I'd like to add an RF stage to an S-38'like receiver.
I'd also like to retain the series filament arrangement, which would have to serve 6 tubes.
Can the 12SA7 converter be replaced by a 6SA7, and can a 6SK7 be used for the RF stage?
The freq range of operation is 5 to 14.5 MHz.

Thanks,
Steve

:I'd like to add an RF stage to an S-38'like receiver.
:I'd also like to retain the series filament arrangement, which would have to serve 6 tubes.
:Can the 12SA7 converter be replaced by a 6SA7, and can a 6SK7 be used for the RF stage?
:The freq range of operation is 5 to 14.5 MHz.
:
: Thanks,
: Steve
:

:I'd like to add an RF stage to an S-38'like receiver.
:I'd also like to retain the series filament arrangement, which would have to serve 6 tubes.
:Can the 12SA7 converter be replaced by a 6SA7, and can a 6SK7 be used for the RF stage?
:The freq range of operation is 5 to 14.5 MHz.
:
: Thanks,
: Steve
:
Steve, the series filament tube complement should add up close to 120 volts, your AC line voltage. If you put two 6 volt filament tubes in, you will change the voltage drop across the whole tube complement causing too high voltage and current in the tube filaments.
Those two tubes will work but, you will need to put a proper dropping resistor(ohm value and wattage in series with the tube filaments.
C Harris

::I'd like to add an RF stage to an S-38'like receiver.
::I'd also like to retain the series filament arrangement, which would have to serve 6 tubes.
::Can the 12SA7 converter be replaced by a 6SA7, and can a 6SK7 be used for the RF stage?
::The freq range of operation is 5 to 14.5 MHz.
::
:: Thanks,
:: Steve
::
Steve,

This would be an interesting project, but there are many factors you should consider.

First, the series string tubes all need to match as far as their heater current requirement is concerned.

A heater string including a 12SA7 will be intended to run at a nominal 150mA. All the tubes will be rated at this current. Power tubes run at increased heater power by dropping a higher voltage, thus a 50L6 or 35Z5. Most common signal tubes for series string radios use 12.6V rated heaters, as this provides them enough heater power to do their jobs at 150mA.

The proposed 6SA7 and 6SK7 types require 300mA heater current. So they can't just be added to the existing 150mA string.

There are a few signal tubes with 6.3V 150mA heaters. These may not be drop-in replacements, but you might get them to work OK in your radio. The loktal tubes, 7A8 and 7B7 might be made to work in place of the 12SA7 and 12SK7. There may be octal and miniature tubes that have the lower heater power requirement, as well. he lower power tubes generally have somewhat lower gain than the otherwise similar full-power tubes.

But before you start this project, there are other issues to ponder.

A main advantage of an RF stage is added selectivity ahead of the mixer. But to get this, the RF stage requires an additional tuned circuit. That is normally done using a third gang on the tuning capacitor. Adding another capacitor gang to your existing radio would not be a trivial undertaking. If an RF stage is added without the extra tuned circuit, it will do nothing for selectivity, and won't even add much gain, even less as the frequency is increased.

I wonder if your best bet is not to make a separate pre-selector to go between the radio and the antenna. This way the radio could be kept original, with no risky modifications to it.

A pre-selector could be housed in a separate box, with a dial and knobs on the front, and be powered from the AC line, or even a battery. No need to be debugging a hot chassis.

The pre-selector could have from one to three tuned circuits at the RF frequency, and possibly an RF amplifier stage to boost the signal. You would have to learn to tune using two knobs, as the pre-selector would have its own tuning knob. The gain stage could use a tube, one or more transistors, or an IC.

Another possibility would be to add a Q-multiplier on the radio's antenna tuning circuit. This could give you much of the benefit an RF stage would provide, but would not require keeping two tuning knobs synchronized.

If you want a real project, you could add all sorts of stuff driven by a microprocessor. An analog output could drive varactor diodes that would allow any additional tuned circuits to track the existing tuning capacitor. It could provide a digital frequency readout, and all sorts of other fancy stuff. You could even use it to stabilize the radio's local oscillator frequency. This would not be a trivial project, but if you are trying to find something to do with your new Arduino, it could keep you busy.

