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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