Are you sure R18 is 16K and not 160K? Should be the higher value. A lower resistance here will reduce gain but not cause other problems. Wattage rating doesn't matter.

C31 wouldn't make a difference.

The 75 usually needs shielding. If there is a spring around the grid wire it needs to be grounded. The 75 should also have a tube shield grounded to chassis.

http://www.nostalgiaair.org/PagesByModel/277/M0013277.pdf

Norm

:Well I switched over to a Philco 39-17 that I have set aside over a year ago to see if I can finally get
:it going. I have gotten to a point where I have reception on two strong stations nearby.
: But I have a pretty annoying hum that varies with the volume control setting but never goes away but does lessen when volume is turned down.
: I have questions that perhaps may help solve this.
:This morning I noticed studying the schematic (both Beitman and Riders) that R18
:in the description says it is a 16K 1/2 watt and on the schematic drawing it says 160K. I did not have time to check which size resistor is actually in the set right now but I have replaced two resistors in the set that tested out of tolerance and I think this was one of them... I will have to check after my
:day job today.
: The other point about this set is that C-31 is listed as two .01 caps in the description as well as on the schematic. But in the set the Bakelite block
:indicates it as being a 3903 ODG rather than the 3903 DG. The ODG calls for a double .015 and the DG is a double .01. Would it make any difference which cap I used in it? I went with the .015's as I was out of .01's and would this cause any problems with hum?
: I also noticed that the grid of the #75 tube indicates that it has a spring on the grid lead. Could
:problems here lead to hum if not grounded properly?
: I think I remember that if certain tubes are not shielded they can cause hum. I do not see any indication on this schematic that shows any shielded tubes. Is this correct?
:Thanks<, Bob
:
:
:

:Hi Bob
:
: Are you sure R18 is 16K and not 160K? Should be the higher value. A lower resistance here will reduce gain but not cause other problems. Wattage rating doesn't matter.
:
: C31 wouldn't make a difference.
:
: The 75 usually needs shielding. If there is a spring around the grid wire it needs to be grounded. The 75 should also have a tube shield grounded to chassis.
:
:http://www.nostalgiaair.org/PagesByModel/277/M0013277.pdf
:
:Norm
:
:
:
:
:
::Well I switched over to a Philco 39-17 that I have set aside over a year ago to see if I can finally get
::it going. I have gotten to a point where I have reception on two strong stations nearby.
:: But I have a pretty annoying hum that varies with the volume control setting but never goes away but does lessen when volume is turned down.
:: I have questions that perhaps may help solve this.
::This morning I noticed studying the schematic (both Beitman and Riders) that R18
::in the description says it is a 16K 1/2 watt and on the schematic drawing it says 160K. I did not have time to check which size resistor is actually in the set right now but I have replaced two resistors in the set that tested out of tolerance and I think this was one of them... I will have to check after my
::day job today.
:: The other point about this set is that C-31 is listed as two .01 caps in the description as well as on the schematic. But in the set the Bakelite block
::indicates it as being a 3903 ODG rather than the 3903 DG. The ODG calls for a double .015 and the DG is a double .01. Would it make any difference which cap I used in it? I went with the .015's as I was out of .01's and would this cause any problems with hum?
:: I also noticed that the grid of the #75 tube indicates that it has a spring on the grid lead. Could
::problems here lead to hum if not grounded properly?
:: I think I remember that if certain tubes are not shielded they can cause hum. I do not see any indication on this schematic that shows any shielded tubes. Is this correct?
::Thanks<, Bob
::
::
::

I didn't see anything on the schematic showing your 75 should be shielded either. Most radios have this tube shielded. If not there will be hum unless the tube is between metal cans in the radio?

Try a shield. Use aluminum foil grounded to the chassis for testing.

Norm

:Norm,
:No I am not sure which value R18 "should" be. I am not sure which one is in there right now either until I get home. I remember having some trouble reading that Philco Cap until I remembered that it had a different coding system and I was confused as to which value it really was until I referred to some info about how Philco coded them back then. Anyway I think it reads Body, Tip, Spot but if you read it Tip Body Spot then those two figures can be 16000 or 160000. I think anyway. Maybe thats where the typo came in on the schematic. I will make sure the 160K
:is the one in there.
: Is there something on this schematic that indicates that the #75 is shielded? If so where do you see that? I think I have seen shielding indications on other schematics but not on this one?
:Thanks again for helping me.
:Bob
:
:
::Hi Bob
::
:: Are you sure R18 is 16K and not 160K? Should be the higher value. A lower resistance here will reduce gain but not cause other problems. Wattage rating doesn't matter.
::
:: C31 wouldn't make a difference.
::
:: The 75 usually needs shielding. If there is a spring around the grid wire it needs to be grounded. The 75 should also have a tube shield grounded to chassis.
::
::http://www.nostalgiaair.org/PagesByModel/277/M0013277.pdf
::
::Norm
::
::
::
::
::
:::Well I switched over to a Philco 39-17 that I have set aside over a year ago to see if I can finally get
:::it going. I have gotten to a point where I have reception on two strong stations nearby.
::: But I have a pretty annoying hum that varies with the volume control setting but never goes away but does lessen when volume is turned down.
::: I have questions that perhaps may help solve this.
:::This morning I noticed studying the schematic (both Beitman and Riders) that R18
:::in the description says it is a 16K 1/2 watt and on the schematic drawing it says 160K. I did not have time to check which size resistor is actually in the set right now but I have replaced two resistors in the set that tested out of tolerance and I think this was one of them... I will have to check after my
:::day job today.
::: The other point about this set is that C-31 is listed as two .01 caps in the description as well as on the schematic. But in the set the Bakelite block
:::indicates it as being a 3903 ODG rather than the 3903 DG. The ODG calls for a double .015 and the DG is a double .01. Would it make any difference which cap I used in it? I went with the .015's as I was out of .01's and would this cause any problems with hum?
::: I also noticed that the grid of the #75 tube indicates that it has a spring on the grid lead. Could
:::problems here lead to hum if not grounded properly?
::: I think I remember that if certain tubes are not shielded they can cause hum. I do not see any indication on this schematic that shows any shielded tubes. Is this correct?
:::Thanks<, Bob
:::
:::
:::

