Thomas Vox - reverb dwell and tone?

[MWaldorf]

As described in another thread http://music-electronics-forum.com/t42902/, I'm trying to restore a 1967 Thomas Vox Cambridge Reverb V1032. In the meantime, I've gotten a 1966 CR V1031, and I'm curious how I might go about adjusting the dwell and tone of the reverb - I want a shorter dwell and a brighter, splashier reverb. Looking at the Fender 6G15 outboard reverb circuit, it looks like the dwell potentiometer is before the tank and the tone pot after. 6G15 circuit link here - https://elektrotanya.com/PREVIEWS/63...6g15.pdf_1.png

I'm not super savvy on the hows and whys of electronic circuits, but in the attached schematic, I'm pretty sure that C14 on the reverb recovery RCA is shunting high end to ground. If that's correct, would I brighten the signal by using a different value capacitor or putting a resistor in line before it?

For the dwell, would I adjust the value of R17, or perhaps the values of the R19/C13 RC circuit on the Q4 emitter? Or would the value of C13 effect the reverb tone? I know that increasing the value of C4, similarly positioned on Q2, makes the tone brighter, like the top boost circuit on the big Thomas Vox amps.

I'm speculating on how to do these adjustments, so any suggestions are appreciated!

Thanks, Mel

vox-berkley-ii-v1082-1032.pdf

[52 Bill]

...I'm pretty sure that C14 on the reverb recovery RCA is shunting high end to ground. If that's correct, would I brighten the signal by using a different value capacitor or putting a resistor in line before it?

The small value of that cap would do little to the overall brightness of the reverb signal. It's likely there to shunt out stray interference signals and oscillations. Reducing the value of C15 will limit the lower frequency response thereby altering the bass/treble balance of the signal, making the reverb seem brighter.

To shorten the dwell or drive to the tank, you could try reducing the value of C13 which sets the ac gain of the circuit.

I'm sure R.G. will have a better answer for you, but it may take him a while to get to it.

[MWaldorf]

Thanks, Bill, I'll give that a shot.

[MWaldorf]

Bill,
I'm happy to report that your suggestions worked - I dropped C13 from 200uF to 150uF and C15 from 0.047uF to 0.033uF and the reverb is now tighter and brighter.
Thanks, Mel

[Mick Bailey]

In a similar manner, many Cascade reverb tanks have a cap soldered across the output. It has a great deal of influence over the sound of the tray, forming a resonant circuit with the output coil - so it isn't as straightforward as just creating a shunt to ground. Experimenting with different caps produces real differences in the sound (somewhat similar to having a wah wired into the reverb output) and I've often though of wiring up a rotary switch to select from a range of values.

[R.G.]

Finally got back to some Thomas Vox stuff - you guys don't need me, it seems! You're doing fine.

Mick - good info on the reverb. I hadn't thought of that. Makes a great deal of sense, just like the loading on a pickup can affect its mids and top sound. Those reverb transducers are inductors!

[52 Bill]

...I've often though of wiring up a rotary switch to select from a range of values.

In my day, that was called a Capacitor Substitution box.

[MWaldorf]

Another interesting tidbit - when I was trying different values for caps in the V1031, I discovered a capacitor under the board! It's 68pf tied from Q3C/Q4B to Q4E. I suspect it might not be original as Q3 and Q4 in the V1031 have been replaced. I was curious if this cap might have an effect on the V1032, so I tried it with some alligator clips and could hear no difference. Odd.

[52 Bill]

A cap that small really won't change the audio signal. Those are usually added to kill a ultrasonic oscillation or to stop radio signal interference. Try searching snubber cap to see if there is a definition out there.

This might be the way for you to tame the reverb oscillation on the other amp.

[MWaldorf]

I thought this little cap might help with the oscillation in the V1032, but it didn't. I'm going to try a 150uF in C13 instead of the stock 200uF and see if that helps.

[52 Bill]

I thought this little cap might help with the oscillation in the V1032, but it didn't.

I didn't mean that cap literally. I meant that you might be able to fix it by adding a small cap in the reverb return circuit to stop it.

[MWaldorf]

Gotcha. I've done a little research on snubbing caps and it looks like it deals with high frequency oscillation? My amp is experiencing low frequency oscillation. It's not affected by the position of the bass control, however. Where would the snubber cap go? There's already a cap (C14/470pF) between the reverb return and ground. FWIW, changing C13 to 150uF didn't change anything about the oscillation - same intensity and same starting point as the reverb knob is turned.

In the meantime I'm also playing with the value of C7 and how it affects the MRB tone. Lowering the value seems to make the MRB effect more intense - stock is 0.068uF, and I've tried 0.047uF and 0.022uF.

[MWaldorf]

I tried lower values for C15 and while delayed the onset of the oscillation only slightly. Then I tried increasing the value of C14, and that did the trick. I started by added 560pF to the existing 470pF, which totally removed the oscillation, as well as much of the "drip" of the reverb tone. Then I backed down the value as much as possible without having the oscillation. 320pF seems to keep the oscillation at bay and still leaves me a little drip.

[Mick Bailey]

In my day, that was called a Capacitor Substitution box.

In my day too, and somewhere I still have one. I was thinking more in a standalone design as a permanent feature. But yes, a sub box would be an easy way to fine-tune the tone of the recovery circuit.

[R.G.]

And would for sure be an interesting experiment.