I think you will find,
The amount of shielding depends on the gain of the tube in question.
The higher the gain, the more chance of pick up.
So power tubes have a dissipation factor, ie they get hot in operation so the trade off is: which is the most important getting rid of the heat from the power section or shielding in a low gain stage?
If the tube overheats this creates its own problems and heat build up reduces service life of components.
Sometimes the use of a screening can, can actually make a circuit sound worse. So its dependant on application and gain of the tube section.
In very high gain stages parasitic oscillation will take place without a screening can. So its more of an engineering issue and application.
NB the shielding around capacitors can also be to shield from heat radiated from components or tubes! Not always just related to pick up.
Regards
M. Gregg
The amount of shielding depends on the gain of the tube in question.
The higher the gain, the more chance of pick up.
So power tubes have a dissipation factor, ie they get hot in operation so the trade off is: which is the most important getting rid of the heat from the power section or shielding in a low gain stage?
If the tube overheats this creates its own problems and heat build up reduces service life of components.
Sometimes the use of a screening can, can actually make a circuit sound worse. So its dependant on application and gain of the tube section.
In very high gain stages parasitic oscillation will take place without a screening can. So its more of an engineering issue and application.
NB the shielding around capacitors can also be to shield from heat radiated from components or tubes! Not always just related to pick up.
Regards
M. Gregg
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Unless you somehow ground the aluminum, it will offer no shielding, and may actually increase pickup.
Given that glass is a poor conductor of heat I would have thought that an external heatsink would need to touch the glass all round, not just in 6 places as that appears to show. A black 'heatsink' which merely surrounds the glass will not cool any better than nothing, as it will just absorb and reradiate infra-red which would have gone off elsewhere anyway.
Worse yet, I think the parts that touch the glass are silicone, so it may actually increase the tube temperature. Caveat emptor.
Hi again, thank you all for your support.
In fact I don't have the EAT dampers in mind for this purpose. I asked them with signal tubes in mind, and from photographs it appeared that it touches the glass at every point.
For larger tubes all I find is something like that:
I have two things in mind:
- shielding from RF
- protection from our scrubwoman who visits us weekly and every time after her visits I find my tubes leaned.
In fact I don't have the EAT dampers in mind for this purpose. I asked them with signal tubes in mind, and from photographs it appeared that it touches the glass at every point.
For larger tubes all I find is something like that:
I have two things in mind:
- shielding from RF
- protection from our scrubwoman who visits us weekly and every time after her visits I find my tubes leaned.
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Several years ago, I built a tube sonic hologram device and horizontally mounted the tubes on vibe mounts with the tubes actually inside of drilled out solid chunks of 1.5" thick aluminum that you could attach external heat sinks to near the tops of the tubes. There are six tubes in each chunk of aluminum, in the left and right sides each one corresponding to a stereo channel, so 12 tubes total in a 1U height chassis about 10" deep. Doing this was to isolate the tubes to the maximum extent from surface and air borne vibrations. I had the aluminum black anodized of course. This all fit in a 1U height chassis that I built. No record of tubes failing from heat, but then this is a line level processor and I wasn't operating any of the tubes near its maximum thermal limits. Of course, this sonic holograph generator doesn't look like has anything to do with tubes - after all, it has heat sink fins on it
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Hi,
Those were about the best around IMHO.
Still, I've never, ever heard or witnessed any improvement in sound or lifespan thanks to any of these devices.
In most cases, adding shielding cans subjectively rolled of the HF response somewhat making for a less open, more "enclosed" sound.
Ciao,
Those were about the best around IMHO.
Still, I've never, ever heard or witnessed any improvement in sound or lifespan thanks to any of these devices.
In most cases, adding shielding cans subjectively rolled of the HF response somewhat making for a less open, more "enclosed" sound.
Ciao,
Given the signal levels present in a 300B no shielding should be necessary - maybe for someone who uses one in a line stage this will be different. (24 300B amps both SE and PP under my belt) Given a dissipation rating of 40W and some designs like mine that run them at 30W or more shielding would be a bad idea.
Note that my 26 pre-amplifier uses ventilated shield cans to keep electro-static coupling to surrounding metal objects to a minimum since they are exposed and they did pick up hum.. (Relatively large plate surface area as compared to more modern types makes them vulnerable to this issue.)
