Biasing Blue Voodoo

[Shotgun]

Plate voltage is just peachy for all four, but cathode current is wanky:

Facing the chassis, left to right: 37ma, 35ma, 10ma, 11ma.

Sure, the 37 is within spec, but I can't figure out why the last (rightmost) two are so dang low. Yes, I've swapped the order of the tubes with no significant change, the first two tubes are just cold.

Oh, these are new matched mesa 6L6’s: the STR 454s.


Keith

[JerryP]

Re: Biasing Blue Voodoo

If you moved the tubes and the values followed the tubes then your tubes are bad, weak, or not matched. If you moved the tubes and the value remained in same place for all the tube positions then your amp has problems.
Jerry

[Shotgun]

Re: Biasing Blue Voodoo

Originally posted by JerryP:
If you moved the tubes and the values followed the tubes then your tubes are bad, weak, or not matched. If you moved the tubes and the value remained in same place for all the tube positions then your amp has problems.
Jerry

<font size="2" face="Verdana, Helvetica, sans-serif">What I should have said was, "Yes, I've swapped the order of the tubes with no significant change: regardless of which tubes are in the first (leftmost) two sockets, the cathode current on those two tubes is always cold: around 10ma to 12ma." But the plate voltage for all four sockets is normal.

K

[Shotgun]

Re: Biasing Blue Voodoo

I meant the rightmost two. Dang, you look at something upside down long enough... [img]images/icons/grin.gif[/img]

k

[jgcable]

Re: Biasing Blue Voodoo

wouldn't that indicate a transformer problem? Pete would know for sure.

[Shotgun]

Re: Biasing Blue Voodoo

Originally posted by jgcable:
wouldn't that indicate a transformer problem? Pete would know for sure.

<font size="2" face="Verdana, Helvetica, sans-serif">That is what I would think, unless there is something goofy about how Crate pairs the power tubes that I don't know about. I've ordered the schematic from them.

I hope Pete chimes in! [img]images/icons/wink.gif[/img]

K

[Shotgun]

Re: Biasing Blue Voodoo

Oh, and the negative bias voltage for each socket is near identical, and well within spec. Forgot to mention that...

K

[dangerbut]

Re: Biasing Blue Voodoo

Is it possible that 2 tubes are for channel 1 and 2 tubes for channel 2??

just a thought

( BTW, i know pretty close to nothing about amps)

[Twisteramps]

Re: Biasing Blue Voodoo

You probably should just take it to a tech. JerryP was right on the money - if it follows the tubes, it's tubes. If it follows the sockets, then it's the amp. Sounds like a problem in the bias supply, which is totally different than the plate/B+ voltage.

Take it to a tech. Just make sure you have a good description of the problem (write it down if it helps) and let the guy know what's up. It could be something pretty minor or major... let a tech have a swing at it. There's not a lot you're going to be able to do there.

Pete

[Shotgun]

Re: Biasing Blue Voodoo

Thanks, Pete - and all.

Keith