It would have to have been in a Scandanavian sauna for a few years since
that transformer is pretty well sealed in heavy transformer varnish.
Carl
KM1H
----- Original Message -----
From: "Vic K2VCO" <k2vco.vic@gmail.com>
To: <amps@contesting.com>
Sent: Sunday, December 18, 2011 11:18 AM
Subject: Re: [Amps] home brew 811a amp - need some advice
> 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@sm0nor.com
>>
>>
>>
>> _______________________________________________ Amps mailing list
>> Amps@contesting.com
>> http://lists.contesting.com/mailman/listinfo/amps
>
>
> --
> 73,
> Vic, K2VCO
> Fresno CA
> http://www.qsl.net/k2vco
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>
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