Another Heathkit conversion project - Questions

[CitizenCain]

Hi, new guy here, but I've been lurking for a while.

A friend of mine sent me two Heathkit AA-23 tube audio amps to see what I could do with them. It's a nice little circuit from what I can tell.

I thought I might use this thread to post a few questions now and again as I progress through this project.

My first question involves the PI. It's a 12AU7 in a cathodyne setup. Is it feasible to sub in a 12DW7 and keep the AU section as the inverter but use the AX section as the driver? Hoping to get a little power tube overdrive going on.

My first mod was to bypass all the input selector stuff and go straight into V1a with a 1/4" jack.

Incredible amounts of gain with a bassy overall sound. My plan as of now is to try and dial back some of the preamp gain, add a little power amp OD and tune the circuit for a less bassy sound. I'd like to keep the overall circuit configuration close to stock, just so I can say it's a modded Heathkit circuit

Any tips, ideas or advice greatly appreciated!

BTW, here's a couple links to schematics. This schematic is from an AA-181 but it's identical to the AA-23 schematic. I got a copy of the AA-23 assy manual w/schematic from Manualman.com, great place.

Stock circuit: http://www.visi.com/~sstolle/heath/aa-181.gif

As mine sits now: http://www.visi.com/~sstolle/heath/aa-23-mod-002.JPG

Thanks for any help and for putting up with a newbie

[CitizenCain]

And a look at the unit as it sits now,

[G-hoppa]

I don't know if you saw the bit in the thread here about my ongoing Heath A-7E conversion, but there are some ideas in it from others (I am a newbie too) that might help you out. One is to bypass V1A completely (only the original phono circuit used it) and wire your 1/4 inch jack directly or though resistors to the second triode. Check my final schematic to see what that would look like.

http://music-electronics-forum.com/s...ead.php?t=7041

This should reduce the gain to a reasonable amount, I think.

As for the tone circuit, have you ruled out the configuration PRNDL suggested? That's the one I'm going with.

[CitizenCain]

I did look through your thread, G-hoppa. That's what inspired me to post my project

I'll have to take a look again and really grok the details you mention though.

[CitizenCain]

I think I'm going to stick with the stock tone stack values for now, just to get this thing flying. I'll maybe tweak them later if needed.

I've had a chance to play through the stock circuit to get an idea of how it works as is. Based on those observations, my plan as of today (it changes ) is:

1) Put a vol pot between V1a and V1b. That'll be a push/pull pot which I can use to disconnect from V1a for a cleaner sound and engage that triode for some OD.

2) Raise the preamp cathode resistors to cut back on the stage gain. This bugger has more gain than I know what to do with right now.

3) Juggle the coupling caps a bit to cut down on some bass and add bypass caps to the V1 and V2 cathodes to tone shape a little.

4) Just maybe on this one - use a 12DW7 for V3. That way I'll get a 12AX7 for the driver and a 12AU7 for the inverter, since it's already setup for that. I thought by reducing the preamp gain a little and increasing the driver gain I could add some power amp OD into the mix.

Anyway, I hope it works! I'm doing all the mods on my buddy's first, which he's having me build in a Princeton reverb repro chassis to fit a cab he has

I'm planning on keeping mine in the Heathkit chassis and making a retro mesh cover for it or something. That, and a custom control panel for the extra and repositioned controls.

[CitizenCain]

Re: part 1 of the plan above. I was looking over the circuit some more and now I don't think my initial idea will work. I had read about how you could internally jumper an amp, like how the 4-input Marshall can use a small patch cord between channels, by tieing the two grids of V1 together with a small jumper. You can then have a vol control for each side of the tube and it's just like jumpering the amp externally.

I now realize this works for parallel channels, but what I was intended was a cascaded gain stage setup, then hook the grid-grid jumper via a push/pull switch. I don't think that plan will work for a cascaded setup, would it?