Best Tube to eliminate RFI

Bruce at BWS should still have the tube shields. If not, I may still have some around here that you could borrow.

The Walker kit is available from Loyd Walker, if not that too is here still in the original box.

You did not mention whether changing tubes had any effect on the RF. Also wonder if signal could come in from the power line. Shielded power cables won't help if the signal is carried along with the AC.

Have you tried an extension cord running from another outlet? Would be interesting to see if that changed anything.

Tubes don't pick up RFI, amps do. If the amp is properly laid out and using proper grid-stop resistors (similar resistors are found in all transistor gear) the amp will not pick up RFI.

I heard of a problem recently where RF was being picked up by gutters in the house! The customer lived near a radio station. Primitive diode junctions caused by rust in the gutters caused them to rectify the RF and vibrate. It took him a while to realize that the sound was coming from *outside* the house rather then from inside....

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.