It sounds like the step start relay did not release and the resistor burnt.
That occurs often in Ameritron amps but should not be necessary for 811's.
Carl
KM1H
----- Original Message -----
From: "Ulf (SM0NOR)" <ulf@sm0nor.com>
To: "Amps reflector" <amps@contesting.com>
Sent: Saturday, December 17, 2011 10:20 AM
Subject: [Amps] home brew 811a amp - need some advice
> Hi all,
>
> I need some advice from you more experienced guys....
>
> I've been building on my 811a amp for some months now and I'm reaching
> quickly the point of the "big" test. Every section of the amp has been
> tested with only low voltages and I basically only had one more test to
> perform before applying HV and some watts of drive. The test I was about
> to perform today was applying only filament to the tubes and no HV. The
> idea was to run the NOS 811:s for a couple of hours with only filament
> before applying the HV.
>
> The test went very well in the beginning; all tubes lit up and I measured
> 6.35 volts at the pins on the tube. I noticed some heat in the filament
> choke, but the temperature leveled out on 38 degrees (Celcius that is,
> sorry F-guys). But after about an hour the amp went dead with blown fuses
> and a burnt step start resistor. Having checked everything in the circuit
> it would appear that the transformer shorted, or broke. The transformer is
> most likely dead as a result of this test. If I apply about 10 volts AC to
> the primary, I get 1,5 volts in the HV secondary instead of 47 volts,
> which I had before, in previous tests.
>
> My questions are:
>
> 1, What would have caused the transformer to melt down? Or perhaps brake
> down? I only had filament and control circuits hooked up. No HV = not
> connected to the bridge board.
> 2, Can old, NOS 811a cause this by shortening its filament? And would you
> advice against NOS tubes? Should I go for chinese instead?
> 3, How can I protect my transformer in the future? Obviously the fuses in
> the primary was not enough to protect the transformer.
>
> The transformer, by the way.. is an original Ameritron 811H.
>
> I can also offer some pictures here:
> http://gallery.me.com/ulf.tjerneld#100183
> The last pictures are a couple of months old. More has happened since
> then. They may not give a clue to what happened but they give you an idea
> of the level of standard with which I have carried out the project.
>
> Thanks! Happy for any advice before I get out for a new transformer!
>
>
> 73's
>
> Ulf (SM0NOR)
> ulf@sm0nor.com
>
>
>
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