Ted

:::I'd like to add an RF stage to an S-38'like receiver.
:::I'd also like to retain the series filament arrangement, which would have to serve 6 tubes.
:::Can the 12SA7 converter be replaced by a 6SA7, and can a 6SK7 be used for the RF stage?
:::The freq range of operation is 5 to 14.5 MHz.
:::
::: Thanks,
::: Steve
:::
:Steve,
:
:This would be an interesting project, but there are many factors you should consider.
:
:First, the series string tubes all need to match as far as their heater current requirement is concerned.
:
:A heater string including a 12SA7 will be intended to run at a nominal 150mA. All the tubes will be rated at this current. Power tubes run at increased heater power by dropping a higher voltage, thus a 50L6 or 35Z5. Most common signal tubes for series string radios use 12.6V rated heaters, as this provides them enough heater power to do their jobs at 150mA.
:
:The proposed 6SA7 and 6SK7 types require 300mA heater current. So they can't just be added to the existing 150mA string.
:
:There are a few signal tubes with 6.3V 150mA heaters. These may not be drop-in replacements, but you might get them to work OK in your radio. The loktal tubes, 7A8 and 7B7 might be made to work in place of the 12SA7 and 12SK7. There may be octal and miniature tubes that have the lower heater power requirement, as well. he lower power tubes generally have somewhat lower gain than the otherwise similar full-power tubes.
:
:But before you start this project, there are other issues to ponder.
:
:A main advantage of an RF stage is added selectivity ahead of the mixer. But to get this, the RF stage requires an additional tuned circuit. That is normally done using a third gang on the tuning capacitor. Adding another capacitor gang to your existing radio would not be a trivial undertaking. If an RF stage is added without the extra tuned circuit, it will do nothing for selectivity, and won't even add much gain, even less as the frequency is increased.
:
:I wonder if your best bet is not to make a separate pre-selector to go between the radio and the antenna. This way the radio could be kept original, with no risky modifications to it.
:
:A pre-selector could be housed in a separate box, with a dial and knobs on the front, and be powered from the AC line, or even a battery. No need to be debugging a hot chassis.
:
:The pre-selector could have from one to three tuned circuits at the RF frequency, and possibly an RF amplifier stage to boost the signal. You would have to learn to tune using two knobs, as the pre-selector would have its own tuning knob. The gain stage could use a tube, one or more transistors, or an IC.
:
:Another possibility would be to add a Q-multiplier on the radio's antenna tuning circuit. This could give you much of the benefit an RF stage would provide, but would not require keeping two tuning knobs synchronized.
:
:If you want a real project, you could add all sorts of stuff driven by a microprocessor. An analog output could drive varactor diodes that would allow any additional tuned circuits to track the existing tuning capacitor. It could provide a digital frequency readout, and all sorts of other fancy stuff. You could even use it to stabilize the radio's local oscillator frequency. This would not be a trivial project, but if you are trying to find something to do with your new Arduino, it could keep you busy.
:
:Ted
:
:
Ted,
You're right about the 150 mA,300ma complication (as I answered in my last response above).
I was planning to run the antenna coupling transformer into the RF stage. Would I actually need another tuned circuit between the RF and converter stages? The freq range of interest is 5 to 14.5 MHz. Could I build a cheesy filter at the output of the RF stage? Could I avoid the filter altogether?
Is more selectivity really useful? I want to listen to low-power commercial shortwave stations.
I specifically chose to have just one band so that the front end doesn't get complicated. The preselector is more than I want to deal with.
I am surprised to hear that the RF stage won't add much gain. Why?
This project is intended as a post-retirement tube tutorial for myself. Maybe some day I'll do digital tuning with a PLL, varactors,a numerical display, etc.. But, its more likely I'll die first.

