Silvertone 1484 OT #4

[LarBal]

Wow it's transformer week around here.

1484 comes in with complaint of weak sounding and weird crackle.
Owner tells me it's been worked on by 1 tech, who replaced the original blown OT. As I understand that's not uncommon.
Owner says the 1st replacement OT blew, so tech stuck a new one in.
Owner plays for 3 hours on amp, 2nd OT blows, so tech puts in 3rd OT, which is what's in it now. It's a Weber unit, WOT1484.
I open it up and confirm that the tech also did a partial cap job, with 220uf/350V caps to replace the original odd arrangement.
I confirm that when amp starts to get loud, there is a crackle through a variety of shop speakers I put on it. Had the sound of an "about to go open plate resistor" type of thing. Both channels.
I check all pertinent resistors and they all read within "acceptable" drifted spec
I do the rest of the cap job for PI and preamp, amp sounds better, louder, punchier, but crackle remains.
I start measuring voltages and everything looks pretty good, except the stage before the PI, the 100K reads fine, but voltage on that plate is 55VDC. I replace it. No change.
Bias voltage good, 6L6s pulling 42ma each with current arrangement
I start to think that maybe my shop speakers are just worn out, so I drag in the original 2x12 w/ the Jensens. Fire that up, and in 30 sec the output goes veeeery quiet. It's still there but obviously wrong. All voltages, etc still good
I fiddle with it for a few hours before patching in a Marshall 50W 4Kish OT and everything works again.

So, I'm not about to just put in OT #4 and hope for the best. I'm trying to come up with a list of possibilities as to why this amp keeps blowing OTs.

Obviously the speaker cab is suspect.
Maybe the Weber OT is straight clone of the originals and is subject to the same faults, and the amp just needs a more robust OT
Maybe the tech made an error rewiring the funky power supply(note that the screens are about 100V below the plates, as I believe they should be). I'm already dreading tracing that out in this rats nest
Bad choke?
Sears_Silvertone_1484_Twin_Twelve_REPRODUCTION_Manual.pdf

[Helmholtz]

Schematic?

Generally when speakers or their connections go open, the OT is at risk (internal arcing caused by kV voltages). Even more so when the fault is intermittent.
Missing load is the main reason for OT defects.

[LarBal]

My bad, I put it in the original post now.

Yes, first thing I'll do is rewire the speaker cab. I've already replaced the jack on the amp w switchcraft . There was cheapy cliff type there for that

[Helmholtz]

except the stage before the PI, the 100K reads fine, but voltage on that plate is 55VDC. I replace it. No change.

The stage before the PI is the upper half of V3, or do you mean V2? The PI is the lower half of V3, a cathodyne.

[LarBal]

Yes, the upper half of V3, the 6CG7, is the one whose 100K Rp I replaced, still only around 55V on that plate. Also, I swapped around the two 6CG7s, thinking maybe a weak tube was causing that, but honestly that all may be a non-issue. But all the other voltages were pretty close to scheme in amp. Cathodes a little low IIRC

[Helmholtz]

Yes, the upper half of V3, the 6CG7, is the one whose 100K Rp I replaced, still only around 55V on that plate. Also, I swapped around the two 6CG7s, thinking maybe a weak tube was causing that, but honestly that all may be a non-issue. But all the other voltages were pretty close to scheme in amp. Cathodes a little low IIRC

Too low plate voltage means too high plate current. Might be caused by a leaky C12. What is the supply voltage at C25A and what's the cathode voltage (pin 8)? Also measure the cathode resistor.

[LarBal]

Well, right now the amp is off my bench, but will get it back up shortly. Does it matter, as far as reporting these voltages, If I use the bad OT or a hacked in good one?

[Helmholtz]

Does it matter, as far as reporting these voltages, If I use the bad OT or a hacked in good one?

Well plate voltage is low by 50% and that might indicate a problem but by itself won't damage the OT.
E.g. it might be a sign of oscillation, though without global NFB it's not very likely.

[LarBal]

Well plate voltage is low by 50% and that might indicate a problem but by itself won't damage the OT.
E.g. it might be a sign of oscillation, though without global NFB it's not very likely.

Ok thanks, I'll report back tomorrow, but to be clear, the plate voltages on the 6l6 plates are fine, 470-480 IIRC. It's just that one triode in V3. This is certainly a red flag area for the crackling, if it wasn't the OT

[Enzo]

I see lots of measurements, but I don't see where you replaced the power tubes with known good ones. Did you?

Random thoughts:
Bad power tubes
An arc on a power tube socket or related connection - try watching the amp with the room light turned off, looking for little blue sparks.
Intermittent connection somewhere between OT and speaker load.

[Mick Bailey]

I wire a protection resistor across the speaker output with some amps. Not low enough to significantly affect the impedance, but enough to prevent oscillation if the speaker is disconnected. Usually a 7W or 10W glazed 220R wirewound (the small-bodied green glazed ones). I think it was pdf64 that put me onto this quite a few years back. It's 'fixed' a few problem situations over the years where I was convinced the speaker was getting disconnected.

[LarBal]

I see lots of measurements, but I don't see where you replaced the power tubes with known good ones. Did you?

Random thoughts:
Bad power tubes
An arc on a power tube socket or related connection - try watching the amp with the room light turned off, looking for little blue sparks.
Intermittent connection somewhere between OT and speaker load.

Sorry, i did leave out a few details accidentally. At some point I replaced every single tube in the amp at the same time(besides the 6GC7, which I switched around a couple times), and nothing changed. I also checked the stanby switch, as it shorts the 6L6 grids together to mute. It was fine.
I will check on the arcing and work on the cab/speaker connections

[LarBal]

I wire a protection resistor across the speaker output with some amps. Not low enough to significantly affect the impedance, but enough to prevent oscillation if the speaker is disconnected. Usually a 7W or 10W glazed 220R wirewound (the small-bodied green glazed ones). I think it was pdf64 that put me onto this quite a few years back. It's 'fixed' a few problem situations over the years where I was convinced the speaker was getting disconnected.

Ok great, thanks. Any reason why a white wirewound cement wouldn't work? I have some 270r 10W here

[Helmholtz]

Any reason why a white wirewound cement wouldn't work?

No. That would make a good base load for OT no-load protection.

[LarBal]

No. That would make a good base load for OT no-load protection.

Ok great, thank you. I'll do that, as well as replace the old harness/jack on the cab. I'm planning to order a HRDeluxe 4K OT, which has had success in the amp, according to the internet. Before I start tracing out this mess of a cap job for the stacked power supply arrangement that was previously done, is there anything here that could conceivably cause the multiple blown OTs? Bad choke? As mentioned, the screens are sitting 100V or so below the plates, which looks to be the intention of the design