Ampeg Reverbrocket 2 OPT Teardown

got a lot of OPT specs for 6L6, EL34, 6BQ5, 6V6, 6550, but the missing link was the 7591.

no more, as we are fixing a Reverbrocket  2 that has a blown output,

open frame design, every transformer we tear down has something not seen in any other transformer.

for this one it was the duel leads for the secondary, two black leads spliced to the same wire and two greens to the other end of the speaker wind, no big deal, but unusual.

this xfmr has an EIA code of 682, hmmm, does not ring a bell, so we looks it up>

http://www.triodeel.com/eiacode.htm

Electrical Windings

Ampeg p/n also stamped>

lams heated with heat gun, small transformers heat up pretty quick,

some lams have a tiger striped look to them if you look close, this is seen from time to time in various dissections,

leads stapled down to press board, you can see the parallel splice job on blk and grn,

easy pickens with the pri-sec structure, just rip off turns and keep track of the counter,

3 layers of 22ish lookin sec wire = 82 turns. we will mic this at work tomorrow,

a layer of maybe 10 mil poly insl between pri-sec and a chunk of anchor tape thrown in for reasons unknown

tearing down the primary, this layered paper job was wound on a multiple winding machine, only way to make a profit as you are competing with a bunch of other companies winding the same thing,

cut apart on a band saw, sometimes they miss the mark and you end up with wire near the surface of the coil which means HV creepage or even a short to the core.

failure location revealed, looks like a shorted turn,  there were many breaks in the wire as we took turns off, could be overheating, bad wire, or a varnish tank that just had solvent added to thin it out, if you leave the coil in the tank for too long, solvents can compromise wire insulation, today, magnet wire uses insulation that is more resistant to solvents,

another view of how close the wire was to the core,

red tape over the glass-cloth anchor to smooth the next layer out as it goes over the top,

looks like 1100T + 1100T = 2200 to 82 turns, so 26.829:1 turns ratio,

let's figure out what kind of a load they are putting on these 7591's with there RCA Tube Manual listed spec of 6600 P-P:

26.829^2 = 719.8:1 Z ratio,  call it 720 and we can get load, 720 x 8 Ohms speaker = 5760 Ohms P-P

ani't gonna argue with engineers at Ampeg, but we could experiment with a 6600 ohm coil,

what would the turns be?  6600/8= 825 Z ratio, turns will be the root which is 28.72 x 82 = 2355

2355/2= 1177 turns, so if we add 77 turns to each pri section, we are at 6600 Ohms.

maybe for a home brew amp but since this is a shop amp, we gotta stay true to the orig.

we need a rough estimate of the AC voltage developed across the pri of the xfmr to determine how hard Ampeg is hitting this core. The owner of the company did not like rock and roll or distortion because it did not swing like jazz, but this is the amp that slipped between the cracks as it had some growl. (apparently he did not listen to Jimmy Reed!)

lets say  20 watts from a pair of 7591's ( we are running 140 ohm cathode resistor)
7591's can handle 19 watts a piece, right in between a 6V6 and an EL34,

so we figure speaker voltage and multiply by the turns  to get voltage on the pri side:

V=root (RP) so V = root (8 Ohms x  20 Watts) V = root  160 = 12.6 pri side, x 26.83=  339 volts RMS

holy cow that works out to about 38 kilo Gauss, our limit is 18 KG with silicon, no wonder this thing barks like a dog, with rabies,  yeah, no, this thing needs about 4000 turns, not 2200, wtf? jus sayin...

with this low turns count, this transformer, if it were cold rolled steel, could safely process about 6 watts without saturation, and that is hitting it pretty hard, especially since it is laced 1 x1 , sheesh, i gotta hear this thing,

ok we are done with the rewind, so got some  numbers for dcr etc.

and by the looks of the Henries (old core steel was used), we have grain oriented steel.

and we are going to put the flux at 15 K Gauss but raising the freq to 40 Hz, which is more reasonable for a guitar transformer.

by raising the frequency, we can now say that the transformer will handle 300 V-rms at 40 Hz and deliver about 15 watts,  bass roll due to saturation of will protect the speaker .

frequency response is very good, flat to 100 K Hz with a peak at 150 K Hz and then dropping rapidly never to return again, (Bessell Curve)

secondary wire was changed to 4 strands of 26 AWG which replaced the single strand of #21, this allows the wire to sit flat and eases damage to the #33 primary wire below due to less tension required.