Verellen amp problem...

[johnnydeformed]

I have a Verellen Skyhammer and it is a fantastic 2 channel, High Gain amp. On occasion, the volume will fade out for a second or two and then comeback. At practice the other day it happened more frequently then it ever did and then last I get virtually no sound out of it except a distorted sound on either channel with both master volumes cranked. I took some measurements and it appears the EL34s are not drawing any current. The Plate voltage is around 505V and is the screen. The bias voltage however is measuring -140mV. I would expect this to be around -40mV. So that appears to be the problem, the power tubes are not really being biased and therefore aren't drawing any current. The issue now is to find the root of problem. I suppose it could be a component in the bias supply or worse yet, the PT itself. I should note that the heaters measure ~6.1V AC.

I have been in contact with Ben Verellen and he has been wonderful. Truly great customer service. I would gladly ship it back to Ben for service but he is in Seattle, I am in Philly and I have a show on the 13th, no back-up amp and my entire rig and sound is built around the Skyhammer so I am stressing a little bit. I will also be out of town a little next week. I could take it to an amp tech but I am not sure a turnaround in less than a week is even possible.

Any thoughts, trouble-shooting ideas or even a tech suggestion for a quick turnaround?

 

[Dave K]

If your bias voltage is reading -140vdc, your power tubes will not be conducting, so you're on the right path.
Is there a trim pot in the bias supply ? If there is and turning it shows no change in your - bias supply rail, the voltage divider has opened up.