Thanks for Your Comments,
Steve

::I'd like to add an RF stage to an S-38'like receiver.
::I'd also like to retain the series filament arrangement, which would have to serve 6 tubes.
::Can the 12SA7 converter be replaced by a 6SA7, and can a 6SK7 be used for the RF stage?
::The freq range of operation is 5 to 14.5 MHz.
::
:: Thanks,
:: Steve
::
:Steve, the series filament tube complement should add up close to 120 volts, your AC line voltage. If you put two 6 volt filament tubes in, you will change the voltage drop across the whole tube complement causing too high voltage and current in the tube filaments.
:Those two tubes will work but, you will need to put a proper dropping resistor(ohm value and wattage in series with the tube filaments.
:C Harris
:

::I'd like to add an RF stage to an S-38'like receiver.
::I'd also like to retain the series filament arrangement, which would have to serve 6 tubes.
::Can the 12SA7 converter be replaced by a 6SA7, and can a 6SK7 be used for the RF stage?
::The freq range of operation is 5 to 14.5 MHz.
::
:: Thanks,
:: Steve
::
:Steve, the series filament tube complement should add up close to 120 volts, your AC line voltage. If you put two 6 volt filament tubes in, you will change the voltage drop across the whole tube complement causing too high voltage and current in the tube filaments.
:Those two tubes will work but, you will need to put a proper dropping resistor(ohm value and wattage in series with the tube filaments.
:C Harris
:
C Harris,
I was thinking of replacing the 12SA7 converter with a 6SA7, and adding a 6SK7 RF stage, so that the total voltage drop along the 6 filament string would be the same as the the drop along the original 5 filament string. But, I had a look at the datasheets and the 6.3v tubes require 300 mA, whereas the 12.6v tubes require 150 mA. So, the substitution probably won't work because I don't want to run 300mA through the four original tubes that would remain in the six tube scheme.

Thanks,
Steve

:::I'd like to add an RF stage to an S-38'like receiver.
:::I'd also like to retain the series filament arrangement, which would have to serve 6 tubes.
:::Can the 12SA7 converter be replaced by a 6SA7, and can a 6SK7 be used for the RF stage?
:::The freq range of operation is 5 to 14.5 MHz.
:::
::: Thanks,
::: Steve
:::
::Steve, the series filament tube complement should add up close to 120 volts, your AC line voltage. If you put two 6 volt filament tubes in, you will change the voltage drop across the whole tube complement causing too high voltage and current in the tube filaments.
::Those two tubes will work but, you will need to put a proper dropping resistor(ohm value and wattage in series with the tube filaments.
::C Harris
::
:C Harris,
:I was thinking of replacing the 12SA7 converter with a 6SA7, and adding a 6SK7 RF stage, so that the total voltage drop along the 6 filament string would be the same as the the drop along the original 5 filament string. But, I had a look at the datasheets and the 6.3v tubes require 300 mA, whereas the 12.6v tubes require 150 mA. So, the substitution probably won't work because I don't want to run 300mA through the four original tubes that would remain in the six tube scheme.
:
: Thanks,
: Steve

Steve:
Think of it like this: The filament of the tube has to be a certain number of Watts...by Ohm's law, 12 Volts at .150 Ampere is the same number of Watts as 6 Volts at .300 Ampere, but the number of Ohms has to be different. So, if you try to replace a 12 Volt tube with two 6 Volt tubes in a series circuit, you are going to get some very bright tube filaments, and some very dim ones. The current in a series string has to be the same for each element in the string. The Voltage can be different, as in 50L6 and 12SK7...different Voltages, but same current.
Lewis
:

::::I'd like to add an RF stage to an S-38'like receiver.
::::I'd also like to retain the series filament arrangement, which would have to serve 6 tubes.
::::Can the 12SA7 converter be replaced by a 6SA7, and can a 6SK7 be used for the RF stage?
::::The freq range of operation is 5 to 14.5 MHz.
::::
:::: Thanks,
:::: Steve

Lewis,
If I do the six tube filament string, with two 6.3 tubes and retain the two 12SK7s, the 50L6, and the 35Z5, and apply the 120v across the string,
is the voltage drop across each filament established by the filament resistance?

