[Amps] input matching question on the YC156?

[craxd]

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 at ezwv.com>
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>>Hal,
>>The plate and a screen voltage if needed was killed the same time as
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>the
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>>antenna relay opened. It was then applied as the antenna relay
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>closed.
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>>Now this was using a smaller tube type (4CX250B) than a 3CPX5000.
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>But if
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>>you have the switching relays in parallel, etc and their switching
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>time
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>>is the same, it will work, or has in what I've done. You could also
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>have
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>>the antenna relay switch in and out another relay to apply the B+
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>and
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>>any screen if needed. There's been some home brew amps wired up this
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>way
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>>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
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>positive
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>>lead in and out for the plate voltage due to arcing. Another way
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>would
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>>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
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>this
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>>is to assure that if the control bias fails, there's not a run-away
>>condition.
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>Will,
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>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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