Creative uses for 6SA7, or hot-rodding a 6 tube radio

[passaloutre]

I have acquired an old radio, the attached Bendix 676D. It appears to be a typical 6-tube radio of its day: 6SK7 RF stage, 6SA7 converter, 6SK7 IF amp, 6SQ7 demodulator and AF stage, 6V6 output stage, 5Y3 rectifier.

Of course I want to make it a guitar amp, and as a learning experience, I want to use what's in there rather than gut it and replace all the tubes with proper octal audio triodes and pentodes. The goal isn't necessarily to have the best guitar amplifier, but to make this thing the best guitar amplifier it can be, and to learn something along the way.

Reading up on such things, I see that the 6SQ7 and 6V6 provide a pretty good foundation for a single ended amplifier. There are certainly two-stage amps out there in the world, but to make something a little more "Champ"-ish I'm likely going to want a little more gain, to make up for the Champ's other 12AX7 triode stage. I've read about running a 6SK7 in triode mode to acheive a modest amount of gain, so maybe I can stick the two in series and come close (or exceed!) the missing high-gain triode. I'm eager to perform some experimentation along these lines, and confident I can come up with something that works...hopefully. Also, why use the 6SK7s as triodes, couldn't they be used as pentodes? What are the funky coils attached to their plates? I assume they need those to operate at such high frequencies...do I need those for audio frequency?

This leaves me with an unused 6SA7: the pentagrid heptode. And I can't just leave it there looking pretty! Now I certainly don't have the electronics background to know exactly why you'd want 5 grids in a tube, but in my research I see that it was used to perform two functions: oscillate at an intermediate frequency, and mix the RF signal with the IF signal (forgive me if I don't have this exactly right, I have no radio background). Well oscillating and mixing seem like useful tasks from an audio standpoint. I've also read about other pentagrids (though not necessarily the 6SA7) being used as voltage-controlled amplifiers, which also seems pretty useful. Maybe I'm just seeing what I want to see, and not really understanding the why, but let me out on a limb here...

Could it be used as a trem oscillator? Or is it only capable of oscillating at much higher frequencies? Could the "mixer" portion be used to modulate guitar signal with the trem oscillator? Could I use it as a cool VCA volume control? Or even a compressor? Would I want to? At a minimum, could it be configured as another modest gain stage? A fuzz? A ring modulator???

Feel free to tell me this is all a dumb idea. Certainly if these tubes were useful from a guitar standpoint, we'd see them in at least one guitar amp, but we don't. But if you're going to call me dumb, at least give me the why. 6SQ7s are cheap, and I could strap enough of those in here to have a high gain firebreather if I wanted, but I have plenty of great-sounding guitar amps already, so help me with my experiment. Even if it makes a bad guitar amp, I hope it will at least teach me something.

[passaloutre]

Here’s a clearer (or at least not sideways) schematic

[PRR]

Radio signals are a thousand times weaker than e-guitars. You do NOT want all those tubes working at audio.

As you say, 6SQ7 and 6V6 is "two-thirds of a Champ". You could put a LPB in front and be playing in minutes.

The 6SK7 when Triode  strapped is a whole lot like a half a 12AU7 only more distorty. That in front of the 6SQ7-6V6 pair is usually all the gain you want. (When Fender used both sides of 12AX7 he had to kill some gain one way or another.)

K.I.S.S.!!! You do not want to get started on "oscillating and mixing seem like useful tasks from an audio". That way lies madness, or at least synthesizers (which are madness). Also the radio "mixer" is not a linear mixer, is very non-linear, and not adapted to good audio levels.

It is a shame. That's about the second best type AM BCB radio ever made, but even out here on the coast of Maine there's not really enough AM worth listening to to justify the electrons. (Modern AM bands are dominated by IM distortion between stations and with all the industrial crap in the air. The leading RF stage's RF filter tanks really cut the hash. But because 5-tube radios ate the market, listeners were trained to listen to the strongest few stations and disregard the rest.)

[WimWalther]

One potential issue will be the qualities of the radio OPT. These are generaly low-bandwidth devices, typically designed for the 300-3KHz vocal range.. much like an analog telephone.

Can you show us a photo of it, maybe with something for scale?

[passaloutre]

Suppose I want to embrace the madness. I’ve made half a dozen amps with noval triodes and 6L6s. You follow the schematics, mix and match circuits, add gain stages if you want, follow decent layout practices and they all sound good.  I want to get weird.

Why run the 6SK7s as triodes? What makes them more distorty? Is it a good distortion?

Can I possibly get the 6SA7 to oscillate in the single digit Hz range? I’m going to try anyways, so point me in the right direction.

I have a similar loctal radio with a push pull 6K6 output section I’d like to mess around with also…

[passaloutre]

One potential issue will be the qualities of the radio OPT. These are generaly low-bandwidth devices, typically designed for the 300-3KHz vocal range.. much like an analog telephone.

Can you show us a photo of it, maybe with something for scale?

The output transformer is definitely small. I don’t have another Champ to compare with, but here’s a photo. It’s larger than a Fender reverb transformer but smaller than the opts in my 4xEL84 stereo.

It also drove a 6 inch speaker of unknown impedance, so yeah there’s no guarantee it’s up to guitar amp duty. If I can get this thing up and running, I might be inclined to spring for a cheap Champ trans.

Not super concerned about sound quality yet (though I’d like to do what I can with what’s at hand), more interested in getting electrons flowing through these tubes again. I have nice amps if I want to sound nice.

I’m a learn-by-doing kind of guy. Please help me make weird noises with this thing.