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Old 4th November 2006, 04:45 AM   #11
vax9000 is offline vax9000  United States
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kevinkr,
How did you achieve the 0.5mV noise with the 300B tube? Did you use DC to heat it? BTW, I heard people saying that AC heated SET sounds better and I really don't know why.

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Old 4th November 2006, 11:04 PM   #12
kevinkr is offline kevinkr  United States
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Yes, dc constant current heating which imho sounds as good as ac heating- at least I have not been able to tell any difference except that there is no residual hum..

I use ac heating on 2A3, 45 tubes, but find 5V and above dht really need dc heating to be hum free except in some pushpull applications where I have used ac heating successfully.

I know Steve Bench and others have deliberately injected hum into the output tube grid circuit to cancel filamentary hum, but the additional complexity and adjustments required discouraged me from following that path. The fact that it did not seem to work that well in a few amplifiers I have heard that used it was further demotivation..
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