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

[Carl]

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
>
>
>
> _______________________________________________
> Amps mailing list
> Amps at contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
>
>
> -----
> No virus found in this message.
> Checked by AVG - www.avg.com
> Version: 10.0.1415 / Virus Database: 2108/4088 - Release Date: 12/18/11
>