Covers on or off?
- Thread starter manu et deo
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triode17
Super Member
Yes 12AX7s or the like. Phono sections especially.
The Premier 11 here has the cage removed. C-J likely knew that many would want to run with the cages off, so the top surface of the amp is finished in gold that matches the front panel. Thing is, the Premier 11 was originally designed for 6550 tubes, and the KT-120s I retubed it with are too tall to fit under the cage, at least until I see if I can get some black standoffs to raise the cage slightly. It is out of the way on the bottom of my equipment rack, but if I put this closer to the speakers in the future where it is out in the open, the cage will go back on it just to protect the tubes.
Most of my receivers and the one integrated amp, plus tuners didn't come with covers, and the one with covers/cabinets get used due to young grandchildren. But I DO use tube shields on all units where the manufacturer mandated. There is a good reason for this. RF/IF interference and in some cases AF interference, can get picked up from Naked tubes. Tube shields will ground out this interference BEFORE it gets to the tube, this is especially true of Phono sections and other moderately hi gain circuits, and Tuner IF Sections where you want each section to not have any interference from the sections before and after. This helps with keeping the RF/IF signal clean and also keeps limiting down to what it's set to, plus sending a clean signal to the MPX unit. .
iplagolf. Read my previous post. manufacturers were being conservative when they put the tube shields on. It's more of a If not When in most cases except the RF/IF tubes. They will interfere with each other and can possibly throw off limiting of the IF stages, plus other tuning issues. The possibility is there so they shielded them. If you want to watch the tubes, pull them from everything except the RF/IF tubes and the Phono tubes. Otherwise it's a 6 of 1 and 1/2dozen the other.
Some things will hum if the shields are off, especially phono preamps.
Anything that came with shields keeps them, and occasionally I've added them. Some things are a bit on the fussy side and you can measure hum or cross-talk from the tube being unshielded. Something I picked up from Dave, my Pilot SA-260 has a very measurable amount of channel to channel cross-talk that almost completely goes away with a shield over the voltage amp tube. The small tube being next to the output tubes gets some signal bleed. I won't tell you I can hear that, but its easy to measure and at some point when I feel ambitious I'll swap that socket out for a shielded one to fix it.
Anything that came with shields keeps them, and occasionally I've added them. Some things are a bit on the fussy side and you can measure hum or cross-talk from the tube being unshielded. Something I picked up from Dave, my Pilot SA-260 has a very measurable amount of channel to channel cross-talk that almost completely goes away with a shield over the voltage amp tube. The small tube being next to the output tubes gets some signal bleed. I won't tell you I can hear that, but its easy to measure and at some point when I feel ambitious I'll swap that socket out for a shielded one to fix it.
triode17
Super Member
Just because you can't hear a difference doesn't mean there isn't any high, ultrasonic interference or oscillation there. It manifests itself as a certain fuzziness in the sound. Without a scope, you just can't 'see' it. Use the shields as the manufacturer meant them to be used.
Just a small tip, swapping out sockets is lots of work. I've made shields for 9 pin tubes by cutting a small rectangle of HVAC sheet metal with tin snips, and then wrapping it around the shaft of a drill bit which is slightly smaller in diameter than the tube, held in a vice. Shield stays on by spring pressure, then just solder a wire to it and attach to whatever convenient ground screw.
