60s Vox AC30 Twin Super Reverb

[sonicmojo]

Aloha,

Just trying to debug this sucker on a low budget and have a couple of questions.

1. How is the selector switch supposed to operate in the back as stock? I can't seem to find any specific info on that.
2. Vibrato/Trem works as do the normal and bright channels. Both Vibrato and Normal appear to have vibrato but not the Bright channel. Footswitch works.
3. Reverb is nothing. I know these have some phono type transducers that are unobtainium. All connections appear intact and I replaced a couple of the electrolytics near the reverb adjust pot board. Just trying to quickly assess whether it is worth debugging further as the owner wants to keep it stock as possible. Trying to replace with a modern tank is out of the question unfortunately. It is what I would do if it were mine but it is not.
4. What would you value something like this in its current state? Speakers are not original. the head cab has a couple of extra vent holes installed.

Also, the EL84 tubes run hot and one is starting to glow. I am assuming I should adjust the cathode bias resistor as EL84s these days are not as robust as once where. Any suggestions?

Thanks!

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[sonicmojo]

Does anyone have a schematic? It looks like there is an extra pot behind the reverb panel and I'm not sure why. I haven't found many pics of these and no schematics with reverb. I saw one site that had the reverb tank picture but this one has a dual set of springs contrary to what I have seen. I guess there weren't to many of these Top Boost reverb ones.

[martin manning]

There is a Vox site with schematics of vintage gear. I don’t have the link handy but you might search for it.

Here ya go: https://www.korguk.com/voxcircuits/

There is an AC30 reverb there; don’t know if that’s the one you have.

[sonicmojo]

There is a Vox site with schematics of vintage gear. I don’t have the link handy but you might search for it.

Here ya go: https://www.korguk.com/voxcircuits/

There is an AC30 reverb there; don’t know if that’s the one you have.

Yep, saw that but I think that is for some later amp mod. The one I have seems to have a transformerless reverb design. Here is a pic of the back panel that has two tube sockets sticking out like ears. Amp works fine without those tubes installed, but of course no reverb......I've fiddled around and one of the cathodes on the right side tube was reading 0 volts as it was directly grounded to what I think was the wrong side of the cathode resistor/cap. No help once I swapped it. The other side tube on the other side has only one triode used and the cathode cap/resistor for that one is grounded on both sides so I have no idea why that would be and can't easily get under that board to see what is wired or grounded there. I think someone tinkered in here before and added that extra pot on the far left (hard to see with the wires). I'll probably get nowhere on this without a schematic and some rare parts to try to fix it so will probably just put it back together without the reverb. I can probably disconnect this entire back panel board and switches and the amp will work fine without it. Heaters seem to be 6.5 without those two tubes installed so I think I am okay running it without them.

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[pompeiisneaks]

If it's the transformerless type, you may want to look at the Vox AC10SRV that was posted here recently https://ampgarage.com/forum/viewtopic.php?f=10&t=31921

Or the AC10SRT both of which have that same circuit https://ampgarage.com/forum/viewtopic.php?f=10&t=31585

It may not match 100% but it's in the same timeline of the build process. that amp seems like a bit of a one off like the Top boost addons they did at that time as well before incorporating it as a standard into later amps.

~Phil

[sonicmojo]

If it's the transformerless type, you may want to look at the Vox AC10SRV that was posted here recently https://ampgarage.com/forum/viewtopic.php?f=10&t=31921

Or the AC10SRT both of which have that same circuit https://ampgarage.com/forum/viewtopic.php?f=10&t=31585

It may not match 100% but it's in the same timeline of the build process. that amp seems like a bit of a one off like the Top boost addons they did at that time as well before incorporating it as a standard into later amps.

~Phil

Ok, thanks, it doesn't seem to compare much to what is in this though. This has a 1Meg reverb pot and some other internal pot tagged onto the board that I am not sure was original or not, maybe a dwell function.

[sonicmojo]

I got it working stable as a standard AC30 Top Boost. I changed out the 50 Ohm Cathode resistor for a 125 Ohm and I think the newer production tubes will appreciate that better. I have read that reverb on these one off models isn't so great anyhow. In all my hacking, I accidentally disconnected pin 5 from one of the tube heaters and that took me a while to figure out. I really don't like working inside this particular style of amp design. It has too many levels and panels to work on and you can mess something up easily! Thanks for the input but this one is done and gone back to the owner. I think he is happy enough without a total redo of the reverb circuit....

[pompeiisneaks]

Good to hear. Seems you're right, I still never found much on the reverb on these online either.

~Phil

[LuciMan]

@sonicmojo

for what it's worth - here are some schematics I found years ago when I started cloning my Vox series.
JMI used some ceramic phono PU systems to get the spring reverb system working - just like on the stand-alone Reverb Unit they made in the Sixties (Cliff Richard Reverb Unit).

Cheers
Claus

[sonicmojo]

@sonicmojo

for what it's worth - here are some schematics I found years ago when I started cloning my Vox series.
JMI used some ceramic phono PU systems to get the spring reverb system working - just like on the stand-alone Reverb Unit they made in the Sixties (Cliff Richard Reverb Unit).

Cheers
Claus

That’s the one! I may see that amp again sometime and take another try at it!

[Stevem]

I would try to implement the reverb from a Ampeg reverbrocket, the models that used no driver transformers and octal tubes have the greatest reverb.

[sonicmojo]

I would try to implement the reverb from a Ampeg reverbrocket, the models that used no driver transformers and octal tubes have the greatest reverb.

I agree the Reverberocket 2 sounds pretty great but the 6U10 in it has 12 pins! I’ve got 2 of them now.....

[Stevem]

No, that's not the model reverbrocket .
The model with the 6u10 is the later model with the front angle mounted control panel , as was referring to the early ones with the top mounted controls and use octal tubes , no compactron types.