Fender Bassman 70 noise

[km6xz]

As a quick check of plate resistors, if carbon comp type, turn up the gain and monitor the noise. Gently squeeze a suspect plate resistor with a pair of well insulated pliers. A little compression should cause the noise to change if the carbon/metal particles have loosened in the resistor body.
This method is pretty sensitive in showing resistor pack density problems that are a common source of random snits and pops.

[Tom Phillips]

...This method is pretty sensitive in showing resistor pack density problems that are a common source of random snits and pops.

Good reminder of this mostly forgotten test. Your comment also made me recall that CC resistors were used as crude pressure transducers in the old days.
Tom

[phantombox]

Pulling V2 kills the noise momentarily. But after a short while it comes back from the grave. Amps output is about half (or less) the original, of cause, and the dynamic range of the whole amp (both channels) is significantly reduced.

[Tom Phillips]

Pulling V2 kills the noise momentarily. But after a short while it comes back from the grave. Amps output is about half (or less) the original, of cause, and the dynamic range of the whole amp (both channels) is significantly reduced.

Keep V2 pulled and, with the amp off, lift one side of the 220k feedback resistor that connects V2 pin6 to the .01uF cap. Now power back up. How does that affect the noise?

This diagnostic test breaks the path that allows signal to bleed through from the preamp to the phase inverter when V2 is out of the circuit. This will help isolate the source of the noise.

[phantombox]

Pulled V2 with amp off and lifted one side of 220k resistor that goes between V2 pin6 and the .01uF that goes to V2 pin7. This totally killed the noise but so did it on the amps output.
However, when I plug a cable to any input and touch the tip with my finger I can hear this on the speaker (with all vols at 10!). But when I plug in my bass guitar (vol all the way up) I hear nothing at all.
When I squeeze the carbon film resistors with some pliers and power on amp turned on, I can hear some change in the noise.

[Tom Phillips]

Pulled V2 with amp off and lifted one side of 220k resistor that goes between V2 pin6 and the .01uF that goes to V2 pin7. This totally killed the noise ...

OK. That verifies that the noise originates before V2 in the signal chain.

… but so did it on the amps output...

That’s expected too because now, assuming the amp is wired as shown in the schematic, we have totally broken the signal path between the pre-amp and the Master Volume / Phase inverter circuit and, of course, the power amp.

The next step is to verify which preamp channel is generating the noise.
Hook the 220k resistor back up and install V2. Power the amp back up and you should hear the same noise again.

Next you will determine if the noise is originating in the Bass Instrument channel or the Normal channel.
Pull V1 and power up the amp. If the noise completely stops this indicates that it is originating in the Bass channel. If there is still noise then either it is originating in the Normal channel OR if there is just less noise then it could be that some noise is coming from each channel. Try pulling V3 and report back on the noise you do or do not hear. You can also operate the amp with both V1 and V3 pulled and all should be quiet based on what we know so far.
Does this make sense to you?

One more thing. If the noise increases and decreases as you change the volume control then we will know it’s coming from the “a” triode section of the tube. If the volume control makes no difference except for the hiss level then the majority of the noise is coming from the “b” triode section of the tube.
It gets more confusing if there is noise being generated in more than one circuit section but these steps will help you track it down. The goal is to track the noise to a specific part of the circuit. We can do this without replacing components.

Let me know how this works out.

Regards,
Tom

[Leo_Gnardo]

There's also the possibilty of DC leakage thru the circuit board, very common with p2p wired Fender amps. Produces the sort of popcorn noise you describe. My solution is to rebuild affected portions of audio circuit on tie-strip above the waxed black circuit board.

[phantombox]

Thanks to everybody for your help.
After figuring out that V3 is the origin of all noise, and replacing almost every component in the whole circuit (it never hurts...), I finally cut out the waxed circuit board of the V3 section (thanks, Leo_Gnardo) and away went the noise.

Phantombox

[Leo_Gnardo]

Thanks to everybody for your help.
After figuring out that V3 is the origin of all noise, and replacing almost every component in the whole circuit (it never hurts...), I finally cut out the waxed circuit board of the V3 section (thanks, Leo_Gnardo) and away went the noise.

Phantombox

Cut the board?!? Well, that works too - glad you got 'er fixed Phantombox!