To: <amps@contesting.com>
> Date: Sun, 24 May 1998 00:47:12 -0500
> From: Jon Ogden <jono@webspun.com>
Hi Jon,
Despite your questions about how to get your 4-1000 to work
right on ten meters, you seem to suddenly now not have problems with
performance.
> >The tube might forgive you, but you might not forgive it! 4-1000A's
> >are notoriously unstable at high anode voltages, and getting a
> >bit scarce.
> I haven't found the tube to be any more unstable at 6 KV.
They absolutely are less stable with more voltage. A tube in
a circuit that is perfectly stable at 3000-4000 volts will often
oscillate like crazy at 5000-6000 volts. If the builder uses enough
conventional non-resonant suppression to stabilize the system at
higher voltages, ten and fifteen meter performance will almost
ceratinly suffer greatly.
> The best thing
> about the tube is that you can really run it hard and it won't die.
Most tubes are that way, as long as you watch and protect the grid
from excess dissipation.
> >You will need about 5-6 kV and a good bit of drive to get 1500 watts
> >out in grounded grid. The driving impedance is high, and that makes
> >the gain low.
>
> Tom is correct here. The tube has plenty of gain on 80 and 40, but
> begins to drop on 15 and 10. I get around 1600 to 1800 on 80 and 40
> while only about 1000 on 10. This is with about 95 watts drive.
Your PA either has severe degenerative feedback on upper HF, or
excessive suppressor losses. Grounded grid gain of the tube itself
does not change much from audio to about 50 MHz.
> >If you do run high anode voltages in an attempt to get
> >enough gain, you will probably run into troubles.
>
> *Knock on wood* but I haven't seen that. Just gotta make sure your stuff
> can handle it.
It isn't a question of if stuff can "handle it". Lower anode
operating impedances makes all the components and stray capacitance
less critical.
If you were using a 3CX800 or 8877, you probably wouldn't be
fighting ten meter efficiency problems AND your you'd be getting a
lot more output power. My 8877 will run 3 kW out with 90 watts of
drive, and still do that on ten meters. A old tube with long
internal leads and high driving impedance, like the 4-1000A, has
much poorer performance.
> >I'd suggest you consider something with a MRI pullout tube Rich, like
> >an 8877 or a pair of 3CX800's. They operate at lower voltages, have
> >better IMD performance, are much more stable, and are more commonly
> >available.
> >
> From what I have heard the 4-1000A has great IMD and audio performance.
Nonsense. The typical 8877 or 3CX800 GG PA will provide -38 dB to
-42 dB third order IMD performance, while the best 4-1000A
in grounded grid (with no screen or grid bias supplies) will only
provide -25 to -35 dB.
4-1000A's, especially when the grids are not supplied proper dc
operating voltages, are notoriously trashy when compared to modern
focus beam triodes.
In the early 70's, I built several 4-1000 PA's and serviced dozens
of others for friends. My last 4-1000A PA was a pair of tubes in AB1.
When I think back on all the work required to get 1500 watts of clean
output with 100 watts of drive, the only thing that made the tubes
worth messing with was my time was "free" and so were the tubes.
There is absolutely no comparison between a clunky old bottle with
long internal leads like the 4-1000 and a modern tube DESIGNED for GG
service.
73, Tom W8JI
w8ji.tom@MCIONE.com
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