5F2A makes weird fizzy fuzz and changes when I adjust tone knob

[Draal]

My 5f2a home built amp makes a crappy fuzzy/fizzy sound when the tone control is low or midway.  Once I crank the tone control, it becomes less fizzy.  Tubes have been swapped and I also replaced the caps in the tone/vol. setup. This is a standard 5f2a build except that I added a choke and filter cap.  There is no NFB resistor either.  I'll post my voltages in a bit after I run some errands. 

The 33k resistor at the input is not shielded as it's proximity to the input jack is about 2 inches away.   Any clues are appreciated.

Pete

[Draal]

Voltages:

12ax7
pin1 - 170v
pin 6 - 169v
pin2 -0v
pin7 0v

6v6
between pin3 and 8 - 330v
pin5 0v
pin 8 - 20v

Dissipation - 13 watts
Shielded cable between volume lug 2 and 12ax7's pin 7.  This is a point to point build with main ground and PT bolt and typical preamp grounding set up.

[eleventeen]

Give us the cathode voltages on the 12AX7. Those voltages, believe it or not, tell the story of what's going on with the tube often better than P or G volts. We really want to see a volt or two developed across those 1500 per-triode cathode resistors. 820 ohms on later amps when two cathodes share 1 resistor. 


Your "fizz" could also be a "no NFB" artifact, sometimes there will be a HF oscillation going on. You can't hear it but it thrashes whatever you try to play through the amp. Got scope? 

[Draal]

Cathodes read 2v each.  I added the neg. feedback resistor to see the results and no change.  Hit a note with volume midway up and get that dirty note that fizzes at the end.  I've changed both pots with spares I have had laying about to no avail.  It is a tight chassis for sure; I'm reusing an old phonograph player chassis. 

[shooter]

Hit a note with volume

what does it do with nothing plugged in?

have you tried another *source* like cabling in an mp3, then playing with the knobs?

[Draal]

With nothing plugged in, it's quiet as a mouse.  I moved a coupling cap further away from the input jack with no change.  I then used shielded wire on the input jack to the 33k resistor going to pin 2 of the 12ax7.  No change.

I will whip out the magnifying glass to go over every connection yet again.  And to answer a previous question: I unfortunately do not have a scope. 

[dude]

Was the amp doing this since day one, did it ever play ok without the noise?

If from day one, check all solder joints, could be a bad joint

[Draal]

Was the amp doing this since day one, did it ever play ok without the noise?

If from day one, check all solder joints, could be a bad joint

Yep, since day one.  I'll be checking for bad solder joints again tonight.  Know I'm gonna find that ghost even if it drives me nuts. 

[Draal]

Found it!  Chopsticks are your friends.  It was the 5k wire from the OT.  Hanging around inside a tight fitting chassis.  Just need to secure it out of the way for good ☺️.

[Ed_Chambley]

Good deal, now time to play.

[Willabe]

Yep, good work! 

[eleventeen]

'nother one bites the dust. Good job, all!