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

[Ulf (SM0NOR)]

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