Buzzing Fender Bassman 50

[blackwing]

Please help! Not much hair left to pull out. This 1968 Fender Bassman 50 has a stubborn noise problem that has been partially reduced. There is that ocean sound "whoosh-whoosh" that goes away a little after the amp warms up, but what's worse is the nasty buzz coming mostly from the normal channel.

So far, the following bad parts have been replaced: 2 70uf filter caps, 5 cathode caps (25uf), the 12AT7 driver tube, the 1meg resistor on the normal input jack, the AC cord. There were some poorly layed out wires, too, which have been moved. These things have reduced the noise somewhat, but all the rest of the parts are within 10-20% of original values

Here's what I don't understand:
As soon as I touch the meter lead to the grid pins of The normal channel preamp tube, BUUUZZZZZ!!!!! and pin 2 reads 0mvdc and .5mvac, and pin 7 reads .4mvdc and 2.6mvac.
Another weird thing (to me) is the high voltage rectifier output, which switches back and forth from 0vac to 115vac.
The bias voltage supply is fine at -61vdc (no ac).

Anyone know what I should do?

Thanks

[corworld]

As soon as I touch the meter lead to the grid pins of The normal channel preamp tube, BUUUZZZZZ!!!!! and pin 2 reads 0mvdc and .5mvac, and pin 7 reads .4mvdc and 2.6mvac.
Another weird thing (to me) is the high voltage rectifier output, which switches back and forth from 0vac to 115vac.
The bias voltage supply is fine at -61vdc (no ac).

Thanks

Me thinks you have a grounding problem. You changed a lot of parts out. Was this problem there before? Recheck the input jacks, particularly the 1meg resistor.

The rectifier changes the alternating current (hence AC) to direct current (DC). if you are looking at the AC side you will see this changing at 60 cycles per second.

[Enzo]

I don't trust your metering. With numbers you are posting it shouldn't work at all, but all you report is some hum on one channel.

ANy measurements you are taking with respect to ground, make sure the ground connection for your meter is solid. In fact, use a clip wire. If your meter has a clip on the black probe, then clamp that thing on a chassis screw - one for mounting the transformer for example. Now use the red probe for readings.

Does the amp have an intact three wire AC mains cord and plug?

Your heaters should be referenced to ground. Ther should be a hum balance pot wired across the heater lines. It is 100 ohms with the wiper grounded. With this pot centered, you should get 3VAC or so on each heater pin.

Oh wait, you meant pins 2 and 7 of th preamp tubes, not the power tubes, sorry. But check the heaters anyway. If that pot fails, you will get hum in the preamp.

Ther should not be any appreciable DC on those grids. And the only AC there will be the noise signal, not much. It is normal to get hum when touching a meter lead to a grid.

Turn the amp off and measure resustance to ground from pin 2 or each input tube. SHould see about 1 meg. if it shows open, your jack is not grounding the tip. And loosen the jack nut, move the star washer on it to one side a little so the teeth see a fresh spot against the chassis, and tighten the nut again. This will make a fresh ground contact for the jack.

ANy chance on of those 70/350v caps is backwards? They aer in series, they don't both connect to ground.

After the rectifier, I expect to see 450VDC, not 115VAC. The 450VDC should have a couple volts of ripple on it.

[blackwing]

Enzo,

Thanks for your input.

Yes, the AC cord is good. I had already changed the plug on it, and tightened it's ground connection to the chassis. That took a little of the buzz out.

Yes, the heaters are getting 3.2VAC (each pin) and the hum balance pot is ok. Interesting is the fact that when hum is minimized, bias is imbalanced (-51V on V5 and -55V on V6). That's where it is now. When the noise is gone, I'll readjust it.

Yes, the 70/350v caps are in series with neg to ground and pos to the HV supply, which is at 468VDC.

I made sure the input jacks were grounded well, and then measured resistance on the two input tubes, and here is what I got:

NORMAL channel (the buzzier one):
jack #1, unplugged: 39k on pin 2, 255k on pin 7.
jack #1, plugged into: 79k on pin 2, 255k on pin 7.
jack #2, plugged into: 1.2M on pin 2, 255k on pin 7. (this is the noisiest jack)

BASS channel jack #1 unplugged: 34.4k on pin 2, 288k on pin 7.
jack #1 plugged into: 68.5k on pin 2, 288k on pin 7.
jack #2 plugged into: 1.02M on pin 2, 288k on pin 7.

AND--- when I touch either of the input leads near the jacks of the normal channel, there is a BIG increase in hum/buzz. I'm an antenna, but that doesn't happen when I touch either input lead on the Bass channel.

Then I plugged the same 1' phone cord into each jack and found:
Bass jack 1 = 137k
Bass jack 2 = .982M
Normal jack 1 = 156k
Normal jack 2 = 1.17M

Does this tell us anything? All the caps and resistors are within 20% of original value.
Thanks for your help.

[blackwing]

Enzo,

Thanks for your input.

Yes, the AC cord is good. I had already changed the plug on it, and tightened it's ground connection to the chassis. That took a little of the buzz out.

Yes, the heaters are getting 3.2VAC (each pin) and the hum balance pot is ok. Interesting is the fact that when hum is minimized, bias is imbalanced (-51V on V5 and -55V on V6). That's where it is now. When the noise is gone, I'll readjust it.

Yes, the 70/350v caps are in series with neg to ground and pos to the HV supply, which is at 468VDC.

I made sure the input jacks were grounded well, and then measured resistance on the two input tubes, and here is what I got:

NORMAL channel (the buzzier one):
jack #1, unplugged: 39k on pin 2, 255k on pin 7.
jack #1, plugged into: 79k on pin 2, 255k on pin 7.
jack #2, plugged into: 1.2M on pin 2, 255k on pin 7. (this is the noisiest jack)

BASS channel jack #1 unplugged: 34.4k on pin 2, 288k on pin 7.
jack #1 plugged into: 68.5k on pin 2, 288k on pin 7.
jack #2 plugged into: 1.02M on pin 2, 288k on pin 7.

AND--- when I touch either of the input leads near the jacks of the normal channel, there is a BIG increase in hum/buzz. I'm an antenna, but that doesn't happen when I touch either input lead on the Bass channel. I also tried substituting shielded wire from Normal input to preamp tube - no difference.
And this hum/buss is LOUD!



Then I plugged the same 1' phone cord into each jack and found:
Bass jack 1 = 137k
Bass jack 2 = .982M
Normal jack 1 = 156k
Normal jack 2 = 1.17M

Does this tell us anything? All the caps and resistors are within 20% of original value.
Thanks for your help.