SF Twin 135 speaker output wiring

[g1]

I have a 135 twin here that wasn't make sound out of the main speaker jack, turned out the shorting switch on the jack was touching the tip even when plugging into it.
Bent it back and it worked.

This is a good point. I had thought about the possibility of the main spkr jack being shorted. The switch could get "welded" shut, or bent like yours did, or in very rare cases, standard 1/4 inch jacks can short from tip to ground.
With this series jack arrangement, if the main spkr jack shorted, you would get no output from main spkr jack but ext.spkr. jack would work properly.
The resistance checks that were done in post #9 imply to me that the windings are ok, but still a good idea to disconnect the windings and check them.

[Zouto]

Great, I'll replace the main spkr. jack and check the OT secondaries next week, hoping to finally fix that §㖹%&?ӞӜ issue.

[Zouto]

Well, I replaced the main spkr. jack, same issue as before. Checking the taps, I found the culprit, it's the green tap (0.6 Ohms to gnd) - no signal, the green/yellow tap (0.5 Ohms to gnd) sounds fine. I soldered the taps to the main spkr. jack successively to check it soundwise. So what to do now? I tend to solder the working green/yellow tap to the main spkr. jack and completely disconnect the green tap/ext.spkr. jack wiring, but I'm not quite sure, whether it can cause damage to the OT due to impedance mismatch: two speakers @ 8 Ohms in parallel (= 4 Ohms) at OT's 8 Ohms tap.

[g1]

Sounds like the grn/yl is shorted to the black in the transformer.
I think it can be safely used this way, older Fender stuff is quite forgiving as far as impedance mismatches go. The older stuff had the ext.spkr. jacks wired in parallel with no impedance switching, so using an ext. cab ALWAYS created a mismatch.
However, I would recommend you move the feedback connection to the green wire.

[Zouto]

I removed the NFB completely for a much better sound, so I'll finally put the amp together again to let him do his duty.

[pdf64]

I thought shorted turns would knacker a transformer?
My guess is that there's something else going on.
Pete

[Zouto]

Sounds like the grn/yl is shorted to the black in the transformer.

I don't think so, because if I use the grn/yel. on + and the black on gnd it works fine. Using the green tap on + and the black on gnd, there's no signal at all, so the green tap must be shorted somewhere. What puzzles me are the impedance readings:
green tap (0.6 Ohms to gnd) and green/yellow tap (0.5 Ohms to gnd). Swapping the OT is a bit complicated, since I want to keep the amp ultralinear, and replacement UL OTs are hard to find and/or too expensive (Merc.Magn.e.g.).

[Leo_Gnardo]

I don't think so, because if I use the grn/yel. on + and the black on gnd it works fine. Using the green tap on + and the black on gnd, there's no signal at all, so the green tap must be shorted somewhere. What puzzles me are the impedance readings: green tap (0.6 Ohms to gnd) and green/yellow tap (0.5 Ohms to gnd).

You're measuring the DC resistance of those output windings, always a fraction of the impedance they intend to drive. It is bizarre that you measure a continuity on the green to black but it delivers no signal. Agreed - best to avoid replacing this OT as it would be deadly expensive.

Running the amp with an impedance mismatch, if you play at high volumes, will shorten your output tube life somewhat.

Have you tried running the black and green wires to the speaker output WITHOUT that switching jack? I've been suspecting that jack is the problem ever since post #2, a couple days ago.

[Zouto]

Yes, as I wrote in post#18, I ran both wires separately to the NEW main speaker jack with the switching jack wiring completely disconnected, first the green, then the yel/grn, black always to gnd., according to the schematic.

[Zouto]

With nothing in the Ext jack, plug a guitar cord into the Main jack.
Now read the resistance across the tip & the ring.
If you get an open circuit reading, then either the tranformer wire is not making a good solder connection at the jack, or the winding is open.

Meanwhile I got the output impedance readings of a friend's working Twin 135:
1.3 Ohms main speaker jack, 1.5 Ohms ext.spkr. jack.

[g1]

Can you measure from green to green/yellow with both wires disconnected?
It is odd that the green is not working now as it is what was running the ext. jack originally, when you had output through it (unless the jacks were wired up wrong).
The green yellow which you say is now working, is what was connected to the main spkr. jack when it did not work.

[Zouto]

Can you measure from green to green/yellow with both wires disconnected?

I get 0.4 Ohms from green to green/yellow with both wires disconnected.

[pdf64]

Try tacking a load directly to the suspect green / 8ohm output to see if you can get a signal out of it, now that the jack sockets/switching are out of the picture.
Pete

[Zouto]

I just connected each of the wires directly to simple single jacks:
green jack- no signal at all (not the slightest hiss, hum etc.) ,
green/yellow jack - full signal as before.
I really don't know, what's wrong with that green tap, it's definitely no jack issue, seems to be some internal short or interruption.