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Question about a Crate VC5115 bias adjustment


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5/25/2000 5:23 PM
Steve Slick
Question about a Crate VC5115 bias adjustment
Okay, okay, so I'm stupid. A guy who has a Crate VC5115, Vintage Club I believe, says you can't adjust the bias. He said, "You would have to derive bias voltage in a different manner in order to vary it, i.e. create a new bias circuit off of the B+"  
 
Okay, help me out here guys. What's special about this amp? I ain't getting it.  
 
Cheers,  
 
Steve Slick
 
5/25/2000 6:10 PM
marleyman
ah...beat me to the punch! i will follow you thread to see what people come up with here!  
 
thanks!  
 
marleyman
 
5/25/2000 6:19 PM
Ray Ivers

Steve,  
 
Do you have a Crate schematic source? Just off the top of my head, I can definitely recommend NOT deriving the bias off the B+, no matter how easy and convenient and gee-I-don't-have-to-find-and-install-a little-transformer that might be, unless you at the very least use a beefy Zener regulator, conducting heavily, with a cap bypass to eliminate the 'hash' noise. Maybe he's referring to the amp being cathode-biased? Does the amp use a bridge rectifier with no PT CT?  
 
Ray
 
5/25/2000 8:08 PM
Steve Slick

Ray,  
 
Yeah, it's Marleyman's (Dreadlocks, M-man?) amp. No I don't have a schematic. But if it is cathode bias, it can still be biased, just in a different way. No?  
 
I'm not familiar with the amp, but I'm looking at it from a theoretical standpoint. How could you hava an amp that could not be biased. Unless Ohm's law is suspended for some reason.  
 
Hey, truth is stranger than fiction. It could happen... maybe.  
 
Cheers,  
 
Steve Slick
 
5/25/2000 9:50 PM
Ray Ivers

Steve,  
 
Sorry - I got shanghaied by the 'derived from B+' part of your post and drifted away from the larger question. Yes, I would have to say that any amp can be biased, or modified to do same if not equipped stock with the capability, except maybe a Class B triode output stage, which usually has no bias at all. I've seen posts here that talk about adjustable cathode-bias, even. Is there anything that the Ampage folk cannot do? - surely not.  
 
All of which doesn't really help you with your situation. I would suggest all the usual don't-have-a-schematic Brownian activity - checking for the presence of cathode resistors and/or tracing backward from the output tube grids if a negative voltage is found there, to the nearly-inevitable voltage divider that will be tweaked. In the meantime, I will check for Crate schematics, and if I find any, you'll be the first to know.  
 
Marleyman - thanks for the post. Remember - if your tech smokes your amp, he owes you a new one. == ;~>  
 
Ray
 
5/25/2000 6:30 PM
marleyman ray, here's a post about this amp fromawhile back
BTW, the amp is mine and was wanting to put new tubes in, and was wondering about re-bias. it has GT EL84S tubes in it now, and was wanting to try something different (i.e. Svetlana, JJ/Tesla, or others) and waswondering about biasing first...the GT's are a hardness rating of 6.  
 
anyways..here is the post from awhile back  
 
 
Mike-  
 
The design of the Vintage Club (i am assuming that this is the fixed bias version) does not allow for the bias voltage to be varied. You would have to derive bias voltage in a different manner in order to vary it. i.e. create a new bias circuit off of the B+...  
 
I worked on one of these for a friend about a month ago, and i was desperately trying to get the bias adjusted differently, but to no luck. Fianally fellow ampagers set me straight as to how this circuit functions.  
 
Unless you want to spend a lot of time messing around with things... I would leave the baby stock and get a fan and blow it into the back of this baby... (the power tubes are being run at about 110%-115% of their rating, i think that my friends ran the EL84's at 375V on the plate, and about 38mA per tube...This cooks them pretty well...(Inspiration to keep this baby stocked with fresh power tubes..))  
 
*end of rant*  
Have Fun!  
Rob W.  
 
Ps.. I think i still have the posts around from my discussion about this amp... Lemme know  
 
 
 
thanks!  
 
marleyman
 
5/25/2000 10:21 PM
eth
Yo! That amp may have a bias servo in it. The bias will always be set and balanced. I'm not positive of this particular amp, but I know Crate has done that on some of theres. If you are swapping in the same type of tube(el84), I wouldn't worry about changing bias until it sounds like crap. If you wanted to trace for a bias servo: look for a current sense and an integrator circuit(op-amp?). _Eth
 

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