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Old 5th March 2017, 11:52 PM   #1
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Default TSE Help!

My buddy and I have built 2 identical TSE boards. Same parts, PT, choke, etc.

My amp works fine but his the FET's will not turn on and bias the output tubes.

We measured static resistance at D, S & G of FET's, as well as all pins on the 45's and they match within a few ohms. Voltage is obviously higher on his because the 45's aren't conducting.

Mine: FET D=254V, G=-240V to -143V (pot), S=-197V
His: FET D=390V, G=-255V to -232V (pot), S=-298V

We have tried a new set of FET's, swapped power tubes, nothing! Driver's seem to be working fine and set to a plate voltage of 175V.

Any ideas?
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Old 5th March 2017, 11:57 PM   #2
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I'm a rookie so take his with a grain of salt. Look over all hook up's. It's probably something real simple.
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Old 6th March 2017, 12:00 AM   #3
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Trust me ... been there done that about 5 times now. Everything is the same which is why we are so stumped.
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Old 6th March 2017, 12:37 AM   #4
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What value R14 and R25 is he using?
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Old 6th March 2017, 01:43 AM   #5
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Quote:
Originally Posted by famousmockingbird View Post
What value R14 and R25 is he using?
36K ... same as mine with same power transformer.
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Old 6th March 2017, 02:20 AM   #6
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Make sure R6 is 10k and measure the voltage drop across it.

To me it looks like your bias supply is too high. Make sure R11 and R22 have a good ground connection, same with R7.
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Old 6th March 2017, 02:27 AM   #7
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Quote:
Originally Posted by famousmockingbird View Post
Make sure R6 is 10k and measure the voltage drop across it.

To me it looks like your bias supply is too high. Make sure R11 and R22 have a good ground connection, same with R7.
Both amps are 10K at R6.

Mine is: -406V -> R5 -> -402V -> R6 -> -243V
His is: -417V -> R5 -> -416V -> R6 -> -333V

I assumed his was higher because no current was flowing through the FET?
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Old 6th March 2017, 02:44 AM   #8
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I just wanted to make sure the bias resistors and r7 were pulling current, 8mA should be right.

Continuity test the MOSFET pins (while off). Put one lead on the source pin and the other lead on r14/r25. Then put one lead on the gate pin and the other lead on r15/r26. The PCB traces can lift and have bad connections. I got my TSE board for free and went through the same issue, bad traces was the problem
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Old 6th March 2017, 12:05 PM   #9
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Quote:
Originally Posted by famousmockingbird View Post
I just wanted to make sure the bias resistors and r7 were pulling current, 8mA should be right.

Continuity test the MOSFET pins (while off). Put one lead on the source pin and the other lead on r14/r25. Then put one lead on the gate pin and the other lead on r15/r26. The PCB traces can lift and have bad connections. I got my TSE board for free and went through the same issue, bad traces was the problem
All good ... have continuity to the FET's.
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Old 6th March 2017, 01:33 PM   #10
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Well at this point can you take some pics of the board for us to look at?
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