:Bob
:
: I didn't see anything on the schematic showing your 75 should be shielded either. Most radios have this tube shielded. If not there will be hum unless the tube is between metal cans in the radio?
:
: Try a shield. Use aluminum foil grounded to the chassis for testing.
:
:Norm
:
::Norm,
::No I am not sure which value R18 "should" be. I am not sure which one is in there right now either until I get home. I remember having some trouble reading that Philco Cap until I remembered that it had a different coding system and I was confused as to which value it really was until I referred to some info about how Philco coded them back then. Anyway I think it reads Body, Tip, Spot but if you read it Tip Body Spot then those two figures can be 16000 or 160000. I think anyway. Maybe thats where the typo came in on the schematic. I will make sure the 160K
::is the one in there.
:: Is there something on this schematic that indicates that the #75 is shielded? If so where do you see that? I think I have seen shielding indications on other schematics but not on this one?
::Thanks again for helping me.
::Bob
::
::
:::Hi Bob
:::
::: Are you sure R18 is 16K and not 160K? Should be the higher value. A lower resistance here will reduce gain but not cause other problems. Wattage rating doesn't matter.
:::
::: C31 wouldn't make a difference.
:::
::: The 75 usually needs shielding. If there is a spring around the grid wire it needs to be grounded. The 75 should also have a tube shield grounded to chassis.
:::
:::http://www.nostalgiaair.org/PagesByModel/277/M0013277.pdf
:::
:::Norm
:::
:::
:::
:::
:::
::::Well I switched over to a Philco 39-17 that I have set aside over a year ago to see if I can finally get
::::it going. I have gotten to a point where I have reception on two strong stations nearby.
:::: But I have a pretty annoying hum that varies with the volume control setting but never goes away but does lessen when volume is turned down.
:::: I have questions that perhaps may help solve this.
::::This morning I noticed studying the schematic (both Beitman and Riders) that R18
::::in the description says it is a 16K 1/2 watt and on the schematic drawing it says 160K. I did not have time to check which size resistor is actually in the set right now but I have replaced two resistors in the set that tested out of tolerance and I think this was one of them... I will have to check after my
::::day job today.
:::: The other point about this set is that C-31 is listed as two .01 caps in the description as well as on the schematic. But in the set the Bakelite block
::::indicates it as being a 3903 ODG rather than the 3903 DG. The ODG calls for a double .015 and the DG is a double .01. Would it make any difference which cap I used in it? I went with the .015's as I was out of .01's and would this cause any problems with hum?
:::: I also noticed that the grid of the #75 tube indicates that it has a spring on the grid lead. Could
::::problems here lead to hum if not grounded properly?
:::: I think I remember that if certain tubes are not shielded they can cause hum. I do not see any indication on this schematic that shows any shielded tubes. Is this correct?
::::Thanks<, Bob
::::
::::
::::

