6g15 buzz

[R.G.]

I typed in an answering post, but it seems to have disappeared. I'll try to recreate bits of it.

EMI comes in two forms: EMI from outside the unit, and EMI/oscillation created inside it by the circuit itself. Outside EMI is dealt with by using an enclosing metal box/chassis as a Faraday cage to block it out, by filtering/choking off EMi conducted on cables (including the AC mains cord), and arranging the cables coming in so they don't act like antennas at some radio band.
This is one reason I always like to make 1.0 connections to the chassis/shield/Faraday cage. It's a shield, period and end. But obviously amps in the past have gotten away with multiple uses to some degree.

Turns out the RF interference (?) is still there, it's just harder to notice if the tone knob is turned below about halfway. The wet signal has a constant crackling sound and some hum/buzz. Additionally, I get radio interference and sometimes full-on oscillation at the output jack of the reverb unit (~100mV at ~12kHz) depending on the positioning of the RCA cable.

One thing that consistently reduces the volume of all of the noise is if I am physically holding the RCA cable and also touching the chassis. This works if I touch the chassis of the reverb unit, amp, or any grounded equipment plugged into the same power strip. I feel like this is a big clue, but I'm too thick to know what it means.

It is a big clue. What you're describing seems to indicate that the AC safety ground is high impedance and not shunting RF into the earth ground. This is at odds with the three wire grounding your diagrams shows. Maybe the power strip has an issue?

Does your ohmmeter show zero-ish ohms between the chassis of the reverb and the chassis of the amp, and then to something else plugged into the power strip? Maybe a high impedance socket in the strip. Does this change if you plug amp and reverb into the same duplex outlet?

I feel like this is a grounding issue. I checked all my isolation washers, everything that is meant to be isolated seems isolated.
Chopsticking and pushing leads around made no difference.

There are other ways EMI could get there, including internal oscillation. Two common ways something can oscillate are if wire like grid wires pass too near high signal wires, and if the grounding wires conduct later and higher current ground return current along the same wires that carry ground return for an input stage. The resistance of the ground wires causes a feedback path into the cathodes and/or gates of the earlier tubes. Which wire exactly carries the output tube current back to the negative of the first filter cap?

[lonote]

Not sure if you tried separating the tank shell from the signal ground & then grounding the shell with it's own conductor?

I think I did this in a very small combo that didn't allow the tank to be all that far away & it helped with noise.

I unsoldered the little ground jumper on one of the RCAs to float the reverb internals & ran a wire from chassis ground to the tank shell so that the shell was just a shield (+ shielding the open side of the tank)

[oxbow_lake]

Just wanted to follow up here, my RF issues seem to have resolved after swapping out the 3 foot long RCA cable (black plastic, like you'd use for a stereo receiver) for separate "vintage" style 15" cables (the braided metal ones: https://www.amplifiedparts.com/products ... one-reverb). I left the grid stoppers in place.

The 120Hz-like buzz is all but gone after swapping UF4007s for the diode bridge, even without snubbers.

So it sounds good, and thanks again!

[martin manning]

The 120Hz-like buzz is all but gone after swapping UF4007s for the diode bridge, even without snubbers.

It’s possible you had a soft failure of the bridge producing the noise. You don’t need snubbers with UF4007, in fact they may do more harm than good.