BF Fender Bandmaster problem

[rocky101]

I have a Blackface Fender Bandmaster(AB763) that works only for a few seconds before the sound fades away to almost nothing. I have to turn the volume around 8 to start getting sound. I have checked all the voltages in the amp according to the schematic and everything is in spec except for the plate voltages of the phase inverter. The plate voltage on Pin 1 should be 230v and on Pin 6 225v, but I am getting 439v on pin 1 and 179v on pin 6. I tested the resistors( I did remove the resistors to test them) and they are within tolerance. I am not sure where to go from here. Any ideas. Thanks

PS I swapped all tubes while testing

[Enzo]

If you are getting 400+ volts on the plate of that phase inverter tube, I think it is a safe bet that half of the tube is not conducting.

DIm the lights and look close at that tube. Are BOTH heaters glowing? If one side is deaqd, either it is the socket/wiring or the tube itself. Since the other half works, we know heater current is getting at least to pin 9 and either 4 or 5. Is the connection between 4 and 5 OK? Is the socket OK, no spread pins on 4 and 5?

If it is fully lit, please tell me you at least tried a different tube in that socket.

If any tube in that socket works the same way - which is to say doesn;t work - then even though your resistors may be OK, that doesn't mean the connections to them are OK.

You gave the plate voltages, but the cathode and grid voltages are important too. The grids are pins 2 and 7. What DC voltages are on each? And cathode pins 3 and 8 should be wired together. Is the same voltage on BOTH?

Right now my future-vision is telling me no voltage on pin 2. (Assuming the tube is lit and a good tube.)

[rocky101]

If you are getting 400+ volts on the plate of that phase inverter tube, I think it is a safe bet that half of the tube is not conducting.

DIm the lights and look close at that tube. Are BOTH heaters glowing? If one side is deaqd, either it is the socket/wiring or the tube itself. Since the other half works, we know heater current is getting at least to pin 9 and either 4 or 5. Is the connection between 4 and 5 OK? Is the socket OK, no spread pins on 4 and 5?

Both sides are lit

If it is fully lit, please tell me you at least tried a different tube in that socket.
YES

If any tube in that socket works the same way - which is to say doesn;t work - then even though your resistors may be OK, that doesn't mean the connections to them are OK.

You gave the plate voltages, but the cathode and grid voltages are important too. The grids are pins 2 and 7. What DC voltages are on each? And cathode pins 3 and 8 should be wired together. Is the same voltage on BOTH?
Pin2 = 24v Pin 7=15v Pins 3 & 8 are 60v

Right now my future-vision is telling me no voltage on pin 2. (Assuming the tube is lit and a good tube.)

Thanks. Answers in bold.

[rocky101]

Update. I replaced the tube socket and no luck, voltage readings still the same. I noticed on the schematic that the voltage off of the filter cap labeled D the voltage is supposed to be 375 but I am getting the entire plate voltage of 440. I'll continue digging to see what is wrong.

[RWood]

the voltage off of the filter cap labeled D the voltage is supposed to be 375 but I am getting the entire plate voltage of 440

If you mean the "D" cap and voltage supply on the schematic.....that cap is as far down the B+ line from the OT voltage as possible, to the tune of one 4.7k resistor plus one 1K resistor plus the choke. It sounds as though you have a short (or several) that is allowing the voltage to go around those voltage dropping resistors and straight to the cap.

Triple check the wiring and connections there for continuity, and I'd remove a leg of each of those resistors to acurately check them, too. Sumpin' ain't right!

RWood

[rocky101]

If you mean the "D" cap and voltage supply on the schematic.....that cap is as far down the B+ line from the OT voltage as possible, to the tune of one 4.7k resistor plus one 1K resistor plus the choke. It sounds as though you have a short (or several) that is allowing the voltage to go around those voltage dropping resistors and straight to the cap.

Triple check the wiring and connections there for continuity, and I'd remove a leg of each of those resistors to acurately check them, too. Sumpin' ain't right!

RWood

Thanks. I checked the resistors and they are fine. I am getting a voltage drop. at cap B I have a plate voltage of 450v, at cap C I am getting 444v, and cap D is 427v. Sorry about the mix up, I wasn't getting the entire plate voltage at cap D like I stated earlier. I will keep poking around. The amp has sound right when I flip off the standby and then fades to nothing. Another thing, when I first fired up the amp after it sat for a few months it played for a few minutes then the sound just faded away.

[RWood]

Have you swapped out the filter caps?

So you are getting some voltage drop. I was going to suggest upping the values of those resistors to get your voltages more inline with the schem, but until you have fresh electrolytics in there, you could be chasing your tail.

[Enzo]

If no voltage drops across the plate load resistor of the triode, then no current is flowing. I still think the problem is in the grid/cathode stuff.

[Robert M. Martinelli]

I'm with Enzo, there's no current flowing through the 1st triode in the (LT)PI section. If your AB763 is stock, you should have some 2.4 mAmps flowing through the first PI triode ( 82K plate resistor ) and some 2 mAmps flowing through the second ( 100K plate resistor ).

If you have 429 V at the 1st triode plate, this is a clear indication no current is flowing through the 1st triode in the PI section, OTOH, if you have 179 VDC at the 2nd triode's plate, this means some 250 VDC are dropping through its plate resistor, indicating that some 2.5 mAmps are flowing through the second triode. Since the cathode resistor and the "tail" resistor of an LTPI are shared, I'd expect them to test good; check the cathode(s) to GND voltage, normally it should be in the 110-120 VDC range, but, since only about half of the current is flowing through them I' d bet you'll find the cathode voltage is now lower than that.

The first thing I'd do, should the above scenario be confirmed by evidence, would be to sub in a new and known-to-be-good tube in the PI slot.

Hope this helps

Best regards

Bob

[rocky101]

Thanks for all the help. Something is wrong somewhere and I can't seem to find it. For laughs I pulled the leg of the 100k resistor and the amp now has sound. I need to find more time to work on this to narrow down what is causing the problem