Thanks,
Steve
::::
:::Steve, the series filament tube complement should add up close to 120 volts, your AC line voltage. If you put two 6 volt filament tubes in, you will change the voltage drop across the whole tube complement causing too high voltage and current in the tube filaments.
:::Those two tubes will work but, you will need to put a proper dropping resistor(ohm value and wattage in series with the tube filaments.
:::C Harris
:::
::C Harris,
::I was thinking of replacing the 12SA7 converter with a 6SA7, and adding a 6SK7 RF stage, so that the total voltage drop along the 6 filament string would be the same as the the drop along the original 5 filament string. But, I had a look at the datasheets and the 6.3v tubes require 300 mA, whereas the 12.6v tubes require 150 mA. So, the substitution probably won't work because I don't want to run 300mA through the four original tubes that would remain in the six tube scheme.
::
:: Thanks,
:: Steve
:
:
:Steve:
:Think of it like this: The filament of the tube has to be a certain number of Watts...by Ohm's law, 12 Volts at .150 Ampere is the same number of Watts as 6 Volts at .300 Ampere, but the number of Ohms has to be different. So, if you try to replace a 12 Volt tube with two 6 Volt tubes in a series circuit, you are going to get some very bright tube filaments, and some very dim ones. The current in a series string has to be the same for each element in the string. The Voltage can be different, as in 50L6 and 12SK7...different Voltages, but same current.
:Lewis
::
:
:

:I'd also like to retain the series filament arrangement, which would have to serve 6 tubes.
:Can the 12SA7 converter be replaced by a 6SA7, and can a 6SK7 be used for the RF stage?

:

This thread seems to have strayed off topic, so I'll cut to the chase:

No, you can't successfully make the change that you are proposing. A series-string set must have tubes that all draw the same amount of current; in the case of the S-38's complement, this is around 150 mA. The 6SK7/6SA7 draw 300 mA.

You can think of each tube as a resistor, calculate its equivalent resistance from its spec, then use that info to play electronic design "what-if" games. The equivalent resistance of any tube will not change from application to application. The amount of current it draws, or the voltage it drops, will.

For the S38 lineup, here are the equivalent resistances for a line voltage of 120 VAC:

50L6: Req= 50/.150 = 333 ohms
35Z5: Req= 35/.150 = 233 ohms
12xxx: Req= 12/.150 = 80 ohms
6xxx: Req= 6/.300 = 20 ohms

So, for a "stock" S38, you have one 50L6 tube, one 35Z5 tube, and three 12xxx tubes in series. This is 333+233+(3*80)= 806 ohms.

Using Ohm's Law, E=IR, and I= E/R, or 120 V/806 ohms= 148 mA.

Again using Ohm's Law, the voltage "seen" by each individual tube will be as follows:

50L6: Req * Istring = 333*0.148 = 49.34 V
35Z5: (same) = 233*0.148 = 34.48 V
12xxx: (each tube) = 80* 0.148 = 11.84 V

So, all is well in S38-land if the correct tubes are used. What if we sub two "six volt" tubes for one of the 12xxx, as proposed?

First, we recalculate the string current:
333+233+80+80+20+20 = 766 ohms (from the above Equivalent Resistance calculations). 120 V/766 ohms =
0.156 amps.

The voltages across each tube in the new string would therefore be:

50L6: Req * Istring = 333*0.152 = 50.6 V
35Z5: (same) = 233*0.152 = 35.4 V
12xxx: (each tube) = 80*0.152 = 12.6 V
6xxx: (each tube) = 20*0.152 = 3.0 V

So, while the "original" tubes in the string would not be damaged by such a modification, the heaters of the "new" 6xxx tubes would be power starved and unable to emit sufficient electrons to operate properly.

