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! |
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. |
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, 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? |
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.... |
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. |
Thank you Albert, I've tried everything else. Made my own shielded and grounded power cord. Changed speaker wires and interconnects. In fact disconnected all of the interconnects from the system and the RF is still there. So the pick up is either in the amp or tubes. I tried the bws site but I haven't heard anything. I've e-mailed them I would be interested in knowing more about your Lloyd Walker product. |
I have had tubes pick up RF in only one situation. However I suppose there must be some instances where it happens or the military would not have manufactured RF shields in such great numbers.
In any case, the shields I purchased were about $5.00 each, and helped to damp microphonics as well as pulling away heat. Pretty cheap experiment for Keithcady even if it fails.
Many suggestions posted here are valid and should be tried as well. Perhaps I misunderstood Keith, it sounded like he had tried various tubes, which improved the rejection of the RF.
When the solution is found I hope the results are posted. Good info for future reference what ever the source of the problem. |
Interesting, Albert, as I've never had a tube pick up RF--glad I saw your post, guess I've been lucky. Keithcady, do you have separate small signal tubes for each channel, perhaps if not in the preamp section maybe the amplifier section (I'm not familiar with the Jolida's circuit)? If so, switching them could probably tell you if the tube is causing the problem. |
Lloyd Walker makes a kit that includes metal wrap and ground wires to place around miniature tubes that eliminate RF and microphonics. This of course, assuming the radio signal is being picked up by small signal tubes and not through your interconnects or electrical system. If these are 9 pin miniature tubes, there are also military RF shields that slip over the tube. The best ones include heat sink design. I have a Walker kit that I bought some time back that I might consider letting you try. It does appear to be quite a bit of work to install, probably the reason I have not experimented with it. The slip on RF shields would be the easy experiment, and I believe that BWS tube services has these in stock. Here is his link: http://www.bws-tube.com/here's hoping that future radio broadcasts be limited to your tuner. |
While not the world's most technically adept guy, I can't see how any tubes could be causing your problem; they're only amplifying a signal, and this isn't microphonics from your description. Bob bundus' suggestions are a good place to start; perhaps switching speaker cable leads could tell you if perhaps one of them is acting as an antenna? I had an Audio Matiere amp in my home for audition once and had a similar problem I couldn't solve with line conditioners, Versa Woodblocks, ferrites or the like; one friend of mine thought it could be a grounding problem within the amp, but since it wasn't mine I didn't follow up on that one. You might ask your Jolida dealer about that, though. Good luck! |
Audioquest makes ferrite RF chokes called RF Stoppers; they come in two sizes, Junior & Regular. The chokes split into two pieces for easy attachment around AC cords & speaker cables. These should help; I thought someone mentioned that Radio Shaft even sells something similar. Use caution if you must filter around your interconnects; I experienced HF degradation doing this around Auqioquest Lapis interconnects, but it might work OK for you? You'll need to experiment, but the RF which is apparently riding into the amp along your cabling can be tamed. |
