JMI Vox AC10 add-on top boost?

[seven]

Hi there,

I am planning out a Vox JMI AC10 build/clone. The normal channel is known for being somewhat whimpy. Had a chance to play an original one recently that was just serviced and I would agree with that sentiment.

So I got to thinking about adding top boost to it, if that would be possible?

Based on some high level research, seems I can add it in a similar location as in the AC30:

JMI schematic:



Thomas schematic:




Top boost schematic, for reference:



What do y'all think?

[J M Fahey]

It should be doable, why not?

Top Boost was not much more than a tone control add-on module, should work on more amps if inserted in a similar position.

[seven]

Thank you JM. Do you think I would be able to have it switchable during operation, say with a DPDT shorting (make before break) type switch?

[seven]

Leaving this here in case someone wants to try something similar...

[Helmholtz]

Leaving this here in case someone wants to try something similar...

So you already built it?
If so, how do you like it?

[seven]

So you already built it?
If so, how do you like it?

Haven't built it yet, just sharing what another forum member shared with me. What do you think of the schematic?

[Chuck H]

Do you think I would be able to have it switchable during operation,..

If you mean to switch it remotely from the amp, like a footswitch, a regular DPDT switch and flying a cable out of the amp won't do. It would be very noisy and lossy in the HF. You would need to do it with a relay and the footswitch would trigger the relay power. You'll also need to add 0DCV reference resistors to the pole circuit on the switch as shown or it will likely make a pop when switched.

[seven]

If you mean to switch it remotely from the amp, like a footswitch, a regular DPDT switch and flying a cable out of the amp won't do. It would be very noisy and lossy in the HF.

Thank you Chuck for your feedback. I was planning on using a DPDT on a push-pull pot... I guess that won't quite work...

You would need to do it with a relay and the footswitch would trigger the relay power.

So, a relay. I presume I can power the relay off the filaments?

You'll also need to add 0DCV reference resistors to the pole circuit on the switch as shown or it will likely make a pop when switched.

Is the "switch" here the switch to trigger the relay or is it the relay switch itself?

Lastly, are there any examples you could point me to that I could reference to see how this is all implemented?

Thank you!

[Helmholtz]

I just noticed: V2-A should have a dedicated 1M grid leak resistor.

[seven]

I just noticed: V2-A should have a dedicated 1M grid leak resistor.

Hi Helmholtz, help me to understand... the original Vox Top Boost schematic doesn't show the 1M grid leak resistor for that triode... is the purpose of the grid leak resistor to reduce (eliminate?) RF? Does it cause a loss of gain?

[Helmholtz]

You're mixing up grid stopper and grid leak.
A grid leak resistor wired from grid to ground is used to make sure the grid sits at 0 VDC.
In the original AC 30 the V2-A grid is hard-wired to the vol. pot wiper, so no extra resistor required.
But when switching the grid might be shortly left without a DC path to ground which could cause popping.

[Chuck H]

Using a plain DPDT for switching at the amp itself will be fine. No need for a relay.

The 0DCV reference resistors would be the same thing as Helmholtz mentioned as the "grid leak" for V2A. That resistor needs to be there to keep bias on that triode. Without it the tube loses bias briefly when the switch is thrown. It will certainly pop and though it might be ok otherwise, it might not. Best practice would be to include a 1M grid leak on V2A

[Chuck H]

the original Vox Top Boost schematic doesn't show the 1M grid leak resistor for that triode...

The grid leak for the stock circuit is the 500k volume pot feeding the Top Boost circuit grid. But this circuit isn't switched so there's no need for the extra 1M to ground. Signal loss will be small. As if you were using a 333k volume pot rather than a 500k. If it bothers you then you could use a 3.3M resistor instead. Making it as if you were using a 430k volume pot. Which I think would be within spec. of the original circuit.

[seven]

Thanks Chuck and Helmholtz for the explanation and guidance.

How does this look?

[Chuck H]

Thanks Chuck and Helmholtz for the explanation and guidance.

How does this look?

Nope. Like this. Notice that it was only the "pole" (and the actual grid of the tube) that didn't have a reference to 0VDC.