:
::I'd also like to retain the series filament arrangement, which would have to serve 6 tubes.
::Can the 12SA7 converter be replaced by a 6SA7, and can a 6SK7 be used for the RF stage?
:
::
:
:
:This thread seems to have strayed off topic, so I'll cut to the chase:
:
:No, you can't successfully make the change that you are proposing. A series-string set must have tubes that all draw the same amount of current; in the case of the S-38's complement, this is around 150 mA. The 6SK7/6SA7 draw 300 mA.
:
:You can think of each tube as a resistor, calculate its equivalent resistance from its spec, then use that info to play electronic design "what-if" games. The equivalent resistance of any tube will not change from application to application. The amount of current it draws, or the voltage it drops, will.
:
:For the S38 lineup, here are the equivalent resistances for a line voltage of 120 VAC:
:
:50L6: Req= 50/.150 = 333 ohms
:35Z5: Req= 35/.150 = 233 ohms
:12xxx: Req= 12/.150 = 80 ohms
:6xxx: Req= 6/.300 = 20 ohms
:
:
:So, for a "stock" S38, you have one 50L6 tube, one 35Z5 tube, and three 12xxx tubes in series. This is 333+233+(3*80)= 806 ohms.
:
:Using Ohm's Law, E=IR, and I= E/R, or 120 V/806 ohms= 148 mA.
:
:Again using Ohm's Law, the voltage "seen" by each individual tube will be as follows:
:
:50L6: Req * Istring = 333*0.148 = 49.34 V
:35Z5: (same) = 233*0.148 = 34.48 V
:12xxx: (each tube) = 80* 0.148 = 11.84 V
:
:So, all is well in S38-land if the correct tubes are used. What if we sub two "six volt" tubes for one of the 12xxx, as proposed?
:
:First, we recalculate the string current:
:333+233+80+80+20+20 = 766 ohms (from the above Equivalent Resistance calculations). 120 V/766 ohms =
:0.156 amps.
:
:The voltages across each tube in the new string would therefore be:
:
:50L6: Req * Istring = 333*0.152 = 50.6 V
:35Z5: (same) = 233*0.152 = 35.4 V
:12xxx: (each tube) = 80*0.152 = 12.6 V
:6xxx: (each tube) = 20*0.152 = 3.0 V
:
:So, while the "original" tubes in the string would not be damaged by such a modification, the heaters of the "new" 6xxx tubes would be power starved and unable to emit sufficient electrons to operate properly.
:
CV,
Thanks for your explanation. So, 150 and 300 mA tubes cannot co-exist on the same filament string. The S-38 owner, Ted, posted a similar response. He went on to mention some 6v tubes that might do the job.
Can you suggest others?

Thanks Again,
Steve

:CV,
:Thanks for your explanation. So, 150 and 300 mA tubes cannot co-exist on the same filament string. The S-38 owner, Ted, posted a similar response. He went on to mention some 6v tubes that might do the job.
:Can you suggest others?
:
: Thanks Again,
: Steve
:
:
One thing that would be rather easy to do is substitute a 35L6 tube for the 50L6, then just add the 12SK7 to the heater string for the RF amp stage. Do the math and I think that you'll find that it works out almost perfectly.

By the way, to get an idea of what additional circuitry is needed to graft an RF stage onto an S-38, take a look at the National NC-57, another "entry level" receiver of the postwar era (although comparing an S-38 to an NC-57 is like comparing a unicycle to a Honda Goldwing, in my humble opinion).

A major complication is the bandspread cap: in the S-38, the main tuning and the bandspread variable caps are both 2-section units. In the NC-57, which has an RF amp stage, they have to both be 3-section units.

:By the way, to get an idea of what additional circuitry is needed to graft an RF stage onto an S-38, take a look at the National NC-57, another "entry level" receiver of the postwar era (although comparing an S-38 to an NC-57 is like comparing a unicycle to a Honda Goldwing, in my humble opinion).
:
:A major complication is the bandspread cap: in the S-38, the main tuning and the bandspread variable caps are both 2-section units. In the NC-57, which has an RF amp stage, they have to both be 3-section units.

:CV,
The S-38'like radio I'd like to build is not intended for communication. Right now, I plan single band operation, 5 to 14.5 MHz. I'd like to add the RF stage so that I can listen to low power commercial stations in eastern Europe. I'm not interested in communicating with hams that might be operating at a few hundred watts.
As you suggested, I looked at the NC-57, which has an additional tuned circuit between the RF and converter stages. I probably don't want to bother with this extra baggage because I won't need the selectivity that would otherwise be needed by hams trying to listen to relatively weak signals with much larger nearby signals.
Then again, I don't have an understanding of the ham spectrum and what's out there.

