|Gil Ayan||Re: Boogie's bias question|
"I'm servicing a Mark I (I guess, I'm not familiar with Mesas). After the royal pain in the ass that is to install a bias pot, I biased the 6L6s at 40mA (80mA per pair)."
Really? Hmmm... in my Mk I all I had to do what ulsoder two resistors (in paralle, the dreaded "factory select") and put a trimmer in place of them.
" Vplate is 400V, so 16W per tube which is fine."
That sounds awfully low for a Mark I. More like 460 and upwards... Older Mark Is used silverface Twin transformers and newer ones had Boogie PTs with about 465VDC on the plates... I think there is something fishy right there.
"This is for the 100W setting. At the 60W setting, current drops a little, 10mA or so per pair."
The current would go up, as the plate loltage will rise on the order of 10V or so. But I have found the idle current to go up a couple of mills (per tube) only, not virtually double like you found!
" For what I infered from the circuit, the "low" power setting lift the cathodes of 2 tubes from ground, thus cutting them, right ?"
That's what I have seen in the past, yes. However, if the amp has been modded, maybe one pair ir wired as a triode and the other as a pentode? Checck to make sure, only one way to find out.
" So, these 70mA are now passing through just one tube ??? (28W!)
What gives ? What is the proper way to bias these things ?"
I don't know how you are measuring the bias, what method are you using? I would suggest doing the 1 ohm cathode resistor trick to be sure. What you can do is measure the plate voltages in 100 VS 60W mode -- should be a bit, not all that much -- and whether the bias supply voltage changes at all when you flip the 100/60 switch -- it shouldn't.