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Old 25th July 2019, 11:15 AM   #1
Mr_Zenith is offline Mr_Zenith  United States
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TSEII: all was going well until...
Default TSEII: all was going well until...

...until I smelled smoke.

This happened three days ago while I was measuring the V1/V2 voltages on pins 3 and 9. The amp is now exhibiting the same symptoms noted in this thread, except that this seemed to occur in both channels simultaneously, and I'm not measuring a short between the drain and source on either MOSFET. Also, R14 and 25 measure their nominal 20K and don't show any obvious signs of overheating.

Some specifics:
  • The CCS ICs for V1 and V2 are apparently fine; I can adjust both to a stable 175 V.
  • V1 and V2 themselves are behaving normally.
  • Adjusting bias pots R12 and R23 has little or no effect on output current.
  • B+ for V3 and V4 is 185 V (measured between ground and plates).
  • V3/V4 current was 167 mA each (!) as measured across R18/R29
So I have several questions:
  • First, I'm fairly certain I've blown the MOSFETs, but why am I not reading a short between pins 1 and 2 (drain and source, if I've read the pinout correctly)?
  • Also, around several of the board's mounting holes I noticed that there is very little clearance between the holes themselves and the adjacent traces. The #6 stainless washers I used in my mounting hardware appeared to be dangerously close to overlapping those traces. Is it possible I've created a short to ground? Do I need to use insulated washers or other hardware?
I've already filled out a shopping cart at Mouser for lots of spare silicon, but I'd just like to know if I've overlooked anything before hitting the "Order" button (I already know better than to ask for a check on my sanity ).

I found it odd how the amp continued to produce somewhat decent audio after the magic smoke escaped. There was a noticeable increase in distortion and a drop in output, but I couldn't hear anything else dramatically obvious. But what really amazed me was how those cheap 2A3B's (more like 2A300B's) ate 30 W each without a burp. Not even a hint of red plating during the time it took me to realize what had occurred - which was embarrassingly substantial.

The magnetic components, however, were very unhappy. They told me not to let it happen again.

Last edited by Mr_Zenith; 25th July 2019 at 11:21 AM. Reason: The missing link...
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Old 25th July 2019, 01:29 PM   #2
Tubelab_com is offline Tubelab_com  United States
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The first question is "what smoked?"

Remove the output tubes and measure the DC voltages. Despite the specifications stating their surge withstanding capability, another builder had an R5 (the 270 ohm in the negative supply) fry on power up. The negative voltage supply needs to come up before the B+ supply to keep the output tubes cutoff until they are warm, so the negative supply is solid state rectified. R5 limits the current and widens the conduction time to reduce noise. It however sees a large current pulse on power up to charge C6. The Vishay part listed in the BOM may not be up to the task. The circuit is the same as on the first gen TSE, but the old part number was no longer available at Mouser.

I have two working boards that have been abused with no issues, but that's two resistors. I plan to test the other 8 I purchased for the original build, and three compatible parts from other suppliers in the next few days. They arrived in a DigiKey box yesterday, but my wife has been in the hospital, so Tubelab activities have been near zero, the box is still unopened.

If R5 is dead, I would try the Yageo RSF200JB-73-270R which is most like the part in the original TSE, or the Ohmite 42J270E which is wirewound, and usually capable of eating such a large pulse.

If R5 is OK, check the negative supply, and then probe the grid pins on the empty 2A3 sockets and verify that the bias voltage can be varied over a range from about -35 to -100 volts. This voltage will vary as lot with different power transformers, but you should be able to hit about -50 to -70 for 2A3's. If this works, the mosfets are fine.