I did try out an IF transformer which I extracted from my GE Model 321 junker. On the primary side, with the secondary open, I saw the expected resonant peak around 455 kHz. Would the S-38, even at the same 455 kHz, need a something less ordinary?

Thanks,
Steve

:

:By the way, to get an idea of what additional circuitry is needed to graft an RF stage onto an S-38, take a look at the National NC-57, another "entry level" receiver of the postwar era (although comparing an S-38 to an NC-57 is like comparing a unicycle to a Honda Goldwing, in my humble opinion).
:
:A major complication is the bandspread cap: in the S-38, the main tuning and the bandspread variable caps are both 2-section units. In the NC-57, which has an RF amp stage, they have to both be 3-section units.

I was given an old N-57 this spring for free. I restored it and can hardly believe how good the RF gain is. It does use the 6SG7 in the RF gain stage. C Harris
:
:

:
:I was given an old N-57 this spring for free. I restored it and can hardly believe how good the RF gain is. It does use the 6SG7 in the RF gain stage. C Harris
::

Yes, I restored a NC-57 several years ago and it's a decent performer. Very sensitive/selective and stable compared to "living room shortwave sets", plus it has a real BFO, unlike the odd IF-feedback kluge used in the S-38 design. It's been awhile since I've worked on it, but I seem to recall that it was a snap to troubleshoot but somewhat tedious/fussy to align- but maybe that's just a subjective impression on my part. I removed the tiny front-mounted speaker from mine and installed an S-meter in its place, purchasing a separate National metal-box speaker that was intended for use with a higher-end set of theirs.

::
::I was given an old N-57 this spring for free. I restored it and can hardly believe how good the RF gain is. It does use the 6SG7 in the RF gain stage. C Harris
:::
:
:
:
:Yes, I restored a NC-57 several years ago and it's a decent performer. Very sensitive/selective and stable compared to "living room shortwave sets", plus it has a real BFO, unlike the odd IF-feedback kluge used in the S-38 design. It's been awhile since I've worked on it, but I seem to recall that it was a snap to troubleshoot but somewhat tedious/fussy to align- but maybe that's just a subjective impression on my part. I removed the tiny front-mounted speaker from mine and installed an S-meter in its place, purchasing a separate National metal-box speaker that was intended for use with a higher-end set of theirs.
:
Do you have the schematic of how to hook up the S meter?
C Harris

:Do you have the schematic of how to hook up the S meter?
:C Harris
:
The factory S-meter was an option that was in a separate small cabinet and plugs into the octal socket on the rear apron of the set. I couldn't find one of these so I made my own. I think that the factory unit schematic is here on NA (with the rest of the NC-57 documentation).

As nearly as I recall, I scoured eBay until I found a circa-1940's NOS DC milliammeter gauge that was suitable. (It didn't take a lot of searching.) I think that the one I used was 1 mA full scale. This was installed in series with the RF amp's plate and is bypassed by a multi-turn potentiometer (1000 ohms, I think- can't recall what I used- may have been a lower value). It works on the principle that an RF amp tuned to resonance has minimum DC plate current at the resonance point. To "calibrate" the meter, you detune the set with the antenna input grounded, then adjust the pot until the meter achieves full-scale deflection (The idea is for most of the plate current to bypass the meter via the pot, with just enough getting through to drive it to 100% of scale deflection). Then, you reconnect the antenna, and tune a station. The stronger the signal, the further toward zero the indicator dips.

A reverse-reading guage was what I was actually looking for, since I believe that is what the factory option unit uses, but those seem to be hard to find (at least on eBay).

I have to admit that this gauge is of low utility, at least to me. About the most useful thing it does is provide visual confirmation that the antenna trimmer control is peaked. But, it looks cool and, as we all know, that's the main thing.

