::
:::::::::Dave
::::::::
::::::::
::::::::If it were mine, I would put it back the way it was designed, then go from there. A resistor across a coil is for dampening; if it was made that way it was necessary for proper operation.
A 1L6 and !R5 have a different pin-outs. But somehow a 1R5 works. The 7F8 and 7AF7 have different pin-outs but the wiring was changed so that it is correct for the 7AF7. It's different. But the gain of the 7F8 was higher than that of the 7AF7. So that's what I'm probably experiencing.
It will oscillate at the SW frequencies. But the capacitor I'm trying to come up with I may never find. It will have to be a compromise. Now I have very good SW but the gain drops off at the low end of the BC band. If I increase the value of the capacitor past 47 pf, SW will get weak but the BC band is nice and strong. Right now it's 37 pf, which I think is the best it's going to get.
The original tube used a 150 pf. That absolutely wont work here.
:Dave,
:
:May be kinda like the saga of replacing the 1L6 with a 1R5, would work at the lower frequencies but struggled at the higher frequencies.
:
:Radiodoc
:***********
:
::Johnnysan,
:: I just checked the tube data. A 7AF7 is the same as a 7N7 which is also the same as a 6SN7 or 12AX7 dual triode. It has an amplification factor of 16. A 7A8 has an amplification factor of 48. So it's 3 times as strong in amplifying than is a 7AF7.
:: 7A8 is the better tube, it seems. If I cannot get SW working, I'll assume that the 7AF7 cannot oscillate fast enough for SW frequencies.
::
::Thanks,
::
::Dave
:::Johnnysan,
::: OK, I will add the resistor. The 7AF7 may be identical to the 7F8 except for Pin-out. So it might work. I'll let you know what happened when the 10K ohm resistor is added.
:::
:::Thanks,
:::
:::Dave:
::::Hello All,
::::: In this radio's past someone replaced the 7F8 with a 7AF7 and did a little re-wiring to make it work. I remember mentioning this here, but I don't remember when. Anyway, AM Broadcast band is working. When I got it, there was a 500 ohm resistor connecting the both cathodes together. This was incorrect. I added the correct 2.2K ohm resistor to the mixer cathode, and disconnected the 500 ohm resistor. So the BC is fine.
::::: The resistor on the SW front end is missing. But I don't see how it would make any difference. It's in parallel with a coil and trimmer capacitor. I get continuity on the SW oscillator coil. Maybe this tube wont work at SW frequencies? It seems that it should. So I'm not going to re-wire for the 7F8. SW does pick up clicks and things but no actual broadcasts.
::::: I haven't tried to put another radio radio near the oscillator coil to see if it's oscillating on the SW band. But I think that it's a good next step, not finding anything else wrong.
::::: What should I look at next?
:::::
:::::Thanks,
:::::
:::::Dave
::::
::::
::::If it were mine, I would put it back the way it was designed, then go from there. A resistor across a coil is for dampening; if it was made that way it was necessary for proper operation.
: A 1L6 and !R5 have a different pin-outs. But somehow a 1R5 works. The 7F8 and 7AF7 have different pin-outs but the wiring was changed so that it is correct for the 7AF7. It's different. But the gain of the 7F8 was higher than that of the 7AF7. So that's what I'm probably experiencing.
: It will oscillate at the SW frequencies. But the capacitor I'm trying to come up with I may never find. It will have to be a compromise. Now I have very good SW but the gain drops off at the low end of the BC band. If I increase the value of the capacitor past 47 pf, SW will get weak but the BC band is nice and strong. Right now it's 37 pf, which I think is the best it's going to get.
: The original tube used a 150 pf. That absolutely wont work here.
:
::Dave,
::
::May be kinda like the saga of replacing the 1L6 with a 1R5, would work at the lower frequencies but struggled at the higher frequencies.
::
::Radiodoc
::***********
::
:::Johnnysan,
::: I just checked the tube data. A 7AF7 is the same as a 7N7 which is also the same as a 6SN7 or 12AX7 dual triode. It has an amplification factor of 16. A 7A8 has an amplification factor of 48. So it's 3 times as strong in amplifying than is a 7AF7.
::: 7A8 is the better tube, it seems. If I cannot get SW working, I'll assume that the 7AF7 cannot oscillate fast enough for SW frequencies.
:::
:::Thanks,
:::
:::Dave
::::Johnnysan,
:::: OK, I will add the resistor. The 7AF7 may be identical to the 7F8 except for Pin-out. So it might work. I'll let you know what happened when the 10K ohm resistor is added.
::::
::::Thanks,
::::
::::Dave:
:::::Hello All,
:::::: In this radio's past someone replaced the 7F8 with a 7AF7 and did a little re-wiring to make it work. I remember mentioning this here, but I don't remember when. Anyway, AM Broadcast band is working. When I got it, there was a 500 ohm resistor connecting the both cathodes together. This was incorrect. I added the correct 2.2K ohm resistor to the mixer cathode, and disconnected the 500 ohm resistor. So the BC is fine.
:::::: The resistor on the SW front end is missing. But I don't see how it would make any difference. It's in parallel with a coil and trimmer capacitor. I get continuity on the SW oscillator coil. Maybe this tube wont work at SW frequencies? It seems that it should. So I'm not going to re-wire for the 7F8. SW does pick up clicks and things but no actual broadcasts.
:::::: I haven't tried to put another radio radio near the oscillator coil to see if it's oscillating on the SW band. But I think that it's a good next step, not finding anything else wrong.
:::::: What should I look at next?
::::::
::::::Thanks,
::::::
::::::Dave
:::::
:::::
:::::If it were mine, I would put it back the way it was designed, then go from there. A resistor across a coil is for dampening; if it was made that way it was necessary for proper operation.
Hi Dave I have the same radio with the same problem, no SW are you refering to cap C5 in the schematic? I might as well check mine also who knows...
E.
As I have understood it, the 1R5 will work pretty well in the place of a 1L6 at the lower broadcast frequencies. It will work at the higher frequencies, but the radio frontend adjustments may have to be retuned and if the frequency is high enough, the adjustments may not compensate. In some circumstances pin 5 of the 1R5 may have to be cut off.
Radiodoc
**********
:Radiodoc,
: A 1L6 and !R5 have a different pin-outs. But somehow a 1R5 works. The 7F8 and 7AF7 have different pin-outs but the wiring was changed so that it is correct for the 7AF7. It's different. But the gain of the 7F8 was higher than that of the 7AF7. So that's what I'm probably experiencing.
: It will oscillate at the SW frequencies. But the capacitor I'm trying to come up with I may never find. It will have to be a compromise. Now I have very good SW but the gain drops off at the low end of the BC band. If I increase the value of the capacitor past 47 pf, SW will get weak but the BC band is nice and strong. Right now it's 37 pf, which I think is the best it's going to get.
: The original tube used a 150 pf. That absolutely wont work here.
:
::Dave,
::
::May be kinda like the saga of replacing the 1L6 with a 1R5, would work at the lower frequencies but struggled at the higher frequencies.
::
::Radiodoc
::***********
::
:::Johnnysan,
::: I just checked the tube data. A 7AF7 is the same as a 7N7 which is also the same as a 6SN7 or 12AX7 dual triode. It has an amplification factor of 16. A 7A8 has an amplification factor of 48. So it's 3 times as strong in amplifying than is a 7AF7.
::: 7A8 is the better tube, it seems. If I cannot get SW working, I'll assume that the 7AF7 cannot oscillate fast enough for SW frequencies.
:::
:::Thanks,
:::
:::Dave
::::Johnnysan,
:::: OK, I will add the resistor. The 7AF7 may be identical to the 7F8 except for Pin-out. So it might work. I'll let you know what happened when the 10K ohm resistor is added.
::::
::::Thanks,
::::
::::Dave:
:::::Hello All,
:::::: In this radio's past someone replaced the 7F8 with a 7AF7 and did a little re-wiring to make it work. I remember mentioning this here, but I don't remember when. Anyway, AM Broadcast band is working. When I got it, there was a 500 ohm resistor connecting the both cathodes together. This was incorrect. I added the correct 2.2K ohm resistor to the mixer cathode, and disconnected the 500 ohm resistor. So the BC is fine.
:::::: The resistor on the SW front end is missing. But I don't see how it would make any difference. It's in parallel with a coil and trimmer capacitor. I get continuity on the SW oscillator coil. Maybe this tube wont work at SW frequencies? It seems that it should. So I'm not going to re-wire for the 7F8. SW does pick up clicks and things but no actual broadcasts.
:::::: I haven't tried to put another radio radio near the oscillator coil to see if it's oscillating on the SW band. But I think that it's a good next step, not finding anything else wrong.
:::::: What should I look at next?
::::::
::::::Thanks,
::::::
::::::Dave
:::::
:::::
:::::If it were mine, I would put it back the way it was designed, then go from there. A resistor across a coil is for dampening; if it was made that way it was necessary for proper operation.
I think that might possibly be attributable to that "plain vanilla" dual triode that is now sitting in the mixer osc position.......see its dupe being used in the tail end audio section of the receiver. Much higher and different interelectrode capacitances / elemental structuring on the "AF" tube. What you might try test evaluating ......considering the retention and use of that current "tube" in the front end is make a test attempt using a mica compression type of variable capacitor [***] temporarily solder tacked in and replacing the mentioned problematic cap. Use an insulated phenolic tool to run thru min / max capacitance adjustments to see where the balance in performance is between BC and SW stability / performance, then being able to select that value of fixed capacitor if all works well with that capacitance value. [***] Such as is used in pairs, for the top variable cap tuning of the larger older generation IF transformers.
|
: : :I think that might possibly be attributable to that "plain vanilla" dual triode that is now sitting in the mixer osc position.......see its dupe being used in :the tail end audio section of the receiver. Much higher and different interelectrode capacitances / elemental structuring on the "AF" tube. :What you might try test evaluating ......considering the retention and use of that current "tube" in the front end is make a test attempt using a mica :compression type of variable capacitor [***] temporarily solder tacked in and replacing the mentioned problematic cap. Use an insulated phenolic :tool to run thru min / max capacitance adjustments to see where the balance in performance is between BC and SW stability / performance, :then being able to select that value of fixed capacitor if all works well with that capacitance value. : : : :[***] Such as is used in pairs, for the top variable cap tuning of the larger older generation IF transformers. : : : : : :73's de Edd : : |
:: :: ::I think that might possibly be attributable to that "plain vanilla" dual triode that is now sitting in the mixer osc position.......see its dupe being used in ::the tail end audio section of the receiver. Much higher and different interelectrode capacitances / elemental structuring on the "AF" tube. ::What you might try test evaluating ......considering the retention and use of that current "tube" in the front end is make a test attempt using a mica ::compression type of variable capacitor [***] temporarily solder tacked in and replacing the mentioned problematic cap. Use an insulated phenolic ::tool to run thru min / max capacitance adjustments to see where the balance in performance is between BC and SW stability / performance, ::then being able to select that value of fixed capacitor if all works well with that capacitance value. :: :: :: ::[***] Such as is used in pairs, for the top variable cap tuning of the larger older generation IF transformers. :: :: :: :: :: ::73's de Edd :: :: |
