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Old 14th April 2005, 09:34 PM   #1
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Default how much above B+ will voltage swing in OPT?

say I have an ouput transformer connected in a SE amp.

B+ on tube is 400v bias is say -20v.

Probably swing down to 100v at clipping... how much above 400v at clipping?

will it swing to 700v and clip or could it swing even farther?

of course i am assuming the tube can easily approach 800v before bias is too low.

does this make sense?

just wondering whaty determines how far an output stage will go above the power supply voltage?

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Old 14th April 2005, 09:46 PM   #2
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You'll be pretty close by assuming that the transformer (or any large inductor as a plate load) will double the effective B+.
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Old 15th April 2005, 04:17 AM   #3
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Voltages are meaningless without specifying a tube type, or at least perveance and Gm

Class A - I wouldn't expect more than 600V peak for a triode. More for a high load resistance. Triodes loaded in the sweet spot of maximum power usually swing something like 1.5 or 1.6 times B+, I forget exactly. Pentodes can go nearly rail to rail, so class A would be somewhat under 2 x B+.

But, if you leave class A - namely grid current to class C, you can easily get peak voltages well in excess. In particular, if "ON" time exceeds "OFF" time, the peak voltage will follow the duty cycle. If done carefully, you can get flyback up to 10-20 times, which would be 4 to 8kV peak for your 400V supply! Add a step-up secondary and you've got an entire television horizontal sweep output and HV supply going.

Fortunately, for a symmetrical sine wave input (this assumes your driver is capable of well overdriving your output ), the extra grid bias produced (across the grid leak resistor) by the overdriving signal pushes conduction to increasingly shorter periods, so you probably aren't going to get much excess voltage beyond 2 x B+.

Oh, and on a related subject, leaving a power amplifier without load and playing music isn't a problem for triodes, but pentodes with gNFB may oscillate. Note that this is a function of being unloaded and probably has little to do with a signal present!

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