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Tubelab Discussion and support of Tubelab products, prototypes and experiments

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Old 29th February 2020, 06:02 PM   #11
Tubelab_com is online now Tubelab_com  United States
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Would the deviation from 36k on R25 be enough to cause a positive voltage at the pot?
If it's conducting current there should be no problem. The absolute value od the resistor is not important for circuit function.

Something is either causing the mosfet to conduct excessive current, or there is an unintended path for current to flow around the mosfet (solder bridge).

What is the part number of the mosfet that you put into the board?
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Old 29th February 2020, 06:32 PM   #12
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What is the part number of the mosfet that you put into the board?
Click the image to open in full size.

Here's the actual one (click picture for better image). The solder pads on the board look pretty torn up, but I've checked that there's no connectivity issues between these pads. The underside of the board looks much cleaner.

Click the image to open in full size.

Here are some additional pictures of the board as it is currently. Tubelab SE - KHG album

Last edited by khg; 29th February 2020 at 06:45 PM.
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Old 29th February 2020, 07:21 PM   #13
Tubelab_com is online now Tubelab_com  United States
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Those are the exact same fets that are in the board that's in my hand now. Even the SPH30 (date code?) is the same.

Try resoldering the end of R25 closest to the pot from the top. The solder joint looks a little odd, at least in one of the pictures. I have seen some odd issues with solder joints that get thermal cycled a lot, and those resistors will get real hot when the fet shorts out.
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Old 29th February 2020, 10:37 PM   #14
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Try resoldering the end of R25 closest to the pot from the top. The solder joint looks a little odd...
Thanks, tried that, but didn't help.

However, I did fix the problem! I broke my own rule and did two things at once, so I probably won't ever know exactly what the issue was. I replaced the fet with yet one more (my last new one) and I decided to remove the chance of a solder bridge. Because the pads on top looked so bad, I broke the path from the center fet pin and bent the center pin of the fet and bridged it directly to the B+ pad. Looks crude, but it works! Really appreciate all the help!
-KHG
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Old 1st March 2020, 04:11 AM   #15
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Well, I finally got the left grid pin to measure a reasonable negative voltage. I completed the checkout process and adjusted the tube bias. All went well until I connected an input signal and listened. The left channel seems fine, but the right channel is acting up. One thing is it "pops" as it powers up (heard through the right speaker only). This has never happened, with either speaker, ever. The second issue, and more important, is that the right channel has no or very little high frequencies coming through. I swapped speakers (left / right) to be sure it wasn't that, and I also swapped the input and output tubes. It's something else. I was going to swap output transformers next, but before I go unsoldering stuff again, I was wondering if there's a more obvious solution - some other component (C11 cap, perhaps?) - that would be more likely to result in no high frequencies? I hope it's not the other mosfet ...

Last edited by khg; 1st March 2020 at 04:14 AM.
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Old 1st March 2020, 05:32 PM   #16
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I switched the 300b tubes (left to right) and played the amp this morning for about 20 minutes - when the right channel popped and then completely cut out (no sound). I measured the voltage of the right channel grid pin and it's reading a high positive value, so same issue I had before on the left channel. I believe the right channel mosfet is toast.

George, is the mosfet you list on the TSE-II BOM (STF3LN80K5) a suitable replacement?

Is it possible that my tube is causing this? Perhaps an intermittent short (grid to plate) within the tube? I'm using genalex gold lions. Just seems strange that the blown mosfets are on the channel using one particular tube.
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Old 1st March 2020, 11:09 PM   #17
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is the mosfet you list on the TSE-II BOM (STF3LN80K5) a suitable replacement?
I haven't actually tried them in an original TSE, but they are in my TSE-II's and the circuit is similar, so they should work.

Quote:
Is it possible that my tube is causing this? Perhaps an intermittent short (grid to plate) within the tube?
A grid to plate short would take a gross failure in the tube since they should be spaced pretty far apart. It would put B+ on the source of the fet, which could cause damage, but I have never investigated this.

A grid to filament short would put ground on the source of the fet, which already has B+ on the drain, likely blowing the fet. I have not seen such a short in a 300B, but I HAVE SEEN it in an 811A, so it is possible.

