Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: MWaldorf on July 09, 2020, 12:09:01 am

Title: Vox AC15CH reverb circuit mods?
Post by: MWaldorf on July 09, 2020, 12:09:01 am

I just scored a great deal on a Vox AC15CH head.  Fun amp, but seriously lacking in reverb quality.  I know I can change the tank out to improve it somewhat, but is there anything in the circuit that would give a more Fendery reverb tone?  I've read that C21, a 560pF cap, can be reduced in value to increase the treble (decrease the non-treble) in the reverb (and ideally the "drip")  If I understand it correctly, this 560pF cap is a high pass filter for the reverb, so reducing the value would let only the higher end treble through - of course, I could be wildly mistaken.

Am I correct that modding this cap will help make the reverb drip?  Are there other components I should change? 

Thanks everyone!
Mel

Here's the thread where I found the cap mod suggestion:
https://music-electronics-forum.com/forum/music-electronics/45585-vox-ac15c1-reverb-seeking-improvement?t=44440 (https://music-electronics-forum.com/forum/music-electronics/45585-vox-ac15c1-reverb-seeking-improvement?t=44440)

Here's the schematic of the amp; fourth image down is the reverb section, and C21 is in the upper right of the image.  For some reason, the circuit in front of the reverb pan is on the right, and the circuit after the pan is on the left. 
(https://i.postimg.cc/5NyyQFc5/AC15-C1-schem-p1.jpg)
(https://i.postimg.cc/BQz6TQsr/AC15-C1-schem-p2.jpg)
(https://i.postimg.cc/8cKCnTvr/AC15-C1-schem-p3.jpg)
(https://i.postimg.cc/HWBjL9Ns/AC15-C1-schem-p4.jpg)
(https://i.postimg.cc/dVT04PVL/AC15-C1-schem-p5.jpg)

Title: Re: Vox AC15CH reverb circuit mods?
Post by: tubeswell on July 09, 2020, 01:54:35 am

If its about not having enough wet signal in the mix, you could increase the reverb recovery gain around U4B (or reduce the attenuation after U4B by maybe putting another 33k in parallel with R93?). At the end of the day, you're only ever going to be able to achieve a limited amount of change by tweaking the existing reverb circuit. YMMV

Title: Re: Vox AC15CH reverb circuit mods?
Post by: MWaldorf on July 09, 2020, 04:09:14 pm

thanks Tubeswell.  I assumed there's a limited amount I can improve the circuit.  I'll try the parallel 33k and see what it does.

Title: Re: Vox AC15CH reverb circuit mods?
Post by: tubeswell on July 09, 2020, 05:55:56 pm

Quote from: MWaldorf on July 09, 2020, 04:09:14 pm

I'll try the parallel 33k and see what it does.

No guarantees implied or given

Title: Re: Vox AC15CH reverb circuit mods?
Post by: MWaldorf on July 09, 2020, 06:24:01 pm

Tubesell - nope, I'm holding you responsible for everything.  In fact I think it's your fault that I don't play as well as Segovia or Hendrix.  Also, didn't I read somewhere that you gave Brutus the dagger he used to kill Caesar?  ;)

Seriously, thanks, and lack of guarantee duly noted.