1930s guitar amp with pentode preamp schematics wanted

[ALBATROS1234]

been having fun with pentodes in v1 for the past 6 months and loving it. guess i have been lucky i have only found one microphonic one a ge 6u8a which sound quite nice in se 6v6 config. did a ef86 in v1 of one channel with a 12ax7 split duty as input for a second channel other half as a driver for the 6G6 in a se with channels set up like a 5e3 so that the 2 volumes and the one tone could be used together so i can blend channels. favorite though is a pp amp with a 6SJ7 v1 set up like a 5c1 champ into half of a 6SL7 with the other half of it a fender deluxe style pi with a pair of 6AQ5s also 5e3ish. been thoroughly enjoying the pentode input tube

i purchased very cheap a basket case 1936 wards airline 62-315 am radio . every side of the wooden case was delaminated which i have since slowly meticulously glued and clamped it back and cleaned up and sealed it.i got it because it had a power tranny, which works,so does the ot. the coils are dead i get no signal but the amp section works.so i am going to make it a small guitar amp. it has an 80 rectifier i will keep, i want to use a 77 sharp cutoff pentode as v1 which feeds a 75 hi mu triode/driver with a 41 power tube se. the 80 is merely a 4 pin 5y3 as i am sure many of you know .a 77 is a 1930s 6 pin 6J7 ,a poular audio tube in the 40s and 50s. the 75 has an a.f. of 100/twin diode ,so basically a 6SQ7. the 41 is electrically a 6K6 power pentode but i am thinking i may get a 42 which is a 6F6. so basically i can cobble together something but i was wondering if any of you guys know of something with that type lineup in a 1930s guitar amp . i guess its like other stuff i have done i can probably set up the 77 like a 6SJ7 the 75 i can treat as half a 12ax7 and ground the diodes and treat the 42 like a 6K6 but i was hoping to find out if there was something similar to use as reference. thank you for any and all responses.

[Papa Jim]

There's some old schematics here. Some Monkey Wards radios also
https://www.americanradiohistory.com/Archive-Rider/39/Rider-1939-MNO.pdf

[ALBATROS1234]

thank you kind sir i will take a look.

[PRR]

The original radio WAS a fine audio amp, from the Volume control onward.

You need a little more gain to spank it with an e-guitar. An ancient trick is to wire the IF stage pentode as triode audio amplifier, preceding the Volume control. Wiring as Pentode is likely to be more gain than you need. But go ahead and try.

Do what Fender did. Find the resistance-coupled amplifier tables for the tubes you propose and copy some workable values. I suggest you take "200K" as load on each stage (even if it looks like a 1Meg pot) to cover stray capacitance. Find some combination of gain stages, maybe with some loss, so 10mV-30mV at the input will push the Power tube to full output.

If it had a '41, stick with it. Ward's designers were not dum-dums. The OT, PT, filtering etc will be scaled to 6K6. 6F6 won't be more power without other changes, and could cook antique parts.

This is Field Coil speaker. Has it survived? Even if so, it won't survive much head-banger playing. It's not just a speaker but (usually) part of power filtering and overall voltage drop. In this case it seems to be back-bias for the '41.

[ALBATROS1234]

thanks so much,great info prr. i didnt think of running the 77 as a triode. i have never done that before hmmm. it came with a 78 which is a remote cutoff and i know audio amps mainly use sharp cutoffs and i have a 77  that came in a tube lot i got for a few bucks. too much gain with pentode v1 , maybe. i like it when the amp naturally crunches up. i dont play metal, but i do love 60s and 70s rock n roll tones. the 6SJ7 ,6SL7 pp6AQ5 i talked about above sounds really amazing, its like sweet harmonic laden cleans and she starts with some blues grit around 12 noon. if i push it a bit more it can really crunch up but it gets super loud. i may try a triode/pentode switch on the 77 which i will prob grid bias and the 75 maybe a 1.8k cathode resistor is my guess doubt i would have to put a bypass cap slamming that 41. oh and yes the original speaker sort of works, meaning hum came out of it and the voice coil as well as the field coil are intact  but the cone is shredded and if you touch it more crumbles away. i only really decided to convert this because something is up with a few of the coils. all i get is hum even though all the filter and paper caps have been changes as well as a few resistors that needed it. i thought of the 6F6 because its electrically similar to a 6k6 from what i have experienced. i have a handful of each in 40s/50s vintage and i like a 6F6 a good bit in a guitar amp it has a nice bite, 6K6 sounds similar it but just a little less of everything ,gain,harmonic content ,character etc.although the rca 6K6s i have actually sound really clear and balanced. i tryed the 41 in one of my se 6v6 type amps last week actually sounds good.very similar to a 6K6. how you say? i have an octal base i removed from a dead tube and resoldered wires in the pins, i use alligator to alligator cables to connect to the 6 pin base and voila a #41 was tested for output in a 6v6 amp. i dont have a tester....anyway stuff to think about. thanks again for your usual informative response sir.

