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Old 22nd February 2020, 08:35 PM   #1
khg is offline khg  United States
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Default TSE - R33 and R29 blown

Hi All,

Looking for a little diagnostic advice. I got my TSE up and working about a year ago and it’s been absolutely flawless. I typically run it for hours on end, and while it does get uncomfortably hot, it’s performed perfectly. Until this morning. I was running it at about mid volume when one of the speakers popped/crackled, then no sound. I ran over to it, cut the power, and noticed some carbon burn marks on the base of the left channel output tube. Opened it up and saw R33 burnt to a crisp. Replaced it with another 1/4w 100 ohm resistor, reassembled everything, and turned it back on. I then noticed a puff of smoke (no sound from speakers, despite active input signal). Shut it down, opened it up and this time I noticed R29 burnt. The picture below is after I replaced R33, but before doing anything with R29.

Click the image to open in full size.

I’m wondering if anyone had seen this pattern of failures? Or if anyone can offer any hypotheses? I wonder if R33 failed for whatever reason and perhaps ruined my left 300b tube? Is that likely or possible? If so, would a bad tube cause R29 to blow?

I suppose my next step is to replace R29, remove tubes, and go through the checkout process again. Unfortunately I don’t have a way to test tubes (short of using a multimeter, if that’s even possible). Any thoughts or advice? Thanks much!
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Last edited by khg; 22nd February 2020 at 08:38 PM.
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Old 22nd February 2020, 08:51 PM   #2
rayma is offline rayma  United States
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Failures of R29 and/or R33 would be symptoms of another problem,
probably a bad output tube, but the fet could also be damaged.
The input tube is not likely to be part of the problem.

Use your other channel's output tube for testing after the repair/checkout,
if you don't have a spare.

Last edited by rayma; 22nd February 2020 at 08:56 PM.
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Old 23rd February 2020, 02:10 AM   #3
Tubelab_com is offline Tubelab_com  United States
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I have never seen an R29 failure. That would take a lot of current through a 300B to blow it. The first thought would be to measure the filament continuity in the tube with an ohmmeter. It's the two fat pins and they should measure 1 ohm or less. If the filament burned out and the filament wire touched the plate or grid these two parts could get burnt.

The more likely failure would be a blown mosfet. R33 is the 'fuse' that protects the tube if the mosfet shorts out. This can be tested with a voltmeter on the grid pin (or R33) with NO tube in the socket. The voltage should be variable by turning the bias pot R23. It should go from a large negative voltage near -100 volts, to a lower negative voltage, maybe -20 volts. A positive voltage that does not adjust, or only moves a little, indicates a blown mosfet.
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Old 23rd February 2020, 01:48 PM   #4
khg is offline khg  United States
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George / Rayma - thanks. Very helpful.

George, the tubes checked out - about 2 ohms across the fat pins. I checked the mosfets as you described. Voltage at the right channel grid pin registered fine - started very negative then stabilized around -45v. Voltage at the left channel grid pin started the same, then shot up to +450v. I ordered a few replacement mosfets this morning. Iíll report back after I replace them. Really appreciate the help!
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Old 26th February 2020, 05:17 PM   #5
Evenharmonics is offline Evenharmonics  United States
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Quote:
Originally Posted by khg View Post
Opened it up and saw R33 burnt to a crisp. Replaced it with another 1/4w 100 ohm resistor, reassembled everything, and turned it back on. I then noticed a puff of smoke (no sound from speakers, despite active input signal). Shut it down, opened it up and this time I noticed R29 burnt.
Whenever such parts failure occur and replaced, the turn on process should revert back to the initial "check out" steps with all the tubes removed so that any other failed parts can be revealed.
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Old 29th February 2020, 03:03 AM   #6
khg is offline khg  United States
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Quote:
Originally Posted by Tubelab_com View Post
The more likely failure would be a blown mosfet. R33 is the 'fuse' that protects the tube if the mosfet shorts out. This can be tested with a voltmeter on the grid pin (or R33) with NO tube in the socket. The voltage should be variable by turning the bias pot R23. It should go from a large negative voltage near -100 volts, to a lower negative voltage, maybe -20 volts. A positive voltage that does not adjust, or only moves a little, indicates a blown mosfet.
I finally received replacement mosfets. I replaced Q2 and unfortunately Iím still getting a high positive voltage reading at the left channel grid pin (no output tube installed). I even tried yet another new mosfet and still saw a high voltage. Iíd think three consecutive bad mosfets is a fairly low probability and so Iím pretty sure my issue is somewhere else. Any other ideas? Iím wondering if either U3 is faulty, or perhaps a bad C11 cap, but I donít know if either of these would result in +450 volts at R33. Iím truly at a loss. Any thoughts?
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Old 29th February 2020, 11:57 AM   #7
Tubelab_com is offline Tubelab_com  United States
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If the voltage at the plate of the 5842 is normal and still adjusts by turning R20, the U3 chip is good.

A shorted coupling cap (C11) could put the plate voltage of the 5842 into the mosfet circuit, but it shouldn't be in the 400 volt range. You could lift one end of the cap to eliminate this possibility.

The source resistor from the mosfet to the negative supply, R25 could be open allowing the voltage to float high.

There is a voltage divider made up of the pot (R23), R22 and R24. These provide the adjustable negative gate voltage for the mosfet. Any of these could cause issues as could R26. You can probe the center pin of the pot or the end of R26 closest to the pot to verify that the negative voltage is present and adjustable.
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Old 29th February 2020, 01:40 PM   #8
khg is offline khg  United States
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Thanks George. Really helpful. Hereís what Iíve found so far ...

Quote:
Originally Posted by Tubelab_com View Post
If the voltage at the plate of the 5842 is normal and still adjusts by turning R20, the U3 chip is good.
I confirmed 5842 plate voltage is good.

Quote:
Originally Posted by Tubelab_com View Post
A shorted coupling cap (C11) ... You could lift one end of the cap to eliminate this possibility.
Lifted one end, still saw a high voltage. Moving on ...

Quote:
Originally Posted by Tubelab_com View Post
You can probe the center pin of the pot or the end of R26 closest to the pot to verify that the negative voltage is present and adjustable.
I checked this with 3 meters. One on B+, one on R26 and one on R15 (both on leads closest to pot). I checked R15 as a reference since the problem seems to be only on the left channel.

B+ measured at +452 volts. R26 measured at +57 volts and R15 measured -42 volts. I would have expected a lower (more negative) voltage off of R15. Could there be some interplay between channels that result in a less negative R15 voltage?

Iím leaning toward just replacing R22 - R26, including the pot, unless thereís a better solution at this point? Also, is there anything on the power side thatís worth checking? Thanks again for all the help!
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Old 29th February 2020, 03:34 PM   #9
Tubelab_com is offline Tubelab_com  United States
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Quote:
I would have expected a lower (more negative) voltage off of R15.
-42 volts on R15 is probably about right. That would translate to about -45 to -50 volts on the grid of the tube in the good channel.

+57 volts on R26 is very wrong since all voltages on the pot should be negative. This could be caused by an open R25. Measure it with an ohmmeter with board power shut off for a few minutes to allow the caps to discharge. You can compare it to R14. I have seen these resistors burn, or silently go open without obvious visual damage when the mosfet blows.
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Old 29th February 2020, 03:55 PM   #10
khg is offline khg  United States
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R25 measures 32k ohms and R14 measures 36k ohms. No visual damage. Would the deviation from 36k on R25 be enough to cause a positive voltage at the pot?

I should clarify that I’m using 36k ohm resistors for R14 and R25.
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