SF Bassman - unwanted distortion

[lowell]

Hi all,
The bassman sounds pretty lame. It is the version w/ the 2 47k plate resistors on the PI - AA371. It is breaking up WAY too much for a SF 70s Fender. It sounds the same in both channels all inputs. The owner of this amp only uses the Normal channel so I'm focusing on that. I have performed all repairs that I would think would fix this problem. I replaced all anode and cathode resistors, all bypass caps, all filter caps and dropping resistors, all coupling caps, screen resistors on power tubes. The bias on output tubes is good around 35ma. I'm thinking there is a problem w/ the p-transformer. Voltages on the Normal channel (tube 3) read: (paranthesis #s are per schematic)

1 180v (220v)
2 0v
3 1.5v (1.9v)
6 220v (270v)
7 0v
8 1.9v (1.5v)

Voltages on the PI read:

1 297v (340v)
3 114v (132v)
6 276v (340v)

When I measured voltages on the top of the anode resistors they are also lowere than the schematics figures. The DC voltage post-rectifier is 405v. Is this low? I the power transformer bad?

[Amp Kat]

Hi all,
The bassman sounds pretty lame. It is the version w/ the 2 47k plate resistors on the PI - AA371. It is breaking up WAY too much for a SF 70s Fender. It sounds the same in both channels all inputs. The owner of this amp only uses the Normal channel so I'm focusing on that. I have performed all repairs that I would think would fix this problem. I replaced all anode and cathode resistors, all bypass caps, all filter caps and dropping resistors, all coupling caps, screen resistors on power tubes. The bias on output tubes is good around 35ma. I'm thinking there is a problem w/ the p-transformer. Voltages on the Normal channel (tube 3) read: (paranthesis #s are per schematic)

1 180v (220v)
2 0v
3 1.5v (1.9v)
6 220v (270v)
7 0v
8 1.9v (1.5v)

Voltages on the PI read:

1 297v (340v)
3 114v (132v)
6 276v (340v)

When I measured voltages on the top of the anode resistors they are also lowere than the schematics figures. The DC voltage post-rectifier is 405v. Is this low? I the power transformer bad?

Not necessarily and one good thing is this is one of the easiest amps to check the secondary voltages on. You can (carefully) measure across the HV red wires using your meter on AC and half of that from the red/yellow to each side of those wires. Then check the green wires at the pilot light for 6.3 VAC and the RED/BLUE wire for your 50 volt bias supply. You could have a bad tube or something shorted bringing those voltages down like a bad power tube. Could be a bad input jack or bad connection also. When I get distortion like that I go from the first tube and connect it to the PI and see if it works right then move down the line until I find the culprit part of the circuit. There is that funky feedback thingy also in this amp so it could be causing some problems. The zero volts your getting on pins 2 & 7 will read about 7 volts if you check across 2 & 7 with you AC meter. For DC, you are using chassis as your refernce point right ?

[lowell]

Yes using chassis as ground for DC measurements. I will pull tubes and check voltages as well as disconnect PT and measure AC on secondary.

So , ie, you hard-wire, say, tube 1 to the PI and see if that fixes it? Then tube 2 to the PI etc...??

[lowell]

Ok I pulled all tubes and most of the voltages returned to normal. I think I narrowed it down the the first preamp tube pulling things down. The big killer was that I had previously removed the coupling cap in series w/ the global feedback so the poor speaker was seeing quite a bit of DC... luckily only for a minute or two. Once that cap was added the voltages returned to normal. I also realized that I had switched the 220k local feedback resistor on the driver tube w/ the 470k series resistor from pin 6 of Normal channel tube accidentally... this cleaned up things a bit more. This thing now sounds a lot more like a Bassman should. I did increase the local feedback resistor to 330k as I felt this livened it up a bit.

[jrfrond]

Those amps (I am assuming it's an AC568) can also be cleaned up at the output by getting rid of the hybrid bias system on the power tubes and running the cathodes straight to ground, then modifying the bias supply to deliver the correct bias voltage for the full fixed-bias operation. That is a mod that actually came out of Fender Service in the late 70's.