• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

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#5
Nice job, you'll need to protect the cathode/filament insulation of the 6922 during warm up though - it won't like a 200V+ differential across it. (Even worse if you need to elevate the filaments and depending on whether your B+ is SS rectified or not.)

You can use a simple NE-2 neon lamp between cathode and ground to do this. It will glow until the voltage at the cathodes drops to about -40V or so and then will extinguish, leaving effectively a very small capacitance to ground at that node. (pFs worth, and less than a few inches of wiring.)
 
#6
All right. Where can I grab a lamp like that? Someone on AudioKarma also suggested using a small switching diode, with its cathode to the 6922 cathodes. I didn't quite understand why before, but does it also protect the tube during warmup?
 
#7
Hi Soren,
You can buy neon lamps at most Radio Shack stores last time I checked. They are also available from Mouser and Digikey, either NE-2 or NE-2H will work fine.

The diode will work too, but I prefer the neon in this application. In the past I have used generally similar designs to what you came up with. In a case where the operating voltage on the cathode is positive enough in normal operation to light the neon I used a normally reverse biased diode in series with the neon. This covers just about all possible scenarios..
 
#14
Thanks, I'll do that. I also notice the screen wires in my 6P14P's glowing a bit in certain places. Is this bad? Could I wire some zeners in along with the 220R 2W screen resistors to reduce current? My operating point seems reasonable, 320V 38mA per tube.. Alternatively, could I just increase the value of the screen resistors?
 
#15
I tend to ignore the glow, it hasn't caused any problems so far with 6P14P in my two infrequently used commercial EL84 amplifiers which both run at way over 400V and as much as 50mA per tube.. (Old Realistic and a modded Scott.) It is probably symptomatic of slightly exceeding the tube's ratings, something commonly done to EL84 in a lot of vintage amplifiers. Your operating point is quite conservative compared to many, but that doesn't mean the tube is running within its max ratings - it probably isn't.. 😀 I'd leave it and wait for concrete signs of trouble which may never come. No I wouldn't use zeners in the screens..
 
#16
All right. I stuck the diodes in there and replaced the run of twisted cat-5 to the left channel with shielded cable and the amp is as quiet as a mouse. I'm blasting Dubnobasswithmyheadman and having a pretty good time. On to the next amp :smash: I have some Z565's, a PA774, a couple 80uF 370VAC motor runs, zero money, and too much time on my hands. 6V6 monoblocks maybe?
 
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