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previous: Roberto Lasco I'm servicing a Mark I (I guess, I'... -- 5/2/2000 9:18 PM View Thread

Re: Boogie's bias question

5/2/2000 9:37 PM
Gil Ayan
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Re: Boogie's bias question
quote:
"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.  
 
quote:
" 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.  
 
quote:
"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!  
 
quote:
" 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.  
 
quote:
" 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.  
 
Gil

 
Replies:
Roberto Lasco Tubes with cathode lifted are supposed to not conduct - yes/no/maybe ? -- 5/2/2000 10:09 PM