Advice needed: unorthodox schem

[Arthur Dent]

Hey guys,

I just need some suggestions on this amp design. I'm pretty confident on the preamps, as they are pretty basic, but I definitely want some advice on the PI. I've never seen pentodes used like that before, and I'm not too familiar with their characteristics. If the heptode driver/ pentode cathodyne PI thing isn't likely to work, I suppose I could stick a 12AU7 LTPI or similar after the 6BE6.

I want the high-gain (top) channel to be close to metal territory (but with tone), with the clean channel capable of some significant crunch.

Any advice on the schematic would be extremely appreciated.

[BackwardsBoB]

Strange to use a variable mu pentode in a cathodyne PI that forces good linearity and unity gain. I think you need a resistor to ground to bias the 6BA6 grid. If you substitute something for the 6BA6 based PI, you'll need to keep the gain low. A triode based cathodyne PI would work.

Do you get the feeling that somebody had a bunch of 6BE6s and 6BA6s laying around?

It does look like an agressive little thing.

[Arthur Dent]

Yeah, I noticed that I missed the grid reference on the PI last night. It wouldn't go to ground though, but instead to the junction between the 100k and the (missing) 68 ohm resistors off the cathode.

Yes, I do happen to have a few 6BA6's and 6BE6's lying around. Even if I didn't, I would probably use them anyway. They're pretty quiet in my experience, and they sound very, very cool. Oh, and they're cheap. NOS for under $6.00!

What would you expect the characteristics of the PI to be like?

[BackwardsBoB]

Oh. I see it now. You do get gain from the PI. It looks like tons of Watts with SE harmonics to me. Very cool. And the non-linearity of the 6BE6s add. Many second-order harmonics and push-pull. Is that where you're trying to go?

[BackwardsBoB]

You know, the more I like at your PI, the less I understand it, and it's not anything you're doing that's special. It looks alot like the PI in an Ampeg SVT. I just can't get my brain around it. I'd have no idea how to size the input coupling cap, the grid bias resistor, etc. I need to think about it a bunch more.

I can understand how you could bias the grid of the PI by DC coupling to the preceding stage's plate, but the AC coupling of the input and biasing to the circuit's non-inverting output ... Isn't that positive feedback? Help! my brain is exploding!

I'm going to have to draw AC and DC models or simulate it or something. I am useless...

[Steve Conner]

I think it's just a regular cathodyne PI, but with a pentode, and also it won't work because there's no DC path to bias the control grid.

[Arthur Dent]

Yeah, I noticed that missing DC reference resistor, as I mentioned earlier. I also forgot the coupling cap after the cathode follower in the high-gain channel...
Hey, that was the first thing I've used that CAD program for, and it was 11:30 at night.

Steve, you're right, it is basically a normal cathodyne PI. All I'm really unsure of at this point is whether or not that PI is going to allow enough signal swing to at least crunch the KT88's.

If the 6BA6 can't provide enough juice, I could simply stick a 12AU7 in there, with a triode feeding each power tube...

[Arthur Dent]

Okay, here's a revised version of the schematic, complete with the components I missed last time.

I also added a 12AU7 after the PI, with a triode amplifying each side. The pots feeding the grids of said 12AU7 will be ganged.

I've discovered that 6BA6's vary a whole lot. I got four NOS ones, three of which are RCA and one of which is GE but clearly made by RCA, and none of them sound much like each other. I've also got three Sonotone 6BE6's, which are made in West Germany and sound incredibly cool.

Backwards Bob, I certainly hope you are correct about the SE harmonics.

Any more comments on the design would be much appreciated.