JMI Vox AC10 add-on top boost?

[fazeka]

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 (red arrow):

JMI schematic:



Thomas schematic:



Top boost schematic, for reference:



What do y'all think?

[Willabe]

Why an AC10?

Why not single channel AC15 or 1/2 of a single channel AC30?

[fazeka]

Why an AC10?

Why not single channel AC15 or 1/2 of a single channel AC30?

Because I'm planning an AC10 build. Been there/done that with both an AC30 TB I've owned and a AC15 I've built.

Not that it matters, but here's a pic of my AC15 build:

[bmccowan]

Nice AC-15! I'm wondering if this Mojo Schematic offers some ideas. I have not built it as is, but used parts of the circuit in a 6973 build.

[pdf64]

Yes, sure a TB section can be fitted in there.
Personally I find Vox designs somewhat half boiled, in how one channel has the trem, another has TB etc.
If it was me, I'd mix the channels then feed them into the TB section.
The unused LTP input cap would then need to be grounded.

[fazeka]

If it was me, I'd mix the channels then feed them into the TB section.
The unused LTP input cap would then need to be grounded.

Hi Pete,

If I'm understanding you here, you would have the two channels both go into the TB?

To be clear, I actually like the EF86 channel as-is. Even the ECF82 normal channel has a place on my sonic palette. I was hoping to top boost just the normal channel but also have it switchable for stock normal channel tones. Best of both worlds (?). DPDT switch in between the normal volume wiper and 0.01 (C12 on the Thomas schematic)?

[pdf64]

If it was me, I'd mix the channels then feed them into the TB section.
The unused LTP input cap would then need to be grounded.

Hi Pete,

If I'm understanding you here, you would have the two channels both go into the TB?

To be clear, I actually like the EF86 channel as-is. Even the ECF82 normal channel has a place on my sonic palette. I was hoping to top boost just the normal channel but also have it switchable for stock normal channel tones. Best of both worlds (?). DPDT switch in between the normal volume wiper and 0.01 (C12 on the Thomas schematic)?

Yes, that's just my preference, if you want to keep the stock circuit then my idea's no use to you.

[fazeka]

Hi Pete, yes OK, understand.

Any problem you see adding in a DPDT to switch the TB circuit in/out of the Normal Channel (see red arrows in schems above)?

[pdf64]

Hi Pete, yes OK, understand.

Any problem you see adding in a DPDT to switch the TB circuit in/out of the Normal Channel (see red arrows in schems above)?

The main issue I foresee is getting the layout and lead dress such that the darn thing keeps stable.

Removing the cathode bypass from the TB's pin3 can help with that. It drops the maximum available gain a tad but the TB adds plenty anyway, so no real downside.

[fazeka]

The main issue I foresee is getting the layout and lead dress such that the darn thing keeps stable.

Understand! I am not married to TB but it would be nice to have it switchable so if I build it and it's unstable, I still have a great (IMO) sounding normal channel.

Removing the cathode bypass from the TB's pin3 can help with that. It drops the maximum available gain a tad but the TB adds plenty anyway, so no real downside.

Help me out with this as I've never unbypassed a cathode... ...would bypassing mean eliminating only the cap or eliminating both the cap and the resistor?

Otherwise, thank you Pete. Very helpful advice!

[Willabe]

Help me out with this as I've never unbypassed a cathode... ...would bypassing mean eliminating only the cap or eliminating both the cap and the resistor?

Yes, only leaving out the cap. The tube stills needs a cathode (K) R to bias properly.

And it's a 'bypass' cap, so what's it bypassing? If you leave the K R out, there's nothing to bypass.

[fazeka]

Yes, only leaving out the cap. The tube stills needs a cathode (K) R to bias properly.

And it's a 'bypass' cap, so what's it bypassing? If you leave the K R out, there's nothing to bypass.

Gotcha, thanks for the explanation.

[fazeka]

Leaving this here for future reference (thanks to sluckey):