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Re: Are you techs experiencing the same problem w/ JCM20


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3/22/2006 2:38 PM
paulh
Re: Are you techs experiencing the same problem w/ JCM20
Biasing the Svetlanas to the factory spec burns them up! I bias mine to 35-37ma per tube. Sounds fine and they don't come back!
 
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3/22/2006 9:04 PM
Roberto Lasco
Great call John, you nailed it !
This version has the 220K grid stoppers indeed. Changed them for 10K, after initial warm-up, up to now (1h turned on), bias seems stable.  
 
Own you a six-pack !
 
3/23/2006 9:56 PM
Jeff Seal
Re: Are you techs experiencing the same problem w/ JCM2000s?
just fwiw..... that stupid V.P.R switch on the front will seriously alter bias settings! the interconnect cable(Con 12 on tube board to Con 20 on Lead board)is so "flaky" that it's not uncommon for bias to change on a whim. I usually strip the wire and solder the connections...Of course heaven forbid, the actual VPR switch should fail to make contact.  
 
"Wave soldering" in these are so bad I would never gig with one of these in my life!  
 
Jeff Seal
 
3/23/2006 10:19 PM
Roberto Lasco
sure
After I finished the DSL, there was a TSL waiting with a bias problem also. And it was dropping voltage indeed on the V.P.R circuit (actually on the balanced lineout), due to a leaking cap on the input of opamp...
 
3/28/2006 1:27 PM
Steve Dallman

I have terrible memory problems, but I believe I had two of these in where the power tube pins went through a double sided printed circuit board where the pins were soldered on one side only. One amp had been arcing on the unsoldered side. I checked the second amp in stock and found the same, although no arcing had happened...yet. I'm sure most make good enough contact to avoid problems, and maybe most are soldered on this side as well.  
 
FWIW from a dementia tinged tech.
 

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