Reverb Circuit Hum

[pompeiisneaks]

Anyone with ideas? Just trying to bump it. Or does the finding of the negative voltage at the grid, and the sounds on the wires mean that the socket is the likely culprit?

~Phil

[pompeiisneaks]

I had mwaller over this last weekend again, and figured out the odd negative bias, this was when the reverb pan wasn't connected, thus the tube couldn't conduct and the negative voltage was probably due to just electrons bouncing to the grid since they had no path out of the tube. once we connected the pan that went away.

Most of the troubleshooting seems to imply that splicing the el34world 10thtx one tube reverb onto this schematic is proving problematic. mwaller is going to potentially try to replace that setup with a high impedance tank and just clone the AC10SRT circuit, but I'm also looking this over and wondering if there weren't a few things that could be sorted.

mwaller, when I look at the ac10srt reverb circuit, the clean signal is actually coming through the 100k/470k resistors coming off of v3b pin 2 . If you didn't include that connection, or even if you did, that's going to potentially create more areas for the circuit to interact. The 1 tube reverb expects you to just tap into a 150 or maybe 220k resistor in the signal chain where you want to add reverb, send from the left side of it, and return to the right side so that resistor prohibits feedback from the reverb back into itself. A mixing resistor basically. Additionally in the one tube reverb there's another 100k resistor between the reverb level pot, probably to also help with mixing and non feedback, I don't know if that was there or not either. If you don't revert it to stock AC10 we could possibly look into that as well.

Let me know what next steps you've taken etc.

~Phil

[mwaller]

I'm currently waiting on parts to convert the circuit as close as possible to the original. I believe the high impedance tank will arrive today, but the resistors are still a few days out.
I'll keep you posted!

I had mwaller over this last weekend again, and figured out the odd negative bias, this was when the reverb pan wasn't connected, thus the tube couldn't conduct and the negative voltage was probably due to just electrons bouncing to the grid since they had no path out of the tube. once we connected the pan that went away.

Most of the troubleshooting seems to imply that splicing the el34world 10thtx one tube reverb onto this schematic is proving problematic. mwaller is going to potentially try to replace that setup with a high impedance tank and just clone the AC10SRT circuit, but I'm also looking this over and wondering if there weren't a few things that could be sorted.

mwaller, when I look at the ac10srt reverb circuit, the clean signal is actually coming through the 100k/470k resistors coming off of v3b pin 2 . If you didn't include that connection, or even if you did, that's going to potentially create more areas for the circuit to interact. The 1 tube reverb expects you to just tap into a 150 or maybe 220k resistor in the signal chain where you want to add reverb, send from the left side of it, and return to the right side so that resistor prohibits feedback from the reverb back into itself. A mixing resistor basically. Additionally in the one tube reverb there's another 100k resistor between the reverb level pot, probably to also help with mixing and non feedback, I don't know if that was there or not either. If you don't revert it to stock AC10 we could possibly look into that as well.

Let me know what next steps you've taken etc.

~Phil

[earl2]

Well personally if i wanted to keep the ac10 srt type reverb circuit I would take the component values from the Ampeg R 12 R schematic which Vox were obviously trying to emulate.
On the other hand having built this amp a year or so ago (and having experimented with it a lot), if I wanted to add reverb to it I would take that one tube reverb circuit from el34 and inject it (with the usual 100 - 200k mixing resistor) between V1a and V4b (on the original Vox schematic) and leave out that V2 mixer altogether. I know colleagues have done just that and it works fine. Basically kind of a 5e3 style amp with el84 power amp and with that one tube reverb injected between gain stages one and two. Tremolo as in the original which sounds good.

[pompeiisneaks]

Well personally if i wanted to keep the ac10 srt type reverb circuit I would take the component values from the Ampeg R 12 R schematic which Vox were obviously trying to emulate.
On the other hand having built this amp a year or so ago (and having experimented with it a lot), if I wanted to add reverb to it I would take that one tube reverb circuit from el34 and inject it (with the usual 100 - 200k mixing resistor) between V1a and V4b (on the original Vox schematic) and leave out that V2 mixer altogether. I know colleagues have done just that and it works fine. Basically kind of a 5e3 style amp with el84 power amp and with that one tube reverb injected between gain stages one and two. Tremolo as in the original which sounds good.

I wouldn't mind seeing a schematic of that, as is right now the setup he's using causes oscillation like mad, so somehow the signal is feeding back into itself, on the reverb only. Thus why he's trying to take it back to stock. If you have a schematic of how that would work in the AC10 SRT I'm sure we'd both love to see what maybe is wrong in this one.

~Phil

[pompeiisneaks]

oh additionally, the problem with that mentioned method is what do you do with V2? It's job is sending to the reverb tube on a, and recovery on b before sending back to the V4 as you mention? It now doesn't have a use for V2B?

~Phil

[earl2]

OK Phil I`ll send you my schematic of that with the one tube reverb.

[pompeiisneaks]

OK Phil I`ll send you my schematic of that with the one tube reverb.

excellent thanks!

~Phil

[mwaller]

I completely rebuilt the reverb circuit per the VOX schematic. Hum and buzz gone!



EDIT: Mod edit to embed youtube