Power transformer fails

[Oddvar R]

I have bought three Weber W022798 power transformers to put in a Dumble 50w ods. The amp itself sounds great, and the transformers works great for a awhile, the it starts blowing fuses. I have tried to search for every possible reason, but the 100w bulb limiter lighst up even when all the secondaries ar disconnected. I have tried to contact Weber, but they never answer anything. Strange that it happens 3 times, works for a while, one month, two months, and the goes the fuse. What can cause a PT to blow fuses. I have done almost anything I can think of, following the advice on robrobinette site on PT failure. I am not an pro amp builder, but I have buildt more than 20 amps that all works great. Anyone shedding a little dimmer bulb light on this?

[martin manning]

What circuit is this 50W built to? Are there any loads on the PT other than the usual high voltage supply and heaters? Do you have a link to the data sheet at Weber?
edit: added Weber data sheet

[Stevem]

Well your using what's stated as a 40 watt PT for a 50 watt amp you know?

It's too late now, but when the second PT failed and when first installing the 3rd one I would have incerted a amp meter to monitor the B+ draw and if that checked out I would have done the same with the heater current draw and what your actual heater voltage is .

How many preamp tubes do you have in this build and what are you using for output tubes, 6L6GCs?

[Oddvar R]

I am using it in the Dumble 50w ODS. Ok, I never thought it would be to lite for the build. Good tip.

[martin manning]

50W Bassman PT is known to be a good choice for 50W ODS. Hammond Bassman replacement 290EX is rated at 275 mA where the Weber would be 218 for the same VA.

[Stevem]

I think the Weber is in fact only 200 ma.

[johnnyreece]

I don't want to insinuate anything, but (at least in the past) Weber transformers were pretty conservatively rated. This PT was used with their 5e3x2, which uses 4 6v6 and 2 12ax7. Not the same, I know (cathode bias, one less preamp tube), but *somewhat* similar. Also, not using the tube rectifier heaters should save you a few mA, too. Just saying that to reiterate what Stevem is saying: It might be worth measuring your current draw to see exactly what you're pulling. I don't really do any Dumble stuff, so I don't know what to expect, current-wise. There might be something besides an underrated PT that's causing you issues, though, and it'd be good to catch it before cooking another PT.

[Oddvar R]

I don't want to insinuate anything, but (at least in the past) Weber transformers were pretty conservatively rated. This PT was used with their 5e3x2, which uses 4 6v6 and 2 12ax7. Not the same, I know (cathode bias, one less preamp tube), but *somewhat* similar. Also, not using the tube rectifier heaters should save you a few mA, too. Just saying that to reiterate what Stevem is saying: It might be worth measuring your current draw to see exactly what you're pulling. I don't really do any Dumble stuff, so I don't know what to expect, current-wise. There might be something besides an underrated PT that's causing you issues, though, and it'd be good to catch it before cooking another PT.

Good advice there. I have changed about everythin partwise, filtercaps, rectifiers etc. Only visible sign in the amp iyself was a burnt grid resistor.

[Stevem]

A burnt grid resistor where, on a output tube?

[Oddvar R]

This one, in blue circle...

[Oddvar R]

I think the Weber is in fact only 200 ma.

Is that to little you think?

[Roe]

it is well known that many weber transformers overheat with 50hz

[pdf64]

… I have changed about everythin partwise, filtercaps, rectifiers etc. Only visible sign in the amp iyself was a burnt grid resistor.

For a 6L6 to cause a 470R 5W screen grid resistor to become damaged, the valve in the socket must either have shorted badly, or have lost bias.
HT fuses are a really good idea

[Roe]

I have bought three Weber W022798 power transformers to put in a Dumble 50w ods. The amp itself sounds great, and the transformers works great for a awhile, the it starts blowing fuses. I have tried to search for every possible reason, but the 100w bulb limiter lighst up even when all the secondaries ar disconnected. I have tried to contact Weber, but they never answer anything. Strange that it happens 3 times, works for a while, one month, two months, and the goes the fuse. What can cause a PT to blow fuses. I have done almost anything I can think of, following the advice on robrobinette site on PT failure. I am not an pro amp builder, but I have buildt more than 20 amps that all works great. Anyone shedding a little dimmer bulb light on this?

classical issue known since 2006 at least. the transformers work perfectly on 60hz, but not on 50hz. Weber claimed they fixed the issue but there are too many reports that suggest otherwise (I have burnt two PTs, a third overheated, while the fourth runs cool)

[Stevem]

Is that 470 5 watt screen resistor going to ground at that terminal strip?

I ask because I do not see a wire feeding the non tube end of it in that drawing.