LarMar MV bias voltage
Posted: Sun Jul 04, 2010 1:21 pm
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Not normal, bias current shouldn't really change with LarMar installed or not. Assuming it biased up correctly before the PPIMV install and you are using the 1 Ohmer biasing method? If so check to be sure your meter batteries (and fuses) are good, that you didn't accidentally set the meter to read milliAmps rather than the desired milliVolts for starters.
Usually a serious mistake in PPIMV install leads to way too hot a bias (loss of - voltage on pin 5's), your problem is opposite so maybe you disturbed something in the bias supply while installing your Larmar. Did you change any values in the bias supply perhaps (like the resistor inline to the bias pot - increasing it would make the amp cold if it was biasing properly before with a lower value, and vice versa)? If so, put the original value/s back (when last known to bias properly). Check your bias supply ground is solid.
If nothing in the above sorts it, what's your B+ (pin 3), pin 4 and 6 readings, and what - DC voltage range do you get on pin 5s of the power tubes? Also are you using EL-34s?
Pics never hurt.