Help with buzzing amp

[cbassss]

Hello,

I have seen this topic covered before, but have not found a solution yet. I have just completed an EL34 amp and while it sounds very nice, there is a very audible buzz at certain volume settings. with the volume at zero, the amp is dead quiet, then from about 1/3 to 2/3 volume a buzz can be heard which gets louder until about 2/3 volume, above that the amp goes dead quiet again.

some information:

- potentiometer is a 100K alps
- issue occurs whether the input source is turned on or not
- shorting the RCA has no effect
- if nothing is plugged into the RCA, the buzz is much louder, and does not get quieter as the volume is turned up
- my power supply is isolated from the signal path
- my signal wires are shielded
- my chassis and circuits are grounded at one point (ie no ground loop)

I would really appreciate some suggestions to help me solve this issue.

Thanks

 

[huggygood]

any schematic ?

 

[JonSnell Electronic]

Place a 10k grid stopper on the input grid from the volume control and fit a 220p 400v with a 10k resistor in series across the first stage anode load resistor.
That should sort it out.

 

[cbassss]

Schem attavhed. Note I am supplying higher voltages, B+ is 425 and preamp is 340v. I am also not grounding with a 1 ohm resistor.

 

[H713]

I'm not a big fan of having the pot serve as the ground reference for your preamp/phase splitter tube.

What I might do is put a 250K resistor from the 1st grid to ground, then a .1uF cap between the wiper of the pot and the grid of the tube.

This may or may not be your issue, but it's something I might be inclined to try.

 

[DAK808]

Maybe it is a transformer that is buzzing? Sometimes that will happen especially if there is an over current problem. Some just have loose hardware or loose laminations that can buzz.

 

[petertub]

I'm not a big fan of having the pot serve as the ground reference for your preamp/phase splitter tube.

What I might do is put a 250K resistor from the 1st grid to ground, then a .1uF cap between the wiper of the pot and the grid of the tube.

This may or may not be your issue, but it's something I might be inclined to try.

Same thought here. When pot is in middle there will be an inbalance between the two tubes in the phaseinverter.

 

[H713]

Maybe it is a transformer that is buzzing? Sometimes that will happen especially if there is an over current problem. Some just have loose hardware or loose laminations that can buzz.

Possible, but that shouldn't change with the volume control.

 

[Diyengineer]

Not sure what the power supply looks like but I had a buzz in a pair of amps I built. It ended up being caused be too many HT rectifier diodes in series on the HT tap. The power trans has a CT secondary and the original print called for 3 1N4007 diodes in series on each leg of the HT tap. Through a ton of experiments with people here I changed to a single UF4007 on each leg and the buzz was gone.

 

[cbassss]

Hi folks thanks for the comments. The transformers are not buzzing, the sound is clearly coming from the speakers. I am going to start with removing the pot as the ground reference as noted by DAK808.

 

[cbassss]

Sorry, meant H713