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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