Enclosing all of the electronics in a metal box will preclude electro-static coupling from external objects, however it will not help if the source is internal to the box.
I tend to use outboard power supplies in my designs, but I make the DC and regulate it externally - the Counterpoint clearly does not do this as indicated by the presence of a rectifier, pass tube, error amp tube and gas reference which is probably a 5651. (Gas reference is not a very good choice in very low noise applications, unless combined with an LPF to remove its noise from the reference - zeners are easier to filter and ultimately quieter if handled correctly at least with simple error amplifier topologies..)
There is lots of AC inside that box with the pre-amplifier, which is why those caps have shielding around them, the large surface area of their electrodes makes them quite susceptible to stray fields from poorly managed wiring, the higher the node impedance the more vulnerable it is to electro-static coupling. The tubes having smaller plates may actually be less sensitive to this pick up mechanism.
Note that my 26 pre-amplifier uses ventilated shield cans to keep electro-static coupling to surrounding metal objects to a minimum since they are exposed and they did pick up hum.. (Relatively large plate surface area as compared to more modern types makes them vulnerable to this issue.)
Enclosing all of the electronics in a metal box will preclude electro-static coupling from external objects, however it will not help if the source is internal to the box.
I tend to use outboard power supplies in my designs, but I make the DC and regulate it externally - the Counterpoint clearly does not do this as indicated by the presence of a rectifier, pass tube, error amp tube and gas reference which is probably a 5651. (Gas reference is not a very good choice in very low noise applications, unless combined with an LPF to remove its noise from the reference - zeners are easier to filter and ultimately quieter if handled correctly at least with simple error amplifier topologies..)
There is lots of AC inside that box with the pre-amplifier, which is why those caps have shielding around them, the large surface area of their electrodes makes them quite susceptible to stray fields from poorly managed wiring, the higher the node impedance the more vulnerable it is to electro-static coupling. The tubes having smaller plates may actually be less sensitive to this pick up mechanism.
There is one aspect of shields which I have not seen mentioned above although Kevin is heading there with the above post.
I have built a number of amplifiers which use the "old" 6J7 pentode. I had HUGE problems preventing these tubes from oscillating until one of the aged tube amp techs of my acquaitance (He is 82 years old) said "You have got shields on them , haven't you? If you don't you are wasting your time." Well I had'nt. I had grid stops, anode stops, screen stops, small caps added from anode to 0V, in fact damn near everything "under the sun" except shields. Putting shileds on them definitely helped significantly with curing parasitic oscillation problems.
I know Kevinkr above has experienced stability problems with 6J7 coz' we had some private correspondence about it.
So if you are at your wits end with a stability problem and nothing you try seems to help at all, I suggest you try a shield on the tube.
Cheers,
Ian
I have built a number of amplifiers which use the "old" 6J7 pentode. I had HUGE problems preventing these tubes from oscillating until one of the aged tube amp techs of my acquaitance (He is 82 years old) said "You have got shields on them , haven't you? If you don't you are wasting your time." Well I had'nt. I had grid stops, anode stops, screen stops, small caps added from anode to 0V, in fact damn near everything "under the sun" except shields. Putting shileds on them definitely helped significantly with curing parasitic oscillation problems.
I know Kevinkr above has experienced stability problems with 6J7 coz' we had some private correspondence about it.
So if you are at your wits end with a stability problem and nothing you try seems to help at all, I suggest you try a shield on the tube.
Cheers,
Ian
For RF amps, the final(s) are almost always contained in their own shielded compartment. That's one reason most RF types have plate top caps, as these allow for the easy isolation of the input and output circuits with the chassis doing double duty as a baffle shield. These shielded compartments being as much for end user safety as for keeping stray RF out of the immediate environment. TV HD deflection circuits are also shielded, and more for safety than anything else.
For audio finals, it's not so much of a concern, since you're not leaking RF. Being low gain devices, 300Bs aren't going to be needing shielding anyway.
Very high gain (pentode, cascode) low level, stages are the ones most likely to be needing shielding, either for stability and/or protection from stray electrical fields that can induce noise into the signal chain.
and as some have found out the hard way (you can just google squeal and howling tube amp) sometimes it doesn't matter what chassis you use, you still have to shield the tubes from each other especially if you have a high-ish gain OR if you make the mistake like i have of cramming all the tubes too close together.
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