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Misc. UL and Bias Questions

7/21/2000 3:26 AM
Misc. UL and Bias Questions
I'm just getting my homebrew amp up and running and I had some questions.  
1. I've got a pair of 6v6's connected in UL. I'm using 470-ohm screen grid resistors. My plate voltage is 378V while my screen voltage is 380V. Is this normal for UL connections? I'm drawing around 2-3 milliamps of screen current per tube at idle; is that about right?  
2. The amp is cathode-biased, currently with a 330-ohm resistor bypassed with a 50 uf cap. I'm getting around 27V at the cathodes. When calculating plate dissapation should I multiply the plate current by the plate-to-ground voltage or the plate-to-cathode voltage?  
3. I've heard that the current in an UL amp doesn't change as much from idle as a straight pentode amp. Is this true, and if so, approximately where should I bias this amp compared to the 14W max dissapation for 6v6's (which I believe I'm very close to right now)?  
One more thing; my heater voltages are running high even with all the tubes installed. 7.1V for the 6.3V heaters and 5.6V for the 5V heaters. How big of a problem will this be?  
Thanks in advance for all of your help!  
7/21/2000 10:38 PM
Ken Gilbert

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,  
7/21/2000 10:43 PM

Thanks for your help guys!

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