[Amps] home brew 811a amp - need some advice

[Vic K2VCO]

My guess is that moisture in the transformer caused an insulation breakdown. I've seen 
recommendations here in the past for how to dry out a transformer before use. If it was 
stored in a damp location, this may be the explanation. It's not the fault of the tubes.

On 12/17/2011 7:20 AM, Ulf (SM0NOR) wrote:
> 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 at sm0nor.com
>
>
>
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-- 
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco