First power up problems w/ recto tube. Opinions

[Mostro]

Hi people, Iīm doing the first tests on my new (first) amp build, and Iīve run into some problems.

Once wiring was finished, I started testing voltages. First I tested all heaters without any tube installed and got this (AC in my country is 220V):

AC: 214V
- Heater pins on all signal tubes (12ax7 and 6V6): 5,4V AC (from each other in every socket)
- Rectifier tube (5Y3GT) pin 2 to pin 8: 4,7V AC
- Rectifier tube pins 6 and 4 w/ respect to ground (trafo is 325-0-325): 329V AC

All well there. Then I installed the recto tube, a Sylvania labeled "5Y3GT - USA". Clicked it on (power on, standby off) and I got this:

http://i68.photobucket.com/albums/i5...t-power-up.jpg

The tube, besides the glow seen in the pic (that by the way "moved around" the plates), made occasional flashes of light. The chassis (aluminium) got REALLY hot very fast, then I touched the power transformer and it was also almost too hot to touch. Remember, no other tube installed. The amp was never on for more than about 30 seconds, just enough to take voltages.

DC to ground was about 35-40V from pin 8 (it wasnīt steady, it fluctuated). Not good!!

When I removed the tube afterwards, a white layer had formed at the top of the tubeīs glass and the plates have sort of "burn marks", darker sections in them.

I never turned the standby on, to protect the rest of the circuit from whatever might have happened.

Questions:

- My conclusion is that the tube shorted and was drawing excessive current, thatīs why the transformer heated. what do you think? By the way, the amp has a 250V 2A slo blo mains fuse, that did NOT blow. Strange... arenīt fuses supposed to blow w/ excess current?
- The white layer that formed in the tube means itīs dead, right? I think Iīve read that this thing happens when the tube loses itīs vacuum and some gas, I think, inside of it gets out
- Afterwards I tested voltages again without the recto tube installed, and they were OK, so I think the transformer was not harmed. Is there anything else that I have to test to make sure the transformer is OK?

Any suggestion is welcome

Thanks

[loudthud]

Suspect either the first filter cap is installed backwards or it is otherwise drawing current. What does your DVM on ohms read at pin 8 of the rectifier to ground? Your 5Y3 is damaged. Was it known good before? Try resistance readings right to the tube pins out of circuit. Any reading besides infinite on pins 4 and 6?

[Mostro]

Suspect either the first filter cap is installed backwards or it is otherwise drawing current. What does your DVM on ohms read at pin 8 of the rectifier to ground? Your 5Y3 is damaged. Was it known good before? Try resistance readings right to the tube pins out of circuit. Any reading besides infinite on pins 4 and 6?

Thanks loudthud for the answer, I never turned on the B+, and since my standby switch is before the first cap, it never got voltage. The 5Y3 I bought supposedly as new, I guess NOS since I donīt believe sylvania even exists anymore... this was itīs first power up.

I get no resistance reading between the tubeīs pins and between the socketīs pin 8 to ground (i.e, infinite, as much as my (cheap) meter can measure.

[loudthud]

That reading in pin 8 tells me you had a bad tube and not some other problem that caused the tube to die. It's a good idea to ohm along the filter caps before you turn the standby on. Your meter may give strange unstable readings as long as there is no short to ground.

[tubeswell]

Also a good idea to protect the HT winding with a 1000V 1A (1N4007) diode in series with each side the HT-to-rectifier anode (banded-end pointing towards the tube anode) This protects the HT winding from shorted rectifier tubes. R.G. has a list of this and other similar handy tips in his series of 'immortal amp' articles.

[Mostro]

Itīs alive, Dr. Frankenstein!
I got a new recto tube, worked OK. Just made the first power up, it makes sound! My first tube amp build, and it didnīt explode or started a fire! C-O-O-O-O-L!!!!
More detailed post later w/ schems, sounds, etc.

I want to thank everyone that helped me.