Marshall problem opinion please

[ckpop]

I have a 50 watt 1987 marshall and having a problem with the current draw on the output tubes. With 1 ohm resistors and a meter I cant get the current draw to swing properly and it really does not really move. When I try to use my compu bias the numbers jump on the B +voltage with little movement on the current draw and not stable.

I can hear a buzz even with no tubes and volumes down. Brand new filter caps and output tubes.

Any ideas

Thanks

[ckpop]

what would cause a a buzz to happen with no output tubes in ?

[LeftyStrat]

I have had a bad output transformer buzz like that. I'd probably de-solder the OT and test it. You want to check for shorts or open connections on both the primary and secondary.

If this started after the new caps or new tubes, I'd check the power supply output. Would also be interesting to see what kind of ripple you had on the b+.

Also check it with known good tubes.

More experienced guys will chime in, but this might give you a good start.

[ToneMerc]

I have a 50 watt 1987 marshall and having a problem with the current draw on the output tubes. With 1 ohm resistors and a meter I cant get the current draw to swing properly and it really does not really move. When I try to use my compu bias the numbers jump on the B +voltage with little movement on the current draw and not stable.

I can hear a buzz even with no tubes and volumes down. Brand new filter caps and output tubes.

Any ideas

Thanks

What type of tubes did you install?

How are you attempting to adjust the bias voltage?

TM

[ckpop]

I installed JJ EL-34's then I had a tried a used set of siemens el34's I that are pulls that I know that work. The same thing

Checked the bias pot, bias caps, and diodes

[Structo]

Pull the power tubes then measure the bias swing voltage while sweeping the pot from stop to stop.

It's really good to have an old analog meter for this as you can see irregularities better during the sweep.

I am thinking the bias pot is probably bad.

[ToneMerc]

I installed JJ EL-34's then I had a tried a used set of siemens el34's I that are pulls that I know that work. The same thing

Checked the bias pot, bias caps, and diodes

Hmm, "the same thing" is relative. It's hard to help troubleshoot when you give very limited info. There's not enough info for me make any type of call.

1. Was the bias system working before you changed the caps(?), tubes or what work did u do period?
2. What was the previous tube type?
3. What value resistors(fixed or otherwise) are in the bias supply?
4. What's the neg voltage after the bias diode?
5. What's the neg voltage at the input to the bias pot?
6. What's the neg voltage at the input to the 220k grid resistors?
7. What's the voltage at the tube grids?

TM

[John_P_WI]

Checked the bias pot, bias caps, and diodes

How about the polarity of the bias cap. Is it installed correctly?

[ckpop]

will start from scratch and provide info

[husky]

what would cause a a buzz to happen with no output tubes in ?

OT orientation to the Power transformer will do that.
Do a turns ratio test on the OT and see if it specs out correctly