jerome schatten wrote:
> Learned gentlemen...
>
> Excuse me if this is somewhat off-topic, but I've been following the
> 811 amp discussion with much interest and have a related question:
>
> I'm currently doing some work my pair of 813's and was thinking about
> the 'parasitic' problem as discussed on this list.
>
> My fear is not blowing the tubes or the band switch, but rather in
> blowing the front end of a modern transceiver driving it. The
> configuration is a pair of 813's truly running zero bias GG (grids all
> tied together and connected directly to ground ~ 50 ma. idle current
> at 2200v). No band switch (rotary inductor pi-network). Manual
> switching tx/rx avoids hot switching. Input to the amp is grounded in
> Rx. Works 80 thru 20 only.
>
> Should I be worried about the parasitic problem taking out what's
> driving it? If so, how to prevent (minimize) that? In the 40 years
> I've been running this beast (still the same set of tubes) I've driven
> it with everything from a Viking Ranger, to a Jupiter and never had so
> much as burp. But one never knows.
>
> Getting nervous in Vancouver,
> va7vv
What a great amp.....I could kick myself for ever selling the pair of
813's I built. For you "new" hams, 2 813's in GG with 2200 to 2700 on
the plates was hard to beat. They didn't like 15 and they definitely
didn't like 10. But from 20 on.....what an amp. If I had some tube
sockets and a filament transformer, I would build a 813 amp in a heartbeat.
I liked the idea that Ameritron uses, instead of floating the B- with a
5 to 10 ohm resistor, they used a diode. So you have B- above ground
enough to run the meter circuit, but the diode doubles as a fuse.
I've been building, repairing, and using amps for 30 years. 99.9% of the
time, any catastrophic event has been operator error or component failure.
I would suggest thinking about using a diode in the B- return and let it
act as a fuse.
Ed W3NR
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