:Bob
:
: I didn't see anything on the schematic showing your 75 should be shielded either. Most radios have this tube shielded. If not there will be hum unless the tube is between metal cans in the radio?
:
: Try a shield. Use aluminum foil grounded to the chassis for testing.
:
:Norm
:
::Norm,
::No I am not sure which value R18 "should" be. I am not sure which one is in there right now either until I get home. I remember having some trouble reading that Philco Cap until I remembered that it had a different coding system and I was confused as to which value it really was until I referred to some info about how Philco coded them back then. Anyway I think it reads Body, Tip, Spot but if you read it Tip Body Spot then those two figures can be 16000 or 160000. I think anyway. Maybe thats where the typo came in on the schematic. I will make sure the 160K
::is the one in there.
:: Is there something on this schematic that indicates that the #75 is shielded? If so where do you see that? I think I have seen shielding indications on other schematics but not on this one?
::Thanks again for helping me.
::Bob
::
::
:::Hi Bob
:::
::: Are you sure R18 is 16K and not 160K? Should be the higher value. A lower resistance here will reduce gain but not cause other problems. Wattage rating doesn't matter.
:::
::: C31 wouldn't make a difference.
:::
::: The 75 usually needs shielding. If there is a spring around the grid wire it needs to be grounded. The 75 should also have a tube shield grounded to chassis.
:::
:::http://www.nostalgiaair.org/PagesByModel/277/M0013277.pdf
:::
:::Norm
:::
:::
:::
:::
:::
::::Well I switched over to a Philco 39-17 that I have set aside over a year ago to see if I can finally get
::::it going. I have gotten to a point where I have reception on two strong stations nearby.
:::: But I have a pretty annoying hum that varies with the volume control setting but never goes away but does lessen when volume is turned down.
:::: I have questions that perhaps may help solve this.
::::This morning I noticed studying the schematic (both Beitman and Riders) that R18
::::in the description says it is a 16K 1/2 watt and on the schematic drawing it says 160K. I did not have time to check which size resistor is actually in the set right now but I have replaced two resistors in the set that tested out of tolerance and I think this was one of them... I will have to check after my
::::day job today.
:::: The other point about this set is that C-31 is listed as two .01 caps in the description as well as on the schematic. But in the set the Bakelite block
::::indicates it as being a 3903 ODG rather than the 3903 DG. The ODG calls for a double .015 and the DG is a double .01. Would it make any difference which cap I used in it? I went with the .015's as I was out of .01's and would this cause any problems with hum?
:::: I also noticed that the grid of the #75 tube indicates that it has a spring on the grid lead. Could
::::problems here lead to hum if not grounded properly?
:::: I think I remember that if certain tubes are not shielded they can cause hum. I do not see any indication on this schematic that shows any shielded tubes. Is this correct?
::::Thanks<, Bob
::::
::::
::::

Maybe the grid spring lug isn't grounded to chassis? Whatever stops the hum should be ok.

Norm

:
:Norm,
:I attached a wire to the body of the shield and attached it to the same lug that the grid spring
:insulation is attached to and it did not stop the hum?!
:I tried the foil trick and noticed that it would only stop the hum on the larger of the two can caps that
:are sitting on top of the chassis...I do not know what this should tell me? Should I run a wire from the shield to the underside of the chassis and hook it to the old can cap workings underneath?
:Bob
:
::Bob
::
:: I didn't see anything on the schematic showing your 75 should be shielded either. Most radios have this tube shielded. If not there will be hum unless the tube is between metal cans in the radio?
::
:: Try a shield. Use aluminum foil grounded to the chassis for testing.
::
::Norm
::
:::Norm,
:::No I am not sure which value R18 "should" be. I am not sure which one is in there right now either until I get home. I remember having some trouble reading that Philco Cap until I remembered that it had a different coding system and I was confused as to which value it really was until I referred to some info about how Philco coded them back then. Anyway I think it reads Body, Tip, Spot but if you read it Tip Body Spot then those two figures can be 16000 or 160000. I think anyway. Maybe thats where the typo came in on the schematic. I will make sure the 160K
:::is the one in there.
::: Is there something on this schematic that indicates that the #75 is shielded? If so where do you see that? I think I have seen shielding indications on other schematics but not on this one?
:::Thanks again for helping me.
:::Bob
:::
:::
::::Hi Bob
::::
:::: Are you sure R18 is 16K and not 160K? Should be the higher value. A lower resistance here will reduce gain but not cause other problems. Wattage rating doesn't matter.
::::
:::: C31 wouldn't make a difference.
::::
:::: The 75 usually needs shielding. If there is a spring around the grid wire it needs to be grounded. The 75 should also have a tube shield grounded to chassis.
::::
::::http://www.nostalgiaair.org/PagesByModel/277/M0013277.pdf
::::
::::Norm
::::
::::
::::
::::
::::
:::::Well I switched over to a Philco 39-17 that I have set aside over a year ago to see if I can finally get
:::::it going. I have gotten to a point where I have reception on two strong stations nearby.
::::: But I have a pretty annoying hum that varies with the volume control setting but never goes away but does lessen when volume is turned down.
::::: I have questions that perhaps may help solve this.
:::::This morning I noticed studying the schematic (both Beitman and Riders) that R18
:::::in the description says it is a 16K 1/2 watt and on the schematic drawing it says 160K. I did not have time to check which size resistor is actually in the set right now but I have replaced two resistors in the set that tested out of tolerance and I think this was one of them... I will have to check after my
:::::day job today.
::::: The other point about this set is that C-31 is listed as two .01 caps in the description as well as on the schematic. But in the set the Bakelite block
:::::indicates it as being a 3903 ODG rather than the 3903 DG. The ODG calls for a double .015 and the DG is a double .01. Would it make any difference which cap I used in it? I went with the .015's as I was out of .01's and would this cause any problems with hum?
::::: I also noticed that the grid of the #75 tube indicates that it has a spring on the grid lead. Could
:::::problems here lead to hum if not grounded properly?
::::: I think I remember that if certain tubes are not shielded they can cause hum. I do not see any indication on this schematic that shows any shielded tubes. Is this correct?
:::::Thanks<, Bob
:::::
:::::
:::::

