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Old 23rd October 2011, 12:06 AM   #1
Jpeg is offline Jpeg  United States
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Default Tubelab SE Bias Issue

Hello again,
My tubelab SE has been up and running nicely for a few weeks now, pretty much a standard build, though I did put 2 100ma panel meters in place of R18 and R29, and possibly a filament voltage switch... I was listening to a pair of vintage 2a3s last night when the music stopped and one of the meters pegged at past 100ma. I shut it off as fast as possible. I suspect the tube shorted out, though that is just a guess. I removed the tube and powered the amp back up. As soon as the B+ starts to register on my mutlimeter, the bias meter pegs.

I have read in some of these forums that the Mosfet often goes bad in situations like these, so I replaced it with a spare. Same thing. I also read that the best way to test it was to remove it from the circuit and see how it behaves, so I did, and it still pegs the meter.

I suspected that perhaps my russian surplus PIO coupling cap might be leaky, so I removed it as well, power on, meter pegs as soon as I have voltage. I did the same with the CSS. So I currently have no output tube, coupling cap, CSS, mosfet, or input tube, but I still get a over 100ma on the bias meter as soon as it starts getting B+. I checked the output transformers, looking for a short, but both sides are the same. I checked a few resisters, but they are the same reading for both the left and right channel.

It looks like I have a short in one channel, but I can't find it. Has anyone else seem this sort of thing? Any ideas? Thanks in advance
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Old 23rd October 2011, 12:19 AM   #2
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Does someone have the schematic they could post?

Working blindly,and taking a SWAG,I'd say you should connect the meters in series with the resistors,not in place of. --Just a wild guess.--
I'm not sure what R18 and R29 are,I'm kind of assuming (probably incorrectly) that they're cathode resistors?

Also,What kind of meters have you got? Do they have built in shunts?
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Old 23rd October 2011, 12:31 AM   #3
Jpeg is offline Jpeg  United States
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The Schematic is here. TubelabSE Scroll down a bit... There are several people on the site that used the meters, George, the disigner suggested the meter in place of R18, and R29. Not sure about the shunt, They are cheepo ebay meters, I did test them for continuity, and both sides tested the same attached to the circuit.
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Old 23rd October 2011, 04:37 AM   #4
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Ah,I see.

So even with no output tubes in the amp,the meters are still pegged at full scale?
If so,I would suspect the output transformer to be shorted,assuming there aren't any stray solder bridges or strands of wire poking out somewhere they shouldn't. Does your output transformer have ultralinear taps? Are they well insulated?

With no output tube in the circuit,there should be zero current flow through the meters,unless something is shorted,or incorrectly connected,or ???
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Old 23rd October 2011, 01:17 PM   #5
Jpeg is offline Jpeg  United States
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There are unused UL taps, I didn't think about those. I will check them.
Thanks
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Old 28th November 2011, 08:01 PM   #6
jrenkin is offline jrenkin  United States
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Any final results?
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Old 6th December 2011, 05:31 AM   #7
Jpeg is offline Jpeg  United States
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It seems to be stable at the moment. I had it all apart when I asked that question, I took care to insulate the UL taps when I put it back together, so it was either that, or knocked loose whatever was shorting it when I had it apart. My 75 year old sylvania 2a3 is sadly not off the hook yet. (I Really wish my tube tester had a 4 pin socket) There is a bit of a hum now, that was not there before. My intention is to take it apart and re-assemple it with more care in wire routing and grounds, but I am enjoying listening too much right now. Thanks for asking.
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