[Amps] Potter & Brumfield Relays

[R.Measures]

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 at 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