Best Tube to eliminate RFI

Ralph, certainly you know as much or more about tubes than anyone at this site. I know this personally from speaking to you on the phone many times. I believe you when you say a tube cannot pick up RF, I had this occur only one time and certainly it could have been something else (grounding, interconnect, etc..) that was causing the problem.

The one curiosity is why did the military manufacture so many RF shields for small signal tubes?

Often tube shields are used to reduce hum pickup, which can still be a problem in RF circuits. Other reasons to install them include reducing noise and other nonlinear effects (especially in areas of strong RF fields). They are also used to reduce RF radiation, as in the case of TV tuners, shielding the local oscillator.

In most audio applications they are used to reduce hum pickup, not RF. We found some years back that when using the Russian ceramic sockets (that had shields) that the sockets and especially the shields did not help things one bit, as they contributed to stray capacitance effects, decreasing the cutoff frequency. Removing the shield was an easy way to improve performance.

If you are having RF problems with your gear, I recommend installing grid stop resistors. They are a series resistance with the signal, but you will find not only does the circuit sound better (and the RF is gone) but also the circuit will likely be quieter. High frequency stability in the circuit will be improved as well. And you won't have to worry about shields (unless you have a magnetic field from a power transformer to cope with).

If you look in older gear you will see that grid stops are quite common. For some reason, many 'new age' designers haven't seen the light....

Ralph, I read your comments on RF problems in amps and just wondering is that the kind of thing a novice can do? How big of a problem is it to solve? Thanks for your comments.

Sorry for my slow response- been out of town.

If you are having RF problems, the first thing to check (IMO) is whether or not grid stop resistors have been used in the design. If not, they should be installed before proceeding with any other solutions.

Grid stop resistors are usually installed as close to the tube socket pins as possible. Depending on the circuit, the value might vary from 150 ohms to 3K. 1K is usually pretty safe. As a grid stop resistor is a series device, any parts that were connected to the grid of the tube will be connected instead to the grid stop resistor, which connects to the grid. So the change is usually not too difficult to perform.

After all the tubes in the amp have been treated this way then it should be tested again for RF. Most likely that will have eliminated the issues, as the primary rectification point for the RF signal will have been reduced significantly. If the problem persists then I would pursue other means, grounding, RF beads, etc.

Good Luck with it!