high plate voltages in blues deville

[MWJB]

Red DMM lead & red wire from OT (the centre tap) both need to be on CP3 at the same time.

Check the mA fuse in the DMM.

"it's the same way I took voltage readings originally"...surely when you took readings before the black DMM lead was grounded, not hooked up to pin 3?

[slapbackdaddy]

readings

Yep, fuse gone, changed that and am getting readings now. What i meant about previous testing but didn't make it clear enough when re-reading my post was that i was aware of CP3, and was sure i had the right terminal....yes, i was gounded before on the previous tests.

OK, first readings were LOW, around 7ma, so have wound up the trim pot to give me roughly 30ma on both tubes.

If i use webers bias calculator and input my current plate voltage (515v) it gives a bias setting 40.7ma for 6L6GC tubes to run at 70% plate dissipation.

You suggested 28-33ma....is it just a case of taste now, listening to what seems better or should i be heading more towards 40ma....or not!?

Thanks a million for all your help so far MWJB, you've been a star!

[MWJB]

EH claim 30W for their 6L6...tube vendors "in the know" have intimated to me that you are better off assuming 25W, which puts your 70% at 33.9mA. 30mA should be plenty to get a regular, Fendery tone.

JJ 6L6s could probably tolerate 40mA.

More current = more grease & more heat.

You're welcome, glad to be of help.

[slapbackdaddy]

I settled on 33.9 on one tube and 32.7 on the other....pretty good match i reckon...but then what do i know! Will see how it sounds over the weekend when i can give it some proper volume.

Thanks for talking me through the whole thing....boy, i've learned A LOT! + having the bias trim will help in the future i'm sure.

[Enzo]

Why make things difficult. The amp ALREADY has a 1 ohm resistor in the cathode leads of the power tubes. It is R66, and is right in the corner of the tube socket board. Looking in the rear of the amp, should be in the near left corner of the circuit board. Same place as the test point for the Hot Rod amps. Ground the meter, and then probe the end of the resistor right in the extreme corner.

[slapbackdaddy]

Thanks Enzo

Sounds an even easier way to do things!....where were you yesterday?

Anyway, everything is fine now, thanks to MWJB.

Cheers!

[MWJB]

Didn't think the '93 BDV had bias test point, thought that came in later (not on the '93 schematic)? Still, the OT shunt method allows you to see how closely matched the tubes are.

[Enzo]

OOOOPS. My apology. I was looking at the reissue - the current version from 2004. No pun intended. The original 1993 did not indeed have the resistor.

My disk has the two versions in two different file folders. Sorry.

I take nothing away from the shunt method, I use it myself, but in the presence of the little test point, I go for convenience.