tube amp bias question can't get a current reading

[hammer99]

I'm working on a Sunn Model T 1974 guitar amp. I removed the center tap of
the OT, connected that to my pos. lead of my meter and the negative end to the
plate of the output tube (tried all plates) and Zero current reading. Checked and re-checked
my meter, using different ranges, but no reading to speak of. I get 570 Volts at the
plates and the amp works, but I can't get a meter reading?

 

[(*steve*)]

Maybe you're trying to measure an AC current on a DC range. The average current may be zero.

Alternatively, you may have damaged your meter.

 

[Resqueline]

What Steve says. Blown fuses in the mA range of DMM's are quite common.
But there are resistors in series with both the plates (47 ohms) and the grids (1.5k), just measure the voltage drop across those and calculate the current.
No need to disconnect anything then.

 

[davelectronic]

It was some time ago, and involved motor current, but the current was described as wattless, any way the book i saw this in suggested using a bridge rectifier package if your meter cant take an AC current reading, ok then you factor the voltage drop and RMS into the equation, as the rectifier alters the results, but if you factor this in, take a DC reading and factor in the parameters that are altered by the rectifier.