:Bob
:
: Maybe the grid spring lug isn't grounded to chassis? Whatever stops the hum should be ok.
:
:Norm
:
::
::Norm,
::I attached a wire to the body of the shield and attached it to the same lug that the grid spring
::insulation is attached to and it did not stop the hum?!
::I tried the foil trick and noticed that it would only stop the hum on the larger of the two can caps that
::are sitting on top of the chassis...I do not know what this should tell me? Should I run a wire from the shield to the underside of the chassis and hook it to the old can cap workings underneath?
::Bob

:Of the 2 lytics according to the print, only the can of the 4uf unit is grounded to the chassis. The 12uf units can is connected to the HV center tap and only connected to chassis ground through 320 ohms of resistors 24 and 25. That may be why only one is effective in shielding.
:
::Bob
::
:: Maybe the grid spring lug isn't grounded to chassis? Whatever stops the hum should be ok.
::
::Norm
::
:::
:::Norm,
:::I attached a wire to the body of the shield and attached it to the same lug that the grid spring
:::insulation is attached to and it did not stop the hum?!
:::I tried the foil trick and noticed that it would only stop the hum on the larger of the two can caps that
:::are sitting on top of the chassis...I do not know what this should tell me? Should I run a wire from the shield to the underside of the chassis and hook it to the old can cap workings underneath?
:::Bob
:

:Of the 2 lytics according to the print, only the can of the 4uf unit is grounded to the chassis. The 12uf units can is connected to the HV center tap and only connected to chassis ground through 320 ohms of resistors 24 and 25. That may be why only one is effective in shielding.
:
::Bob
::
:: Maybe the grid spring lug isn't grounded to chassis? Whatever stops the hum should be ok.
::
::Norm
::
:::
:::Norm,
:::I attached a wire to the body of the shield and attached it to the same lug that the grid spring
:::insulation is attached to and it did not stop the hum?!
:::I tried the foil trick and noticed that it would only stop the hum on the larger of the two can caps that
:::are sitting on top of the chassis...I do not know what this should tell me? Should I run a wire from the shield to the underside of the chassis and hook it to the old can cap workings underneath?
:::Bob
:

:Is it because it would then be connected to the most negative connection on the set being the center tap?
:Still thinking here....
:Bob :O)
:
::Of the 2 lytics according to the print, only the can of the 4uf unit is grounded to the chassis. The 12uf units can is connected to the HV center tap and only connected to chassis ground through 320 ohms of resistors 24 and 25. That may be why only one is effective in shielding.
::
:::Bob
:::
::: Maybe the grid spring lug isn't grounded to chassis? Whatever stops the hum should be ok.
:::
:::Norm
:::
::::
::::Norm,
::::I attached a wire to the body of the shield and attached it to the same lug that the grid spring
::::insulation is attached to and it did not stop the hum?!
::::I tried the foil trick and noticed that it would only stop the hum on the larger of the two can caps that
::::are sitting on top of the chassis...I do not know what this should tell me? Should I run a wire from the shield to the underside of the chassis and hook it to the old can cap workings underneath?
::::Bob
::

Hee..hee…considering that unit to be using the older canned electrolytics, the last response may be the logic of the loss of the hum, since you

typically would have shorted out a bias supply when your foil shielding bridged over to chassis ground from the ‘lytic can [Item # 27].

Whenever one sees a power supply of that design , and you see that the center tap of the high voltage winging is NOT going directly to ground,

you can figure that they are wanting to acquire a derived NEGATIVE voltage by holding that centertap away from ground level by the insertion

of a selected value of resistor in that node……in this case two series resistors, since they are wanting to derive TWO different levels of voltage.

Consulting the mark-up you can see the series 250 and 70 ohm power resistors are acquiring negative bias for the AF front end stages at

reference point B [within the YELLOW buss mark up route] and also bias for the AF output stage at point A [within the GREEN buss markup route]

In operation, the total B+ drain of the set flowing thru the power supply will be passing thru those two resistors, so, by using initial data of current

consumption on hand they computed the value of resistance for each resistor to be… in order to have that level of voltage being developed across

each of those two nodes…..in this case since it is merely a biasing voltage …the extra drain diverted off from those nodes is ultra- infinitesimally small.



I suspicion the integrity of your input electrolytic [Item # 27] in the set as being your problem , with some degree of additional possibility of the second unit

[Item # 28] also having some degree of effect.


Initially power up the set for warm-up and take DC metering in hand and power up the set and stab the neg probe to chassis ground to make one

connection, and then bite into the external aluminum can of [Item # 27] with the metering positive probe and confirm that you are then metering a negative

voltage there. Confirming that, take a fresh dry electrolytic of adequate voltage rating in the10-12-15-20-22 ufd range and it would only need to be ~50VDC

range, since the next testing would be merely further filtering of the bias voltage being developed . ….no hundreds of volts are going to be HERE.
You would then have the sub electrolytics POSITIVE lead going to chassis ground and its NEGATIVE lead would go to the external aluminum can of [Item # 27].

