First power up problems w/ recto tube. Opinions

[Mostro]

Hi people, I´m doing the first tests on my new (first) amp build, and I´ve 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/Mo ... wer-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 wasn´t steady, it fluctuated). Not good!!
When I removed the tube afterwards, a white layer had formed at the top of the tube´s 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, that´s 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... aren´t fuses supposed to blow w/ excess current?
- The white layer that formed in the tube means it´s dead, right? I think I´ve read that this thing happens when the tube loses it´s 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

[Structo]

Hmmm, I'm kind of new to tube rect but I don't think the purple glow thing is normal.
I've seen some power tubes get kind of a blue glow.

Was this a NOS tube or used one?

Not sure why the fuse didn't blow but it probably would if given enough time.

Tranny getting hot is not a good sign though is it?

[M Fowler]

Filament heaters are 6.3v (3.15v +3.15v)

Rectifer heater is 5v to 2 and 8.
Pins 4 and 6 go to PT 325-0-325 as stated which is high voltage for a 5Y3 or near the end limit. You should use a GZ34.

If you powered up with no load and only the 5y3 rectifier tube and things heated up then you need to start over at the beginning and figure out where you went wrong. What amp are you building?

Mark

[Mostro]

Structo, I´m pretty sure the tube is/was NOS, although I can´t be sure it wasn´t a pre-used one, the place I bought it from is, let´s say, not a customer-satisfaction-oriented place (you should try to get tubes in Argentina! not an easy task).

M fowler, thanks for your answer.

Since my mains V was a little low when I measured, I guess my heaters voltages were OK. Or not?
I don´t get it when you say that 325 is high voltage for a 5Y3. Are you talking about plate voltage? I´m building a mutant 5E3 Deluxe, same power section, different pre, same tube set, the transformers I have are made for cloning that amp.
GZ34? Hmm, I´ll investigate that one

[M Fowler]

No the voltage isn't excessive for a 5Y3 and pretty much in the right ball park for the 5E3 Deluxe. The plans I have are for 320 ac on #4 and #6 with 360 DC on #2 and #8.

Yeah I was think to use the 5y3 in a lower power amp with lower plate voltages.

Are you sure your using the 220/240 voltage taps correctly? With no load your fuse should have went first.

Mark

[Mostro]

My power transformer only has 220V, no different primary voltage taps. the fuse thing also baffles me...

[M Fowler]

Ok kick me if I get too sickening here.

AC recepticle line load to fuse ?2A, then wired to the on/off power switch then to the tranny 220/240 mains.

AC receipticle line neutral to tranny mains.

E to chassis ground.


PT 5V tap to 5y3 pins 2/8
PT secondary 325V to pins 4/6.
PT 6.3V tap to filaments of other tubes.

Should not have any problems. Told you this would be sickening, sorry. Just want to help.

Mark

[Mostro]

M Fowler, every bit of help is most welcome, thanks for taking the work to answer in such a detailed way. Yes, my wiring is -just- as you described it.
I guess I´ll have to buy another recto tube...
On a related note, I´ve been looking at the Fender 57 Deluxe Reissue schem, and for the mains fuse it has a 2A L 250V for 100-120 mains, and a 1A L 250V for 230-240 mains. Since I have 220 V mains, I ´m guessing that I shouls use a 1A fuse.. now I have in there a 2A one. Maybe that´s why it didn´t blow? Too high for my mains?

[M Fowler]

I would use the 1 amp fuse next time and put in another rectifier tube and try it again. I have seen some 220/240 mains using 1A and others 2A.

Mark

[Mostro]

OK, I got a new rectifier and everything works good now, the only problem is that it´s a 5AR4 and so it has less voltage drop than a 5Y3, so I now have to reduce the B+ a bit, cause it´s around 450V unloaded, and my filter caps are 450V ones, too close to the limit, AND my 6V6´s are russian ones (6P6S), and I´ve read that they don´t take plate voltages over 350V very well, so...

Isn´t building this things fun?

Thanks all for the help

[Structo]

I recently had the same problem with a Hammond organ amp conversion to Tweed Deluxe type amp.

Except that transformer was at 465v unloaded.

I installed a 130v 50w zener stud mount diode connected to the center tap of the HT winding.
This is a reverse voltage diode with the cathode to the case.

It dropped the voltage to 322v with tubes in.

I need to get a 150v zener to get it where it belongs but the local store did not have one of that voltage.

I mounted the zener diode near the power supple ground, used heat sink compound when I installed it.

[Mostro]

It´s alive, Dr. Frankenstein!
Just made the first power up, it makes sound! My first tube amp build, and it didn´t 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.

Structo, When I put the tubes in, the power dropped to 345V, so the problem solved itself! Thanks for the zener idea, sounds interesting.

[Structo]

Wow!
It dropped 105 volts?!
Man, that doesn't sound like a very stiff transformer if it dropped that much.

5Y3 supposedly drops 60 volts and the 5AR4 only drops around 17 volts according to my data.

5U4GB drops 50 volts.

What's the deal with the direct and indirect heated cathode rectifiers?

Seems I read that one is slow start and you don't need a standby switch.

Which one is it?

[M Fowler]

You have 325-0-325 VAC secondary and rectifer 5u4B you now have 345v DC. 325v x 1.2 full wave rectifer would be 390v. The voltage you are getting is good if you are reading it correctly. Should be about 365v with a 5Y3, 390v with 5U4B and 422v with GZ34.

Conversation factor for 5Y3 is 1.13 x the secondary voltage = 367.
GZ34 is 1.3 = 422. The 5U4B is 1.2 = 390.

I would read your specs for your tranny and see if it calls for 2A or 3A filament. The 5U4B requires 3 amps and your transformer may not handle the current draw for 3 amps. Either the 5Y3 2A or GZ34 1.9A.

You are going to get a higher current draw reading with all the tubes in.

Mark

[Mostro]

Let me detail a bit how I got to this B+ voltage (my previous post was mostly a "it-works-I´m-jumping-in-joy" post):
I added a 100 ohm 10W resistor between each power transformer HT outs and each recto tube´s plate. I measured the tranny´s HV secondaries, and it gives me around 200 ohms from each side to grounded center tap. This was measured in circuit.
I haven´t done the numbers, I just guessed the R´s value, knowing I may have to change them, but they were fine at first try. I got the idea mainly from AX84´s october amp schem. My only concern was that they could create too much sag, but in the little testing I did so far it´s not noticeable.
The recto tube is a Sovtek 5AR4. As I´ve been reading, new rectos are really not the same as old ones, they are mostly different tubes (russian in this case) with the pinouts adapted and characteristics similar to the old ones, so that may be a factor... I think using the old datasheets as reference for this tubes can be misleading.

Structo, I don´t really know what kind of heated rectifiere this is, I -think- directly heated. My standby switch is installed already, so it´s staying there.

M fowler, the tranny was made for a 5E3 Deluxe, so it´s meant for a 5Y3 recto, so 2A filament. It´s a handmade tranny (not by me), not a commercial one, I don´t have a spec sheet... I could ask the guy who made it.