Mod'ing a tube circuit (from early 60's stereo turntable)

[TubeNoob]

@eschertron: Wired up the tone-stack bypass - really added a nice warm distortion to things (and bumped the volume up). Not at all over the top, in fact it's pretty laid-back, so i'd definitely need to find a way to crunch it up further. But a really, really nice sound option for sure. Thanks for guiding me on that one.

Aside from pedals, is there anything further that can be done to the circuit (without major changes) to get some more dirt?

A V1 cathode bypass cap would give it more headroom (and less crunch), would it not?

[eschertron]

A pair of cathode bypass caps on V1 (a and b) will increase gain and reduce input HR. By about a factor of two. Definitely noticeable, if it's already getting 'warm'. The other thing to do is investigate the NFB loop on the output stage. Hi Fi usually has more NFB than guitar amps, so you could relax that a bit to get more crunch. I didn't look at the values, so can't tell you what to expect at this point. But from your description, it appears you will be happy with modding this thing. Won't be a metal monster, but could get nice and bluesy!

[TubeNoob]

Fantastic, thank you Eschertron - so adding those cathode caps will overdrive the power tubes more? Going for it!

Just ordered up the parts from Tayda Electronics, a good cheap place to go!

Will have the following: Multiplier, Muff Boost (clean and dirty on a switch), Tape saturation simulator, FX Send section, 4 channel mixer with pan and level controls, mini-jack and two mono 1/4" inputs direct to amp.

Should be a cool little color box when its done.

[Chuck H]

Sorry I'm late to the game. I sketched up the changes "I" would make, short of anything too crazy. Some have already been covered but I included them to consolidate everything on one schematic. These would make it pretty much a pair of 2W Champs Probably with a little more crunch though.

Changes in red:

[TubeNoob]

Hi Chuck! You've helped me before, really glad to have your input here and thank you so much for mocking up that schematic!!!

Is there any way i can trouble you to walk me through what these changes do in particular? It's a lot to throw on a switch (so i can revert back to the 'clean' sound) and i do have a home-built Princeton 5f2a, but i'd really love to understand what these changes do and implement any and all options i can.

[TubeNoob]

...by the way, Chuck - i just ordered up all the parts you suggested just in case!!

dang i love forums like this. makes an agoraphobic tinkerer like me feel like a part of the world! heheh! and the best part is communicating with some really brilliant people who share a common interest...

[Chuck H]

There's no need for a switch to get clean. The amount of grind you'll get from the above changes isn't dramatic and the clean tone at lower volumes (around 4 or 5 probably) will be as loud as the amp will get anyway. So a switch buys nothing in headroom. The amount of distortion will be a little greater than a BF type Champ. Well into a rock grind, but not metal by any stretch. The actual number of gain stages is the limitation here. Something you COULD put on a switch would be the classic tone stack bypass. The normal circuit for this "mod" just lifts the tone stack ground resistor and disables it. That would buy a lot of gain. Maybe too much, but that'll be up to you. I'll draw it up and re post, along with some explanation of what the changes I proposed above do.

P.S. Since you've already ordered parts it may be too late, but you DO want log pots (or a log dual ganged pot) for the new volume controls.

[Chuck H]

As promised:

[TubeNoob]

Chuck you're a monster!

1) I did order log pots and not linear.
2) So you're saying these mods wouldn't need to be on a switch since i could basically back the volume off enough so that the 'clean' sound is still an option? I want to use this for re-amping stuff and warming things up sometimes (not just for guitar crunch - i'm trying to have the crunch be optional).
3) How much of a difference would the replacement of balance controls with volume controls make? That's the only mod i'm a little hesitant on.
4) I did try bypassing the tone stack and it added a really lovely (yet subtle) tube dirt flavor. Really, really nice.

Thank you so much for taking your time to do this.

You should do a book with common schematics and then have 'overlays' for your suggested changes!!!

[TubeNoob]

Chuck - can you explain what's happening on that tone-stack bypass? The original value was 10k, but it looks like you've got a 2.2M there 'normally' and then switch in a 15k?

The way i tested the tone-stack lift was completely bypassing the whole thing - is there a problem in doing that way?

I'm curious how your version sounds compared to the way i did it (and i do love the way it sounds).

Thanks man!!

[Chuck H]

The 2.2M resistor is there to keep the caps in the tone stack connected to ground when the REAL load (the 15k) is lifted. This keeps the switch from going POP! when you flip it.

The actual difference in gain between actually bypassing the TS and just lifting that resistor is really small and simply lifting the resistor only requires a SPST switch. Along with the other changes it'll be PLENTY of dirt. I promise.

Changing the balance controls to volume controls is an imperative part of this mod. It allows you to bypass the (very non guitar amp like) existing volume control. This is where a lot of your added gain will come from. An extra few dB at the input triode is reamplified by the next making it exponential. Relocating the volumes to replace the balance control also puts the volume controls in the circuit where every other instrument amp does. Making their use more intuitive.

The way the balance control will be wired has one channel like a typical volume control and the other wired opposite, as if rotating the knob counterclockwise turns the amp up. So this one gang on the pot is wired "backwards". This is the only thing you'll need to wrap your head around. If you still want "balance" control you'll need to use two 1M pots (and drill a hole I suppose). If you're alright with the stereo channels going up and down in gain simultaneously you can use a dual ganged pot (and skip drilling a hole).

The amp should be well behaved used as a typical "vintage" style amp. It'll be clean with most guitars up to around 4 or 5 on the volume control and should be real crunchy at 10. The actual location on the knob where the amp will start to break up will, of course, depend somewhat on pot taper. If you ordered the typical Alpha pots the taper on their 1M is 15% I think. That should be fine.

[TubeNoob]

Ok, excellent explanation, Chuck. Thank you!

No probs on doing the balance controls as two volume pots.

Just as a note, i don't want to use it all for guitars, i want to have the option of it being 'clean' enough to just warm up some mixer buses and let me re-amp anything from keys to vocals (and have these grittier options available!).

So now i guess i need to look into how much the power lines to the turntable's motors have got for me so i can hopefully power these solid state circuits from that.

I should probably have the tube amp's power on a separate switch if possible so that i can just use the SS effects without running the tube circuits. Thoughts on this, anyone?

Otherwise i would probably need to go with an adapter.

[Chuck H]

since you'll be using it for things like vocals and keys as well you'll probably want more mid control. I'm thinking a SPDT on/off/on make before break toggle on the mid control for normal/tone stack bypass/mid cut selection. It WILL be a guitar amp and it IS a little amp. That means it's going to sound a bit boxy or lack headroom in the midrange for vocals or other instruments without a mid cut. I'll draw it up

[TubeNoob]

ooooh i can't wait to see it! thank you for your suggestions and expertise, Chuck.

if i run vox or keys through it, it's going to be for some saturation, so i think i do understand the sonic trade-offs.

i guess i'm just going for overall flexibility wherever possible.

[Chuck H]

i guess i'm just going for overall flexibility wherever possible.

Not asking too much amplifier duty from a sixty year old turn table, are you?