::: ::: :::I think that might possibly be attributable to that "plain vanilla" dual triode that is now sitting in the mixer osc position.......see its dupe being used in :::the tail end audio section of the receiver. Much higher and different interelectrode capacitances / elemental structuring on the "AF" tube. :::What you might try test evaluating ......considering the retention and use of that current "tube" in the front end is make a test attempt using a mica :::compression type of variable capacitor [***] temporarily solder tacked in and replacing the mentioned problematic cap. Use an insulated phenolic :::tool to run thru min / max capacitance adjustments to see where the balance in performance is between BC and SW stability / performance, :::then being able to select that value of fixed capacitor if all works well with that capacitance value. ::: ::: ::: :::[***] Such as is used in pairs, for the top variable cap tuning of the larger older generation IF transformers. ::: ::: ::: ::: ::: :::73's de Edd ::: ::: |
:::: :::: ::::I think that might possibly be attributable to that "plain vanilla" dual triode that is now sitting in the mixer osc position.......see its dupe being used in ::::the tail end audio section of the receiver. Much higher and different interelectrode capacitances / elemental structuring on the "AF" tube. ::::What you might try test evaluating ......considering the retention and use of that current "tube" in the front end is make a test attempt using a mica ::::compression type of variable capacitor [***] temporarily solder tacked in and replacing the mentioned problematic cap. Use an insulated phenolic ::::tool to run thru min / max capacitance adjustments to see where the balance in performance is between BC and SW stability / performance, ::::then being able to select that value of fixed capacitor if all works well with that capacitance value. :::: :::: :::: ::::[***] Such as is used in pairs, for the top variable cap tuning of the larger older generation IF transformers. :::: :::: :::: :::: :::: ::::73's de Edd :::: :::: |
::::: ::::: :::::I think that might possibly be attributable to that "plain vanilla" dual triode that is now sitting in the mixer osc position.......see its dupe being used in :::::the tail end audio section of the receiver. Much higher and different interelectrode capacitances / elemental structuring on the "AF" tube. :::::What you might try test evaluating ......considering the retention and use of that current "tube" in the front end is make a test attempt using a mica :::::compression type of variable capacitor [***] temporarily solder tacked in and replacing the mentioned problematic cap. Use an insulated phenolic :::::tool to run thru min / max capacitance adjustments to see where the balance in performance is between BC and SW stability / performance, :::::then being able to select that value of fixed capacitor if all works well with that capacitance value. ::::: ::::: ::::: :::::[***] Such as is used in pairs, for the top variable cap tuning of the larger older generation IF transformers. ::::: ::::: ::::: ::::: ::::: :::::73's de Edd ::::: ::::: |
:::::: :::::: ::::::I think that might possibly be attributable to that "plain vanilla" dual triode that is now sitting in the mixer osc position.......see its dupe being used in ::::::the tail end audio section of the receiver. Much higher and different interelectrode capacitances / elemental structuring on the "AF" tube. ::::::What you might try test evaluating ......considering the retention and use of that current "tube" in the front end is make a test attempt using a mica ::::::compression type of variable capacitor [***] temporarily solder tacked in and replacing the mentioned problematic cap. Use an insulated phenolic ::::::tool to run thru min / max capacitance adjustments to see where the balance in performance is between BC and SW stability / performance, ::::::then being able to select that value of fixed capacitor if all works well with that capacitance value. :::::: :::::: :::::: ::::::[***] Such as is used in pairs, for the top variable cap tuning of the larger older generation IF transformers. :::::: :::::: :::::: :::::: :::::: ::::::73's de Edd :::::: :::::: |
::::::: ::::::: :::::::I think that might possibly be attributable to that "plain vanilla" dual triode that is now sitting in the mixer osc position.......see its dupe being used in :::::::the tail end audio section of the receiver. Much higher and different interelectrode capacitances / elemental structuring on the "AF" tube. :::::::What you might try test evaluating ......considering the retention and use of that current "tube" in the front end is make a test attempt using a mica :::::::compression type of variable capacitor [***] temporarily solder tacked in and replacing the mentioned problematic cap. Use an insulated phenolic :::::::tool to run thru min / max capacitance adjustments to see where the balance in performance is between BC and SW stability / performance, :::::::then being able to select that value of fixed capacitor if all works well with that capacitance value. ::::::: ::::::: ::::::: :::::::[***] Such as is used in pairs, for the top variable cap tuning of the larger older generation IF transformers. ::::::: ::::::: ::::::: ::::::: ::::::: :::::::73's de Edd ::::::: ::::::: |
