PT Heater Center Tap Meltdown

[Morgan]

Hi guys. Had a weird situation last night at band rehearsal. We just picked up a keyboard player and he didn't have his amp, so I plugged him into my 5E7 clone. About two hours in, the amp started smoking - smoking enough that I pulled the plug from the wall socket and rushed it outside before it caught my house on fire.

I opened it up today to have a look at what went, and the heater circuit center tap wire is very melted/charred, the two heater wires coming from the PT are melted, and the indicator assembly where the heater wires from the PT shows signs that it got very hot as well.

The heater CT wire is the most damaged - the actual heater wires are less melted. It looks like the CT wire got extremely hot and where it contacts the two heater wires as they leave the PT end bell, it looks like the CT wire melted through its insulation, then melted through the heater wire insulation and possibly caused them to heat up through contact. The three wires are sort of fused together where they leave the PT end bell. As it stands now, the whole heater circuit is shorted to ground.

I've built around 30-40 amp and worked on a whole lot more. I've seen a share of PT meltdowns, but nothing I can recall where it seems like a CT wire started a chain reaction.

Anyone have any ideas of why this might have happened? I'm having a hard time coming up with a plausible scenario.

[trobbins]

Can you separate the PT heater connections from the rest of the heater wiring, and confirm there is no inadvertent short to 0V for the general heater wiring? Perhaps prod around the wiring and valve base terminals whilst checking for low resistance.

You could disconnect the heater CT, and tack in a fixed humdinger to 0V, and then turn on amp and check if the humdinger legs each have correct balanced voltage across them.

[Morgan]

That's a good idea - thanks! I'm figuring the PT is gone, but that may show something.

[pdf64]

Maybe there's a power tube short, sending B+ via the heater CT?
Has the B+ been fused?

[Leo_Gnardo]

Anyone have any ideas of why this might have happened? I'm having a hard time coming up with a plausible scenario.

Saw similar in a 60's Fender Bassman head where there was a short in the pilot fixture. Burnt insulation off CT as well as one of the 6.3V leads. I fixed that amp @ 25 years ago, and it showed up again just a couple weeks ago with its new owner. Amazing - the PT was still working. Double layer of heat shrink tubing over the afflicted 6.3V wire. The CT wire was so brittle it fell apart & I put in a pair of 100 ohm hum balance resistors.

[Morgan]

Maybe there's a power tube short, sending B+ via the heater CT?
Has the B+ been fused?

No - it's a straight clone. Only fuse is a 3 amp on the mains.

[Morgan]

Anyone have any ideas of why this might have happened? I'm having a hard time coming up with a plausible scenario.

Saw similar in a 60's Fender Bassman head where there was a short in the pilot fixture. Burnt insulation off CT as well as one of the 6.3V leads. I fixed that amp @ 25 years ago, and it showed up again just a couple weeks ago with its new owner. Amazing - the PT was still working. Double layer of heat shrink tubing over the afflicted 6.3V wire. The CT wire was so brittle it fell apart & I put in a pair of 100 ohm hum balance resistors.

Wow - that's an interesting one!

[ToneMerc]

Anyone have any ideas of why this might have happened? I'm having a hard time coming up with a plausible scenario.

Saw similar in a 60's Fender Bassman head where there was a short in the pilot fixture. Burnt insulation off CT as well as one of the 6.3V leads. I fixed that amp @ 25 years ago, and it showed up again just a couple weeks ago with its new owner. Amazing - the PT was still working. Double layer of heat shrink tubing over the afflicted 6.3V wire. The CT wire was so brittle it fell apart & I put in a pair of 100 ohm hum balance resistors.

Look at the rear of his lamp assembly and you see it was definitely was hot.

TM

[Structo]

I know how you feel.

Big time bummer.

A couple possibilities, you exceeded the current rating of the power transformer or a power tube shorted and fried the heater wires.

Some people like to install small wattage (1/2 watt or less) 100-120R resistors to create an artificial ground for the heater supply.
The idea being that the resistors will fry before the winding does.

My preference is to let a fuse take the current dump, not the transformer or the power tubes.

The mains fuse in the power supply, is only a stop gap measure to prevent a fire if a catastrophic failure occurs,
with little to no protection against an internal short in the amp.

Remember, current is a savage beast and always tries to find the shortest path to ground.

Some high end amps have a HT fuse on just about every secondary winding
on the power transformer.

[R.G.]

I think you're getting good advice - something created a soft short across the heater winding, and the increased primary current was not enough to pop the primary-side fuse. The lamp holder definitely looks involved, but it will take, as already noted, unwinding and dissecting out the heater wires to find out what might have done it.

I had an ... um, animated, I guess, discussion on another forum with a professed experienced amp builder who refused to accept that a fault on a heater would not pop the mains breaker. He argued that for a week or so, and finally just quit, but never believed.

The comments here are correct - the AC mains fuse is there to prevent fires and other disasters for AC mains wiring faults. If you want to protect the secondaries or anything on them, you need fuses out there that are sized to the loads the secondaries carry.

It gets tedious, as a CT winding needs two fuses, but it helps to remember that barring museum-stock tubes from the La-De-Dah Tube Exhumation Society, the PT is probably the most expensive single part in an amp.

[sluckey]

I've seen more than one of those Fender style lamp holders cause this exact problem. The way they are built, it's easy for one of the electrical contacts to touch the mounting frame which is connected to chassis.

[dorrisant]

Don't consider the PT to be toast just yet. You can remove the end bell and repair it all the way to the connection to the enameled wire there... Like Leo said, you could always forgo the CT wire and use a virtual CT. Easy enough to investigate and save some $$$>

Tony

[JMFahey]

With due respect, if current through external wires was enough to melt insulation, the actual enamelled wires must certainly be charred inside too.

Maybe not *yet* shorting, which might give you a false sense of security, but definitelly toasted, lost all flexibility, etc. so later heat/cold cycling (simply using the amp) can get through.

And, as the OP said, maybe the next time the house can catch fire

I woudn't gamble with that.

Sorry because it's a relativeluy expensive part, it's a mess ordering and replacing it, etc. , but better safe than sorry.

And yes, the pilot lamp holder is a big suspect.

using 2 x 100r resistors is fine and safer.

[Morgan]

Thanks for all the input guys.

I had big plans to go through the amp over the weekend, but naturally life got in the way - hopefully will get into it this week. I plan to dissect the PT, but I don't plan on reusing it.

RG - first thing I thought of when I rushed the amp outside was your Premier Guitar article on fusing the secondaries. Seemed like a good idea when I read it...and of course I never did anything about it. And I was just inside of a Rivera M60 the other week thinking "man - all these fuses are a good idea..."

Some people like to install small wattage (1/2 watt or less) 100-120R resistors to create an artificial ground for the heater supply.
The idea being that the resistors will fry before the winding does.

I hadn't though of using a virtual center tap as a safeguard before; just sort of used it on smaller amps when the PT didn't have a CT for the 6v circuit. Makes a heck of a lot of sense now!