My amp keeps blowing power transformers

[mfreqmaster]

I have a Weber kit 6M18 amp that I built a year ago. I worked great for an enitire year until a few weeks ago. The amp would not turn on, fuses would be blown, even on standby, rectifier removed. I replaced the power transformer with an exact duplicate. The amp worked and sounded great for one week until last night when it did it again. Am I missing something here? If there were other bad parts down the power supply chain wouldn't that be immediately apparent when I power up the new transformer? This amp worked for an entire week. No other electronic devices in my house have experienced these problems, I don't believe there is a problem in my house electric. Thanks for any help.

[R.G.]

It is always possible that you're simply getting bad power transformers.

That being said, it's the very least likely thing to be happening.

If there were other bad parts down the power supply chain wouldn't that be immediately apparent when I power up the new transformer?

Possibly not. Transformers die young from only a few causes. Most likely among these are
1. over temperature operation caused by overloads
2. Punctures by over voltage spikes

Number 1 could happen if the rest of the power supply was fine for the load, which was larger than the transformer was intended for. The rest of the power supply would be wondering why the transformer couldn't keep up. It's also possible that there is an almost-short somewhere that is overheating the power tran. You can find these out with a power meter on the front/primary of the transformer (Kill-A-Watt is cheap and good for this) and maybe measuring the temp of the transformer. An near-short or a major overload on the filament windings will kill a transformer without blowing the mains fuse.

Number 2 could happen if your electrical mains are noisy, with repeated transients that puncture the winding insulation. Generally, OTs are destroyed by transients on the output stage. I can't think of a good way to get punch-through failures on the PT that doesn't come from the primary side. but it could happen.

This amp worked for an entire week. No other electronic devices in my house have experienced these problems, I don't believe there is a problem in my house electric.

That's a good thing to believe.

The effects of overheating near destruction are cumulative as the insulation degrades.

How are the ratings of the PT versus the amp it's driving?

Are all the components new?

[mfreqmaster]

The power transformer was the one provided by Weber for it's 6M18Vib kit. All parts were selected by Weber for their design. Even the replacement power transformer was an exact duplicate of the original. All parts were purchased new from Weber.

[R.G.]

The power transformer was the one provided by Weber for it's 6M18Vib kit. All parts were selected by Weber for their design. Even the replacement power transformer was an exact duplicate of the original. All parts were purchased new from Weber.

So did they get hot?

Do you still have the corpses? The obvious thing to do is an autopsy.

[MWJB]

Go for the Hyboer version of the PT.

Check bias current to ensure that you're not pulling too much plate current, check power tube socket wiring & heater wiring for any shorts that could be stressing the PT.

Check current draw accross any power supply/screen grid resistors etc to see if anything untoward is going on.

[J M Fahey]

What I find strange is the mains fuse not blowing !!!
Fuses don't protect SS components, which die *very* fast, but transformers have a big mass which takes more than a few seconds to dangerously overheat, letting the fuse do its job.
If you get a new one, after replacement hook the amplifier to a series lamp, without tubes in the amp, and leave it on a long time, to see whether series lamp glow increases along time, which should show unexpected losses in the transformer or somewhere else, in-circuit.
After ,say, 6 or 8 hours, plug the rectifier tube and re-check, repeat with power tubes, and so on.
Along that path *something* strange must appear.
It's a very slow checking system but the only way to catch random gremlins.

[mfreqmaster]

Weber has agreed to replace the transformer I bought recently. When I get the replacement I will proceed with caution, testing everything I can think of along the way. I will keep you posted.

[R.G.]

Weber has agreed to replace the transformer I bought recently. When I get the replacement I will proceed with caution, testing everything I can think of along the way. I will keep you posted.

While you're waiting for the replacement, buy and wire in a fuse holder so you can put a fuse in series with every independent winding on the transformer. You'll need at least three and possibly four fuses. One goes in each high voltage red/white lead to the rectifier tube. Another (at least) goes in the filament winding. It's better to use two in the filament winding if you ground the centertap, because either half shorted to ground will kill the transformer. This can't happen if you open the centertap and insert a "resistor centertap" because the resistors - 100R, usually - are big enough that shorting one side does not let enough current flow to kill the trannie. If you use a resistor centertap, one fuse will protect the heater winding.

The resistance in series with the bias tap is big enough not to cause the dying transformer syndrome.

