----- Original Message -----
From: "craxd" <craxd1@ezwv.com>
> 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!
Yes, it depends on the kind of tube.
I run several different homebrew amps:
Two with glass envelope tetrodes in highµ, those are instant on with
HT
and heater applied the same time.
On 23cm I use a TH-338 triode with all voltages supplied at the same
time,
but bias switching and coaxial relays disabled for 3 minutes.
I built two with the tetrode YL1056 for 2m and 70cm, there the screen
is
grounded and screen voltage applied to the cathode. A string of diodes
from anode to ground makes shure I have screen voltage if the anode
voltage failes.
The tube is 3 minutes preheated and disabled for that time.
Screen and anode current is electronically controlled as well as
presence
of anode voltage to disable screen voltage if absent.
At present I am building one with two of those tubes for HF
There is one with the 8171, same practise as the YLs.
Another two with 4CX3000/5000A, but currents controlled by relays.
All amps with ceramic tubes switch off without blower pessure after 30
seconds and screen voltage is not switchable without anode voltage.
I never had the idea to switch the B+, that is switched on the primary
of
the transformer only
73
Peter
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