Input buzz!

[jkey04]

So.... this amp is a hybrid Fender-esque circuit that I designed and have built several of now. It’s essentially a single channel 35w blackface vibrolux in a Princeton sized package with cathode biased 5881s. Has reverb but no trem.

This particular one though is buzzing right at the input and I can’t seem to figure out the cause. It only buzzes with a guitar plugged in and my hands off the guitar. If I touch anything metal on the guitar or the amp chassis the buzz quits. If I ground the grid to V1a the buzz stops. It’s not standard single coil buzz either. If I rotate my tele around 90deg it doesn’t do anything to the buzz. I’m sure its something simple, but can I get a little help before it drives me crazy?

Things I’ve tried:

- shielded wire from input jack to grid of V1a with 33k resistor right on the grid pin
- rolled 4 or 5 tubes through each preamp position
- isolated the input jack with a rubber washer and grounded it to the star ground where the rest of the preamp is grounded
- reflowed all grounds points even the CE Manufacturing cap can
- reflowed all pins on V1 and V2
- reflowed all eyelets on the board
- replaced input jack

None of these things improved the buzz.

[xtian]

Confirm some resistance readings with your meter, with amp off. Plug in your instrument cable, and measure tip to shield resistance (1M). Measure tip to 12ax7 pin 2 resistance (33K).

[jkey04]

Alright - done. The 1M and 33k resistances check out.

[pompeiisneaks]

also the buzz dissappearing with you touching the strings means it's a noisy guitar/guitar cable, not the amp itself. you are providing an earth for the hum from the guitar when you touch it.

Have you tried other guitars? Especially a humbucking one?

~Phil

[jkey04]

Phil,

Thanks for the comment. It’s not the guitar or cable. Those were the first things I ruled out. It buzzes just the same with humbuckers and single coils. I also tried a couple different cables.

Also, it’s not dirty power either. I tested on different circuits in my house as well as at a friends house.

** This is me thinking aloud here (Feel free to critique my thoughts.) **
Since wiring in the shielded wire from the input to V1a grid didn’t clear it up, I’m thinking I should start looking at other components associated with V1a? The 1.5k cathode resistor reads fine on my meter, but maybe something is up with the bypass cap and it ground connection? Maybe I should just go ahead and change the plate resistor as well.

[pompeiisneaks]

Well it seems that with the jack grounded, and no hum that indicates the noise is coming in from the input section for sure. the first triode. Otherwise you'd have the noise when it wasn't inserted. Try tapping the components and soldered connections with a wooden chopstick, and try moving the leads around a bit as well to see if a specific location resolves it.

~Phil