If that hum abated appreciably, or disappeared, a high ESR of that sets electrolytic [Item # 27] would be suspect. At the same testing period flip the chassis to get

access to node B at the top of the 70 ohm resistor and shunt the test filter across that point with the POSITIVE lead going to chassis ground and its NEGATIVE

lead going to the resistor and listening for hum abatement.

If these two tests give degrees of improvement, you would then want to replace those two [Items # 27 & # 28] by replacement with individual electrolytics, and with them being
of the larger ~450VDC rating with the old cans being OUT of circuit connection wise….but possibly being left mechanically for appearance sake, as is probably desired.


Referencing:



73's de Edd

:That might be the reason although I would have thought the chassis would be the best spot for a shield. I like your logic.
:
::Is it because it would then be connected to the most negative connection on the set being the center tap?
::Still thinking here....
::Bob :O)
::
:::Of the 2 lytics according to the print, only the can of the 4uf unit is grounded to the chassis. The 12uf units can is connected to the HV center tap and only connected to chassis ground through 320 ohms of resistors 24 and 25. That may be why only one is effective in shielding.
:::
::::Bob
::::
:::: Maybe the grid spring lug isn't grounded to chassis? Whatever stops the hum should be ok.
::::
::::Norm
::::
:::::
:::::Norm,
:::::I attached a wire to the body of the shield and attached it to the same lug that the grid spring
:::::insulation is attached to and it did not stop the hum?!
:::::I tried the foil trick and noticed that it would only stop the hum on the larger of the two can caps that
:::::are sitting on top of the chassis...I do not know what this should tell me? Should I run a wire from the shield to the underside of the chassis and hook it to the old can cap workings underneath?
:::::Bob
:::

C27 is now a 22 mfd and C28 is a 4.7 mfd.

The radio plays beautifully the way it is right now.

The thing I was wondering about was why I had to connect a wire (or tin foil) from the "old" C27 can
to the tube shield which I robbed from another set (so it does not have a base to slide over and make connection with).

If I ground that shield to the chassis it will not stop the hum.

There is still a connection to the nut that holds the old can to the chassis. This is (1 meg) R19 and then there is a jumper wire from that same connection to
a lug where it connects to the (250 Ohm) R24 and also the center tap.

A connection to the #75 tube shield from these connections is what stops hum and makes the set sound great!

Now I do not know if this correct or incorrect of if perhaps dangerous in some way?

...or if I should just leave it alone and and call it finished?

I have a suspicion that
there could be a problem in wiring it this way but I do not know what to do to mend it.
Thanks, Bob

:
:
:
:
:Hee..hee…considering that unit to be using the older canned electrolytics, the last response may be the logic of the loss of the hum, since you
:
: typically would have shorted out a bias supply when your foil shielding bridged over to chassis ground from the ‘lytic can [Item # 27].
:
:Whenever one sees a power supply of that design , and you see that the center tap of the high voltage winging is NOT going directly to ground,
:
:you can figure that they are wanting to acquire a derived NEGATIVE voltage by holding that centertap away from ground level by the insertion
:
:of a selected value of resistor in that node……in this case two series resistors, since they are wanting to derive TWO different levels of voltage.
:
:
:Consulting the mark-up you can see the series 250 and 70 ohm power resistors are acquiring negative bias for the AF front end stages at
:
:reference point B [within the YELLOW buss mark up route] and also bias for the AF output stage at point A [within the GREEN buss markup route]
:
:In operation, the total B+ drain of the set flowing thru the power supply will be passing thru those two resistors, so, by using initial data of current
:
:consumption on hand they computed the value of resistance for each resistor to be… in order to have that level of voltage being developed across
:
:each of those two nodes…..in this case since it is merely a biasing voltage …the extra drain diverted off from those nodes is ultra- infinitesimally small.
:
:
:
:I suspicion the integrity of your input electrolytic [Item # 27] in the set as being your problem , with some degree of additional possibility of the second unit
:
:[Item # 28] also having some degree of effect.
:
:
:Initially power up the set for warm-up and take DC metering in hand and power up the set and stab the neg probe to chassis ground to make one
:
:connection, and then bite into the external aluminum can of [Item # 27] with the metering positive probe and confirm that you are then metering a negative
:
:voltage there. Confirming that, take a fresh dry electrolytic of adequate voltage rating in the10-12-15-20-22 ufd range and it would only need to be ~50VDC
:
:range, since the next testing would be merely further filtering of the bias voltage being developed . ….no hundreds of volts are going to be HERE.
: You would then have the sub electrolytics POSITIVE lead going to chassis ground and its NEGATIVE lead would go to the external aluminum can of [Item # 27].
:
:If that hum abated appreciably, or disappeared, a high ESR of that sets electrolytic [Item # 27] would be suspect. At the same testing period flip the chassis to get
:
:access to node B at the top of the 70 ohm resistor and shunt the test filter across that point with the POSITIVE lead going to chassis ground and its NEGATIVE
:
:lead going to the resistor and listening for hum abatement.
:
:If these two tests give degrees of improvement, you would then want to replace those two [Items # 27 & # 28] by replacement with individual electrolytics, and with them being
:of the larger ~450VDC rating with the old cans being OUT of circuit connection wise….but possibly being left mechanically for appearance sake, as is probably desired.
:
:
:Referencing:
:
:
:
:73's de Edd
:
:
:
:
:
:
:
:
::That might be the reason although I would have thought the chassis would be the best spot for a shield. I like your logic.
::
:::Is it because it would then be connected to the most negative connection on the set being the center tap?
:::Still thinking here....
:::Bob :O)
:::
::::Of the 2 lytics according to the print, only the can of the 4uf unit is grounded to the chassis. The 12uf units can is connected to the HV center tap and only connected to chassis ground through 320 ohms of resistors 24 and 25. That may be why only one is effective in shielding.
::::
:::::Bob
:::::
::::: Maybe the grid spring lug isn't grounded to chassis? Whatever stops the hum should be ok.
:::::
:::::Norm
:::::
::::::
::::::Norm,
::::::I attached a wire to the body of the shield and attached it to the same lug that the grid spring
::::::insulation is attached to and it did not stop the hum?!
::::::I tried the foil trick and noticed that it would only stop the hum on the larger of the two can caps that
::::::are sitting on top of the chassis...I do not know what this should tell me? Should I run a wire from the shield to the underside of the chassis and hook it to the old can cap workings underneath?
::::::Bob
::::