Since B+ is present on both terminals of the meter, I made certain that the the "mod" was well insulated (and HIGH-VOLTAGE labeled) to give a future owner fair warning.

The S-meter schematic is shown in Fig. 8 of Section 6 of the National NC-57 data pack available under Resources.

I was incorrect about the monitored tube- it is actually the 2nd IF amp plate that is being measured, not the RF amp plate current. And the bypass potentiometer is 50 ohms, not 1000.

It is much more practical to monitor the 2nd IF stage plate current than that of the set's RF amp, since the frequency is so much lower than that of the incoming antenna RF. However, the operating principle is the same.

:The S-meter schematic is shown in Fig. 8 of Section 6 of the National NC-57 data pack available under Resources.
:
:I was incorrect about the monitored tube- it is actually the 2nd IF amp plate that is being measured, not the RF amp plate current. And the bypass potentiometer is 50 ohms, not 1000.
:
: It is much more practical to monitor the 2nd IF stage plate current than that of the set's RF amp, since the frequency is so much lower than that of the incoming antenna RF. However, the operating principle is the same.
:Thanks much
C Harris

:
::CV,
::Thanks for your explanation. So, 150 and 300 mA tubes cannot co-exist on the same filament string. The S-38 owner, Ted, posted a similar response. He went on to mention some 6v tubes that might do the job.
::Can you suggest others?
::
:: Thanks Again,
:: Steve
::
::
:One thing that would be rather easy to do is substitute a 35L6 tube for the 50L6, then just add the 12SK7 to the heater string for the RF amp stage. Do the math and I think that you'll find that it works out almost perfectly.
:
CV,
Yes. One of my standard broadcast junk radios is a GE Model 321, with a 12SK7 RF stage. And,as you suggest, the 50L6GT has been swapped out for a 35L6GT. So,I guess the all six tubes are running somewhat cooler.
Is anything sacrificed?
Does the 12SK7 do the job in the 5 - 14.5 MHz range?
There also appears to be some RLC filtering between the RF and converter stages.
I'm mildly surprised that an ordinary broadcast band table radio of 1948 vintage would have an RF stage.
The radio seems to be a prime candidate for cannibaliztion. I'll have to destroy it in order to save it.
I appreciate your inputs.

Thanks,
Steve
:

Always like playing with the good old S-38's
You can use a 12SG7 in place of the 12SK7 for twice the gain. You can copy a Fada 1000 That has an RF am with a two section tuner. I have an S-38 transformer operated, full wave power supply. Three stage IF. Pulls in the world SW and Ham radio.

:Always like playing with the good old S-38's
:You can use a 12SG7 in place of the 12SK7 for twice the gain. You can copy a Fada 1000 That has an RF am with a two section tuner. I have an S-38 transformer operated, full wave power supply. Three stage IF. Pulls in the world SW and Ham radio.
:
Warren,
Thanks for the suggestions.
I'll look at the 12SG7 and the Fada.
Did you modify an existing S-38 to add an extra IF stage?
It sounds like you listen to commercial SW.
Do you routinely pick up low-power stations?
How low do these get to be?

Thanks Again,
Steve

:Always like playing with the good old S-38's
:You can use a 12SG7 in place of the 12SK7 for twice the gain. You can copy a Fada 1000 That has an RF am with a two section tuner. I have an S-38 transformer operated, full wave power supply. Three stage IF. Pulls in the world SW and Ham radio.
:
Warren,
I looked at the Fada 1000 schematic.
The wave trap seems to be a low Q filter.
Is this typical of standard broadcast radio with an RF stage? It might be useful, and straighforward, for my S-38, which will be set up for 5 to 14.5MHz.
Why does the plate operate at only 24v?
Is the RF input on the 12SK7 at 0v?
Can you name any other six-tube radios with an RF stage?

Thanks,
Steve

The wave trap is not needed. It's a 455KC trap. A good 6 tube radio would have a 3 section tuner. One section is to tune the RF front end. With your Hallicrafters it only has 2 sections. That's why I picked the fada 1000 to copy it's front end. Here is a link you can play with.

http://www.wa2ise.com/radios/12ba7.html