Express won't bias - reads 0mA on probe

[jakehop]

Hi all,

I got my Express up and working with 2xEL34 about a month ago. I've been playing it regularly, and yesterday it stopped working - not sound at all! Everything lights up as it should, the voltages on the B+'s and the bias-circuit are all as they're supposed to be, and no resistors on the tubes (the 1K5 and the 1K0) all measure fine.

I tried different tubes, with no avail. The bias-probe works fine on my other amplifiers - in this one, it just shows 0mV (it's the 1 Ohm-resistor thing).

Any help would be greatly appreciated!

Kind regards, Jake

[jakehop]

Ohh, by the way, sound is coming through if I turn up the amp and hammer the strings. It's gated and distorted and very nasty sounding, and I can see the bias-reading move a couple of mA, if that's any help. I tried replacing the preamp tubes as well, just to find out if I'm insane or not. Still no luck!

- Jake

[bnwitt]

Jake,
do us a favor and fill out a voltage chart on the amp.

[bnwitt]

Even without a signal from the preamp section of the amp you should have quiescent current in the power tubes flowing if every thing is good there. So, something is wrong in the power section. Check all connections from the power tube sockets, the OT primary connections including the center tap ground, the PT center tap ground connection, the power tubes cathode ground connection etc. Chopstick those connections with a signal going into your amp and the volume set high. If you hit the bad connection, the volume will blast.

[skyboltone]

first thing I'd be lookin' at would be the power tube cathode grounds.

[rooster]

Pretty clear from what you said that your Presence control is disconnected as it leaves the 10K resistor and attaches to the Presence pot - or the ground attachment from the Presence pot is not attached.