OK on the pristine 'lytics.

I was just working with the info on hand.... in this particular thread.... and that fact just might have been touched on in another post on a separate thread in the past.

Now in just looking at the portion where you mentioned:

I also noticed that the grid of the #75 tube indicates that it has a spring on the grid lead. Could
problems here lead to hum if not grounded properly?




Since I am more into, and have more 6BE6-6AV6-6AQ5 and 6SA7-6SQ7-6V6 oriented sets than the mentioned 6A7-75-41 sets, I don't remember the precise mechanics on the way that 75 wiring of the 1st grid is handled from under the chassis at the connection point of the 4meg and .01 coupling capacitor and that wires routing upwards from circa-center chassis and on up to connection of its grid cap that connects to that 75 tube.
Is that wire concentrically centered within a long tightly closewound coil spring, that extends from coming out of the hole in the chassis, where it is soldered to chassis ground, and covers all the length of the grid wire length up to ~ 1/8<--->1/4 inch of that grid cap connector ?

OR is the wire one that is shielded with a common tight braided metal mesh shielding that covers/shields it in a like manner. I just didn't remember how Philco handled their shielding in that particular sets appliction...and don't have one here to eyeball.
I just now want confirmation that there is no cotton/(or plastic) covered wire that is going from the connection node under chassis and on up to the 75's grid cap connector, and thus, with that line being unshielded.

I do not see any indication on this schematic that shows any shielded tubes. Is this correct?

Most typical schematic notations have a broken dotted line/arcs encircling the periphery of the envelope symbol with a ground symbol connected to that shield symbolization...and this units schema does not show that.

Tony J.:

Certainly couldn't hurt on the filtering aspect of that bias source ....considering that there were no actual deficiencies in the prime 2 main electrolytic filtering components.
I just have not fully compared and related the situation to all of the other chassis variants that they eventually used.




