Sound City 50 PA. Tube bias question

[bubbastain]

Just picked up an old Sound City 50 PA Plus. It works but has a few issues. It’s a 50 watt all tube PA running a pair of EL34’s. As soon as I got it home I pulled it from the chassis to check a few things. It’s running a plate voltage of 507. I checked the bias on the left tube and it measured 35ma. I lowered it to 30 which was as low as the bias pot would go. The right side tube measured 48ma! Running near 100%. I immediately lowered the bias trim for that tube (each of the two power tubes has its own) as low as it would go, but that only brought it down to 45ma.
So, do I have the worst matched pair of tubes with one being incredibly hot to begin with? Or is there something else I should check?
I do intend to take it to a competent tech for a thorough inspection and replacement of suspect parts. Just not yet.

 

[JerryP]

Swap the tubes and see if the bias numbers follow the tubes or stay with the socket. If it moves with the tubes then the tubes are mismatched.

 

[bubbastain]

Swap the tubes and see if the bias numbers follow the tubes or stay with the socket. If it moves with the tubes then the tubes are mismatched.

I was just thinking the same thing on my way home. I’ll try that first. Thanks Jerry.

Update: I just tried switching the power tubes and it indeed is that one tube that is extremely warm to begin with. Another thing I found out is that even though each power tube has it's own bias trim pot, they do work together too. I would actually prefer they didn't, but they do. So when I raised the trim pot for the tube not connected to the bias probe, the number went down slightly for the one connected to the probe. Unfortunately I don't have two bias probes so I can't see if one goes up and the other goes down proportionately. But that tube is still way too hot. I'll get a matched set that are on the colder side.