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 Post subject: Huge drop in amp volume
PostPosted: Tue Sep 25, 2018 6:31 pm 
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TRIWATT's been running great for a year or two, the other day I turned it on the the volume is <10% of what it should be. Took a look at the tubes and V4, one of the 12AX7, was glowing like the sun, compared to V1, V2 and V3 at least.

I swapped out V4, now the tube glow is inline, but it hasn't sorted the volume issue. Thoughts?

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PostPosted: Sat Sep 29, 2018 8:18 am 
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First measure all he voltages and compare to before. There may be a clue in that. that's the starting pont.

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PostPosted: Sat Aug 17, 2019 4:13 pm 
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Finally got around to pulling this amp and checking voltages, something definitely up with V2.


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PostPosted: Sat Aug 17, 2019 4:25 pm 
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Hrm, replaced V2 and I got my volume back but V2 is glowing maybe a little too much? Here are V2 voltages after the tube swap,

Pin 1 - 186
Pin 2 - 0.007
Pin 3 - 1.3
Pin 4 --
Pin 5 --
Pin 6 - 112
Pin 7 - 0.008
Pin 8 - 1.6

Anything to be worried about with these readings, or the glow? V4 seems quite a bit off (by 70 VDC on Pin 6).


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PostPosted: Sun Aug 18, 2019 12:23 pm 
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You might want to check the value of the 1K cathode resistor. 1.6V is quite high compared to the sheet.
Touch up the solder joints there. Rest looks OK.

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PostPosted: Sun Aug 18, 2019 2:56 pm 
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Cathode resistors test fine. Preamp tubes are a mismatch of brands and ages now. Does it make any sense that the 12AT7 on V4 would go, I swapped that out for a 12AX7 and then V2 seemed to have gone, now I've swapped V2 for a 12AX7 and I've got volume back but I'm concerned I'm just going to burn through tubes. I'll give it a run through on the bench here at operating temp and see what happens.

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