Hey Enzo!

[Freecrowder]

I'm working on a Bassman AA864. Looks to be in pretty good shape. Before I did anything to it It had a hum/buzzy sound form the normal channel and the bass channel. It gets louder as you turn up either channels volume pot. Not way loud but definitely noticeable. Anyway just changing out all electrolytics not directly to fix that issue but just going through the amp. Also all preamp level plate resistors and cathode resistors. Changed both volume pots and 68K input resistors. All voltages look close to spec. After all that still have the same level of noise, no change. I notice that when I get my finger close to either volume pot or the wire connect that pot to the gain stage the same noise gets louder. Of course this is an open chassis so might just be normal. I also notice that when I measure the filament voltage either leg and other probe on chassis ground I get 2.3 and 4vac so doesn't look balanced. I'm not sure which wire is the grounded centertap for the filament winding but assume it is referenced to chassis. One more thing death cap has been removed and three wire cord added with green gong to chassis. I tried removing the ground from chassis and it makes no difference to the hum/buzz. is this just normal? Sorry long winded but all pertinent info.

 

[turk 182]

sorry not Enzo but have you looked at the input jack?
i'm assuming this noise/buzz condition is with no input plugged in? if the contacts that normally shorts the input to ground are dirty it may be leaving the input floating and susceptible to noise.

 

[Freecrowder]

Thanks for the input. I have had the input jacks out and thoroughly cleaned and checked for spring tension. They are good quality Switchcraft heavy duty. I don't believe that they are the problem.

 

[Freecrowder]

I'm really questioning the unbalanced heater voltage. Although schematics show a centertap I'm not seeing a centertap wire. My reading says that should be a green with white stripe. Might try the artificial centertap with 2 100ohm resisters and see if that helps?

 

[Freecrowder]

So one other clue is that if I pull the input or first gain stage tube from either channel the noise all goes away for the corresponding channel. I'll try grounding the input jacks with clip wire while I'm at it.

 

[Freecrowder]

Just so that others might learn from my experience. My XFMR did not have a centertap wire for the heater winding even though it showed one on the schematic. I added the artificial centertap of 2 resistors and the midpoint tied to chassis ground. Now measuring from each winding to ground it is balanced at 3.3vac. The drop in noise level was amazing around 20-30db! I did not know of this issue until now.

 

[Enzo]

Is your transformer the original Fender part? You definitely always need to reference the heaters to ground either by center tap or the two resistors (virtual center tap), or other means.

 

[Freecrowder]

Thanks Enzo. This was a learning experience for sure on this amp. I dont own the amp not sure if it is original or not.

 

[Enzo]

Look on the transformer for the part number.


And production runs varied, Not all Fender amps had center tapped windings.

 

[Freecrowder]

So am I correct Enzo. There is no current flowing through the resistors to ground? But there would be current through the 2 resistors in series?

 

[turk 182]

? there should be approximately 60ma going through the resistors to ground.

 

[Enzo]

Two 100 ohm resistors from either end of the 6v winding to ground effectively forms a 200 ohm resistor across the 6.3v. I get 32ma flowing through the resistors. I don't see that changing with a ground connection. I don't see a circuit for current to ground. Current through the resistors, yes, but not into ground.