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

[Thomas Carlsson]

Hej,

Några av kommentarerna följer mina tankar,

Fukt i trafon som orsakat kortis mellan primär o sekundär, i så fall ny
Trafo

Det är inte röret som behöver step start utan det är mest för att skydda HV
dioderna.

73,
Thomas, PY2ZXU, SM0CXU

On Sun, Dec 18, 2011 at 5:32 PM, Ulf (SM0NOR) <ulf at sm0nor.com> wrote:

> 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 at 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 at sm0nor.com>
> > To: "Amps reflector" <amps at 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 at sm0nor.com
> >>
> >>
> >>
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-- 
Thomas, PY2ZXU/SM0CXU
Member of the PW2D Team