Did a Buyer Kill My Amp?

[JKDenver]

I recently sold a Van Alstine Ultravalve Tube Amp to a buyer in a different state. I purchased it new in 2015 and took good care of it and used it pretty lightly. Before listing it I put in new EL34 tubes and a new GZ34 rectifier. I biased it and put it on my oscilloscope and it measured beautifully. Upon first trying the amp the buyer contacted me to say the the EL34 tubes were not lighting and wondered if I knew why. Then he told me that the rectifier tube had come apart. Finally he said that he put it a different set of tubes and the left channel wasn't working. I told him to send it back to me for a refund and I've just received the unit.

The base of the rectifier tube was indeed damaged and now there is only bare glass on the bottom. I put in a used JAN-CG-5U4G rectifier and all the working tubes that I had previously used before. I ran it on a dim bulb tester and guess what? It's drawing high current non-stop! It draws current even without a rectifier tube installed.

I have a hypothesis that I'd like to share and would like feedback. I'm guessing that the buyer accidentally put an EL34 in the rectifier socket and the rectifier in the EL34 socket. I'm wondering if doing that could cause the problems that I'm seeing now, and what the problem might be. Thoughts?

Many Thanks,

Jonathan

 

[danadak]

Compare the pinouts which will provide insight as to what could happen.

 

[JKDenver]

I'm not sure that I'm interpreting the pinouts correctly but here goes: There are no common pin pairs between the tube types. GZ34 uses pins 2 and 8 to power the heater and cathode (with no connection on pin 7). EL34 uses pins 2 and 7 for the heater and pin 8 separately to the cathode. So, no overlap between the two types.

GZ34 uses pins 4 and 6 to power the plate. EL34 uses pin 4 for the screen grid with no connection on 6. Again, no overlapping connections on any pin pairs. Does that mean that no harm comes from mixing up these tubes in their respective sockets? That makes it less likely that the buyer messed thing up, right?

I did some more testing and found that the current draw isn't excessive. It's only about 1.5 amps. But there is a BIG PROBLEM. I tested the bias points and the right channel was under 2V but the right channel measures 395VDC! Between 1.5 and 1.8VDC is the ideal bias setting.

Does that information provide any further hints to what might be wrong, how it could have happened, or how difficult/expensive it might be to repair?

 

[JKDenver]

From the owners manual:

"The correct setting of the bias provides a total cathode current for each pair of 6CA7 or EL34 tubes of 100 ma. This current through the precision 16-ohm resistors produces a voltage drop of exactly 1.6vdc."

Could one of those resistors have fried? Seems like that would account for the high reading, but may not explain why it shorted out.

 

[ZipZapOuch]

Did he kill it? Maybe.

Was the tube defective? Maybe.

Did something happen in transit? Maybe.

Regardless of what happened, is there a satisfactory way to diagnose it that will make both buyer and seller happy? Unlikely

Solution:
- buy it back (refund buyer),
- tell buyer (no refunds, it worked when it left here)
- partial refund