Odd filament circuit in an AA165 Bassman

[Randy Magee]

I have a AA165 or AB165 Bassman on the bench with an odd filament circuit that I've not seen before. The PT has not center tap on the filament circuit and there is no artificial center tap using 100K resistors either. Rather on V1 pin 4, there is a .05 600V capacitor going to ground. Though I plan to clip that and install an artificial tap, I was wondering if anyone might be familiar with this?

[martin manning]

Never seen or heard of that before. Do you think it's original? Can you date the amp? I agree a direct-coupled artificial CT is a better idea.

[NickC]

That's weird.

Both the AA165 and AB165 schematics show the PT heater winding having a center tap.

My 1966 AB165 (has an AA165 sticker in it) has the heater winding center tap. Heater CT was factory soldered to the chassis next to the PT (picture attached).

Looked like maybe your PT isn't original?

[Randy Magee]

I'm thinking that the PT must have been replaced a very long time ago as the area where the center tap was soldered to the chassis looks to have been poorly done... in fact the center tap only had one strand of wire left and it was causing some very strange voltage readings... voltage lower than expected and bias voltage all over the place, never steady. I have never seen a capacitor used like that in a filament circuit. The tube chart ws missing in this amp and it is extremely dirty, so reading anything is difficult. The PT has been very hot at some point as there is varnish covering the numbers on the PT. It is an AB165 as it has that stupid bias balance that the AA165 doesn't have.

[Firestorm]

You see that cap to ground off the filaments occasionally; a couple of Ampegs, Dynacos, others. Usually it comes off the center tap or there is a cap from each side. I'm not sure how it works, but I am pretty sure Fender never did it.

[NickC]

I'm thinking that the PT must have been replaced a very long time ago as the area where the center tap was soldered to the chassis looks to have been poorly done... in fact the center tap only had one strand of wire left and it was causing some very strange voltage readings... voltage lower than expected and bias voltage all over the place, never steady. I have never seen a capacitor used like that in a filament circuit. The tube chart ws missing in this amp and it is extremely dirty, so reading anything is difficult. The PT has been very hot at some point as there is varnish covering the numbers on the PT. It is an AB165 as it has that stupid bias balance that the AA165 doesn't have.

I changed the bias circuit, and a few other changes (AA864 and AB165 hybrid), as per this article:

[martin manning]

It is an AB165 as it has that stupid bias balance that the AA165 doesn't have.

It's easy to add a 10k trimmer to the bias board and have both level and balance adjustments.

[Randy Magee]

I thought about adding a trimmer, but elected to just make the bias adjustable. I can't get the damn thing to clean up for anything though. Bias is just below 60%, but the amp just sound dirty... driving me crazy! I'm wondering if I have a failing coupling cap that is causing this ugly distortion...