Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Draal on December 08, 2015, 02:22:18 pm
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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
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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.
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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?
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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.
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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?
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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.
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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
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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.
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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 ☺️.
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Good deal, now time to play. :wav:
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Yep, good work! :bravo1:
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'nother one bites the dust. Good job, all!