Yes, I guess I could do without the step start. I may try without it when I get
a new transformer. I just need to find a way of protecting it better. I would
probably put in a fast acting circuit breaker and not relay on the glass fuses.
Thanks
73's Ulf
Sent from my iPad
On 18 dec 2011, at 17:56, "Carl" <km1h@jeremy.mv.com> wrote:
> 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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