plate voltage/bias variation on Bogner xtc 20th help please!

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RG955TT

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I have posted this in the Bogner section with no response so thought I'd post it here too so i could ask for some opinions from all of our resident amp guru's/builders.

I am trying to bias up a set of GLKT77s in my XTC 20th and I am getting variation in plate voltage from 498 to as high as 533, does this seems normal?

Bias was also moving from 30-33 (over the 4 tubes-this is where I'd like to keep it, Jorg at Bogner said ok to go as high as 35) to as high as 37 then would go back down eventually but oddly didnt always seem to correspond to the change always in plate voltage. This is after an hour or more (several hours) of being on and running amp off of my Furman voltage regulator.

I am using my Compubias to make these measurements as it reads plate voltage, bias and tube output (watts) at idle all at the same time for two tubes at a time. I dont remember this being an issue in the past and is drving me kind of nuts so I was looking for any input. Thanks

Sound wise my issue is it sounds great one day and the next is thinner and somewhat fizzy and colder (almost like it's underbias'd) Thanks
 
are you saying your plate voltage fluctuates that much between like 2 different days or even same day a few minutes apart???? on all four tubes does it fluctuat. try measuring plate voltage with a meter to make sure the compubias is ok.. I use those and love them and have not had that type of problem but ,,, we can try to eliminate some things '
 
Same day, within minutes at times and some times I can watch it move. I biased other amps s I dont think Compubias is the issue. I only can read two tubes at a time but yes it seems its on al 4. Thanks
 
bump up for more ideas/help. Thanks guys. Just trying to determine if this issue warrants a trip to Bogner for the amp.
 
Is the plate voltage different for one power tube vs. another? Or are all the tube seeing the same plate voltage, but that voltage fluctuates?

Plate voltage should be the same for every power tube. And if your B+ is fluctuating that much, it is most likely a problem with your mains (the wiring in the building). But it could also be an issue with the amp. I would grab a real multi-meter and test your power outlet before doing anything else. If your mains power is fluctuating, then that's your issue right there. Unless you have other amps that don't fluctuate when plugged into the same outlet.

If that's the case, and the issue is the XTC, then it could be any number of issues from the power transformer to a bad component on the board.

In any event, I would use a real mutli-meter instead of those plug-n-play things that go into the tube sockets. You'll get more accurate results and you won't have to keep pulling tubes.
 
Seems like its on all 4 tubes, home is 12 years old with good power and I have other amps that have not displayed this much of a range, amp is plugged into Furman voltage regulator AR1215.
 
RG955TT":3szpyma0 said:
Seems like its on all 4 tubes, home is 12 years old with good power and I have other amps that have not displayed this much of a range, amp is plugged into Furman voltage regulator AR1215.
Click to expand...

Then, yeah. Send to Bogner.
 
RG955TT":1b6fk5ju said:
Seems like its on all 4 tubes, home is 12 years old with good power and I have other amps that have not displayed this much of a range, amp is plugged into Furman voltage regulator AR1215.
Click to expand...
Have you tried it without the Furman?
 
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