[Amps] HV Relay

Jim Garland 4cx250b at muohio.edu
Sat Feb 6 14:48:10 PST 2010

Allan, I have no doubt that your sequencer using Russian vacuum relays (B1B
and B2B) will work fine in normal operation.  As I understand Mike's
posting, however, he was interested in flashover protection, and had
proposed using vacuum relays to interrupt the HV in the event of a fault,
such as might be caused by an internal arc in a tube, or a short circuit in
the HV circuitry. Unless the B1B and B2B are specifically designed for
hot-switching (and perhaps they are -- I've not used them), they shouldn't
be counted on to protect your amp and power supply under fault conditions. I
have a Jennings application note that specifically warns against using
(Jennings) vacuum relays in that kind of application.

Also, looking through the F1FRV website that you flagged, I notice several
circuit diagrams where the HV is dropped with a voltage divider string down
to 1 or 2 volts, which is then sampled by an op amp whose output sequences
various control relays. To me, this is risky design practice, since if the
bottom resistor in the divider string opens up, the full 4kV will appear on
the control circuitry and destroy everything in sight.  

To my mind, keeping it simple and minimizing active solid state components
in a power amplifier results in greater reliability. Harris amps, for
example, strike me as relatively unreliable; the ones I've repaired
inevitably have a failure in a FET, diode, transistor, or IC. I know it's
tempting use use sophisticated control circuits when building an amplifier
(and I've succumbed to the temptation more than once), but there's a price
to be paid in terms of overall amplifier reliability.

Jim Garland W8ZR

> -----Original Message-----
> From: vk3pa at vk3pa.com [mailto:vk3pa at vk3pa.com]
> Sent: Saturday, February 06, 2010 2:27 PM
> To: 4CX250B
> Subject: Re: [Amps] HV Relay
> Quoting Jim  Garland <4cx250b at muohio.edu>:
>   I USE this system "Sequencer for HV & G2 Voltages" on
>   http://f1frv.free.fr/ it uses b1b es b2b UA vac R/Ls
> it also cut power if HV drops below a pre set voltage.. i set it so if
> HV goes below ht but well before G2 voltage.. gud luck Allan vk3pa
> > Mike KC7NOA writes:
> >
> > I'm changing my 2 at 3-500Z amp to a single GS-35B and in the spec's for
> > tube it states that the warm-up time is Aprox~ 2 minute. So instead of
> > 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
> > 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
> > the relay when the relay opens. This destroys the relay and doesn't
> > interrupt the circuit. There are vacuum relays intended for
> > HV, but they're rather uncommon.  If a vacuum relay's rating is, e.g.,
> > kV, that means that an open contact inside the relay can withstand that
> > 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
> > 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
> >
> >
> >
> >
> >
> > _______________________________________________
> > Amps mailing list
> > Amps at contesting.com
> > http://lists.contesting.com/mailman/listinfo/amps
> >

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