Doug, they changed the front end a bit. Would your mods still apply? My friend has already purchased the iron and built the amps (just finished the other mono block last night) so I doubt he would be willing to go to a different OPT. But he might consider some front end changes or something like that. Can you give me a hint about the mods?

Russ

But I want to have a nice chassis built, make this one a show piece. So I guess I better wait 'til the details are ironed out.

The most critical thing left to set its value for is the dropping R for the front end g2, and the size of the capacitor for bypass to ground. Since it's a PP stage, the bypass cap size is a bit less critical.

Got some cool frame-grid, high-gm pentodes for $2 each at Radio Electric Supply...looking good from here.
cheers,
Douglas

I have a matched pair of Electro-Harmonics 2A3 tubes, so I'll get another pair coming. Guess I better make time to do a shopping list. But the chassis fabrication is probably the thing that will take the longest. I'd like to have it built from steel plate, bent, welded and ground smooth, then sent to be chromed.

If I didn't like a straight diff amp, as drawn for Galahad, it is an attractive option. The stage as drawn also has room for improvement( read that: well tweaked complication ). It has to look possible for the lower skill/confidence levels too. ONe can either spend a lot of time with support, or a lot of time simplifying. I think the end result of the former results in better amps...:) The end result of the latter is greater profit, so that's a non-issue in this forum.
cheers,
Douglas

Don’t lose heart.

A P-P 2a3 is a good thing and this circuit look like a winner.

It may look complicated at first glance, with it’s pos and neg supply’s, grid choke and feedback.
But there is no reason why it could not be built in stages.

Start with the basic front end, hold off on the CCS for the moment. Leave out the grid choke and feed back for now and use cathode bias for the first cut.

This way we don’t need a neg power supply to get music going. and we have something that looks like the attached. I did not try to redesign the input section for single supply operation. Dougles is the pentode expert. This may not be Douglas’s amp, but the bones are there.

My apologies to Douglas for diddling with his design, he puts this stuff in for good reasons but how good does an amp sound that doesn’t get built?

At the last NE Bottlehead meet, I got to listen to Zacster’s 6b4 amp. Which is a cathode bias with a Curcio Front end, built on a ST70, no feedback.

It sounded outstanding. Has me dragging my ST70 out of the dungeon, for yet another flogging.

I think this is a good approach. Listen for a while, then add the feedback circuit or the grid chokes.

Next we need to address the Neg supply, not a big deal by it’s self. Just leave a bit of room under your chassie for a 3"x3" or so transformer. Now we can add the CSS (BTW Jim McShane Has a kit of CSS parts for the "bargain basement amp that they are working on over at the DEC forum. They lifted it form Guienevere).

With the Neg supply in place you can try fixed bias for the outputs if you wish.
I like cathode bias but if you have a neg supply it is easy enough to try it.

This way you can try one thing at a time and see if it is your cup of tea, with no waste save for a few resistors.

The Seth is a simple platform that offers lots of room for eventual improvements. Everyone is most welcome to offer their ideas about how to improve it.

"One can either spend a lot of time with support, or a lot of time simplifying. I think the end result of the former results in better amps...:) The end result of the latter is greater profit, so that's a non-issue in this forum."

It appears that you obviously miss the original point of the Seth by a mile or two. The main goal was simply to offer a simple PP design, easy to build for the first timer in amp building from scratch. Nothing more. I'm a strong believer that you have to walk before you can run. In my case I think it's much better for a rookie to start simple than to start with something complicated. Feel totally free to think otherwise.