5f2 pRINCETON BIAS QUESTION

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trescat
Junior tube assistant
Posts: 12
(3/7/04 9:32 am)
 5f2 pRINCETON BIAS QUESTION
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 mY Princeton 5f2 has the 6V6 running at 49 mA at the output tube, is this current to high??? as I have heard it reccomended that a 6V6 shouldn't run to high if you want good tube life, also I have a plate voltage of 407 VDC a cathode voltage of 23.5 VDC and the cathode resistor is 470 ohms, . This works out to be running at 134% of max. rated 6v6 GTA dissipation, what happens if you push the tubes this hard, can you get away with such a high dissipation , or does it end up lessening the life of the tube, All feedback greatly appreciated. Thanks in advance

Trevor
 
6G6
Junior tube assistant
Posts: 113
(3/7/04 10:10 am)
 Re: 5f2 pRINCETON BIAS QUESTION
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 If you are doing it, and it's working, then it is possible.

Without doing any research at all, I'd have to say that running anything at more than 100% of what it's rated for is going to cause it to have a short life. Maybe days instead of months or years.

70% seems to be the accecpted ballpark bias. YMMV.

Is the PT hot?
Is the tube glowing real bright?

Mostly though, how does that puppy sound???  

Edited by: 6G6 at: 3/7/04 10:12 am
 
HotBluePlates
I only work on Fender's
Posts: 704
(3/8/04 12:53 pm)
 5F2 Princeton Bias
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 The 5F2 Princeton should have about 320v on the plate instead of 407v. Is this an old Princeton, where the higher voltage may be caused by higher line voltage than what there used to be?

If not,maybe using a custom copper cap to lower the voltage will prevent you from needing a different transformer.

In any event, this is a single-ended amp, and would be biased class a. The idle dissipation is high now, but would be alleviated by lowering the plate voltage. At 320v, you would bias the amp to be close to 100% dissipation at idle, since the current draw will actually lower somewhat at full tilt. About 40-43mA at 320v on the plate would be about right. The old schematic implies 40mA, as it shows 19v being measured on the cathode side of the cathode resistor.

You could simply raise the cathode resistor value, but the voltage is higher than what would be good for class a, and by the time it was large enough to tame the idle current the amp probably wouldn't sound right. Class ab plate voltage biased for class a just doesn't really work.
 
 
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