Recently I have been overhauling an Ampeg V4B bass amp, and after all the necessary repairs had been made, the matter of checking the bias current raised itself. The bias probes I put together from Bruce Hoffman's parts do not, unfortunately, work very well with the owner's chosen 7027A power tubes. So....what to do?
After a little thought a solution suggested itself. There are four 5w cement resistors marked as PR1 on the schematic in series with the plates on each power tube. Measuring the resistance of them yielded a value of about 4.5 ohms more or less although they are nominally 3.6 ohms. They're in the upper left hand corner of the picture-the grey blocks.
After writing the values down in my shop notebook I proceeded to set my multimeter to millivolts and CAREFULLY! CAREFULLY! read across each resistor. I ended up with measurements of about 70 mv each.
Why carefully? Because your probes are touching a measured 536 plate volts, doofus, and you can easily damage things here including yourself.
Dividing each reading by its attendant resistance value and calling it MA gave me the plate current numerical value, which, it seems, is a bit low. Because we're not accounting for screen current here, adding in 3-4 ma as a safety factor won't hurt anything and can avoid a costly episode of tube cookery.
I'm waiting for the mailman to arrive with a package of parts from Mouser Electronics that contains a couple of 100k trimmer resistors, one of which will replace the 75 k bias set resistor located to the left of the two small electrolytics in the image.
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