Another Radio Interference Thread

[wingobingo]

My Alessandro Doberman has Radio Interference - both local and on stage. I use a pedal board with multiple effects. Doesn't happen when not plugged in. Whats puzzling is that I also have a Silverface Twin and this never happens to this amp. I also have a Marshall 2104 which does pick up RF but to a less degree.
Where do I begin?

[plexitone]

I'm not familiar with the Alessandro but the first thing to do, if you haven't already, is to see if you have RF problems when plugging straight into the amp. If not, then try different cables and pedal combination's until you find the source on the pedal board.

RF problems in an amp can usually be fixed by using shielded cable from the input jack to the first tube and maybe between it and the next gain stage, depending on how "high-gain" the amp is.

Adding grid stopper resistors on the tube socket of the first couple of stages also helps. If the amp already has grid stoppers but they are not mounted on the tube socket either move them (if practical) or add another resistor on the socket. You can also try increasing the resistor value (say from 33k to 82k) to lower the low-pass corner frequency.

[selloutrr]

set it all up and make it happen. recreating it is key to resolving it.

basically you are checking for which piece of gear is not properly grounded and acting as an antenna.

start at the guitar and work toward the amp. replace yes / no? restore and continue in toward the amp. every pedal, every cord.

or bypass the pedal board and go guitar to amp.. is the problem solved?

if not is the amp properly grounded. are you sure the power supply you are running it on is grounded.

[FUCHSAUDIO]

Does it have a series input grid resistor ? Is the input jack ground floating ? If not, maybe float it and ground to chassis through a .01 disc cap. Is the input grid lead shielded ? You could add some shielded wire there, even a small cap (100-pf) plate to grid or plate to cathode. Is the BOX shielded ? If it does not have foil on the bottom open end of the chassis side of the box, add some....try another input tube, just for fun. Got pics ?

[drz400]

My Alessandro Doberman has Radio Interference - both local and on stage. I use a pedal board with multiple effects. Doesn't happen when not plugged in. Whats puzzling is that I also have a Silverface Twin and this never happens to this amp. I also have a Marshall 2104 which does pick up RF but to a less degree.
Where do I begin?

Very Stupid thing to check but many people have made the common mistake of using Speaker cable (since it looks like guitar cable these days)

Make sure you put a .01 (optional a 51ohm in series) to chassis ground to the input jack ground. Grid resistor right at the tube grid on the first stage is also a good call like 15~33K

Pretty much what Andy said !

[Kagliostro]

You can do many things to prevent RF income

very good to trim the grid resistor value and to connect it directly to the tube socket instead to the input jack

first thing you can simply try is to recover a ferrite "clamp" from a computer cable and to put it on the input cable the near you can to the input jack

working inside to the amp you can also add a ferrite bead (look to some Mesa Boogie schematics)

se the attached images

Kagliostro

[Super_Reverb]

You can do many things to prevent RF income

very good to trim the grid resistor value and to connect it directly to the tube socket instead to the input jack

first thing you can simply try is to recover a ferrite "clamp" from a computer cable and to put it on the input cable the near you can to the input jack

working inside to the amp you can also add a ferrite bead (look to some Mesa Boogie schematics)

se the attached images

Kagliostro

This is good advice in general for guitar amps. Considering that the highest fundamental frequency on a guitar is 600-700 Hz with harmonics on the order of 8-10 khz, it's a good idea to low pass the 1st preamp stage, and maybe every other preamp stage and the PI and power amp. This will go long way toward keeping your amp stable. You see this in some high gain amps. At times parasitic oscillation be ultrasonic and could be 100s of kHz or in the mHz.

Grid stopper resistors create a low pass filter working with tube grid/plate capacitance (remember Miller effect) and caps across plate load resistors work by decreasing stage output impedance, and hence gain. Can also place cap across lower resistor in a divider(used to attenuate signal from previous stage)

Wanted to mention this because a recent build of mine had measureable frequency response to about 5 MHz. I placed a small cap across the plate load resistor to set a pole at about 10 kHz.

Add caps or grid stoppers one at a time: you'll know if you have overdone it, as the amp will lose treble response and sparkle.


cheers,

rob

[ForcedFire]

I used a grid to chathode capacitor on v1 with no grid stopper. I have 2 560pF 50V caps in parallel, amp is still super bright.