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Old 14th July 2012, 01:21 AM   #11
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Originally Posted by boywonder View Post
The B- voltage for a 300B config is typically more negative than -300V. I'm no expert, but the only real worry here is that you exceed the drain-source voltage of your mosfets. If they are 900V rated parts, then no worries.

for Question #2, yes, exactly

If you can crank the pot to get that negative for bias voltage, that's fine. With the bias voltage more negative than about -150V, the 300B's will be completely cut-off, i.e. not conducting.

do you have access to a variac? If so, you can bring up the amp slowly. It makes the process much less white-knuckle.
Ah, I feel so much better after reading your post! Thank you very much! You have put my mind at ease.

I do have a Variac, and I had planned on using it. But then I read a post on here that dramatically showed how much tube life was degraded by being as much as 10% outside of the filament voltage. Normally I wouldn't care, but 300b tubes are $$$! Am I being paranoid? All my voltages come from a single transformer, so I didn't know any way to split just the heater volts away from the Variac. What do other peeps do?
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Old 14th July 2012, 01:44 AM   #12
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Default I'd be a little lost without a variac....

Use the variac to bring the amp up slowly and get the bias dialed in and the voltages for the 5842's. You typically need to get above about 60VAC out of the variac to get the rectifier going, so crank it up slowly with a couple of voltmeters on the 10R bias R's and the anodes of the 5842's.

I usually bring the variac up to 60-70VAC or so, then adjust the bias pots to bring the 300B's out of cutoff to 10ma or so. Then increase the line voltage slowly while watching the bias voltmeters and adjust as required so you end up with the 300B's at your target bias at full mains voltage. Same procedure with the 5842 anode voltage.

Even if you exeed the bias voltage for a few seconds while adjusting nothing bad should happen.

I think your worry about filament voltage applies to a chronic condition of over/under voltage. For few minutes to get things dialed in I can't imagine that it matters much.

That's pretty much how I've brought up both of my TSE's without issue (assuming that there are no other issues along the way).

Use a multimeter to monitor the B+ voltage as well; that way you can convince yourself that things are behaving properly.

The variac will minimize letting the smoke out of components and blowing fuses, etc.
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