Quote:
how those cheap 2A3B's (more like 2A300B's) ate 30 W each without a burp.
I am running mine at 380 volts B+ and 70 mA each current. That's 27 watts and they don't seem to mind. They are 10 year old cheap Chinese units. I think they were under $20 each when I got them.
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Old 25th July 2019, 03:18 PM   #3
Mr_Zenith is offline Mr_Zenith  United States
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TSEII: all was going well until...
Thanks, George. I didn't think about R5, and I didn't perform the "tubeless" tests because the event occurred at night when I was tired and frustrated - precisely the wrong time to go poking about a high voltage circuit. I'll check it out tonight when I get the time. I'll still place an order for more "sand" though, mainly because I'd like to have some spares on hand "just in case".
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Old 25th July 2019, 06:40 PM   #4
Grumpish is offline Grumpish  United Kingdom
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Quote:
Originally Posted by Tubelab_com View Post
The first question is "what smoked?"

another builder had an R5 (the 270 ohm in the negative supply) fry on power up
That would have been me - I have got a 3W metal oxide (TE Connectivity I think) in there now, it has been running nicely on the bench for quite a few hours now without any problems at all.
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Old 25th July 2019, 07:34 PM   #5
nigelwright7557 is offline nigelwright7557  United Kingdom
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Mosfets can blow between any of the terminals.
Use fibre washers on the mounting screws to stop shorts with tracks.
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Old 26th July 2019, 01:16 PM   #6
Mr_Zenith is offline Mr_Zenith  United States
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TSEII: all was going well until...
Default Give the man a cigar...

...R5 was indeed the culprit. At George's suggestion, I examined it visually and was a bit disappointed - the resistor looked absolutely normal. But when I measured it, my DMM read 3.1M(!).

Thinking this surely must be an error, I broke out the opti-visors to get a closer look and sure enough, there was a small crack almost perfectly bisecting the purple color band. No charring, burned traces, melted solder - nothing. I've never seen a resistor fail like this before, and without magnification the thing looked so normal I never would've seen it here if I hadn't known to look.

I searched my parts bins and found a 250 ohm 5W cement resistor, so I soldered that in as it measured within 5% tolerance of the original. It works perfectly.

In retrospect, I'd been goosing the power switch quite frequently during fitting and testing phase, and that must've done it. Lesson learned. I'll still place that Mouser order to lay in a supply of spares and to safeguard against part extinction (another lesson learned).

Thanks to everyone for your input!
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Old 26th July 2019, 01:36 PM   #7
Tubelab_com is offline Tubelab_com  United States
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These two failures have led me to discover that plenty of resistors are "suitable for high surge currents," "rated for high inrush current," or "surge capable," but virtually none actually have real specs for the amount or duration of such current. Some however have ratings for time and amount of overload duration.....like 5X rated power for 5 seconds or 10X rated power for 5 seconds.

This means I get to test a bunch of resistors with a BIG cap to see which ones can eat the most current for a few milliseconds.

The Mouser part number for R5 is being removed from the BOM. I will post a new one once I have blown up some resistors. Meanwhile a wirewound or metal oxide part is probably the safest choice.
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Old 6th August 2019, 06:10 AM   #8
quinine72 is offline quinine72
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Add me to the list for the blown R5.
I did add a CL-90 Thermistor to the power input and have had no issues thus far. (80 hours or so, 5-6 at at a time).
Otherwise, thank you again, George. Lovely design and beautiful sound.
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Old 6th August 2019, 11:07 AM   #9
Mr_Zenith is offline Mr_Zenith  United States
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TSEII: all was going well until...
When min blew I was using a CL-60 that just happened to be handy. I'm just glad I didn't have to rip everything apart to change out the MOSFETs as it's a fairly complex build, mechanically speaking. Sounds like you were able to get yours up and running well.

And you're right, it is an elegant, beautiful-sounding design. Mine is in the final stages of completion (90% finished, 90% to go - pictures coming soon), but it's going slowly. I can't stop listening to it long enough to get any work done!

That's a good place to be.
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Old 7th August 2019, 12:34 AM   #10
quinine72 is offline quinine72
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I definitely had a moment of panic when I saw current on the 45s shoot up, but didn't have to change anything but the single resistor.

Not quite at 90% here, with same issue. It wants to play.
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