It sounds like the cathode circuit has a short to ground.
73
Gary K4FMX
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
> Behalf Of w2cqm@juno.com
> Sent: Friday, September 07, 2007 8:30 PM
> To: amps@contesting.com
> Subject: [Amps] Need some help
>
> I'm having a problem with an amplifier project that appears to defy a
> solution. Hoping someone can suggest an answer that apparently I'm not
> seeing. Except for a rewound HV plate choke, the amp is a basic Drake L4B
> with a pair of 3-500's. It's been mated to a custom built outboard
> Hipersil, step start, metered supply making about 4.2KV plate voltage. RF
> deck primary voltage circuit changes were made (done this before
> successfully with several Drake 8877 retrofits) to control the power
> supply with the reconfigured RF deck on/off switch controlling 110VAC.
> On this retrofit, the grids were grounded and the filament voltage CT
> biased through a zener to the B-. Pretty straight forward modification.
> On the initial start up, half power HV was applied via a Variac and the
> (no signal) idle current tested. All was well and indicated an
> appropriate idle current simultaneously on both the deck and power supply
> plate current meters. I slowly increased the plate voltage to full o
> utput. After a short time at this power level, there was a "bang" in and
> around the HV choke/blocking cap area. As a result of the arc over, no
> fuses let go but the zener was replaced along with a small choke in the
> bias line. To prevent a recurrence of shorting, the under rated OEM Drake
> blocking capacitor component package was replaced with a more substantial
> 500pFd/20KV unit and repositioned with greater clearances to ground. The
> flash over was eliminated but other problems developed from this
> experience. After making all the circuit changes, testing began again and
> at Variac controlled half power levels, the Eimac tubes began glowing red
> with no excitation. The remote power supply has its own 220VAC plug so
> there's no interaction with the RF deck (which has its own 110VAC plug).
> Oddly, there is no plate current meter indication as the plates glowed!
> With the primary Variac controlled voltage quickly shut down the voltage
> on the plate meters were immediately reduced
> to zero. In other words, The plate meter(s) in tandem followed the
> removal of primary voltage clearly indicating a current draw on the DC
> supply. To diagnose the strange occurrence I rechecked all the wiring
> (keeping in mind that it was working OK initially).I replaced tubes with a
> brand new set;installed new zener(s); checked/removed all by pass caps,
> ensured that the bias relay contact set was not welded,checked the
> parasitic choke assembly, etc. etc. Subsequently, I even disconnected the
> filament CT from the zener with no result. In desperation, I removed the
> plate current/HV meters from the circuit with no change.Currently, the
> only way I can get the HV to bleed off normally is to disconnect the HV
> from the tubes' plate caps. Obviously, something is drawing current
> through the tube that continues to elude me. Before I give up and lose
> forever the "glow in the dark" tubes (in favor of an 8877 retrofit),
> perhaps someone can see the error of my ways and save the da
> y. Any help would be appreciated. Ron W2CQM/3
>
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