[ALBATROS1234]

oh and by back bias for the 41 do you mean that odd little array of 3 resistors in the filter section?

[TIMBO]

A few I have done...

[ALBATROS1234]

excellent timbo, sort of like your second schematic except one triode stage instead of 2 like your 6SN7. i have a 6" 8ohm speaker from a ss combo amp i will use for the internal with a parallel output jack. that way if i plug into another 8 ohm speaker load i will get 4ohm which is the value of the original speaker impedance and what the ot wants.i am going to completely strip the chassis down and paint it in a hammertone brownish/bronze then build this simple little tone machine.

question, the last one with the 2 6J7s does it not have too much distortion? i find using just a pentode falls flat with a guitar amp. but i have also had problems trying to squeeze to much gain from a dual triode which ends up squealing and oscillating. i imagine where it branches before the 6V6 into the 220k resistor leading to the tone cntrl helps reduce overloading the pt grid.

thanks again sir

[nichesound]

This is a 6J7 Pentode front end with two 6SN7's into push pull 6V6's. The first 6SN7 is a cathode follower so I can add a volume pot / tone / volume

 David Pippen lives in Georgia and has a studio, I made this especially for his sound.These are the two Little Big Jam videos fortunately that were made at 3am on Oct. 5th 2019 when I gave it to him after the days shows.
Initial impression ffffaaaaaaaaaaaa

David does a BB King number....

These little 1940's amps from Bell and Howell revoiced are amazing!!
Johnne in Seattle

[isaac_teller]

http://prewaramps.org/schematics.htm

[scstill]

A few I have done...

I am in the process of trying to use a 6D6 as a guitar preamp tube. Your diagram shows a mic input. How would that work with guitar? I see that the 6D6 has many similar values to the 6SJ7 which Gibson used in their BR-6F (and I also used), but the amplication factor of the 6D6 is huge at 1200 while the 6SJ7 is 19. Will that add crazy signal to the rest of amp?

[sluckey]

the amplication factor of the 6D6 is huge at 1200

Where did you find that info?

[PRR]

>> the amplication factor of the 6D6 is huge at 1200
> Where did you find that info?

True, as defined, and at high current (8.2mA, 250V). Gm is 1600uMho or 625 Ohms, rp is 800,000k. The quotient is 1280.
https://frank.pocnet.net/sheets/127/6/6D6.pdf

But (as you know) we almost never see this quoted on datasheets because amplification-factor of a pentode is never a useful number. Especially in audio. The external resistor to feed DC to the plate is always much less than the pentode rp.

Also the datasheet numbers are show-off values, and especialy this RF/IF tube which can use like 8mA. In Timbo's plan with 250k plate resistor under a maybe 250V-350V supply the tube current is more like 1mA. Gm is likely to be near 500-600uMho or 1800 Ohms, rp well over 1Meg, so stage gain may be (250k||500k)/1.8k or 91.

Is that "crazy gain"? Maybe.

[scstill]

Excellent info thanks PRR.
A couple of attachments of older datasheets that also boast 1280.
91 for the 6D6 is very nice. Like a 12AX7.
I'm generally living in the clean & quiet area anyway

[isaac_teller]

http://prewaramps.org/

[scstill]

A few I have done...

Hey Timbo, do you have the C voltage measured? I think it might be as high as 400v.
I might have to adjust dc plate resistors on my 6D6 preamp tube since my design is 275v before plate resistor.

[TIMBO]

Hey mate,
Voltages as requested
A - 303v
B - 368v
C - 364V

[HotBluePlates]

>> the amplication factor of the 6D6 is huge at 1200

True ... at high current (8.2mA, 250V). ... 1280.

... In Timbo's plan with 250k plate resistor under a maybe 250V-350V supply the tube current is more like 1mA. ... stage gain may be (250k||500k)/1.8k or 91. ...

... A couple of attachments of older datasheets that also boast 1280. ...

Use Ohm's Law for a moment scstill:  the 250kΩ of Timbo's schematic times 8.2mA ---> 250,000Ω x 0.0082A = 2,050 volts (!)

We don't have a 2kV power supply.  And 250v dropped across 250kΩ implies 1mA current through the resistor.
Gm falls off rapidly when plate current falls.  These preamp pentodes have less-impressive gain when the load is a resistor rather than a coil or transformer.

We have to take data sheets with a big grain of salt.