New CG build. Smoking PT

[DocJames]

So I've just finished a CG build, using 18 watt Heyboer iron from turretboards. Everything seemed to be going fine, bulb test looked good, great sound coming out of it. Today I was playing it for an extended amount of time, and I suddenly lose all sound and its smoking from the PT area. I immediately shut it off, found the fuse popped. There's an oil-like substance dripping off the PT right now. Is it pretty safe to say that the PT has gone kaput!?

The real question is why...... I don't want to turn it on right now, but I'm going to look at it later. The voltages looked good on everything, and the sound was not dampened in any way.

I'm sure I can't be the only one that this happened to before.

[Blackburn]

No, you aren't the first this has happened to...

Sounds like something pulled way too much current and killed your PT, which I'd wager is gone. This is when you are thankful for B+ fuses. I'm guessing you just have one for the mains? Need to check those power section tubes.

[JazzGuitarGimp]

Another possibility is an overcurrent condition on the heater winding.

[mark enger]

I've had a PT get hot and still be ok, but if she spewed goo, thats bad, I wonder if it's more cost effective to have it rewound? sorry! let it cool and take your meter too it, it was'nt MM was it? mark

[DocJames]

No, you aren't the first this has happened to...

Sounds like something pulled way too much current and killed your PT, which I'd wager is gone. This is when you are thankful for B+ fuses. I'm guessing you just have one for the mains? Need to check those power section tubes.

Yeah I just had one fuse on the mains .

What would cause the overcurrent condition on the heater winding?

[M Fowler]

To rule out a defective PT you need to go over your wiring very carefully.

I would request your supplier to accept a return on the PT and let Heyboer take a look at it. Never know they might standby the product they did for me on a TW PT that was incorrectly built during a production rush period. At one time about 2 years ago Heyboer brought on a bunch of new builders and quality suffered.

[Phil_S]

While not 100% guaranteed, on any build I do of serious wattage (over ~10W), I like to put a 500mA fast blow between the CT and ground. It's cheap insurance.

(On the low wattage builds, I usually ignore this because I often use recycled iron that is over-rated for the requirement.)

I'm wondering what others do to fuse the secondary.

[TUBEDUDE]

i think most people use the fusible winding technique.

[Smokebreak]

Can someone explain the scenarios that a B+fuse protects against, versus scenarios mains fuse protects against? It seems ,based on historical location of each(b+ after rect, mains at wall), that mains only protect agains rectifier failure, but I know this isn't the case. Also, if fusing for rect failure, it seems you'd want to fuse between PT sec and rectifier as well. You could go crazy with fusing I imagine .

[Blackburn]

Can someone explain the scenarios that a B+fuse protects against, versus scenarios mains fuse protects against? It seems ,based on historical location of each(b+ after rect, mains at wall), that mains only protect agains rectifier failure, but I know this isn't the case. Also, if fusing for rect failure, it seems you'd want to fuse between PT sec and rectifier as well. You could go crazy with fusing I imagine .

A B+ fuse will save your PT in the event of something happening to your power tubes. For full insurance you'd need a fuse on the heaters too, but I just stick with one on the B+. The mains fuse takes much more than what will cause your B+ fuse to blow. Something like a blown PT. I just had an instance where my B+ fuse saved my amp because of my stupid ass mistake. I was very happy having that in there that day!

[Blackburn]

Is this crazy enough for you, Smoke? Take a look at the pic for Immortal Amp Mods #3.

http://www.premierguitar.com/articles/t ... ods-pt-3-1

[Phil_S]

The mains fuse is there first to protect your house, not the amp. Amp protection is secondary.

Nice article on the immortal amp. IMHO, too much work to do all that, but not a bad concept. You'd want to use a 4P or 6P switch (2 for each winding) for the secondary, LOL, pretty chunky I expect.

[Smokebreak]

Yea! Nice article. So basically, there is never a reason not to have a B+ fuse in any amp? Smaller transformers cost $$ too

[gldtp99]

This: http://www.classictone.net/40-18035.html

Use a fuse on the B+----- find the reason the first PT failed, probably an output tube failure---- but could be a wiring fault----- if you just install the new PT in a defective amp all you will accomplish is to burn up the new PT.

[DocJames]

This: http://www.classictone.net/40-18035.html

Use a fuse on the B+----- find the reason the first PT failed, probably an output tube failure---- but could be a wiring fault----- if you just install the new PT in a defective amp all you will accomplish is to burn up the new PT.

Yeah and that's my concern. I have another 18 PT hanging around, but I need to figure out why this darn thing toasted like it did. All the voltages seamed with normal limits, although I don't have an expected sheet of voltage values for a CG. I've combed over this thing twice now, and I don't notice anything out of place. I'll upload some potato shots of it later, for some more eye's on the project, and I'm going to get a B+ fuse in there I think as well. Space has been a limiting factor for this build, as it's a shallow box.

I was making this for my little nephew, hah. I'm glad this happened now, and not after I gave it to him. I can imagine the look on his face, as smoke pours out of the amp!