New power tranny: rectifier and power lamp blow (Silvertone 1482)

[slidincharlie (Carlo P)]

Hello fellow ampagers!
I have had the power tranny of my mid-60s Silvertone 1482 amp rewound.
First thing: there is a new wire that wasn't there in the original tranny. The tech called it "grid". Any idea of what this wire is for? I left it unhooked for good.
Second thing: the heater wires have slightly different, and higher than stock, tensions: about 6.6 and 6.4 Vac, measured with no tubes in place.
Third - and most important - thing: I turned the amp on for a first check, with only the rectifier tube (an RCA 6X4) on. After less than one minute there was a weak crackling noise and the tube lit off. The heater and AC voltages are still there, but the tube shows no sign of life (no dc high tension). Also, the power light shut off shortly after the recto.

What can cause a rectifier tube and the power lamp to die when the ac voltages are fine?
I'd love to find the cause of failure before putting a new 6X4 in the amp.

[52 Bill]

The tech called it "grid". Any idea of what this wire is for? I left it unhooked

Internal shield?

the heater wires have slightly different, and higher than stock, tensions: about 6.6 and 6.4 Vac, measured with no tubes in place.

What are these voltages measured in reference to, ground? From wire to wire?

The heater and AC voltages are still there, but the tube shows no sign of life (no dc high tension). Also, the power light shut off shortly after the recto.

Does the filament still light up? Read the ac voltage across the socket for the filament (pins 3 and 4), and across the pilot lamp socket.

[trevorus]

I would guess that your newly wound PT is over-volting the recto. it's only rated for 325v a side, by the datasheets. My 1483 came with a 5y3GT in the recto spot, but that is with a stock transformer. Measured potentials were in the 700 volt range peak to peak, no load. That is skirting the edge of the data sheet's ratings as it is. You might give a 5U4 a try (you'll have to change the pinouts when changing tube types), as it can handle 450 volts a side, but before anything else, I would measure voltage on you HV winding with no load. Be careful measuring it, though, that crap hurts, and can kill... Also, you may look into your filter capacitors to make sure they are rated for whatever voltage your PT is kicking out.

[Enzo]

Yes, what is the HV AC from the transformer?

Why does the 6X4 blow? HArd to say. What had the tube experienced from whatever caused the original transformer to need replacement? The tube could have been damaged, just waiting to die.

The 6X4 is a small 7-pin miniature, it would take some serious effort to stuff a 5U4 in there, aside from its needing a 5v heater winding. And I don;t know if there is room in the cab for a tall 5U4.

Rectifiers can arc over if the voltage is insane, but usually it is excess current draw through it that kills them. COUld be from leaky or shorted caps, bad tubes, shorts. Maybe whatever killed the first tranmsformer is still wrong in the amp.

[slidincharlie (Carlo P)]

Here's a combined reply (a schematic is attached).

The AC HV from the tranny is 350 V on each end (referred to ground), which is pretty normal since the schematic calls for 335 Vac (I've been told here that I can expect quite a drop in AC HV from a 6X4 when all tubes are in place). All filter caps and most other caps are fresh (yet I don't know if one has developed a short somewhere).
Also it is possible that the 6X4 was already damaged from the previous tranny failure, but I wanted to eliminate all possible causes of damage before using a new tube.
The heater voltage is 6.6 and 6.4 V referred to ground (plain 13 V across heater pins and across pilot lamp legs).
There IS heater voltage throughout the heater circuit now (no tubes in place for the moment).

This amp quit working all of a sudden several months ago after a couple years of normal operation. Only evident symptom of death was a shorted power tranny. That tranny had been rewound, too (by a different person). Since the first 'rewinder' had no experience with musical applications of a tranny, I thought that the tranny died due to poor craftsmanship. The second 'rewinder' does build trannies for a local boutique amp manufacturer.

[Robert M. Martinelli]

Hi Carlo,
the schematic you posted calls for 6,3 VAC PT sec. and heaters, this means that you should not have 6,3-6,4 VAC between each PT heater winding leg and GND, but rather 3,15 VAC.

With some 12,6/12,8 VAC across them, your heaters are bound for destruction, as, getting twice the voltage, they will have to dissipate four times their rated power.

Another point I can think of...An 6X4 can withstand 450 VDC between the heater and the cathode, and you don't have a separate winding for the rectifier's heater, this means you MUST NOT connect the rectifier's cathode together with one heater's leg, as by doing so you're creating a short to ground ( from the heater to the center tap of the PT heaters' winding ).

Hope this helps

Best regards

Bob

[Enzo]

You think he's going to exceed 450VDC on that cathode, Bob? That is 100v over the circuit spec.

