PT trouble

[Lowcurrent]

I recently built an amp, and it's misbehaving. It's a 6G2 with deluxe-sized transformers, the PT is a Weber 025130 that has been sitting on a shelf for years. The amp blows fuses from time to time. I've been over the circuit with a loupe, and I can't find any errors (and believe me, I always suspect myself first).
Connected the amp to a light bulb current limiter, the bulb glows, but it's not full brightness. Not a dead short. So I tried to find out where all the current goes. Even with all tubes out, the light bulb limiter glows orange. And even when I unsoldered all the PT's secondaries, it keeps on glowing, just as orange.
Is there something I'm missing here, or should I ditch this PT before it's toast?

[eschertron]

How much current? Every transformer has an idle current to maintain the flux.
The glow of the lightbulb will depend on the wattage rating of the bulb, compared to the size of the transformer.
If you can get engineering unit numbers, someone will chime in with advice.

[Helmholtz]

What's the bulb wattage?
Any transformer draws some idle/no-load current.
If that's say 50mA, the lamp will get up to 6W (assuming 120V mains).

What's the rating of the fuse?

What input AC current do you measure without limiter (Caution! Meter leads at mains voltage!)?

[Lowcurrent]

Thanks for swift replies!
The lightbulb is rated 150 w. The fuse is 1 amp (I'm in Europe with 220 v mains).
I should mention that I did pull all the tubes from a 5e3 Deluxe and 5f6 Bassman clone and plugged them to the current limiter for comparison, and then the bulb didn't glow at all.

[Helmholtz]

Thanks for swift replies!
The lightbulb is rated 150 w. The fuse is 1 amp (I'm in Europe with 220 v mains).
I should mention that I did pull all the tubes from a 5e3 Deluxe and 5f6 Bassman clone and plugged them to the current limiter for comparison, and then the bulb didn't glow at all.

That actually points to a PT problem, though not a hard short (more like a creeping short).

What are PT secondary voltages with tubes in and without limiter?

Generally with (small) tube amps a 40W to 60W limiter lamp is recommended.

[Mick Bailey]

A PT that will light a 150w bulb (even orange) with the secondaries disconnected must be dissipating heat internally - does it get warm to the touch? There could be a hotspot that's developed due to the insulation breaking down, becoming carbonised and conductive. You could use the neon test trick across the HT secondary (with the amp unplugged and all secondaries disconnected) to see if there's a shorted turn (even if the 'short' is fairly high resistance)

[Lowcurrent]

Helmholz: the plate voltage was 327v with the output tubes biased at aprox 70% plate dissipation.
Mick Bailey: when the amp was running properly, the PT would get warm after a while.

Anyway, the PT is toast now. I plugged it into the mains, still without the secondaries connected, and tried different fuses to see what would happen. I started with slow blow 100mA, then 200mA, 300mA, 500mA, 1mA. They all blew.

But that wasn't the end of my bad luck. I had another identical PT in my stash. New and unused. Taking it step by step, I connected the primaries first, and left the secondaries unconnected. Plugged it into the current limiter, and watched the bulb glow dimly orange. Again. What are the odds? Connected to the mains, fuses blow, same as the other PT. The shorted PTs are starting to pile up.
So I am wondering if those W025130 PTs are more prone to failure than others. I bought six of those PTs seven or eight years ago, and in addition to the two recent failures, I've had one more fail a couple of years ago. Back then I couldn't find any error with the amp, but I naturally assumed I had made some kind of mistake that caused the PT to fail.
So that's a 50% failure rate, and then those tranformers aren't so cheap anymore.

[glebert]

Just to rule out everything, did you disconnect the primary side and verify no glow?

[Randall]

I wonder if the shellac, or varnish, or whatever it is called is breaking down. I would call Weber and hold his feet to the fire. Although, since Ted is gone, I don't know how far you would get with that.

[Enzo]

Well, Ted Weber is a company, so Ted himself doesn't have to come to the phone...

[Lowcurrent]

Glebert: yes I did try the limiter with the primaries disconnected. I even swapped the on/off switch.

[Lowcurrent]

So. I emailed Weber two months ago and told them about the failing PTs and to ask if this is a known problem with this PT. No answer yet, so I take that as an affirmative.

[Enzo]

Just my opinion here...

So. I emailed Weber two months ago and told them about the failing PTs and to ask if this is a known problem with this PT. No answer yet, so I take that as an affirmative.

May I suggest CALL them on the phone. Emails, you never know if they got it or it went into a spam folder.

And may I further suggest when talking to a company, referring to something as a "known problem" implies that the product systematically fails. It implies a lot of people are getting defective product. Much nicer to simply describe your problem and ask how best to handle it. Yours could well be the only ones that failed.

[J M Fahey]

1) Ted Weber canīt answer the phone, he passed away a few years ago.

In any case, itīs a pretty small "Family" operation

2) the fuses mentioned were all too small; 1A might be marginal, try 2A

Also plug PT primary in series with a 100W bulb, how bright does it light?
What voltage do you measure across PT primary ends, still through bulb limiter?