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Old 16th December 2010, 03:00 AM   #11
rknize is offline rknize  United States
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Sounds like an adjustable NLD would be the perfect candidate.
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Old 16th December 2010, 02:49 PM   #12
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Thanks! I'll bump up R10 & R21 a bit. Pretty obvious now that you mention it.......
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Old 21st December 2010, 12:59 AM   #13
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Well, I bumped up R10 & R21 a little, and that fixed the problem of running out of pot adjustment on the 5842's.

The amp is presently functioning, although my B- is around -300V. I smoked R5 once already, so I replaced it and swapped out the MOSFETS, replacing the 2SK3563 with the 900V rated parts (2SK3564 IIRC).

I replaced R5 with a 3K R in an attempt to reduce the B- voltage, but then I ran out of pot adjustment on the 300B's......so I put a 270R back in for R5.

My transformer is a Hammond 273BX 350-0-350 and runs a bit high. I've added another RC stage in the PS; instead of just a choke in place of R4 I put the choke, then another RC stage. My B+ is about 400V with a 5AR4 when running about 65ma through the 300B's. But the excessive B+ is making the B- very -ve.

I've been calculating the various expected voltages down the bias circuit to ground, but I don't really know how much current is flowing through the MOSFETs to determine whats good and whats bad.

I just placed an order with Edcor today for a XPWR131, so the project goes on hold...

so....a few questions.....

What are the symptoms of blown MOSFETs?

Whats a reasonable B- voltage with the Edcor 330-0-330 transformer? -300V seems like a long way from -150V....I'm assuming the -150 is with a transformer suitable for 45 outputs.

Last edited by boywonder; 21st December 2010 at 01:02 AM.
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Old 21st December 2010, 03:38 AM   #14
rknize is offline rknize  United States
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I dusted off my TSE (still on the breadboard) to check. I have the XPWR131 in there, obviously, and with it set in 300B mode I see about -270V of B-. This is with no tubes, so it will sag as the B+ rail loads the transformer.
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Old 21st December 2010, 04:08 AM   #15
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My TSE has -275VDC for B- under load and around -290 without load.
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Old 21st December 2010, 01:02 PM   #16
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Speaking about pots. Is 50k the max pot value the 5842 can support ? I have 100k on my TSE 45, is it worth to replace it ?

Thanks,

Davide
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Old 21st December 2010, 02:04 PM   #17
rknize is offline rknize  United States
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Oh, I just realized that the -150V number was coming off the schematic. Yeah, that would be for 45s...the original application of this design. What are you using for R14/R25? The values on the schematic are too low for the 300B setup because of the higher B+ and lower B-. The FETs have to deal with too much dissipation.
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Old 21st December 2010, 03:54 PM   #18
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Quote:
Originally Posted by rknize View Post
What are you using for R14/R25?
This quote is from tubelab website. "R14 and R25 should be 36 K on boards that operate at 350 to 400 volts" Link
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Old 21st December 2010, 04:10 PM   #19
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I just wanted to make sure you noticed that as it is not part of the instructions.
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Old 21st December 2010, 06:24 PM   #20
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I'm using 36K.

I've been contemplating a small separate transformer to power the bias circuit, say a 120:240 tiny thing as an experiment......One problem is that my power transformer is on a separate chassis and my umbilical does not have enough wires to bring the mains voltage into the amp chassis.

I would then play with the various bias circuit R values to get the bias pots back in their happy place. Perhaps another RC stage for the bias as well. I suppose that adding more RC sections even off of the power transformer secondary would be interesting; burn some voltage and get more filtering.

Last edited by boywonder; 21st December 2010 at 06:30 PM.
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