[AMRadio] Tube Voltage ratings Question

[Brett Gazdzinski]

I think you can safely run much more voltage on most tubes than
the rated plate voltage.
WA3JVJ was running 811a tubes at 1750 volts or higher without problems,
as long as you don't go over the plate dissipation.
I run 811a and 812a tubes at 1500 volts or more in my homebrew
push pull rig, which is over the recommended voltage for plate modulation
I think.

I am not sure just what sets the high voltage spec, insulation
between the plate and other tube bits (not spacing), X ray generation,
or just what.
The tubes all have vacuum in them, so act like a vacuum variable cap,
the spacing on those is quite close, as it is on tubes like the 4-125,
4-400 tubes, the 4-400 can run 4000 volts on the plate with plate
modulation I think.
The spacing on those tubes are much closer than something like the 811, 812,
572b tubes I think.


I can run the 813 tubes in my big RF deck at 2500 volts
and modulate it well over 100% positive without problems,
even running it at 400 ma.
That is over voltage and over plate dissipation, but with forced air
cooling, the tubes seem to laugh at the overload.

I have never had a tube go bad in over 10 years, except those
worthless pull out 4cx250b tubes that arc over inside because
of the stuff from the cathode that builds up on the inside
of the tube.

4-125, 4-400, 813, 100th, 811a, 812a, if they work when I get them,
they seem to work forever, I never had one fail.
I changed the tubes in my 813 rig after 10 years to get a more
matched set, not because the old ones went bad.

I don't know if you will get any benefit of running 572b tubes
higher than 2750 volts!

I don't think its wise to try to push tubes beyond their limits
except as an experiment.
Parallel them up or go to bigger tubes if you need more power.

I much prefer running moderate voltages at higher currents,
its safer for the mod iron and other components.
1500 to 2000 volts is nice.

There are loads of easy to get tubes that will get you
over the legal limit at moderate voltages, 813, 572b, 100th/tl,
4-125/250/400, 4cx250b/4x150a, 811a/812a.

I have a pair of 4x150 tubes that gives 600 watts
of audio out in AB1, no driving power, low distortion.
I can variac the plate voltage from 1000 volts to 2000 volts
without any change in other voltages!

I also have 4 100th tubes that will give 900 watts of audio out!


Once you get around or over 2500 volts, you REALLY need to
pay attention to things, and use huge insulators and very
high voltage components.
Spikes can go way high at those voltages.

Brett
N2DTS


> -----Original Message-----
> From: amradio-admin at mailman.qth.net
> [mailto:amradio-admin at mailman.qth.net]On Behalf Of John
> Sent: Wednesday, October 29, 2003 10:44 PM
> To: amradio at mailman.qth.net
> Subject: [AMRadio] Tube Voltage ratings Question
>
>
> I have a question to the tubers out there. What determines
> the Anode voltage
> rating on a tube ?? For instance ....I have a amp with 3-500z
> in it.... the
> space between the anode and grid wire cage is very small
> ...Yet is has a
> high rating for maximum plate volts..... My SB-200 with 572B
> has a huge
> space between the plate to grid....But it has a much less
> voltage rating of
> 2750 V ...I know current and anode dissipation play into
> this. But could the
> 572 B's run on higher voltage as long as the total plate
> dissipation is in
> spec ?? because with all that space it looks like there would
> be no flash
> over problems or is there more to it ???
>
> thanks John, KI4NR
>
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