Peavey Valveking Pre PI Master Volume mod Thoughts?

[torcamaniac]

Hi guys.
I'm thinking about installing a Master Volume to my VK 112 before the PI.
It uses the second half of V3 as PI.
It is cathodyne circuit like old Fender Deluxe.
I was going to just replace R155 1 Meg grid leak resistor with a 1Meg log pot with wiper to grid of V3B pin 7.
What do you think?
Here's an original piece of schematic and beside it the modded version.


I guess I could do a Post PI MV by connecting one end of 1meg pot to grid of the first power tube and the wiper to the grid of the other one, but I tought with PRE MV it sounds better on circuit like VK and Deluxe, and would have a bit more control at lower volumes.

[torcamaniac]

Anyone with suggestions?
Would this work?

[potatofarmer]

Yep; works fine. I'd also recommend adding a hefty grid stopper between the MV wiper and the grid: The Valve Wizard

[torcamaniac]

Thanks man but are you sure I should add it to wiper and not before the upper end of the pot?

It says on wizards site to use a 100K to 1M grid stopper, but on the picture the 470k form a voltagr divider with 47k. It is like this:

[robrob]

I typically don't recommend adding a grid stopper between a master volume and cathodyne phase inverter grid because if you add one you are altering the amp circuit for good. But without one you can simply dial down the master volume and the resistance above the wiper becomes grid stop resistance (resistance between the grid and coupling cap). So it's the best of both worlds: at max master volume the circuit is unchanged but dial down the MV to add some grid stop resistance while attenuating the input signal and the cathodyne will be happy while being heavily overdriven.

[torcamaniac]

Thanks guys.
Still waiting for the orders (pot and some other stuff) to come and hopefully I'll try it on weekend. I'll also be adding adjustable bias and bias measure points.
Than also add a choke on my JTM 60...
Lots of work and drilling

[potatofarmer]

Thanks man but are you sure I should add it to wiper and not before the upper end of the pot?

Yes; that 47k (and the diode) is an arc-protection circuit that's effectively out of the circuit once the tube has warmed up. Adding a stopper to the "upper end" of the pot would permanently attenuate the master volume.

I typically don't recommend adding a grid stopper between a master volume and cathodyne phase inverter grid because if you add one you are altering the amp circuit for good.

See, I advocate for doing so for exactly the same reason. I suppose it depends on how you like the amp's current performance dimed. It'd probably be inappropriate on a 5E3, for example. I suppose it's a matter of taste, but it's more needed when the master volume is all the way up - at lower MV settings, it's less likely the cathodyne will be overdriven.

Maybe I shouldn't have suggested it, but I thought "while you're in there..."

[torcamaniac]

Yes; that 47k (and the diode) is an arc-protection circuit that's effectively out of the circuit once the tube has warmed up. Adding a stopper to the "upper end" of the pot would permanently attenuate the master volume.




See, I advocate for doing so for exactly the same reason. I suppose it depends on how you like the amp's current performance dimed. It'd probably be inappropriate on a 5E3, for example. I suppose it's a matter of taste, but it's more needed when the master volume is all the way up - at lower MV settings, it's less likely the cathodyne will be overdriven.

Maybe I shouldn't have suggested it, but I thought "while you're in there..."

Okay, I guess I will try both setups and see what I like better.
Thanks guys...