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Plate voltages on 6BQ5's...

4/30/1999 7:03 PM

Plate voltages on 6BQ5's...
In my home-brew AC-30 I'm hitting pins # 7 & 9 at @ 390 volts... estimate ma = @ 35 ma...TONE=AWESOME!...Is this O.K.?...6BQ5's are SOVTEK...100ohm resistor is bias value (power supply)...X4 220's are doing the screens...I've been pumeling amp for @ 1 hour and the plates are not yet glowing!...Is this recommended, or am I GONNA DAMAGE SOMTHING?...Frank.
4/30/1999 9:33 PM
Bob S

If they get too hot they'll just burn up but you shouldn't hurt anything else. Your tube life maybe shorter. That's an awful lot of plate voltage on those tubes. Stock AC-30's run about 330 volts on the plates with about 40 to 45 ma. per tube. To continually run that high of a plate voltage you should reduce screen voltage at pin #9. You might try a 2.7k at the screens. Less voltage there will run the tube cooler, and with that kind of plate voltage you'll need to run them cooler.  
Bob S.
5/2/1999 6:37 PM

ANother thing is to measure the plate to cathode voltage not plate to ground.  
In this type of amp there probably won't be much more then 10v or 12v difference but you should check and see where it lies anyhow.  
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