On Dec 25, 2004, at 12:22 PM, Harold B. Mandel wrote:
Dear Bob,
You want to be very careful about relay-switching high voltage.
Despite the open-frame design of the Magnacraft W158HVX-1,
rated at 200mA @ 5.0KV, the possibility exists for an "anode event"
whereby a pulse whose amperage far exceeds the relay contacts
will weld shut such contacts or vaporize them completely.
The best solution is to choose another secondary tap from the
Anode transformer, and to use a suitably-rated relay for the
switchover operation, a relay whose contacts are rated at
or above the anode a.c. RMS value. In the case of the Alpha 77-series
amplifiers, the above Magnacraft relay chose the low or high
transformer
taps.
If you are firmly committed to inserting a dropping resistor in series
with the Anode circuit your best choice is a vacuum relay akin to the
Jennings RB-2A, DPDT, or to a Kilovac SPST high voltage, high amperage,
26 volt coil model.
The Anode Plus Polarity (B+) lead should have a glitch resistor in
series
with
the anode RF choke and bypass capacitor so in the event of an arc the
resistor
absorbs the current pulse where all the stored energy in your filter
capacitors
will attempt to travel through the amplifier tube.
The typical arc occurs before the tube. An arc from the anode cooler
to the roof of the RF deck is a apparently a favored spot.
Some people use a
high-voltage
fuse at this point and some people use a vitreous-enamel resistor that
will act as a fuse,
but when an event occurs, this fuse or resistor, if not suitably sized
as
to
wattage, will explode. The bits of ceramic and debris will shatter
glass
tubes,
vacuum caps, other resistors, etc.
The bad news is that an exploding glitch R fails to do its job of
limiting I.
This can also happen to the dropping
resistors
you envision reducing the plate potential.
Be very careful about what component you choose and where you mount it.
Sincerely,
Hal Mandel
W4HBM
On Sat, 25 Dec 2004 14:44:51 -0500 "Bob Maser" <bmaser@tampabay.rr.com>
writes:
I am converting an old Collins 20V-3 to use on 75 and I want to use
the power cutback circuit. This involves switching in some high
wattage
resistors into the HV line. I have 2 P&B PRD type DPST relays that
look like they will work fine. I am concerned about using a relay
that
is only rated to 277VAC, 20A. Since I will be switching up to
3200VDC, is
there a safety potential here? I do not intend to hot switch them,
turn off
the plate voltage, then switch. The bases on these relays are made
from
Bakelite. They are NOS and in good condition. A friend of mine
pointed out that with the HV there might be a problem with the
proximity to the
24V coils. The P&B relays are part number PRD60055. The alternative
is
to switch the cutback resistors with a hefty ceramic rotary switch,
or
a pair of vacuum relays.
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Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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