Uncle Doug Tremo-verb circuit question

prongster

TDPRI Member
Joined
Jan 11, 2021
Posts
3
Age
57
Location
California
Just built a Tremo-verb and about to begin modding it for my own education, but first I have a question about the stock circuit:

What is the purpose of the network of the .015uF cap & the .005uF cap in series with the 470k resistor to ground between them? These components are between the 1st triode and the volume control.



I assume they act as both coupling capacitors and an RC filter, but I’m not used to seeing multiple capacitors in such an arrangement. The amp has less low end than I would like, but I want to know what these components are doing before I remove them.

thanks
 

FenderLover

Poster Extraordinaire
Joined
Jun 11, 2009
Posts
6,351
Location
Minnesota
They are high pass (C-R) filters. Two in a row (blocking only the lowest sub-guitar frequencies) and a third one feeding the reverb circuit.
Notice the Trem circuit connects to stage 1 at "C". Those high pass filters are to keep the Trem LFO from bleeding into the audio. In cases without the filters, you will see the speaker breathe to the pulse of the Trem.
Leave them in, get it working first, experiment later.
 

FenderLover

Poster Extraordinaire
Joined
Jun 11, 2009
Posts
6,351
Location
Minnesota
Just noticed you already built it... Those filters should not neuter your low end, only filter out the LFO. You could temporarily short the 0.005uF cap to test its effectiveness in filtering LFO and/or effecting bass in the dry side.
 

rdjones

Tele-Holic
Joined
Nov 6, 2010
Posts
730
Location
Music City, TN
Note that these circuits perpetuate the "rectifier pin 2 mistake".
B+ shoukd always be taken from pin 8 on guitar octal rectifiers.
Otherewise, it stresses the tube and creates additional hum and voltage drop.

If you are trying to increase "sag" this is not the way to do it.
Better to just use a resistor.
 

prongster

TDPRI Member
Joined
Jan 11, 2021
Posts
3
Age
57
Location
California
Oh wow, glad I asked...I will try temporarily shorting the 0.005uF cap to see what that does. I was also planning to increase the coupling cap going to the 6v6 to .02uF to see if that helps.

FWIW, I did not build this as an exact copy of the UD Tremoverb...I used Tubenit's 1 tube reverb circuit...and I didn't even notice the rectifier pin 2 B+..I was making my own layout in a Princeton Reverb chassis and used pin 8 as I had w the Princeton. Good thing I wasn't paying attention!

The rest of the amp is pretty much the same tho I think.
 

prongster

TDPRI Member
Joined
Jan 11, 2021
Posts
3
Age
57
Location
California
OK, so I’ve made some modifications that I’m happy with…I increased the coupling cap going to the 6V6 to .022uF, and that seemed to give me some more low end. Next I replaced the tone circuit w the tweed Bassman tone stack…again an improvement, giving me more tonal control, but I think i will continue to tweak it just to see how the changes affect it.

I have a question tho regarding how the tone control works in the original circuit linked above…it appears to be a low pass filter/ variable treble bleed, but I’m confused as to how it works with the resistors on either side of the cap to ground..can someone explain that please?

Also, I moved the trem to the 2nd triode of the 12ax7, like in a vibrochamp, and I like the trem better now.
 




Top