Peter,
Yes your correct. I mentioned doing this because I had success with it
using 4CX250B's. But, when you get up into the big tubes, there's a lot
of current with the anode voltage to deal with. If the control voltage
was monitored through a safety circuit, and if it failed (went open)
then a secondary negative voltage could be applied or a relay opened to
prevent B+ or the cathode being connected. An error lamp would show this
and the amp would be shut down until repaired. What works for one tube
don't necessarily work for all. Most all this type of switching I've
seen has always been in home brew amps. Most all commercial amps have
been built one after the other, the same for years. I hate to say this
but they look at it, if the amp blows due to no bias, it's our problem
not theirs. Plus, anything that will run the cost up and come out of
their pocket wont happen. I set down once and tried to think of every
scenario I could for failures in amps. This was about #2 on the list.
The first was having screen voltage applied before B+. That's another
point, that if bias is missing in a tetrode, the screen can become toast
quickly. So, screen voltage should never be applied before plate voltage
or have screen voltage without the proper cutoff bias. You really think
about it, in a triode without bias, thats like a big diode shorted
across the B+ and in a tetrode, like a boosted diode. Just like turning
a valve wide open in a high pressure hydraulic line. It gets hot pretty
darn quick!
Will Matney
DF3KV wrote:
----- Original Message -----
From: "craxd" <craxd1@ezwv.com>
Hal,
The plate and a screen voltage if needed was killed the same time as
the
antenna relay opened. It was then applied as the antenna relay
closed.
Now this was using a smaller tube type (4CX250B) than a 3CPX5000.
But if
you have the switching relays in parallel, etc and their switching
time
is the same, it will work, or has in what I've done. You could also
have
the antenna relay switch in and out another relay to apply the B+
and
any screen if needed. There's been some home brew amps wired up this
way
in the past, but not as many as leaving the B+ on the anode all the
time. It's better to kill off the B- lead than to switch the
positive
lead in and out for the plate voltage due to arcing. Another way
would
be kill the cathodes connection to ground on idle. Keep in mind that
even though I done this successfully with 4CX250B's doesn't mean it
would work with your tube as well. The only real reason for doing
this
is to assure that if the control bias fails, there's not a run-away
condition.
Will,
I think there will be less dangerous solutions to prevent runaway of a
tube.
Remember a floated cathode without ground connection keeps full
anode voltage!
To my knowledge no commercial communication transmitter switches
ht+ or screen voltage together with the t/r switch.
Instead, they sense over- and undercurrent and disable transmit, or
limit
excessive current, to prevent damage to the tube this is a much
better
engineering approach.
73
Peter
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