Scott, Jim and all, it certainly is not as simple as grounded grid B minus
protection but there are a couple of options.
There are some hefty surge limiting devices available. One series was made
by Harris (not sure if they still make them) and sold through Newark
Electronics Supply and others. These limiters apparently are sold for AC
applications as the ratings are in rms voltage. I have used one that was rated
for
480vac and it actually started conduction at 480 times 1.414 volts dc.
Limiters are available from tiny to huge sizes. Pick one for the number of
Joules you are trying to hold back.
If you include the recommended surge limiting resistor in the B plus lead,
and pick to correct surge limiter, it will survive a B plus flashover
event. Like all surge limiters, it dies a little during every protection event
and eventually becomes too leaky for service, requiring replacement.
There are other devices designed like a spark gap, arcing over at a preset
voltage and shunting the overvoltage to ground. It has been too long since
looking into this for past projects so my information is skimpy. You can be
sure though that some google searching will turn up much detailed info.
By the way, an option for tetrode operation is grounded grid. In that case,
the single diode from B minus to ground still works. The cathode is
positive by an amount of voltage equal to the bias voltage. Polarities are such
that the single diode still does the job.
73,
Gerald K5GW
In a message dated 9/12/2010 11:01:31 A.M. Central Daylight Time,
4cx250b@muohio.edu writes:
> With all this talk about B- diodes etc., I wonder what the recommended
> safety practice would be for an amp/supply arrangement where B- is not
near
> ground potential; for example, a grounded-screen tetrode amplifier where
B-
> is several hundred volts below chassis ground.
>
>
> Scott Townley NX7U
> Gilbert, AZ DM43di
> mailto:nx7u@cox.net
> http://members.cox.net/nx7u
>
I think that's a very interesting question, Scott, and I'm looking forward
to hearing other folks' opinions. I don't see any obvious way to protect
such a circuit, other than fusing the HV line with, e.g., a safety resistor
that will explode in the event of a flashover.
The problem I see with grounding the screen grid, so that the cathode is
several hundred volts below ground potential (as is done with, e.g., the
Collins 30S-1), is that a flashover will basically dump the full B+ on the
screen, risking immediate destruction of the tube as well as the screen
power supply components.
73,
Jim W8ZR
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