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previous: morris I'm just getting my homebrew amp up... -- 7/21/2000 3:26 AM View Thread

Re: Misc. UL and Bias Questions

7/21/2000 10:38 PM
Ken Gilbert
Re: Misc. UL and Bias Questions
hey morris.  
brian's right about the higher g2 voltage in UL operation... there's less resistance between the UL screen tap and the center tap, so there's less voltage drop.  
i WOULD increase the screen stoppers a bit to 1k or so. that can only help those 6v6's live a bit longer and shouldn't affect tone at all. i personally go even higher, up to 2k2.  
2-3 mA of Ig2 is fine, well within reason.  
to calculate the power dissipated in the tube, at the tube's plate, you need to know the voltage from plate to cathode, since that's what the tube "sees." anything outside the tube, like voltage across the OPT primary, or across the cathode resistor, doesn't factor into the voltage that the tube "sees."  
in other words, measure the voltage from plate to cathode.  
peak currents in a UL amp are a bit lower than for pure pentode operation, so it does cut down on the change in average plate current between no load and full load--IF the amp is NOT class A.  
regardless of the connection, class A has the distinct honor of drawing very LITTLE additional plate current as the signal is increased. so if you've got a class A amp, there's very little change in average Ip, doesn't matter if it's pentode, triode, or UL.  
chances are you're working in class AB, so there WILL be a change in average Ip. don't worry too much about it, though. if you're really curious, run the amp into a load while you monitor the voltage across the cathode resistor.  
i WOULD run the 6v6's at some reasonable dissipation, say 10W, for longevity. you could bump that up a BIT if you increase that Rg2, btw.  
hope it helps,  

morris Thanks for your help guys!... -- 7/21/2000 10:43 PM