Fender Princeton Reverb OPT

[CJ]

we hacked an aftermarket a while back, let us see if it matches up to a real vintage Fender OPT,

the victim has been tried and convicted and is awaiting punishment for the sins therein...

 

[CJ]

the 2 outside lams are called "Keeper" lams, they extend further to hold the I bar to the core during assy,  the tongue is curved so it does not tear the cardboard former as the last lams are a tight fit.

 

[CJ]

this coil is still good, we took some Henries for the old core, we stuff it with modern lams, then re measure Henries to see what is shakin,

old lams H value mentioned first, they have more juice at the low end, but match up with the new lams a bit up the freq spectrum, in fact, they dip a bit lower,

20 Hz  old: 446 H new: 403
40 Hz    306  H 294 H
60 Hz  242 H  240 H
80 Hz  213 H  217 H

 

[CJ]

shim it up on the mandrel and tear it down and i mean hard.

 

[CJ]

let's photo-doc the pri leads, what say?

 

[CJ]

10 KP wrapper off,  (.010" kraft paper)

 

[CJ]

secondary is easy,  two chunks, parallel,  pretty lite,  both #25

sec DCR = 0.44 ohms, good enough, this is a small coil so wire length is short so DCR does not build up as fast as with say, a 125 EI Ampeg V4 OPT monster,

MLT (Mean Length Turn) for 75 EI is 4.17"

 

[CJ]

they used red and green wire, probably to avoid confusion during hookup,

this splice is between the red and the blue lead, (plate to CT)

the red to brn wind is in the middle of the coil and is all in one chunk of 1280 turns of #36,

 

[CJ]

outer layer is 111 as opposed to 128 average ,  this winding length is 9/16", full layer winding length is 13/16".

9/16" / 111 = 0.0506" per turn, wire measures 0.005" , so this layer has very little space between turns,

13/16" / 133 = 0.0059"  so a little more space between turns on the the full layers,

 

[CJ]

half the layer wound over piece of tape, finish layer and put another piece of tap over the whole wind,

 

[CJ]

do not know the purpose of the number 24,

 

[CJ]

what's up with your funky blue glue?

pri-sec  insl. is 1  L0 .002"  poly + 1 L  0 .003" KP

 

[CJ]

one of two secondary winds, first layer has a few more turns (41 vs 39) so that the layer after that does not fall off the edges,

 

[CJ]

ok so the blue glue is used to hold the poly insl tape down,  they probably did not have yellow mylar tape back then, regular tape would probably slip off the poly insl and tape also adds lumps to the coil,

 

[CJ]

ok that glue is to fill the gap caused by the anchor so as to keep the coil surface flat as it builds up,

 

[CJ]

middle primary anchor

 

[CJ]

inner secondary,  note offset margins, 

 

[CJ]

it is hard not to deform inner layers when putting on the thick wire which requires more tension to keep coil build in tolerance so that the lams clear the coil,

 

[CJ]

and finally down to the first wind,  641 turns of green #36,  5 layers, 

so we have 2562 T : 80 T = 32:1 = 8.2 K to 8 Ohm voice coil

 

[CJ]

why does the HP former have a layer of poly? we are starting with the hi voltage primary instead of the more common low voltage secondary,  so we need to keep this wind from shorting out to the core which represents ground when bolted to the chassis,

but we do start with the center of the wind instead of the blu plate lead, which cuts down on the hi freq AC which is highest near the plate side of the wind,

do transformers violate Kirchoff's Laws?  is current always the same in a series circuit? think about it. the CT has a big filter cap attached to it. so mostly DC at that point of the wind, but the plate side of the wind has big time AC whipping around, causing some big time AC current, but it averages out so Kirchoff's Law still holds,