73's de Edd






:Edd,
:Thank You for your extensive response. Do you understand that I have already replaced the electrolytic caps in this set?
:
:C27 is now a 22 mfd and C28 is a 4.7 mfd.
:
:The radio plays beautifully the way it is right now.
:
:The thing I was wondering about was why I had to connect a wire (or tin foil) from the "old" C27 can
:to the tube shield which I robbed from another set (so it does not have a base to slide over and make connection with).
:
: If I ground that shield to the chassis it will not stop the hum.
:
:There is still a connection to the nut that holds the old can to the chassis. This is (1 meg) R19 and then there is a jumper wire from that same connection to
:a lug where it connects to the (250 Ohm) R24 and also the center tap.
:
: A connection to the #75 tube shield from these connections is what stops hum and makes the set sound great!
:
:Now I do not know if this correct or incorrect of if perhaps dangerous in some way?
:
:...or if I should just leave it alone and and call it finished?
:
:I have a suspicion that
:there could be a problem in wiring it this way but I do not know what to do to mend it.
:Thanks, Bob
:
:
:
:
:
:
:
:
::
::
::
::
::Hee..hee…considering that unit to be using the older canned electrolytics, the last response may be the logic of the loss of the hum, since you
::
:: typically would have shorted out a bias supply when your foil shielding bridged over to chassis ground from the ‘lytic can [Item # 27].
::
::Whenever one sees a power supply of that design , and you see that the center tap of the high voltage winging is NOT going directly to ground,
::
::you can figure that they are wanting to acquire a derived NEGATIVE voltage by holding that centertap away from ground level by the insertion
::
::of a selected value of resistor in that node……in this case two series resistors, since they are wanting to derive TWO different levels of voltage.
::
::
::Consulting the mark-up you can see the series 250 and 70 ohm power resistors are acquiring negative bias for the AF front end stages at
::
::reference point B [within the YELLOW buss mark up route] and also bias for the AF output stage at point A [within the GREEN buss markup route]
::
::In operation, the total B+ drain of the set flowing thru the power supply will be passing thru those two resistors, so, by using initial data of current
::
::consumption on hand they computed the value of resistance for each resistor to be… in order to have that level of voltage being developed across
::
::each of those two nodes…..in this case since it is merely a biasing voltage …the extra drain diverted off from those nodes is ultra- infinitesimally small.
::
::
::
::I suspicion the integrity of your input electrolytic [Item # 27] in the set as being your problem , with some degree of additional possibility of the second unit
::
::[Item # 28] also having some degree of effect.
::
::
::Initially power up the set for warm-up and take DC metering in hand and power up the set and stab the neg probe to chassis ground to make one
::
::connection, and then bite into the external aluminum can of [Item # 27] with the metering positive probe and confirm that you are then metering a negative
::
::voltage there. Confirming that, take a fresh dry electrolytic of adequate voltage rating in the10-12-15-20-22 ufd range and it would only need to be ~50VDC
::
::range, since the next testing would be merely further filtering of the bias voltage being developed . ….no hundreds of volts are going to be HERE.
:: You would then have the sub electrolytics POSITIVE lead going to chassis ground and its NEGATIVE lead would go to the external aluminum can of [Item # 27].
::
::If that hum abated appreciably, or disappeared, a high ESR of that sets electrolytic [Item # 27] would be suspect. At the same testing period flip the chassis to get
::
::access to node B at the top of the 70 ohm resistor and shunt the test filter across that point with the POSITIVE lead going to chassis ground and its NEGATIVE
::
::lead going to the resistor and listening for hum abatement.
::
::If these two tests give degrees of improvement, you would then want to replace those two [Items # 27 & # 28] by replacement with individual electrolytics, and with them being
::of the larger ~450VDC rating with the old cans being OUT of circuit connection wise….but possibly being left mechanically for appearance sake, as is probably desired.
::
::
::Referencing:
::
::
::
::73's de Edd
::
::
::
::
::
::
::
::
:::That might be the reason although I would have thought the chassis would be the best spot for a shield. I like your logic.
:::
::::Is it because it would then be connected to the most negative connection on the set being the center tap?
::::Still thinking here....
::::Bob :O)
::::
:::::Of the 2 lytics according to the print, only the can of the 4uf unit is grounded to the chassis. The 12uf units can is connected to the HV center tap and only connected to chassis ground through 320 ohms of resistors 24 and 25. That may be why only one is effective in shielding.
:::::
::::::Bob
::::::
:::::: Maybe the grid spring lug isn't grounded to chassis? Whatever stops the hum should be ok.
::::::
::::::Norm
::::::
:::::::
:::::::Norm,
:::::::I attached a wire to the body of the shield and attached it to the same lug that the grid spring
:::::::insulation is attached to and it did not stop the hum?!
:::::::I tried the foil trick and noticed that it would only stop the hum on the larger of the two can caps that
:::::::are sitting on top of the chassis...I do not know what this should tell me? Should I run a wire from the shield to the underside of the chassis and hook it to the old can cap workings underneath?
:::::::Bob
:::::

