59 Fender Super amp, replaced PT heater question

[centervolume]

I got an old tweed super amp in that has been shotgunned but still worth getting straightened out if for nothing else to hear it work again.

The heater scheme factory installed calls for a non-center tapped heater winding with one lead going directly from PT to ground and the other
running along 1 heater connection of the tubes (except the 5u4) with the other heater connection of the tubes going to ground.

There is a replacement PT in there with a GRN-YLW center-tap on the heater that has been isolated and the other 2 green leads are hooked up as described above.

The PT however is not happy with this setup, it is overheating and I am wondering what the best solution is. I am not sure what model of PT is in there so I have no idea if this setup delivers the 6 VAC to the 6L6 as well as the 12 VAC to the 12AX7s.

It appears to have been run like this already, checking for continuity between the heater winding and the cover of the PT shows approx 8 ohms, which seems to indicated the winding is shorting to ground through the body of the transformer and the transformer needs to be replaced

[g1]

There are several windings coming off the PT. How have you determined that the heater winding is the one responsible for the over-heating? If the grn/yel is not connected, there is no reason the factory winding should cause this issue.
All heater leads from the transformer would have to be disconnected to properly measure resistance to the cover of the PT.

As you are unsure about the type of PT replacement, perhaps it is not big enough for the amp, which would cause over-heating.

[centervolume]

G1 -
The replacement came out of a brown 6G4 super amp where it was delivering only 425 of b+. Underpowered in that application but a good fit for the tweed 5F4 super inasmuch as that amp's schematic calls for approx 430VDC.

You are absolutely correct that I needed to pull the whole thing out of the chassis to double check, which I have done to find that none of the windings are shorted to each other or to the casing of the transformer. It doesn't *appear* to be fried but still not sure why it was getting hot (sounded great, but getting hot and emitting a worrisome smell, shut off after a few notes).

The leads are typical: black primaries, red high voltage w/ red-yel center tap, yellow rect. heater, red/blue bias and green heaters (with grn-yel center-tap).

While the fact that it was operating without issue (just slightly underpowered) in the previous application does lead me to believe that it is generally a good fit, I have to remain open that the heater winding is not the only possible culprit.

How to troubleshoot this though?

thanks in advance for any dialog on this!

[DRH1958]

It could be the 5v winding that is causing problems. The 5U4 draws 3A and the 5AR4/GZ34 draws 2A. If the tranny you put in is for an amp that has 2A draw for the 5v winding as the 6G4 has and you have a tube that draws 3A, you are 1A over with the 5U4. I don't know if this is enough as they sometime over engineered for things but worth a look.

[centervolume]

I hooked it back up again to measure heater voltages - the 5 VAC rectifier volts were present (not sure if that was taxing the tranny too much or not just yet). But the green heaters were only putting out 5.4 VAC rather than the 6.3 (this in an open circuit, that dropped to 4.8 with the heater circuit hooked up). So that's an clear problem.

I then unhooked everything again to remeasure resistances between leads since the tranny had gotten warm again. This time I found the center tap to high voltage and the fixed bias lead were only 4 Ohms apart, which put the fixed bias (blu-red) lead only 33 ohms of resistance away from the high voltage. That would strike me as too close for comfort if these were not part of the same winding

[g1]

The bias tap is on the same winding as the HV, so I wouldn't worry about that for now.
But the unloaded heater voltage looks like a problem. Is that 5.4V across the greens with everything disconnected, even pilot lamp?
And I guess that voltage was not measured in the old amp, even though it seemed to work?

[g1]

It could be the 5v winding that is causing problems. The 5U4 draws 3A and the 5AR4/GZ34 draws 2A. If the tranny you put in is for an amp that has 2A draw for the 5v winding as the 6G4 has and you have a tube that draws 3A, you are 1A over with the 5U4. I don't know if this is enough as they sometime over engineered for things but worth a look.

This is a good point, we are just speculating without a proper part number for the PT. If it is actually original from 6G4 super (5AR4), then that is the same PT that was used in the 5F4 super (5U4), and should be ok. But it should be verified.

[centervolume]

Yes, those measurements were taken with the heaters floatin in the air. When I put them in circuit, it went down to 4.8 I think. So that transformer must be a loser, regardless of the 5VAC issues. Right?
It worked in the 6G4 super, but the amp was not loud enough, tone not that great

[DRH1958]

Yes that's too low for sure. Under no load conditions, it can be as high as 7v or so depending on the % regulation, but the full load voltage shouldn't be lower than, say, 6v at the most. I would replace the tranny. Don't even worry about the 5v readings. I would guess the tone was not good and had low output in the 6G4 because the heaters were under-voltaged.
Just for future reference, g1 made a good point, the schematics do spec the same tranny for both amps.

[g1]

If it's not overheating with the 6.3V winding disconnected, you might get away with using an external small 6.3V transformer just for the heaters.

[Leo_Gnardo]

How to troubleshoot this though?

Run the transformer with nothing loading any of the secondaries. If it heats up all by itself, it's toast. Sometimes old transformers do that. Good idea to fuse the AC line while doing this test of course.

You should see minimal current draw on the AC line, maybe 50 mA or so. I've seen PT's that draw upwards of half an amp, get roasty hot all by themselves and even start to smoke with that sickening hot transformer smell.