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grounded grid

kevin gilmore

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So to expand on Dr. GIlmore's point.  With few exceptions, tube amplifiers use output transformers.  An audio output transformer with a very high turns ratio is more difficult to design and build, so for output tubes you want a relatively moderate voltage (say 300-550 volts) with relatively high current (say 80-150 mA).  That way a moderate turns ratio will supply a reasonable output voltage with adequate current.  For a stat headphone amp, on the other hand, you want a high voltage to supply enough voltage swing, but you don't need nearly as much current (10-20 mA is enough).  As spritzer has noted, you seldom if ever approach the full voltage swing that a stat amp is capable of - enough to fry the diaphragms in all likelihood.  But having that large voltage swing means that at tolerable dB levels, the outputs are staying well away from the voltage limits where device linearity goes to pot.

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  • 1 month later...
On ‎2016‎-‎02‎-‎15 at 1:21 AM, nopants said:

any ac hum problems?

I can’t hear any AC hum – doesn’t mean that there isn’t any though.

The filament transformer gives [email protected] out. In order to reduce voltage for tube heating (5.0V +/-5%) I’ve put wire wound resistors in series with heater.

Below shows the small board with terminals and resistors mounted to Kevin’s main board on two stand offs.


For info I followed this link  and bought some SiTubes filament supplies. They work OK for a quarter of an hour or so than the voltage drops to a few tenths of a volt. After some cooling down they are fit for fight again. I should have red data sheet.

Maybe this would do if you really want DC heating. Probably overkill??

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I’m actually reading the information Emission Labs sent me with the tubes.

Regarding filament they say:

“The filament voltage must be exactly as specified, under all circumstances. Slow-start filament electronics taking longer than 10 seconds, are not allowed, otherwise this may damage the tubes”

Stand-by mode should not be used with directly heated tubes.

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