[Enzo]

Hey Bob, you get all over this forum, so you;re the perfect guy to tell. I'm on vacation until tuesday, if anyone misses me, tell them. I'm too lazy to turn off my email link, I'll probably get a ton of that.

[Robert M. Martinelli]

You think he's going to exceed 450VDC on that cathode, Bob? That is 100v over the circuit spec.

Hi Enzo!
No, quite the contrary, I was rather concerned about a "short to ground" issue, probably I didn't manage to explain it very clearly, I cited the 450 V spec to highlight that the 6X4 rectifier tube doesn't need one heater's leg to be connected to the cathode as it can withstand very high cathode-to-heater voltages...Thanks for asking though, because it gave me the chance to re-read my post and to understand how badly I expressed my thought....Hope this time I' ve managed to be clearer....

Enjoy your vacation, everyone here will miss you, so be back to us ASAP ( or bring a laptop with you to keep in touch with us.....just kidding! Get some well deserved rest and forget about us ! )

Cheers

Bob

[slidincharlie (Carlo P)]

Jesus... something odd is going on here...
I can't believe the guy rewound the tranny the wrong way...
Back from work I'll check the amp and post again.

[52 Bill]

With some 12,6/12,8 VAC across them, your heaters are bound for destruction, as, getting twice the voltage, they will have to dissipate four times their rated power.

Yes, this is why I asked if the filament was still lighting up.

I could imagine that an old tube would die, but when he said that the tube and the pilot lamp died at the same time, I thought that there might be a problem with the filament voltage.

[slidincharlie (Carlo P)]

I have re-read the heater VAC with the secondaries and the heater CT on air:
there are about 6.4V between either heater secondary and its CT, and about 12.8V across the two secondaries: same as with the PT fully hooked.
Also, the heater CT is made of two wires inside the same tubing: is this normal? Actually it happens the same in my 1472 amp, which is quite similar to this 1482.
EDIT: I made a mistake in the starting post. The green (currently unused) wire is labelled "screen", not "grid" as I stated above.
Can this wire play a role in the "double heater voltage" issue?

Now, where is the problem? Did the tech rewound the heater coil the wrong way? Can this be corrected? May I use a resistor to get half the heater voltage?

[Robert M. Martinelli]

Hi again Carlo,
As I stated in my previous post and as Bill suspected, your heater's winding is wound wrong, as it gives you twice the voltage you'd need out of it.

Now, for the possible solution :

You stated that the heater's winding center tap has two wires tied together. This center tap consists of the "end" of the first 6.3 VAC winding tied together with the "start" of the second 6.3 VAC winding.

If you separate these two wires, you now have two separate 6,3 VAC windings ( a simple measurement will confirm this ), and you can try to use only one ( if its current rating is enough ) or, better yet, put the two windings in parallel ( the "start" of the first tied together with the "start" of the second - the "end" of the first together with the "end" of the second ) Should you connect them "wrong" ( out of phase ) you won't have any voltage out of them; should this be the case, simply swap one winding's connections and everything should now be ok.

This way you'll get a 6.3 VAC winding with twice the current capability.

You' ll no longer have a center tap, but this can be very easily remedied by adding a couple of 100 Ohm resistors ( or a hum balance pot ) between each 6.3 VAC leg and GND ( for hum-canceling purposes ).

Hope I've made things sufficiently clear, if not, don't hesitate to ask for further help!

Best regards

Bob

[Steve Conner]

Great idea

Just be careful, if you parallel the windings wrongly, you'll not just get no voltage, you'll short out the whole transformer and might burn it up. (Don't ask me how I know this...)

If you think about it, the two wires that were previously inside the same tube should NOT end up connected together again. Following this rule, there are fewer ways left to screw up.

Of course, if one half reads 6.6 and the other 6.4, there's always the possibility that our foolish transformer winding friend has used one extra turn in one of the two sections. (0.2v is about right for one turn.) Paralleling them will end in disaster. I recommend connecting one end, then rubbing the free ends together with the power on and looking for sparks (there should be none) Also ask the guy who wound it if it's OK to parallel the windings. Better still, send it back and get him to parallel them for you and test it

In fact, maybe when all's said and done, it's less hassle to separate the windings and run half of the tubes off each one.

PS, a wire marked "Screen" is a Faraday screen between primary and secondary, and should be connected to mains ground/chassis. It helps to stop noise from the mains getting into your amp.

[Robert M. Martinelli]

Hi Steve,
Thanks for appreciating my idea and for the additional "caveat".

Carlo,
I have attached a ( poorly drawn ) drawing to make things clearer.

Cheers

Bob