Been building one of the Tubelab SE amplifiers for 300b power tubes. I've got Plitron OPTs, Mundorf silver coupling caps, etc etc. I've got some money in this build and perhaps my reach has extended my grasp. I have successfully assembled one of the Dynaco VTA kits, so I assumed things would be fine here, but I ran into a problem during checkout procedure accompanied by the usual sinking feeling, existential dread, and so on that go with seeing good parts lunched.
1. Everything assembled, no tubes: filament power checks ok on all tube sockets
2. Rectifier tube in: filament glowing, B+ present at ~171vDC
3. 300b power tubes in: filaments glowing, biased to lowest -vDC
4. all tubes in: all filaments glowing nicely. getting 90vDC from coupling cap/driver plate to ground, but the trimpots don't seem to be changing the voltage.
and that's where things got problematic
I stopped, removed my meter and said "hmm". then moved the meter to check mA, grounded black sensor again and touched the coupling cap lead closest to the left 5842. a loud pop through the speaker, quick flash in the 300b for that channel. I pulled everything away and turned the amplifier off. Visual check doesn't look like anything is wrong.
After a couple of minutes turn the amp back on. Noticed the slightest wisp of smoke from one of the two semiconductors in the top corner of the board, I believe the 5-pin 5 volt U1 regulator.
Could anyone please suggest what I should do now to determine what I have done? Any help appreciated. I hate to say it, but there is some nervousness associated with this project
1. Everything assembled, no tubes: filament power checks ok on all tube sockets
2. Rectifier tube in: filament glowing, B+ present at ~171vDC
3. 300b power tubes in: filaments glowing, biased to lowest -vDC
4. all tubes in: all filaments glowing nicely. getting 90vDC from coupling cap/driver plate to ground, but the trimpots don't seem to be changing the voltage.
and that's where things got problematic
I stopped, removed my meter and said "hmm". then moved the meter to check mA, grounded black sensor again and touched the coupling cap lead closest to the left 5842. a loud pop through the speaker, quick flash in the 300b for that channel. I pulled everything away and turned the amplifier off. Visual check doesn't look like anything is wrong.
After a couple of minutes turn the amp back on. Noticed the slightest wisp of smoke from one of the two semiconductors in the top corner of the board, I believe the 5-pin 5 volt U1 regulator.
Could anyone please suggest what I should do now to determine what I have done? Any help appreciated. I hate to say it, but there is some nervousness associated with this project
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What about input tubes between sequence number 2 and 3? What are the voltage numbers for step 3?1. Everything assembled, no tubes: filament power checks ok on all tube sockets
2. Rectifier tube in: filament glowing, B+ present at ~171vDC
3. 300b power tubes in: filaments glowing, biased to lowest -vDC
4. all tubes in: all filaments glowing nicely. getting 90vDC from coupling cap/driver plate to ground, but the trimpots don't seem to be changing the voltage.
You need to follow Tubelab's "check out" steps by the letter. If anything doesn't match each step, DO NOT MOVE FORWARD or you will risk damaging the parts and speakers.
I am doing my darndest to keep it by the book here 
Tested very slowly today. Here are the results:
WITH NO RECTIFIER
Filaments are measuring well (filament supply OK!)
R7/B- at -298vDC
R30/B+ at -6.88vDC
Looking ok. Powered off for 5 min and then inserted 5ar4 rectifier.
WITH RECTIFIER
Rectifier heats ok
B+ at 470vDC (I was looking at the wrong resistor before. Oops.)
On for about a minute before I recognize smoke, turn the amp off and find a wisp from R25 which appears to be be in the bias supply for the left channel.
What else can I check? What else may have failed?
Tested very slowly today. Here are the results:
WITH NO RECTIFIER
Filaments are measuring well (filament supply OK!)
R7/B- at -298vDC
R30/B+ at -6.88vDC
Looking ok. Powered off for 5 min and then inserted 5ar4 rectifier.
WITH RECTIFIER
Rectifier heats ok
B+ at 470vDC (I was looking at the wrong resistor before. Oops.)
On for about a minute before I recognize smoke, turn the amp off and find a wisp from R25 which appears to be be in the bias supply for the left channel.
What else can I check? What else may have failed?
An externally hosted image should be here but it was not working when we last tested it.
ANSWER to my question: with no load, this B+ seems about right (http://www.diyaudio.com/forums/tubelab/256509-high-b-initial-checkout-phase-se.html#post3931947)
sorry folks
So still the question is, if I have a burned R25, which other components should I watch out for?
Well, if it is smoking, it is drawing too much current, obviously. What could make it draw too much current - the only active device in that area is the MOSFET.
What is the dc voltage at the junction of R25, R33, and the source of the MOSFET?
I would think it should be strongly negative, and adjustable - if it is not, or if it is positive, I would suspect the MOSFET as having failed.
I sort of already suspect that, but would at least try some diagnostics before ripping it out and replacing it.
I have a TSE that I have hacked up, but I am far from expert on the TSE. Maybe someone else has a suggestion for you.
Win W5JAG
What is the dc voltage at the junction of R25, R33, and the source of the MOSFET?
I would think it should be strongly negative, and adjustable - if it is not, or if it is positive, I would suspect the MOSFET as having failed.
I sort of already suspect that, but would at least try some diagnostics before ripping it out and replacing it.
I have a TSE that I have hacked up, but I am far from expert on the TSE. Maybe someone else has a suggestion for you.
Win W5JAG
R25 and q2 have been replaced. Q2 measured totally open from source to drain, which I understand to mean that it was fully passing b+ voltage through R25.
I will be going through the tubelab checkout procedure with painstaking attention to detail later today.
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I will be going through the tubelab checkout procedure with painstaking attention to detail later today.
Sent from my ASUS_Z00AD using Tapatalk
Today the amplifier made it all the way through the checkout and bias procedure with correct measurements.
It got through 1 song at low volume levels and distortion, and then the 300b filiments went out.
My life unexpectedly got a lot busier in the middle of this build, and I don't know if I have it in me to keep on trying. May be time to part things out and take the financial bath
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It got through 1 song at low volume levels and distortion, and then the 300b filiments went out.
My life unexpectedly got a lot busier in the middle of this build, and I don't know if I have it in me to keep on trying. May be time to part things out and take the financial bath
Sent from my ASUS_Z00AD using Tapatalk
Yes, big pins on the right. They're oriented correctly
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Went for a bike ride and let it cool off. Just got back -- with a fan on, the amp passes signal. However, it's still very fuzzy and low volume
I had a supro champ style amp that did this, and it was an undergrad cathode that caused the same type of distortion. Maybe the filiments aren't getting enough current.
I did test the board with the 300b out, and the filiments are indeed getting 5v. I suppose I should check current across the two heater pins too?
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