Carvin vintage 33 - rebuild - what circuit?

[hasserl]

I think that schematic is drawn wrong, I think those screen grid resistors supply both tubes. If not I would wire it up that way, or provide each tube it's own resistor.

Typically I see these run about 390 to 405 B+, depending on line volts, the screens run about 7 volts below that.

[überfuzz]

I think I'm gonna go for the one screen resistor to each power tube alternative. In the scheme their rated at 5W. Do I really have to beef them up that much? In my 6L6 builds I've used 1W or 2W metal film resistors. Voltage usually drops a volt or two, is it going to be more with EL84s?

Oh by the way. I looked at the pcb board. There are just two screen resistors, tube v5 and v8.

[hasserl]

Interesting, I checked out a few other schematics just to see what others have done, I noticed Peavey Classic 30 and Classic 50, both using EL84's, have the same thing, screen grid resistors on two of the power tubes, none on the other two. Also, Carvin did change that when they redesigned the amp and renamed it the Nomad, giving all 4 tubes their own screen grid resistor (350 ohm). Marshall didn't use screen grid resistors at all on the 20 watt, and a single 100 ohm resistor for both power tube screen grids in the 18 watt amps. Vox used a 100 ohm screen grid resistor for each power tube in the AC30 and the AC15.

I think the actual current draw isn't that much, but in the case of a tube failure the draw can escalate drastically. With 1 or 2 watt resistors they will act like fuses, opening up under the increased current draw. So you'll have to replace them along with the tubes. Some people like that, others don't. I guess you have to make up your own mind where you fall on that issue. You might want to start a new thread in the design and theory forum about screen grid resistors and see what kind of advice you get there. I'm sure a lot of the guys have already tuned out on this thread, you're likely to get a lot more activity on a new one specifically about screen grid resistors.

[überfuzz]

I guess you're right about this thread, it's more or less coming to an end. As of now I've got a circuit I'm satisfied with. There might be some minor changes here and there though.

Thank you for al your help Hasserl. I'll let you know when I fire it up. (It will probably be a couple of month...)