Fender 125P34AX

[Esquirefreak]

So, I'm confused...

I noticed that in my '68 SF AB763 Showman, there are two 100Ω resistors forming an artificial CT for the heater winding.

I hade a vague memory, reading on this forum, that those transformers have an internally grounded filament CT. Checked the AB763 schematic and sure enough, my memory was right.

The AC568 schematic also uses the 125P34AX PT, however that one is not internally grounded.

Theres no doubt mine has had the 100Ω resistors from the factory. So my best guess is that my AB763 has the "later revision" 125P34AX that the AC568 uses.

My question is, it couldn't be using both an internally grounded CT and 100Ω resistors, right?

/Max

[sluckey]

My question is, it couldn't be using both an internally grounded CT and 100Ω resistors, right?

It's possible to use both but not likely. No harm done if using both. It's also possible that the two transformers are different even though they have the same number.

[Esquirefreak]

So, upon closer inspection, this amp seems to be a transition between AB763 and AC568.

Reason I was asking about the PT was that I have more hum than I'd like to. It's not really bad, but I think the noise floor could be lower.

I lifted the 100Ω resistors for the heaters and found that I still have continuity to ground.

Also, there's a green/yellow lead from the PT to ground. Or at least I think it is, I'm partially color blind.

I haven't found a schematic of either AC568 or AB763 Showman with a heater CT lead to ground. The AB763 shows an internally grounded CT, whereas the later model is grounded through two resistors.

Can anybody shed some light on this? I am a bit confused, to be honest.

/Max

[Esquirefreak]

Resistors

[Esquirefreak]

PT stamp/codes

[Esquirefreak]

I have basically the same resistance reading between GREEN - GREEN/YELLOW as I have between GREEN and ground/chassis. I get that my Fluke probably can't measure such low resistances accuracy.

But I shouldn't get any readings at all if they weren't connected, right?

[sluckey]

The grn/yel is the center tap for the two green wires. That explains the continuity from green to chassis.

[Esquirefreak]

Yeah, so I thought...

I disconnected the green/yellow from chassis and I still had continuity between green and ground. Since the amp has worked fine for all these years, I don't suspect a shorted winding.

I should add, though, that I haven't yet disconnected the green wires from the lamp holder. I'll do that first thing in the morning.

The thing that I don't get is that I haven't seen a filament CT on these transformers. Might be due to the amp being an export model.

Any danger in trying the amp with or without grounding the green/yellow?

It feels like the 100Ω resistors are probably unnecessary, though.

Thanks a bunch for helping me!

[sluckey]

Any danger in trying the amp with or without grounding the green/yellow?

no

[Esquirefreak]

Some further investigation regarding the PT in my Showman.

I lifted both the GRN/YL and RED/YL prior to taking the readings.

RED -> RED 28 Ω
RED -> RED/YL 14.8 Ω / 13.3 Ω

GRN -> GRN 0.5 Ω
GRN -> GRN/YL 0.5 Ω

GRN/YL -> RED/YL 2.1M Ω
GRN -> RED 10M Ω

GRN/YL -> CHASSIS 1.0 Ω
RED/YL -> CHASSIS 10M Ω

My multimeter can't measure low enough resistance to get accurate readings on the filament windings. But it appears that I have continuity between GRN and CHASSIS even with all 3 filament wires detached.

Could this be a result of bad insulation or might the PT (like the earlier 125p34AX) have an internally grounded filament CT? It does not make sense to me why there would be both an internal and external CT. Or is my multimeter fooling me? 

/Max