Eric Barbour's 6BM8 One-Tube Reverb Imagined...

[uneumann]

.... By leaving out the other side, it forces the amp modder to come up with a solution for recovery and mixing on his own. In other words, Barbour's "one tube reverb" isn't a complete reverb--it's a "one-tube half-reverb" or "one-tube reverb driver."

Maybe this is a solution... The Vox V125Lead has a cascode (JFET/12ax7) input. The gain of the tube hardly matters in this configuration. You can use a lower u triode (12at7) if that's all you have. This circuit can be tweaked (see below) to give you a true voltage gain of 1000x. That gives you 5V out from 5mV in. Distortion is actually pretty low too - the 2nd harmonic is down ~50dB.



Anyway - I suspect you could mount the little J113 on the tube socket, so maybe this counts as 1.1 tubes...

Something to consider ...

[dchang0]

Maybe this is a solution... The Vox V125Lead has a cascode (JFET/12ax7) input. The gain of the tube hardly matters in this configuration. You can use a lower u triode (12at7) if that's all you have. This circuit can be tweaked (see below) to give you a true voltage gain of 1000x. That gives you 5V out from 5mV in. Distortion is actually pretty low too - the 2nd harmonic is down ~50dB.

[ATTACH=CONFIG]58314[/ATTACH]

Anyway - I suspect you could mount the little J113 on the tube socket, so maybe this counts as 1.1 tubes...

Something to consider ...

Very cool! This could be a fantastic and easy-to-fit-in solution. I could replace the daughterboard I have in my amp with a newer one that has space for the JFET on the daughterboard. Thanks for the suggestion!

[SoulFetish]

Maybe this is a solution... The Vox V125Lead has a cascode (JFET/12ax7) input. The gain of the tube hardly matters in this configuration. You can use a lower u triode (12at7) if that's all you have. This circuit can be tweaked (see below) to give you a true voltage gain of 1000x. That gives you 5V out from 5mV in. Distortion is actually pretty low too - the 2nd harmonic is down ~50dB.

[ATTACH=CONFIG]58314[/ATTACH]

Anyway - I suspect you could mount the little J113 on the tube socket, so maybe this counts as 1.1 tubes...

Something to consider ...

This was my solution for the recovery amp in the 6U8A reverb amp I designed for my buddy's Silvertone 1474. I used an LSK170 for the JFET for its very low noise characteristics.
.

[dchang0]

Awesome--thank you!

[SoulFetish]

One other thing: If you go with the hybrid cascode, I found it much easier to add a potentiometer for the jfet source biasing resistor. This is the most critical part of the circuit to it's operation, and the differences in jfet tolerances can make it unstable to go with one resistance figure.
It's easy enough to bias a particular fet. Adjust the source resistance for minimum current and gradually decrease the resistance while measuring the upper triode plate voltage until the plate voltage gets to the desired idle voltage set by the load line.

you dig?

[dchang0]

I dig--thanks!

[Splash Zone]

Appreciate the knowledge shared in this thread. I am looking to implement this circuit in a build of Gibson GA-5T. I’d like to understand a little better how to implement the schematic in terms of wiring the RCA jacks. If I am using grounded jacks, must I also ground the ground tab of the jack to the circuit, or is the chassis ground provided by the jack itself sufficient? So for the below circuit which I grafted into the Gibson schematic:

So for this schematic, my layout for these jacks currently looks like the following:



Thank you for looking in advance!

[Chuck H]

Appreciate the knowledge shared in this thread. I am looking to implement this circuit in a build of Gibson GA-5T. I’d like to understand a little better how to implement the schematic in terms of wiring the RCA jacks. If I am using grounded jacks, must I also ground the ground tab of the jack to the circuit, or is the chassis ground provided by the jack itself sufficient? So for the below circuit which I grafted into the Gibson schematic:

So for this schematic, my layout for these jacks currently looks like the following:



Thank you for looking in advance!

You never want to ground things twice because this can create ground loops. So in your case the chassis ground will have to be sufficient. I'd suggest the MOD tank because it has a small PCB for the connecting jacks that can be modified for shield grounding. This may or may not be important as many implemented reverbs need grounding modification to avoid ground loop hum.

[Splash Zone]

You never want to ground things twice because this can create ground loops. So in your case the chassis ground will have to be sufficient. I'd suggest the MOD tank because it has a small PCB for the connecting jacks that can be modified for shield grounding. This may or may not be important as many implemented reverbs need grounding modification to avoid ground loop hum.

Thank you! Much appreciated! I bought the Switchcraft rear-mount naturally insulated jacks. So I think these will be grounded via the chassis and so therefore I should leave the sleeve terminals unwired.

I have Mod tank on the way, so thanks for the tip.

[Chuck H]

Correct. The jacks ground via mounting and the little tab that sticks off can be used as a ground point for other circuits.