Rebuild into a Dumble

[Littlewyan]

I am going to be rebuilding one of my amplifiers into a Dumble, likely the 102 circuit but I'll be subtracting/adding some things due to chassis space and what I need.
- Accent switch instead of pot (with internal trim pot adjustment)
- No mid switch
- Built in buffered effects loop
- 6V6 output stage
- No PAB Boost
- Boost

With the boost my idea was to switch the cathode bypass cap on the second gain stage in and out. It'll give a nice full range 3-6dB boost, for both clean and distorted. I'm wondering if anyone knows if this is how Andy Fuchs does it on his amplifiers? I don't think he's using a FET boost and he has a separate PAB boost, so I can't think how else he achieves this. His manual states it's a 3dB boost, factory adjusted, so all I can think of is either a cathode bypass cap switch or an LNFB switch?

[samtech60]

It sounds about exactly like what I am in the middle of doing a small minimumal 6V6 powered tone machine . Excellent!

[rootz]

In a 102 style amp this could be lnfb on v1b. That should be good for 3 dB at higher frequencies, a bit more in the bass.

Cathode bypass boost? Not likely, as in a 102 all cathodes in the preamp are already almost fully bypassed.

Maybe Andy partially lifts the trigger trimmer? I don’t think that would be the best sounding solution, but possible though.

[Littlewyan]

Yeah I was going to lift the bypass cap on the second stage but I think this may change the tone and gain structure too much. Found out Andy uses a FET boost. I may just revert to the PAB boost, was just trying to find a more elegant solution.

[Littlewyan]

Well it's only taken 8 months, but I've finally done it! It's the 102 circuit but scaled down with a 20W output stage. Thoughts so far are, it sounds great! It has just enough drive to sit behind a vocalist when covering pop songs and that's kind of what I built it for. Clean tone is lovely, very full and present. It's not 100% finished as I'm adding an effects loop, hence why the master volume isn't wired in yet and why there are a few holes in the back!

As you can see I didn't bother with the rock/jazz or mid switches. I ended up adding the PAB boost but used a 2.2Meg resistor, very pleased with this. Didn't have enough room for a presence/accent pot so I put it on a switch with a trimpot inside for adjustment. It's currently set to give me 3dB which works quite nicely.

I've adjusted the PI Trimmer using the power valve cathode method whilst using a spectrum analyser on the speaker output and you can see the even harmonics decrease as you balance out the power section, which was pretty cool. OD trimmer has been set to 26K, tried it higher but at this setting I like the smooth drive I get. Not too in your face.

So overall, dead pleased.

[erwin_ve]

Lovely build, neat wiring. Am I seeing 2 different amps/faceplates?

Erwin

[Littlewyan]

Thanks! Yes, one is how the amp was before I gutted and rebuilt it. I'll remove the old amp pics.