[TenTec] Ameritron amp issues and looking for a Centaur

[Gary Hoffman]

Well this sounds like a classic case of VHF parasitic oscillation in the
amp.  I'm sure you thought of this already.

As you may know, these so called glitches frequently take out the bandswitch
as well.

What is never clear to me (or to anyone apparently judging by the
impassioned arguments on this subject) is why amps go along fine for years,
and then suddenly explode multiple times.

73 de Gary, AA2IZ


----- Original Message ----- 
From: "Rick Denney" <rick at rickdenney.com>
To: <tentec at contesting.com>
Sent: Monday, March 31, 2008 2:54 PM
Subject: [TenTec] Ameritron amp issues and looking for a Centaur


> I'm having a persistent problem with an AL-811 and now I'm wondering
> if the way my Omni V is keying it might be part of the problem.
>
> Synopsis: When working Ducie Island, I tried to tune on 10 meters and
> blew the finals in my AL-811. I replaced them with 572b's, which
> worked fine for about ten minutes of testing into a dummy load, at
> which time I heard arcing and smelled smoke. Inspection revealed blown
> parasitic suppressor resistors in the old-style suppressors. I
> replaced the suppressors with Ameritron's newer style, which is a PCB
> that sits atop the plate choke and provides a resistor protected from
> DC by caps, and an inductor to bypass low frequences around the
> resistor. It also provide new tube ceramic caps, etc.
>
> Everything worked fine with no smoke smell for about 30 minutes of 30
> or 40% duty cycle testing on all bands.
>
> Then, the amp stopped putting out any power at all. Nothing. The
> filaments are still lit, the high voltage reads correctly. When keyed,
> the amp shows that it's keyed, and the plate and grid meters both read
> about 10% of scale (100 and 20 ma or so, respectively). These readings
> remain completely unchanged for any setting of input drive power or
> plate and load. Of course, there is no ALC hooked up.
>
> I have checked the bias voltage filter cap on the output of the
> anodes, and it checks out okay (.001 uF with my Fluke DMM). I can find
> no shorts anywhere, and there is continuity in the entire RF chaing
> from input to output where there is supposed to be. The capacitors
> that pass RF from the input to modulate the filament biasing voltage
> check out okay, too. The input band switch is undamaged and seems to
> work fine.
>
> I have spent a lot of time with the T/R relay in the amp, and it's
> fine. It's working per spec.
>
> The Omni V reads high reflected power when attempting to excite the
> amp, but when I put the amp in bypass, the Omni happily puts out full
> drive power into my dummy load. It's almost as if the exciter's output
> is not connected to anything when the amp is not in bypass. All this
> tells me that it's gotta be on the input side of the tubes. The
> behavior is identical on all bands, telling me that it's not because
> of the tuned input components.
>
> The input band switch shows appropriate continuity. But if it is the
> input bandswitch, I'm wondering if it was damaged as a result of
> hot-switching. I'm keying the amp with the Relay output of the Omni V,
> and using the Tune button to put out keyed CW. Maybe I should have
> been testing with a whistle into the mike, heh, heh.
>
> I'm at a loss. Any ideas of things to check?
>
> And--I'm in the market for a Centaur if anyone has one they want to
> sell.
>
> Rick, KR9D
>
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