Mike KC7NOA writes:
I'm changing my 2@3-500Z amp to a single GS-35B and in the spec's for the
tube it states that the warm-up time is Aprox~ 2 minute. So instead of using
a power SCR in the mains to switch on the HV bank( and probably darn slow to
disengage if a fault occurs) I would like to use a Vac relay - probably
close to the Plate choke since i have room in that compartment and a delay
circuit is darn easy to DIY.
Mike
KC7NOA
I'm afraid ordinary vacuum relays can't be used for your intended
application, Mike. Relays such as the RF1D, RJ1A, RB2, RF3A, etc. aren't
intended for hot-switching high voltage. If you try to use one in that
application (I'm speaking from experience, now!), an arc will form inside
the relay when the relay opens. This destroys the relay and doesn't
interrupt the circuit. There are vacuum relays intended for hot-switching
HV, but they're rather uncommon. If a vacuum relay's rating is, e.g., 12
kV, that means that an open contact inside the relay can withstand that much
voltage. It does NOT mean the relay can interrupt a high voltage short
circuit!
In my amplifiers, I normally protect against a flashover by using the
tried-and-true methods: a 25 ohm resistor in the HV lead, in front of the
filter capacitor, and a small wire wound resistor (e.g., 0.82 ohms, which is
what the L4B used) or a 10 ohm 10W ceramic WW resistor. The later is
available from Radio Shack. Also, in a grounded grid amp, you should have a
safety diode between chassis ground and B-.
73,
Jim Garland W8ZR
### what he wanted the vac relay for was to interrupt B+ [with NO load]..
while the fils warm up. During warm-up, there is no current flow, so no big
deal. Moot point, since he can have B+ ON.. while fil warming up.
## I just looked at the W8ZR 8877 amp in Orr's last book [23rd ed]
## I was also going to use the vac relay as intended... to open off
B+ UNDER LOAD... . In that case... it was part of the W8ZR
plate over current setup. IE: If plate current exceeds say 1000 ma
during tune up, vac relay opens off the B+. Problem is, during
a fault condx.. and only a 25 ohm WW for a glitch R... fault current
is 4000 V /25 = 160A !! Which will FRY vac relay contacts !
## In the very 1st month of AMPS ... [ Jan 1997] It was explained
the vac relay concept won't work for fault condx. I got into it
with engineers from Kilovac. They told me the typ vac relay's
would open off maybe 1-2 A at most.. with NO guarantee how
many times you could do that. [ in the..'mis tuning scenario]
## for a fault current condx... like B+ to grid.. or B+ to chassis
= a disaster for typ vac relay's. The correct component is called
a 'vac interrupter'.. about the size of a 813... designed to interrupt
50-1000A at 5-50 kvdc. Trbl with em is... WAY too slow.. like
100 msecs operate time. [ per kilovac engineers]
## I opted for a 50 ohm Glitch R... PRE-ceded with a sand filled
Buss HV fuse. On a 7700 B+ supply, fault current is 7700/50=
154 A. Sand fuse blows open in < 2 msecs every time. I also
use a mag-hydraulic breaker in the 240vac line.. with an instant trip
rating [no oil in reservoir]. I also install a 2nd HV Sand fuse..located
between SEC of plate xfmr [one leg only] and input of FWB.
## depending on the B+ current used, typ 50 ohm glitch R consist
of 2 x 100 ohm @ 100-225 watt WW's in parallel. In some commercial
applications, 4 x 200 ohm @ 225 watt WW's are in parallel.
Globars, type SP ,and also type AS will also work. For you YC-156 folks
4 x 200 ohm, 225 watt WW's [ cheap at Mouser] make an excellent glitch
resistor.
## In my tests, with the big WW's, B+ can be cro-barred repeatedly, with no
ill effects. The secret is the HV fuse b4 it. Also, several 6A10's
in parallel, and also RVS connected, are wired between chassis
and B- [like 4 each way, 8 in total] B- can never wander more than
+/- .7 v More RVS connected diodes across the plate and grid meter.
## The big 50 ohm glitch R.. LIMITS the fault current. The HV fuse,
INTERRUPTS the current ! Event is over < 2m secs. So deep 6 the
vac relay in the B+ line..doesn't work, and too slow anyway.
later... Jim VE7RF
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