:::::::: :::::::: ::::::::I think that might possibly be attributable to that "plain vanilla" dual triode that is now sitting in the mixer osc position.......see its dupe being used in ::::::::the tail end audio section of the receiver. Much higher and different interelectrode capacitances / elemental structuring on the "AF" tube. ::::::::What you might try test evaluating ......considering the retention and use of that current "tube" in the front end is make a test attempt using a mica ::::::::compression type of variable capacitor [***] temporarily solder tacked in and replacing the mentioned problematic cap. Use an insulated phenolic ::::::::tool to run thru min / max capacitance adjustments to see where the balance in performance is between BC and SW stability / performance, ::::::::then being able to select that value of fixed capacitor if all works well with that capacitance value. :::::::: :::::::: :::::::: ::::::::[***] Such as is used in pairs, for the top variable cap tuning of the larger older generation IF transformers. :::::::: :::::::: :::::::: :::::::: :::::::: ::::::::73's de Edd :::::::: :::::::: |
:::::::: :::::::: ::::::::I think that might possibly be attributable to that "plain vanilla" dual triode that is now sitting in the mixer osc position.......see its dupe being used in ::::::::the tail end audio section of the receiver. Much higher and different interelectrode capacitances / elemental structuring on the "AF" tube. ::::::::What you might try test evaluating ......considering the retention and use of that current "tube" in the front end is make a test attempt using a mica ::::::::compression type of variable capacitor [***] temporarily solder tacked in and replacing the mentioned problematic cap. Use an insulated phenolic ::::::::tool to run thru min / max capacitance adjustments to see where the balance in performance is between BC and SW stability / performance, ::::::::then being able to select that value of fixed capacitor if all works well with that capacitance value. :::::::: :::::::: :::::::: ::::::::[***] Such as is used in pairs, for the top variable cap tuning of the larger older generation IF transformers. :::::::: :::::::: :::::::: :::::::: :::::::: ::::::::73's de Edd :::::::: :::::::: |
::::::::: ::::::::: :::::::::I think that might possibly be attributable to that "plain vanilla" dual triode that is now sitting in the mixer osc position.......see its dupe being used in :::::::::the tail end audio section of the receiver. Much higher and different interelectrode capacitances / elemental structuring on the "AF" tube. :::::::::What you might try test evaluating ......considering the retention and use of that current "tube" in the front end is make a test attempt using a mica :::::::::compression type of variable capacitor [***] temporarily solder tacked in and replacing the mentioned problematic cap. Use an insulated phenolic :::::::::tool to run thru min / max capacitance adjustments to see where the balance in performance is between BC and SW stability / performance, :::::::::then being able to select that value of fixed capacitor if all works well with that capacitance value. ::::::::: ::::::::: ::::::::: :::::::::[***] Such as is used in pairs, for the top variable cap tuning of the larger older generation IF transformers. ::::::::: ::::::::: ::::::::: ::::::::: ::::::::: :::::::::73's de Edd ::::::::: ::::::::: |
:::::::::: :::::::::: ::::::::::I think that might possibly be attributable to that "plain vanilla" dual triode that is now sitting in the mixer osc position.......see its dupe being used in ::::::::::the tail end audio section of the receiver. Much higher and different interelectrode capacitances / elemental structuring on the "AF" tube. ::::::::::What you might try test evaluating ......considering the retention and use of that current "tube" in the front end is make a test attempt using a mica ::::::::::compression type of variable capacitor [***] temporarily solder tacked in and replacing the mentioned problematic cap. Use an insulated phenolic ::::::::::tool to run thru min / max capacitance adjustments to see where the balance in performance is between BC and SW stability / performance, ::::::::::then being able to select that value of fixed capacitor if all works well with that capacitance value. :::::::::: :::::::::: :::::::::: ::::::::::[***] Such as is used in pairs, for the top variable cap tuning of the larger older generation IF transformers. :::::::::: :::::::::: :::::::::: :::::::::: :::::::::: ::::::::::73's de Edd :::::::::: :::::::::: |