tubes redplating when signal applied

[twilightofthedogs]

Howdy folks, i have a magnatone 442 with two el84 and ultralinear OT. All the voltages, bias, etc. check out when the amp is idling, but when I run a 1000Hz signal through the amp the tubes begin to redplate. The bias drops about 15 volts and the plate voltage drops about 40 volts. Any idea what might cause this?

[Steve Conner]

What are you using for a load? If it's a speaker, what does the amp sound like when it redplates? Have you tried new tubes? Have you scoped it for high frequency oscillations?

[twilightofthedogs]

Thanks Steve. Have tried a coupe different sets of tubes. Same issue. I have been applying the signal with a dummy load, but I have also played a guitar through a speaker. When I play it through a speaker, it sounds relatively quiet and distorted. It redplates when I run a signal through it and crank it.

I replaced the caps associated with the bias circcuit and checked the diode and resistors. The coupling caps also check out for leakage. I have not checked for oscillations with a scope, and to be honest I have a scope, but would not know exactly what to look for. Thanks!

[g1]

Why is the bias dropping 15V ? Sounds more like the cause of your problem than a symptom.
Do you have a schematic? Has it been modified for EL84's or are there other versions of this amp?
The only schematic I have seen shows a single ELL80 twin pentode power tube, cathode bias: http://www.magnatoneamps.com/schemat...natone_422.jpg
Is this a new problem for the amp?

[twilightofthedogs]

Apologies, I whote 422 above, it is the 442. Starlite bass. Originally had 7185 tubes, but switched to el84s. I did jumper the 1&2 pins and 6&9 to accomodate the tube switch. I have attached the proper schemo.

[Steve Conner]

Given the info above I'm suspecting a bad OT.

[g1]

Originally had 7185 tubes, but switched to el84s. I did jumper the 1&2 pins and 6&9 to accomodate the tube switch.[ATTACH=CONFIG]21953[/ATTACH]

Do you mean you removed the jumpers? They are shown as connected for the 7189's. Pins 1,6,&8 are internal connections for EL84's and should not be connected to anything.
Was the amp working correctly before? When did it start having the problem?

[twilightofthedogs]

The schematic shows them jumpered but they were not. I had read that some came this way. I got the amp in the condition it is in. I think the previous person had tried to just drop in EL84s, and become discouraged when it didn't work. I had suspected the OT, but was unaware that they could fail in this way. I thought they pretty much either worked or they didn't. I could definitely be wrong.

[g1]

7189A can be wired either way, EL84's can not. We can guess at what pins 1,6,&8 are internally connected to but there are no guarantees.
Remove the jumpers and wire it to 2 and 9.
Other than that, I would agree with Steve's suspicions but am wondering whether that would cause the bias drop.

[Joe L]

Apologies, I whote 422 above, it is the 442. Starlite bass. Originally had 7185 tubes, but switched to el84s. I did jumper the 1&2 pins and 6&9 to accomodate the tube switch. I have attached the proper schemo.

It isn't surprising that you are redplating because your plate voltage is too high for EL84s. With fixed bias you should be in the 325vDc range.

Either reduce the HV by 90 volts with a variable voltage regulator or find some 7189As.

JJ 6v6s could be made to work but you would have to come up with higher bias voltage and change out the sockets.

[g1]

It isn't surprising that you are redplating because your plate voltage is too high for EL84s. With fixed bias you should be in the 325vDc range.

Doh! Thanks Joe L, didn't notice that 410V shown at the plates on the schematic.
EL84 not an acceptable sub in this case.
MagnatoneAmps.com

Edit: EL84M (Sovtek) or 6P14P are both supposed to handle the higher plate voltages and be acceptable subs for 7189's