Uberschall Rev 2 KT88 Bias Problem

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loganflynn294

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So I'm not new to biasing tubes but I've never swapped from EL34 to KT88 before and I'm having a weird problem. Amp in question is an Uberschall Rev 2 serial #220 that currently has EL34's in it biased at 36ma with 470v measured at pin 3 with the amp running (not in standby). One of the factory installed bias resistors has a resistor piggybacked on top that someone soldered in. It's a 5 band resistor, the bands are so close together that I can't tell if it's 100k or 130 ohms (brown, orange, black, black, brown). Anyways, popped in a matched set of Gold Lion KT88's and I cannot get the bias low enough. With the trim pot set to where the 34's biased at 36ma (almost all the way counterclockwise) I'm getting 120ma. If I turn the trim pot clockwise all the way the lowest it will go is 80ma. How can I get the bias lower? I thought if anything I wouldn't be able to get the current high enough not have it way too high. What am I missing?
 
loganflynn294":2cmdcfq6 said:
So I'm not new to biasing tubes but I've never swapped from EL34 to KT88 before and I'm having a weird problem. Amp in question is an Uberschall Rev 2 serial #220 that currently has EL34's in it biased at 36ma with 470v measured at pin 3 with the amp running (not in standby). One of the factory installed bias resistors has a resistor piggybacked on top that someone soldered in. It's a 5 band resistor, the bands are so close together that I can't tell if it's 100k or 130 ohms (brown, orange, black, black, brown). Anyways, popped in a matched set of Gold Lion KT88's and I cannot get the bias low enough. With the trim pot set to where the 34's biased at 36ma (almost all the way counterclockwise) I'm getting 120ma. If I turn the trim pot clockwise all the way the lowest it will go is 80ma. How can I get the bias lower? I thought if anything I wouldn't be able to get the current high enough not have it way too high. What am I missing?
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Well, if you're not scared to give it try. Remove the resistor that is piggybacked. See where that takes you. Then get some alligator clips and some different valued resistors and start experimenting. As far as what values at what you need that is out of my area of expertise. Just an idea. PErhaps someone more proficient can chime in.

Disclaimer:
If you do decide to do it, be careful!
 
That sounds simple enough, don't know why I didn't think of that first lol. So, snipped the added piggybacked resistor and the bias current shot up to 140ma! The piggybacked resistor is definitely 100k dropping the resistance from 33k down to about 25k. So it looks like I need to get the total resistance down below 25k and see what happens. Thanks for the advice! :thumbsup:
 
Thanks for the help guys, replaced the piggybacked resistor and got it to bias fine. Having another issue though; first time I fired up the amp with the KT88's there was a faint static/crackling sound. Even after getting the bias set, the sound is still there. It does it with all knobs set to 0 and doesn't change at all when turning knobs. Replaced the new power tubes with known good tubes, no change. Replaced preamp tubes one by one, noise still there. Pulled PI tube and noise got much louder. . All of this troubleshooting was done on the high gain channel, when I switch to the clean channel the noise gets much louder as well. The noise doesn't get louder as the volume is turned up, but at home volume it's pretty unbearable. Any ideas? Most of the caps had the markings scraped off when the amp was built so replacing those could be difficult...

Also, mods feel free to move this to the tech corner if that might help
 
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