Fender Hot Rod DeVille Output Transformer Teardown

[CJ]

that's right, back in action,

a pair of 6L6 tubes in this amp, and a 12" speaker, gonna need something in between me thinks,

but this OPT thinks it is an incense burner, every time you crank the Variac past 60 volts AC, the the 3 amp current meter needle starts dancing around like a skeleton on top of a Jimmy Swaggart confessional booth. emitting a fine aroma of baked enamel and burnt cardboard.

replace it with the same thing oh hell no. this is a small town. everybody knows everyone else. if an amp repair blows up onstage, my rep takes a hit. and it might even be that i might be at the same show now how is that going to feel?
run for the exit, that's how.so we rewind it to bulletproof specs, but first we need the turns and structure. so we do a tear down.

this guy right here, with the EBL AA batts charging in the background which is a story in itself. How do you get 1.5 volts from a 4.2 Li-Ion cell? internal circuit, that's how. Trip me out, looks like a normal cell, i hope they don't explode like those vape pipe batts.
ok back to this, EIA code says 606, Schumacher?>

 

[CJ]

lead breakout, do not like it, HV leads on same side as grounded big wire secondary.
asking for trouble if somebody is having a bad day with the soldering iron.

 

[CJ]

chucky's in love, get it, chuck key?
use the counter on the DIY winder, looks like we lucked out and de-spooling makes the counter go 1,2,3 instead of 99,999,99,998,99,997

 

[CJ]

if this is like 99 percent of all other failures, the trouble will be at the first layer where all the heat and pressure is,
outer layer is a primary section, looks good from here,

of course the lams have been removed, 13/8" stk of 1 x 1 Ei 100,the cheap stuff, non grain 0.0185"

 

[CJ]

lead cover, blue stuff, 15 PB which means 0.015" press board,
some people call it fish paper, either way, it is treated with a chemical to improve the voltage rating.

 

[CJ]

when you crank out a lead above the12th fret , you might generate about 600 volts of flyback AC between that red wire and the green wire at 700Hz, square wave, which jumps gaps a lot easier than 60Hz with all those harmonics being generated.
better to put the secondary leads on the other side of the coil.

 

[CJ]

look how close the primary center tap (red) is to the business end of one of the primaries,
which is another good reason to put the secondary leads on the back. you can put more distance between the primary leads also.

 

[CJ]

these coils are wound 10 at a time, all on the same stick. so the insulation is applied in one big sheet that covers all the coils at the same time. you take the finished coils off the stick, which is usually two long wedges stuck together such that you just pull the wedges apart and the 10 coils are free to cut apart on the band saw. but if you do not know where to cut, you end up with bad, uneven margins on the ends of the coil which will deduct volts from the hi-pot test after final assembly.

 

[CJ]

measuring the outside dims of the coil as we tear it apart.

this way we can write up a "Build" chart which has the dimensions of the coil , layer by layer, so we can track our winding as we go along and increase wire tension to bring the coil back into tolerance. sometimes wire changes diameter as it comes off the spool, one minute it is running at 0.0080, the next minute 0.0075 or 0.0085" . These small increments can add up and make it impossible to get the coil on the core without ripping into the outside cover.

 

[CJ]

Good Lordy look at this, do not have an explanation for how that happened, all i know is that those turns are at high voltage and are going to be right next to the laminations after assembly. The lams are at ground potential when the transformer is bolted to the chassis.
Sometimes there is a layer of tape or insulation, a core pad we used to call it, between the coil and core. I do not know if there was anything between this coil and the core. Enameled copper wire has at least a few KV of voltage insulation, but that does not mean you want to lay the winding on the grounded core. Creepage is the word used by transformer engineers. They do not want the volts to creep across between points of different potential.

 

[CJ]

pri-sec insulation- 2 wraps of 5 KP (0.005" Kraft Paper) and one layer of 5 mil poly. The poly has better voltage insulation properties than the kraft paper. The kraft is used for additional voltage protection, as well as setting up the leakage inductance for high frequency response, although i do not think this coil is that hi-tech, like a Pearless or a Jensen transformer. The kraft paper also provides a smooth bed for winding the smaller primary wire.

 

[CJ]

some 5 PB for keeping those secondary leads where the belong during hookup,

 

[CJ]

One layer of the inner primary, notice how the margins on the ends of the wind are more even, this leads me to believe that the wire traverse became mis-alligned somehow during winding such that the outer primary wire was put down at a different place. The traverse is the part of the winding machine that holds the wire as it traverses back and forth across the coil.

Sometimes a layer is put down and the traverse is brought back to where it started and a new layer is putdown, as opposed to back and forth winding. this is called "fold back" winding and is used to improve capacitance between layers, as the starts of each layer will be at half the potential as if the coil were wound in the standard back and forth direction.
since C=V/q, the less voltage per unit charge, the less capacitance, thus the foldback technique.

An example of an audio transformer that uses this foldback winding technique is the infamous UTC A-10 used in the LA2a compressor and other audio equipment.

 

[CJ]

This coil is wound primary-secondary primary. a bit simpler than he standard Schumacher like you might seen in the Twin Reverb. this structure is plenty adequate for a guitar amp as the guitar has limited bandwidth compared to what is needed for audio reproduction.

We are reaching the last layers of the coil, the inner primary, which is where we expected trouble.
sho nuff, tell tale signs of a Bar B Q are starting to show upon the insulation>

 

[5v333]

C = q / U

 

[CJ]

Looks like trouble on aisle 5,

 

[CJ]

Would you like that Medium Well or Well Done?

 

[CJ]

yes, C=q/U thanks!

Those turns burned clean through to the core, which is why we had excess current draw with no power tubes installed. The primary DCR from center tap to the Brn lead acted as a current limiting resistor instead of a dead short. A nylon bobbin will be used to wind a replacement. And Nomex 410 insulation.
Turns go 2140 to 89 (8 Ohm full secondary) for a Z of4.6K to 8.

 

[CJ]

making a 1 x 1 3/8" bobbin out of two 1" bobbins,

what, did you think i was gonna short stack the guy like Classic Tone?

 

[hitchhiker]

Love this thread CJ! and BTW Happy New Year!!!