I converted an old radio into a guitar amp

[jewishjay]

I got an old RCA radio (1940s?) from my inlaws, and built this little princeton-esque amp with the unusual tube compliment. the pair of 6SK7 in the preamp are pentodes running in triode mode by jumping 3to5 and 6to8. the 6K6GT power pentode seems very similar to a 6V6GT. I just took the 5F2A schematic and made some minor tweeks. So it operates and sounds half decent, but I would like you more experienced techs to take a look and let me know if there's changes I should make.

1. the 6K6 is running 300v to the plate and 290v to the screen, the plate dissipation is ~12w
2. the 6SK7s are only seeing 50v to the plate, but the sheet says 250 max
and the plate dissipation is less than 1w but when I changed the cathode resistor the plate voltage dropped to 22v and the overall sound and volume got worse, so I changed it back
3. the tone control is very subtle

thanks for any input

[shooter]

2.  50vdc seems maybe low, what are the cathodes at?


you can "get more" plate by changing the PS 10k 1W to 5k 1W, but I'd verify the cathodes 1st, double-check wiring.


the 6K6 is an anemic 6V6, I just did a PP build with them, they were ok, met my design "small house" volume.
I eventually went with 6V6, tweaked and got bigger house volume   

[PRR]

The 6K6 is a low-power low-gain ancestor of 6V6. Note low plate dissipation limit.

6SK7 as triode is much lower gain than 12AX7. With low gain in power stage, this won't be as sensitive as the maximum-overdrive headbanger amps. Maybe 90mV instead of 20mV for many BlackFace amps. At home this may be ample.

In retrospect: the radio surely had a diode-triode ahead of the 6K6 and the team worked together very well. I would not modify the audio. Interrupt in front of the radio Volume control and add a triode-strap IF tube as preamp.

You "never" run resistor-loaded voltage amp triodes at maximum rated plate voltage. You typically make some clean B+, like 250V, pick a plate resistor appropriate to what it drives (100k-330k), and trim the cathode resistor so the plate sits at 50%-70% of the supply, 125V-175V. The low-Mu 6SK7 may want Rk 3x or 4x higher than the 1/5k we use with hi-Mu 12AX7; try 4.7k.

[sluckey]

I would simplify by using a single 6SL7 rather than two 6SK7s. I know they work well in that preamp circuit.

[jewishjay]

I would simplify by using a single 6SL7 rather than two 6SK7s. I know they work well in that preamp circuit.

thanks, but this time I'm really trying to use whats here. the other tubes in that radio were 6SA7 and 6SQ7. for my next little conversion project I have a Victor phonograph amp with several 6J5 and a pair of 6V6gt, so I came up with this idea.....

[sluckey]

Got it.

[AmberB]

That 6SQ7 is a twin diode-hi mu triode tube.  The triode section would be more useful than one of those 6SK7s for voltage gain.  You could use that instead of the 6SK7 to drive the 6K6 output tube.

[jewishjay]

That 6SQ7 is a twin diode-hi mu triode tube.  The triode section would be more useful than one of those 6SK7s for voltage gain.  You could use that instead of the 6SK7 to drive the 6K6 output tube.

ok, i will try that! this project is all about experimenting with the unusual.

[tubeswell]

... You typically make some clean B+, like 250V, pick a plate resistor appropriate to what it drives (100k-330k), and trim the cathode resistor so the plate sits at 50%-70% of the supply, 125V-175V. The low-Mu 6SK7 may want Rk 3x or 4x higher than the 1/5k we use with hi-Mu 12AX7; try 4.7k.

@jewishjay ^this is good advice^

[PRR]

That 6SQ7 is a twin diode-hi mu triode tube. ...

It is in fact essentially half a 12AX7. So paired with a 6V6 (or 6K6) you have 2/3 of a Champ. Champs with simple tone controls have twoo much gain with another 12AX7 section; the IF pentode triode-strapped is a great fit.