Transformer short

[lowell]

Got a Bassman 50 head on the bench. It sounds cold biased... kinda low volume and distorted. Measuring bias I get 0ma on my meter. I tried adjusting the bias and it didn't have any affect. I do have screen voltages. All voltages look good. ~ -50mv on grids. I'm thinking maybe the OT is bad. I get 50ohms from CT to both primary ends and 100ohms from end to end. I get about 6-8 megohm when I measure from primary to secondary. Any thoughts? Would a slight short between secondary/primary cause this symptom of not seeing any current on my meter? I'm using the transformer shunt method of bias measurement.

[g1]

Do you mean -50VDC on the grids? What range of DC do you get on the grids when you move the bias pot from one extreme to the other? Or is it a bias balance/output tubes matching control?

[Chuck H]

Try a different method of reading current. I have a Marshall OT on the bench now. Same primary ohms as you have. I didn't measure primary to secondary. Gimme a sec...

I get infinity, UNLESS I'm holding the bare wire leads to the probes with my fingers. Then the resistance across my body moves around between 6 and 8 megs

[g1]

Make sure your meter fuse for current range is good.
If you've used shunt method before, you should be ok with that method.
If this is a new symptom for the amp, then something must have went bad in the bias circuit, or power tubes have been replaced and bias is too far out.
From the looks of the schematic, it is just a balance, not adjustable bias circuit. If you can't find a defect, you can adjust the bias by increasing the value of the 3.3K coming off the bias diode, or install a pot in series with the 3.3K.

[R.G.]

Remember that you can pretty thoroughly test out a trannie with a multimeter, a 6V lantern battery and a neon bulb.

[Chuck H]

deja vu