What often kills power transformers is a low-voltage fault. This produces a huge current but the change in primary current is not large enough to blow the primary fuse. The local current does burn out the wires it's on, though. A shorted tube heater, or a shorted filament wire to chassis or the other side of the heater winding will do this neatly. Fuses stop this for external faults.]

While you're putting on suspenders with that belt, solder in two solid state diodes. 1N4007s work fine in that amp. Put one in series with each anode of the rectifier tube. Now a shorted rectifier tube will not kill your power transformer and your filter caps. The solid state diodes keep the AC from getting to the filter caps if this happens.

Anticipating your next question, no, you can't hear solid state diodes (0.7V) added to the 15-50V drop across the rectifier tube, and no the solid state diodes don't make any more noise or distortion than is already there. They just keep a dead rectifier from killing your PT.

[mfreqmaster]

Closer to issue

OK, I received the new transformer. I fused both the 6.3V filament wires and the B+ wires. Upon power up the indicator light flashed once then quickly died. Thanks to the filament fuses I was able to see one of those fuses start to glow before I powered down.

I removed the fuses and checked the transformer by itself. Whew, the transformer was still putting out the proper secondary voltages. I removed the tubes, except for the brand new rectifier tube (I was previously using a Weber Copper Cap solid state rectifier). I was able to power up no problem, the tube rectifier was doing it's job fine. I then started to add tubes, starting with the power tubes. The power tubes caused the problem again, even in standby. I took them out and tried the preamp tubes, they did not cause the indicator light to go out like the power tubes but the indicator light became very dim and one of the filament fuses started to glow. t did not have any replacement tubes available except for one 12AX7 lying around that was never used in this amp. I plugged the 12AX7 in the phase inverter slot, powered on and everything was working fine. So, it looks like somewhere along the line a fault occured that messed up all my tubes? If that is the case then fresh set of tubes should do the job. Does anyone concur with this analysis?

[WholeToneMusic]

Install the power tubes one at a time to see which one is bad.

[mfreqmaster]

Correction

I pulled another 12AX7 from a different amp and added to the one working 12AX7 and as soon as a second tube was added, the problem surfaced again (indicator light dimmed, filament fuse glowing). It doesn't matter which of the 3 sockets the tubes are in. As soon as a 2nd tube in in, this seems to happen. Any ideas of what might be going on?

[R.G.]

I pulled another 12AX7 from a different amp and added to the one working 12AX7 and as soon as a second tube was added, the problem surfaced again (indicator light dimmed, filament fuse glowing). It doesn't matter which of the 3 sockets the tubes are in. As soon as a 2nd tube in in, this seems to happen. Any ideas of what might be going on?

Your heater wiring is hosed somehow. Trace it out. The indicator runs on the heater 6Vac. That thing dimming means the heater winding is being overloaded.

There's no magic trick to locating where. Trace it out.

And... how much are those fuses worth to you now? 8-)

[mfreqmaster]

ghost in the machine

By taking the fuses out, I can measure the continuity of each filament wire without the low resistance of the power transformer getting in the way. I have confirmed that the filament wires are not touching each other or any other pin on any socket. Each filament side is continuous from the point of the fuse holder to the last preamp tube. Neither of the filament wires are touching ground. Everything appears to be hooked up correctly. I even touched up all of the connections by reheating them with my soldering iron. I get approx 3.1V on each side of the heater secondary until I add a 12AX7. Each subsequent 12AX7 drops the heater voltage until the voltage gets real low, approx 1.4V per side, then the fuses start to glow, I quickly power off to avoid damage to the power transformer. I don't know what I could be missing.

[Enzo]

Don't measure heater voltage by taking readings to ground from each side. Measure ACROSS a tube socket. What matters is getting 6v across a tube's heater. It is tempting to see 3v on either side and infer that 6v must be there, but there are other ways to get 3v readings.

Pop your heater fuses out, you have already checked your heate4r wiring, but now check the transformer. First see if 6v is coming out, but then powr off and measure for continuity through the 6v winding. And I mean end to end, not ground to each end.

From your earlier posts, I get the impression a power tube shorted B+ to heater internally (or the index pin was broken off a tube and it was inserted in teh socket facing the wrong way), and caused heater trouble.

[Steve Conner]

What rating of filament fuses are you using? Are they slow or fast blow?

Tube heaters draw a big surge current when they're cold, so hefty slow-blow fuses might be needed.