Mig 100 - Transformer Potted With Wax?

[earache]

I am looking at a Mig 100 that the owner says "blows the standby fuse". I took out all the power tubes., made sure it had the correct value fuses, and powered up thru the light bulb limiter. It dims the bulb and doesn't blow the fuses. The power tubes seem ok after testing them to see if they light up with a Conar tube tester (they light, no shorts). Looking over the tube sockets it's possible there was arcing because some of the pin holes look black. There's no scorching on the bottom of any tube, however. There is a yellow waxy substance on the top of the chassis, thick enough to scrape off especially near the power tranny.

Are Mig 100 transformers potted in wax? Should I risk cleaning the tube sockets (which are ceramic), put the tubes back in and trying to power this amp up again? Maybe the tranny got warm enough to melt the wax which dirtied the socket?

The amp has a service sticker in it, dated April 2006. The caps were replaced with IC branded caps.

Thanks in advance!

[mozz]

Could be the company lets the power trans run warm and the wax dripped out. Or, it could have been drawing excess current and caused damage. Either way, i would change the sockets. Do you have a variac?

[earache]

I do have a variac.

Are these transformers known to be potted in wax?

Can ceramic tube sockets be cleaned? I know phenolic ones, once scorched, need replacing.

[The Dude]

With the output tubes removed, I would check the bias voltage also and make sure it is holding and staying there consistently. A loss of bias would cause red plating and/or a blown HT fuse.

[Enzo]

Try different power tubes anyway. Most tube testers put relatively low voltage on tubes and don't often challenge them with current.

Tube testers will verify bad tubes are bad fairly often, but they can never tell you a tube is "good".

Any evidence of your wax on the top vent? Up under it inside?

[Mick Bailey]

I always suspect IC caps. Do they have any crusty deposit around the +ve lead? An arced ceramic socket is just as bad as a phenolic one. If there's just a soft, sooty deposit that easily cleans off you may be lucky. Sometimes arcing produces a conductive metallic deposit (it can melt the glaze) or fractures that cause future problems.

Sometimes a shorting tube can be intermittent and only fail once in a while, leading you to think they're OK.

[Chuck H]

+1 to all!!!

Suspect tube sockets should be monitored, bias voltage should be monitored, tube testers don't always mean a green light for guitar amps and IC caps are too often suspect. I could have posted any one of these things if everyone else hadn't already.

[earache]

Got back to this today. Monitoring pin 5 of all power tube sockets for about 20 minutes now. Voltage reading is at -53 VDC and holding steady.

Enzo - the wax has dripped down the wire bundle underneath the power transformer a little bit. I'm not sure what vents you are referring to. There are no topside vents on this amp head like an AC30 has.

I have a set of 6L6's I can try instead of these 5881s.

The IC caps show no outward problems - no bulging, no leaking goo, no crusty stuff.

I'm measuring the transformers outside temp with an infrared thermometer and it isn't any warmer than ambient temperature.

I'm going to put the tubes back in, fire it up and see what happens

[earache]

Well that was interesting! When powering up with the original tubes, I flicked off of Standby and got a blue flash in V7, a quick hum and a blown main fuse.

[Chuck H]

Testing transformer temp with no tubes in is sort of moot unless you suspect the transformer is shorted across some windings. No current through the amp isn't going to heat the transformer much.

It's possible the tube was just ready to do that and was drawing too much current before and causing excess heat in the transformer.?. Probably just time for new tubes. Do check the screen grid resistor for V7 since it's pretty common for that resistor to be taken out with a tube failure. Check voltages again before installing the new tubes and monitor bias voltage again after the tubes are installed. If anything looks hinky, shut the amp down and post your observations before anything else goes kaplooey.

[Enzo]

The vent I refer to is the hole in the top of the cab with a screen cover. Like something could have dripped in the top. My MIG30 has a top cab vent, I thought my MIG 60 did as well. I don't know what the 100 is laid out like.

A flash inside a power tube is almost always a sign of death.

From what you have told us so far, my surmise is one of your power tubes failed and took out the fuse. I have not heard anything yet to make me think it is more than that.

[earache]

I get you now Enzo. This particular Mig 100 doesn't have any vents along the top of the case.

I'm not sure anyone answered my initial question - are Mig transformers potted in wax, and could it have overheated and the wax have melted out of it? I'm familiar with transformers being potted with a kind of tarry stuff, and that other coils like guitar pickups can be potted in wax.


I was curious (and perhaps foolhardy) so I changed out the fuse, swapped the tubes at V7 and V6 and the same thing happened again but the flash occurred in the same tube.

[earache]

I have a new matched quad of JJ 6L6s so I tossed them on in and have been playing them for about 20 minutes or so. No redplating or hum or odd noises (other than my playing!)

The 6L6GC is pretty much a straight replacement for the 5881, isn't it?

[J M Fahey]

I'm not sure anyone answered my initial question - are Mig transformers potted in wax, and could it have overheated and the wax have melted out of it? I'm familiar with transformers being potted with a kind of tarry stuff, and that other coils like guitar pickups can be potted in wax.

Itīs very unlikely.
Wax and relatives melt between 60C and 70C and quickly flows like water, very unwise choice for a tube amp power transformer and even for an OT.
Tar softens at higher temperature, not actually *melting* .

Wax is fine for ambient temperature stuff, such as guitar pickups, transformer microphones and such.

Transformers are submerged in transformr varnish, natural or vacuum impregnation, let to drip excess and usually oven cured, do not re-melt later even when hot.

[earache]

Thanks for that quick reply, JMF!
Still gotta wonder where that wax came from...

I'm gonna check the bias now and see if it's in the ballpark. This amp doesn't have a bias adjustment pot ��