Low volume and red plating (bugera 1960)

[Zozobra]

Evening folks,

I was gifted a troubled bugera 1960 that has an intermittent fault and has been bounced off every amp tech in Manchester at least twice. The last guy couldn't find anything wrong with it, my friend got sick of the hassle, gave it to me to tinker with and then bought a sound city 120 and lived happily ever after.

Today is the first bit of time I've had to look over it since it came into my possession. The symptoms are that it is pretty quiet, even at full bore. With the volume up full all of the tubes will red plate when a chord is strummed. First thought was that the bias might be screwy but the bias is right on queue at -44VDC. The plates and screens are 490VDC and nothing in the poweramp looks immediately suspicious. Any ideas? My second suspicion is that the OT is goosed. I've not had the pleasure of checking for a dead OT before so before I figure out the best way to do that is there any other low hanging fruit that I may have missed that I should check?

[TigerAmps]

Hi - Do you have a schematic and what tubes are in the amp now? Red plating is usually a high current condition caused by low or no bias.
Do you have a bias probe that you can monitor the current at idle and while playing?
I would check for loose connections in the output stage and the bias circuit.
Also - if it has shorting output jacks, make sure they are making the correct connections.
Bad jack issues range from no output to red plating, low distorted output.
The original Peavey Butcher had a crazy jack scheme to accommodate different speaker loads.
They were a messed up (not opening and/or closing correctly) and drove me crazy. If you have a spare output transformer,
I would disconnect the current output transformer and wire in the spare to rule out the jacks and the transformer.
Connect the output of the spare transformer directly to a known good speaker.
Try these one at a time - as this may help isolate the problem or shed additional light on the issue.

[nickb]

If the output trannie is bad you won't get anywhere near full power out. Hook a sine wave source and measure the output using a scope.

The tubes do dissipate more power at full output, therefore if the idle dissipation is very high then that would cause the problem-measure it.

Ultrasonic instability is another idea - watch the scope.

[Zozobra]

Hi - Do you have a schematic and what tubes are in the amp now? Red plating is usually a high current condition caused by low or no bias.
Do you have a bias probe that you can monitor the current at idle and while playing?
I would check for loose connections in the output stage and the bias circuit.
Also - if it has shorting output jacks, make sure they are making the correct connections.
Bad jack issues range from no output to red plating, low distorted output.
The original Peavey Butcher had a crazy jack scheme to accommodate different speaker loads.
They were a messed up (not opening and/or closing correctly) and drove me crazy. If you have a spare output transformer,
I would disconnect the current output transformer and wire in the spare to rule out the jacks and the transformer.
Connect the output of the spare transformer directly to a known good speaker.
Try these one at a time - as this may help isolate the problem or shed additional light on the issue.

No schematic unfortunately. It has bugera branded EL34s, which are probably shuguan EL34bs. I do have a Weber BiasRite that I bought a number of years ago. 3 of the tubes are biased at ~ 30mA at idle and the 4th at ~10mA. This follows the tube when moved to a different socket. The cathode current exceeds 150mA for each tube when playing with the volume/gain low. I don't want to push it too far as I don't want to bugger up the probe. This certainly looks rather high, especially for the lack of volume as that should be stripping paint off the walls.

The amp does have a rather flimsy impedance selector switch. Problem persists on both jacks and I've tried both a 16 and 8 ohm load and tap to the same effect.


I don't have a suitable spare OT to tack in unfortunately.

If the output trannie is bad you won't get anywhere near full power out. Hook a sine wave source and measure the output using a scope.

The tubes do dissipate more power at full output, therefore if the idle dissipation is very high then that would cause the problem-measure it.

Ultrasonic instability is another idea - watch the scope.

I will have to dig my scope out and run through your suggestions. I fear it might be dead as wasn't looking great the last time I used it. But hey, it was free so I can't complain too much.

[TigerAmps]

Can you bypass the output board and go directly to a speaker? That would help rule out at least one part or isolate the problem to the board.

[mozz]

Last bugera i worked on had 50 volt caps and was running 40 something. Check or change those caps.
Most are clones so check the schematic for Marshall 100 watt SL. Also, if it's factory tubes, they are garbage.

[Zozobra]

I'm going to pull the OT to test it, rather than try to work out what is going on with the output ans loop board. If it's okay I'll toss the boards and build something new. If not then I guess it'll sit on the junk pile until a suitable OT drops in my lap.

[g1]

If it's intermittent, it will be tough to test the OT out of circuit. You will probably need to use a load and deliver some power through it.
Maybe the neon bulb tester will show it, but not necessarily with an intermittent fault.
But from your description of the symptoms, it sounds like the fault is always there and not intermittent?

[Zozobra]

If it's intermittent, it will be tough to test the OT out of circuit. You will probably need to use a load and deliver some power through it.
Maybe the neon bulb tester will show it, but not necessarily with an intermittent fault.
But from your description of the symptoms, it sounds like the fault is always there and not intermittent?

I have a spare neon and a 6vac transformer to test it with. The fault is certainly consistent at the moment. The last tech to look over it gave it a clean bill of health so I'm not sure what's gone on. I can only assume that there has been some sort of user error and it's been incorrectly loaded at some point since and the ot hasn't liked it.