Soundtronics Custom300 7027 screen resistors?

[IFixAudio]

Hey Group,
Is anybody familiar with Soundtronics? I can not find anything on them.
I have burned screen resistors on the four 7027's.
Anybody know what a standard screen resistor value would be for a quad 7027? I can get some voltages after I replace the caps.
Also, why are they usually two different values for the two tubes?

Thanks

Jim

[Gtr_tech]

470 ohm is fine. What is 2 different values......the screen resistors? They are always the same for all output tubes in a given amp.

[IFixAudio]

470 sounds good to me but I have seen this value higher and lower. The Traynor YBA3 uses resistors on two of four 6CA7 tubes but not on the other.

http://www.lynx.bc.ca/~jc/721005_YBA3.gif

Also the Ampeg VT22 uses 7027's and has two different supplies for each pair.

Schematics :: Ampeg V4.com

Jim

[Steve Conner]

In the case of the YBA3, the designer probably wanted to use no screen resistors at all for maximum power output. But I guess he had trouble with parasitic oscillations, and found that two screen resistors was the minimum that would stop them.

In the VT22, both of those supplies connect to point "E". So they're connected together. It has no screen resistors, but it has grid stoppers and plate stoppers.

If all four screen resistors are burnt, the amp was probably cranked up into an open circuit, and the screen grids may be shot too.

[Gtr_tech]

The VT22 schematic denotes the screens running from the "E" supply....both pairs. The traynor has all the screens tied together with a common 1K 20w resistor. Why they put 47s on one pair's screens I don't know. I don't care for that method, although Traynor did it alot back in the 60s/70s. Individual non wirewound screen resistors make a good fuse in the event of tube failure. They open up and essentially take the tube out of the circuit by cutting off the screen supply.

Screen resistors aren't super critical as far as value.....they're there to keep the screen grid from over dissapating. So you use whatever value keeps screen current within the safe area. 7027s are just a slightly uprated and repinned 6L6, so treat them as 6L6s. 470 1 or 2w is fine.