Peavey 5150 Output Transformer

trying to fix oscillation problem on a 5150

tried everything, tweaked preamp, no luck,

last thing to try is rewinding the OPT

don't have to twist my arm,   

nice chunk o iron, 1  3/8" tongue, 2 inch stack

wow, layered paper, no bobbin, looks hi fi,

pri inductance is about 50 Henries at 60 Hz, 100V-rms input

freq response is 10Hz to 100  K hz,  lots of interleaving of coil layers,

no wonder this amp oscillates, mega-gain preamp into a hi-fi OPT,

so we will dumb it down like Fender and wind pri-sec to get some roll off and hopefully tame the beast,

here is low level ind. vs freq:

let's get a template going...

CJ said:

pri inductance is about 50 Henries at 60 Hz, 100V-rms input

freq response is 10Hz to 100  K hz,  lots of interleaving of coil layers,

no wonder this amp oscillates, mega-gain preamp into a hi-fi OPT,

so we will dumb it down like Fender and wind pri-sec to get some roll off and hopefully tame the beast,

here is low level ind. vs freq:

Click to expand...

Won't that change the design and alter it from the original "sound"?

Maybe call it a 5149.95 after that.

JR 

well the Hi E on a guitar is 329.63 Hz, that makes the 12th fret about 659 Hz,  so we really do not need 100 k Hz,

thanks to Adolph Sax, who lowered A to 440 from 445,

James Brown is not gonna like this, so we will keep it on the QT, 

> Won't that change ....the original "sound"?

A 5150 is a Chevy 454 with hi-rise carb and headers. Jericho trumpets in a box. Total sensory overload.

I would not worry about fine points of sound.

I do think any squeal could be damped with small caps at key grids. But don't object to another CJ teardown.

this transformer sits directly over the preamp tubes so we miught try a lacation change,

those grid caps do work well, you need a drawer full in the 5pf to 100 pf range to find the values that will squelch the circuit without stealing tone,

CJ said:

well the Hi E on a guitar is 329.63 Hz, that makes the 12th fret about 659 Hz,  so we really do not need 100 k Hz,

thanks to Adolph Sax, who lowered A to 440 from 445,

James Brown is not gonna like this, so we will keep it on the QT, 

Click to expand...

I know James and I'm pretty sure he doesn't care one way or the other... (now if you were messing with his pedals that's another matter). 

Some day a 5150 might be collectable in original trim.

As PRR hinted that design was an exercise in excess.

JR

we had a wire break in our hand while unwinding the inner primary, (it is always the inner primary, heat and core expansion)

so there was a partial connection to one half of the power tubes,

very well engineered transformer,  sec-pri-sec-pri-sec-pri-sec-pri-sec

or 5-4 in xfmr lingo,

bi fi sec winds,  this way you can use smaller wire which makes coil easier to wind and does not stress the finer pri layers as much,

coil not impregnated with varnish, first time i have seen a dry coil, maybe factory defect, maybe called out for by engineers in order to keep capacitance down,

what nation is the transformer capitol of the world?

Delamination.

these guys just fell right out, thanks to my friends in the processing dept.

there is one half of your wiring structure right there,

there is one half of your winding structure right there,

notice the fuzz from the dull band saw blade,

tape job gets an A+

were gonna snip some leads so we can get down to it,

8 leads for the price of 1,    middle speaker wind uses thicker wire, probably for the 4 ohm section,

16 ohms at 4 amps = 64 watts   
4 ohms at 16 amps = 64 watts,

so the 4  ohm tap seeds 4 times the current to make the same watts,

1/10 ohm of DCR  at 16 amps = 1.6 volts
64 watts = 16 amps at 4 volts,

so if the wire is small, we lose voltage to the speaker, thus the bigger wire for 4 ohms,

margins for speak wind should be equal,

i bet this poor guy had a mean supervisor who wanted coils quick,
i bet the lighting could have been better also,

notice how dry the paper is,

outer primary layer was squeezed by the tension of the big sec wire, you can fix this with tape anchors when you wind, or wrap a strip of yellow poly around the ends of the coil,

this is not a problem on a bobbin wound coil, but with layered paper, you have to watch out,
old school winders knew how to avoid this, but modern layered paper coils seem to have this problem,

why is this bad?  there might be 600 volts of AC going through that wire, which no sits a few mil from the core, which represents ground to the 600 volts,
turn out the lights and you might see some corona,

inner secondary taped good,