:
:
:
:
:OK on the pristine 'lytics.
:
:I was just working with the info on hand.... in this particular thread.... and that fact just might have been touched on in another post on a separate thread in the past.
:
:Now in just looking at the portion where you mentioned:
:
:
:
:I also noticed that the grid of the #75 tube indicates that it has a spring on the grid lead. Could
:problems here lead to hum if not grounded properly?
:
:
:
:
:Since I am more into, and have more 6BE6-6AV6-6AQ5 and 6SA7-6SQ7-6V6 oriented sets than the mentioned 6A7-75-41 sets, I don't remember the precise mechanics on the way that 75 wiring of the 1st grid is handled from under the chassis at the connection point of the 4meg and .01 coupling capacitor and that wires routing upwards from circa-center chassis and on up to connection of its grid cap that connects to that 75 tube.
: Is that wire concentrically centered within a long tightly closewound coil spring, that extends from coming out of the hole in the chassis, where it is soldered to chassis ground, and covers all the length of the grid wire length up to ~ 1/8<--->1/4 inch of that grid cap connector ?
:
:OR is the wire one that is shielded with a common tight braided metal mesh shielding that covers/shields it in a like manner. I just didn't remember how Philco handled their shielding in that particular sets appliction...and don't have one here to eyeball.
: I just now want confirmation that there is no cotton/(or plastic) covered wire that is going from the connection node under chassis and on up to the 75's grid cap connector, and thus, with that line being unshielded.
:
:
:
:
:I do not see any indication on this schematic that shows any shielded tubes. Is this correct?
:
:
:
:Most typical schematic notations have a broken dotted line/arcs encircling the periphery of the envelope symbol with a ground symbol connected to that shield symbolization...and this units schema does not show that.
:
:
:
:Tony J.:
:
:Certainly couldn't hurt on the filtering aspect of that bias source ....considering that there were no actual deficiencies in the prime 2 main electrolytic filtering components.
: I just have not fully compared and related the situation to all of the other chassis variants that they eventually used.
:
:
:
:
:73's de Edd
:
:
:
:
:
:
::Edd,
::Thank You for your extensive response. Do you understand that I have already replaced the electrolytic caps in this set?
::
::C27 is now a 22 mfd and C28 is a 4.7 mfd.
::
::The radio plays beautifully the way it is right now.
::
::The thing I was wondering about was why I had to connect a wire (or tin foil) from the "old" C27 can
::to the tube shield which I robbed from another set (so it does not have a base to slide over and make connection with).
::
:: If I ground that shield to the chassis it will not stop the hum.
::
::There is still a connection to the nut that holds the old can to the chassis. This is (1 meg) R19 and then there is a jumper wire from that same connection to
::a lug where it connects to the (250 Ohm) R24 and also the center tap.
::
:: A connection to the #75 tube shield from these connections is what stops hum and makes the set sound great!
::
::Now I do not know if this correct or incorrect of if perhaps dangerous in some way?
::
::...or if I should just leave it alone and and call it finished?
::
::I have a suspicion that
::there could be a problem in wiring it this way but I do not know what to do to mend it.
::Thanks, Bob
::
::
::
::
::
::
::
::
:::
:::
:::
:::
:::Hee..hee…considering that unit to be using the older canned electrolytics, the last response may be the logic of the loss of the hum, since you
:::
::: typically would have shorted out a bias supply when your foil shielding bridged over to chassis ground from the ‘lytic can [Item # 27].
:::
:::Whenever one sees a power supply of that design , and you see that the center tap of the high voltage winging is NOT going directly to ground,
:::
:::you can figure that they are wanting to acquire a derived NEGATIVE voltage by holding that centertap away from ground level by the insertion
:::
:::of a selected value of resistor in that node……in this case two series resistors, since they are wanting to derive TWO different levels of voltage.
:::
:::
:::Consulting the mark-up you can see the series 250 and 70 ohm power resistors are acquiring negative bias for the AF front end stages at
:::
:::reference point B [within the YELLOW buss mark up route] and also bias for the AF output stage at point A [within the GREEN buss markup route]
:::
:::In operation, the total B+ drain of the set flowing thru the power supply will be passing thru those two resistors, so, by using initial data of current
:::
:::consumption on hand they computed the value of resistance for each resistor to be… in order to have that level of voltage being developed across
:::
:::each of those two nodes…..in this case since it is merely a biasing voltage …the extra drain diverted off from those nodes is ultra- infinitesimally small.
:::
:::
:::
:::I suspicion the integrity of your input electrolytic [Item # 27] in the set as being your problem , with some degree of additional possibility of the second unit
:::
:::[Item # 28] also having some degree of effect.
:::
:::
:::Initially power up the set for warm-up and take DC metering in hand and power up the set and stab the neg probe to chassis ground to make one
:::
:::connection, and then bite into the external aluminum can of [Item # 27] with the metering positive probe and confirm that you are then metering a negative
:::
:::voltage there. Confirming that, take a fresh dry electrolytic of adequate voltage rating in the10-12-15-20-22 ufd range and it would only need to be ~50VDC
:::
:::range, since the next testing would be merely further filtering of the bias voltage being developed . ….no hundreds of volts are going to be HERE.
::: You would then have the sub electrolytics POSITIVE lead going to chassis ground and its NEGATIVE lead would go to the external aluminum can of [Item # 27].
:::
:::If that hum abated appreciably, or disappeared, a high ESR of that sets electrolytic [Item # 27] would be suspect. At the same testing period flip the chassis to get
:::
:::access to node B at the top of the 70 ohm resistor and shunt the test filter across that point with the POSITIVE lead going to chassis ground and its NEGATIVE
:::
:::lead going to the resistor and listening for hum abatement.
:::
:::If these two tests give degrees of improvement, you would then want to replace those two [Items # 27 & # 28] by replacement with individual electrolytics, and with them being
:::of the larger ~450VDC rating with the old cans being OUT of circuit connection wise….but possibly being left mechanically for appearance sake, as is probably desired.
:::
:::
:::Referencing:
:::
:::
:::
:::73's de Edd
:::
:::
:::
:::
:::
:::
:::
:::
::::That might be the reason although I would have thought the chassis would be the best spot for a shield. I like your logic.
::::
:::::Is it because it would then be connected to the most negative connection on the set being the center tap?
:::::Still thinking here....
:::::Bob :O)
:::::
::::::Of the 2 lytics according to the print, only the can of the 4uf unit is grounded to the chassis. The 12uf units can is connected to the HV center tap and only connected to chassis ground through 320 ohms of resistors 24 and 25. That may be why only one is effective in shielding.
::::::
:::::::Bob
:::::::
::::::: Maybe the grid spring lug isn't grounded to chassis? Whatever stops the hum should be ok.
:::::::
:::::::Norm
:::::::
::::::::
::::::::Norm,
::::::::I attached a wire to the body of the shield and attached it to the same lug that the grid spring
::::::::insulation is attached to and it did not stop the hum?!
::::::::I tried the foil trick and noticed that it would only stop the hum on the larger of the two can caps that
::::::::are sitting on top of the chassis...I do not know what this should tell me? Should I run a wire from the shield to the underside of the chassis and hook it to the old can cap workings underneath?
::::::::Bob
::::::