Tweed Twin power reduction

[Helmholtz]

One problem I can see with low MV pot settings is bass roll off (because of the coupling caps).

Good point.

[Pedro Vecino]

I only use it on Vox type amplifiers. I usually install a limiting resistor in series to establish margins for good operation. That way nobody can say "it sounds really bad with the master volume at 1"

[Randall]

"That way nobody can say "it sounds really bad with the master volume at 1"

IMHO all MV's sound bad at 1 or so. I don't think that is really how this will be used. It's not a bedroom gain type of deal, it is just to try to make a Twin not so loud at gigs. Granted, it is a "high powered Twin", which makes little sense to me to try to fit it in a low to moderate volume gig, but hey?

[The Dude]

What I didn't get from the start is, why doesn't he just turn it down? I can't see that it would be much different than adding a MV (to a Twin, anyway).

[Randall]

What I didn't get from the start is, why doesn't he just turn it down? I can't see that it would be much different than adding a MV (to a Twin, anyway).

He is turning it down to a ridiculous amount. Last gig he reports to 1.5 - 2 on volume. Getting no preamp love at those levels. I think we can all agree this is not ideal, and why not get a different amp, but as I have told him, I am his amp tech, not his therapist. His money is good, if his knowledge and judgement is not.

[Chuck H]

What I didn't get from the start is, why doesn't he just turn it down? I can't see that it would be much different than adding a MV (to a Twin, anyway).

My thought too. It's not as if he's ever had the chance to crank it into clipping for practice or gigs. If he has then maybe.?. But otherwise it's the same dirt box no matter where you set the volume control on the amp.

[Chuck H]

He is turning it down to a ridiculous amount. Last gig he reports to 1.5 - 2 on volume. Getting no preamp love at those levels. I think we can all agree this is not ideal, and why not get a different amp, but as I have told him, I am his amp tech, not his therapist. His money is good, if his knowledge and judgement is not.

Put a 10% taper volume pot in his amp. He'll think your a magician

[nickb]

Isn't the output impedance of the "circuit" the PI output impedance parallel with the 220k bias resistance? And where is the signal "divided" to? A more easily understood voltage divider divides to 0V, but in this case it would have to be divided to the opposing phase. That seems like local NFB and should reduce the circuit impedance IIRC. I'm seeking clarity because I always saw this circuit as phase dependent like Rob's site says.

Sorry for slow response. Notifications are a feature of the site that could do with tweaking as they stop if you don't keep checking back to see what it happening.

There is nothing magic about 0v, it's just a reference. If it helps you can call the bottom end of the MV 0V and you'll see it more clearly. Local (voltage) NFB (i.e. on the PI stage) would lower the output impedance but would not change the divider operation. That said the NFB would stop when you hit clipping on the PI.

If you put a pot bewtwen the two phases and looked at the wiper I'd happily call that phase cancellation. But now as I say it, it seems like it's mostly semantics.

[Helmholtz]

But now as I say it, it seems like it's mostly semantics.

I would say there is not any more phase cancellation/mixing involved than in any PP stage where the (reflected) load is connected between opposite phase plate signals.
In fact a LTPI can be viewed as a PP amplifier and this MV would be its main load. Volume reduction is achieved by brute force downloading the PI.

[Zozobra]

I'd probably give trying a bigger PI bias resistor a shot if he comes back to you.

[Randall]

Text from customer today. "Randall, I love the master volume. Thank you".

Halelujah!