Look carefully into the top of the tube and see it all of the filament wires are still in hooks in the support springs, and all 4 of the springs are the same height. I had several early vintage Sovtek 300B's fail with HALF of the filament going open. The tube still played reasonably well, but wouldn't make as much power as it should. This was a pretty common failure mode for the Sovteks about 15 years ago. Sovtek would not replace them, just told me, and other buyers to buy their new EH 300b's since they were better....I bought cheap Chinese tubes instead, and they are still good today.
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Old 2nd March 2020, 12:44 AM   #18
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Originally Posted by Tubelab_com View Post
Look carefully into the top of the tube and see it all of the filament wires are still in hooks in the support springs, and all 4 of the springs are the same height.
Thanks. The filament wires are all still in their hooks. I'll also order some replacement fets this week. Again, I appreciate all the help.
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Old 10th March 2020, 01:23 PM   #19
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Last night I went through a series of attempted fixes. I replaced the right channel mosfet (Q1) using the one recommended for the TSE II (STF3LN80K5). The left channel still has the one referenced above (NDF04N60ZG). I tested the voltage at the grid pin and experienced a high positive voltage. So, I skipped ahead and did the same thing I did on the left channel -- replaced the mosfet again, "wiring" the center pin above the board directly to B+, and severed the copper path on the board to try to eliminate any potential shorts between the mosfet pads. I re-tested the grid pin voltage and this time it held a lower negative value. Good. Put the output tubes in (with one new tube), powered it up, set the bias (without input or speakers connected). Still good. I let it run for about 30 minutes, then reassembled the amp back into the chassis and connected an input signal and speakers. I immediately heard a pop from the right and no music.

Okay, back to the bench. Replaced the fet again, replaced R18 for good measure, checked resistance of R34 and R14, checked grid pin voltage, commenced checkout procedure (no input or speakers connected), everything good. Reassembled chassis, connected input and speakers, pop / fizz from the right channel - this time R18 burnt.

One more time - replaced R18, checked grid pin, measured other resistors, checkout process was fine on bench. Reassemble, connected speaker and input, powered it up, heard an immediate pop from right channel.

I unplugged everything and went to bed.

As I slept, I started thinking of everything that could be causing this and potential troubleshooting next steps:
- Do the mosfets interact at all such that they need to have similar characteristics? In other word, do the mosfets need to match?
- Could there be a short somewhere on else on the board - perhaps at the line-in or left channel speaker terminal (or in the output transformer?!).
- Could there be a short in the right speaker itself (old Klipsch Heresys)
- Maybe the issue is still at the mosfet. Perhaps it's worth attempting to wire it point to point above the board and just sever all the paths on the board that lead to/from that mosfet?
- Maybe I just order a TSE II and rebuild everything re-using my transformers and tubes, this time with better ventilation (By the way, F-4V / F-6V work perfectly to power a small 40mm 12v fan).

Any thoughts / ideas / help are greatly appreciated!

Last edited by khg; 10th March 2020 at 01:26 PM. Reason: spelling
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Old 13th March 2020, 01:25 AM   #20
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Well, I'm back up and running (for a few hours now, anyway). Just wanted to bring this thread to a close in case others experience the same issues.

I replaced R34, since it had blown. I also replaced the right channel MOSFET again, using the one recommended for the TSE II. It seems to work fine. Upon replacing the fet, I did what I did on the left channel and bridged the center pin above the board to the B+ pad immediately in front of it and severed the path on the board leading to the center pin.

I also severed the board between the center fet pin and the left fet pin (the one closest to Q2 if looking down onto the board). I did this on the left channel as well. The board almost seemed hollow between these 2 pads. When I was scraping away all the crud, the knife blade sunk easily into the board at that point. I wonder if this small area is susceptible to shorts - perhaps promulgated by the heat buildup over time? The same happened between the same 2 pins at Q2.

I added a small fan and raised the chassis up to create a bit more airflow. Hopefully this will help keep the temperature down a bit.

Thanks again for everyone's help here.
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