Hi Ray,
Sorry to hear about your problem. It sounds like a flashover in the HV
circuit, with the current surge burning out the plate choke. It might it
have been an internal short in one of the tubes, especially since you've not
been using the amp much. With all due respect to the Ten-Tec service guy, it
doesn't sound like a bias problem to me, although the flashover may have
damaged the bias circuit.
I wish you hadn't applied drive to the tubes without any HV on them, because
that can quickly ruin a tube by destroying the grid. The excess grid current
heats up the grid and boils off the gold coating, which shortens tube life
to a few hundred hours. There's no way to tell at this point if the tubes
are damaged, but in the future be careful never to drive the tubes without
plate voltage.
It sounds like you're now getting HV on your multimeter, which is good
because it means the rectifiers and filter caps are okay. The question is
whether the short is still in the RF compartment, but isn't blowing fuses
because the plate choke is open. If you're lucky it might be just a shorted
bypass cap. After you repair the plate choke, and assuming an ohmmeter test
doesn't show a short circuit, try turning on the HV with no tubes in the
amplifier. If nothing pops, then I'm guessing a tube may have arced
internally. That can happen with tubes that haven't been used for awhile,
but isn't common with 3cx800a7s. It might not be a bad idea, however, to run
the filaments on the tubes for a day or so without any HV, just to condition
them again.
73,
Jim W8ZR
>
>
> Need some help trouble shooting my Titan amp.
>
> I bought a Titan 425 amp about a year ago and have fired it up only 5 - 6
> times. It has always worked fine with full output. Last week I fired it
up,
> but when I tried to tune the amp, there was no plate current or output. I
> keyed the amp several more times and tired to adjust for output from its
> preset set dial values. Nothing happened, no plate current and no output.
> All of a sudden, 'BANG' and the amp went dead.
>
> It turned out that one of the 20 amp fuses had blown. I replaced it and
> waited for the delay relay to turn off. I then moved the multi-meter
switch
> to 'Grid' and when keyed, saw that I was driving the amp way too hard. I
> backed off the drive to 50ma grid current and tried to tune the amp again.
> Still nothing, no plate current and output. I shut it down. Later in the
> day, I removed the top cover and found that the plate choke had blown. I
> looked around more but could not find any other obvious damage. Hope I
> didn't blow a tube!
>
> Today, I ordered a replacement plate choke. I'll rewind the old one as a
> spare. While talking with the customer service at Ten-Tec the guy said the
> bias board could be bad. They do not have any replacements.
>
> Does anyone know these amps and offer any reason for the cause of this
> failure? Could you offer any other trouble shooting advice. Later, without
> the B+ connected and no drive power, I will key the amp and see if the T/R
> relay is working. Other than that, I'll just have to take the schematic
and
> start looking at everything. Oh, something interesting, the customer
> service guy said that the Titan 425 schematic does have some known errors
> in it. Wonder if anyone has run across them?
>
> Any advice or comments will be appreciated.
>
> Tnx,
>
> Ray,
> N6VR
>
>
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