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Tube Shieds??

sled108

sled108

Super Member
I've been Pondering this for a while now, but have not seen much mention of Tube Shields around here. I think of this because of my Fisher KM-60 Tuner has a Bunch of them...And, those Shields are over Fisher/ Telefunken 6DJ8's, 12AX7's,12AT7's and 6AQ8's...Now, the thing is that Shields shorten a Tube's Life with Heat. So, my question is...Is the Benefit of a Shield worth the Price of it's Tube? And if so, which ones in a KM-60 would Benefit the Most with a Shield?
I have not been able to here any difference With, or Without them....The FM Stereo Hiss has always been the Largest Factor...
 
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sled108 said:
I've been Pondering this for a while now, but have not seen much mention of Tube Shields around here. I think of this because of my Fisher KM-60 Tuner has a Bunch of them...And, those Shields are over Fisher/ Telefunken 6DJ8's, 12AX7's,12AT7's and 6AQ8's...Now, the thing is that Shields shorten a Tube's Life with Heat. So, my question is...Is the Benefit of a Shield worth the Price of it's Tube? And if so, which ones in a KM-60 would Benefit the Most with a Shield?
I have not been able to here any difference With, or Without them....The FM Stereo Hiss has been the Largest Factor...
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1) So you hear no difference or your hear hiss without them?

2) Can you point me at the facts behind your statement that shields shorten tube life? Assuming those Fisher/Tele's have been in there for a long time and have been in use they certainly have not had a short life.
 
Well, heat will Shorten a Tube's Life, and Shields will make them a lot Hotter...I've been Rolling mine for a while, so I cannot be sure of their Longevity...And, I hear Distant Station FM Hiss either way...When I switch to Mono, it's as Clean as a CD..
 
Kinda got that bass ackwards.

Sure, a tube shield's primary purpose is to keep interference from entering the tube it's mounted over, but they also act as sinks and help dissipate heat. They also serve to maintain core temp on idle or standby for better startup and stability.

If the shield is hot, that's a good thing ... just means its doing it's job. That's all heat that's been drawn from the tube itself. Same principle as any heat sink.
 
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Well, that is why I've been Pondering this...I can see the Shielding Effect that they might have....But, not the Core Temp thing....They get Hot...And, there are Plenty O' 12AX7's out there without Shields..
 
It really depends where in the circuit they are used. 12AX7's did a lot more than just audio amp duty. Also consider that in something as tightly packed as a Fisher receiver, coupling from adjacent tubes was a very real thing. In a more spread out layout, its probably far less of a concern.

I would also question that tube shields do any significant harm to lifespan. I've got some really old tubes that have been shielded their entire lives that still work fine. And when I say really old, I've got some dating to the late 1920s. My TV set is packed full of tubes, almost all dating to 1959, and better than half are shielded.
 
Worth mentioning - just touching the shield or the tube isn't gonna be much of an indicator of any temperature difference with or without. I mean ... hot is hot, no matter what.

Fun filled fact - the emitter itself is running anywhere from 1000-2000C once it's fully "warmed" up depending on the tube type and how it's driven. Yeoch!!! That'll leave a mark!

So, why do some tubes have shields and some not? Cost, I expect. If a design is a bit dirty, the shields can help contain EFI, and with decent air circulation, the additional heat dissapation may not be a factor. If some tubes are shielded and some not, chances are good the shielded ones are in the pre-amp section to keep your low level signal pure.

Not to forget either, all tube shields aren't created equal. Most just use close contact with the glass to help cool them (once again, relatively speaking), but there's lots of variety. Some have vertical ridges to channel heat up and away, some are layered so they act as chimneys, some are six or eight sided to achieve the same purpose, and some just get weird. There, you're mostly talking milspec stuff ...

tua-tr5-5015-2.jpg


Thing looks deadly, don't it <G>

And before running off to upgrade, it's also worth noting that for tubes, heat is a GOOD thing. Too cold, and they don't develop their ratings ...
 
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I'm using a pair of tube shields with my 12AT7 (Ei gray plate) tubes.
Helps keep the noise level down and....so far....they've been running fine.

Steve
 
Also, some datasheets will list tube ratings with and without shields--shields, being metal, can add parasitic capacitances to the tube elements! Not much, 0.1~0.5pF from what I've seen, but it's there.
 
gearhound said:
I'm using a pair of tube shields with my 12AT7 (Ei gray plate) tubes.
Helps keep the noise level down and....so far....they've been running fine.

Steve
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Steve,
I run the same tubes in one of my amps, so I'm just curious -- what kind of shields are you using succesfully with them? Thanks

Batch
 
Here's another little tidbit to consider: I've found that in some amps, the way it's designed can affect the tube's propensity to benefit from a shield or not. In some guitar amps I've worked on in the past, the 'gain' was so high on the tube that ANY little bit of RF would feed through an un-shielded tube. Even the overhead florescent lights! But, with the shield in place, noise was gone. Yet, the same tube in an audio pre-amp for home stereo....quiet as a church mouse. The Phono Sections of many integrated amps have shields on the driver tubes, while not in place on the pre-tubes or phase splitters. Same tubes....all 12AX7s in some amps like the Fisher X-100. But, the gain in the phono section merits the RF shielding. So, that may have some application in your considerations. Also, I support that the shield does NOT affect a tube overheating. In fact, I'd add that the shield adds an additional pathway for the tube's heat to conduct to the chassis and away from the tube itself. So, in that sense, I think any shield will help. If you look at the old Dynaco PAS series of Pre-amps, those all had closed boxes with NO venting on the chassis or internally. The four 12AX7s and one rectifier tubes all lived well in those old amps.....and when the occasional rare find of an OE PAS amp with OE Telefunkins in it comes up......many of those tubes will still test as well as when new.....and perform that way as well. So....FWIW....but I don't think just pulling shields for the sake of being able to view your tubes and 'think' they're cooling better is necessarily a good idea. On the other hand, if you do pull 'em and things sound fine, then I doubt you'll do any harm by doing so. Tom D.
 
Batch,

I purchased the tube sockets and shields from Angela Instruments in Maryland many years back.
I....think....they were listed as NOS Fender-issue?
But, they have NO manufacture or country of origin markings.

Steve
 
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