Question about the metal preamp tube cover thingies

LnTH

Member
Messages
63
What do they protect the preamps from? shock or something that I'm not aware of? Are they absolutely necessary? Cause they piss me off big time. either I'm very unlucky or this is a general BAD DESIGN! cause in any amp i've owned, without exception, I push the covers in and try to turn so they lock in place but they won't turn all the way. They just hold on barely and at some point in time they just pop off their place making a big sound and scare the **** out of me cause I always think a tube or some other component just blew up.

Can I just take them off and live happily ever after?

Thanks

/rant
 

Blue Strat

Member
Messages
30,722
They're supposed to shield the preamp tubes from picking up Electro Magnetic Interference (EMI) from surrounding electrical fixtures, lights, etc.

If the amp sounds ok without them it's fine to take them off.
 

LnTH

Member
Messages
63
They're supposed to shield the preamp tubes from picking up Electro Magnetic Interference (EMI) from surrounding electrical fixtures, lights, etc.

If the amp sounds ok without them it's fine to take them off.
Ah! I'll try it without. Just can't get them to hold in place for some reason. Maybe I'll just put them on without the spring so they don't shoot out of place for no good reason.
 

NOVA70

Silver Supporting Member
Messages
239
Wipe the inside by the threads with a cloth with some WD40 on it and also the base. Just a little, and try to turn them on.
Pat
 

dorianadams

Member
Messages
587
Also the higher quality heavier chrome ones actually ring if you tap them. If your preamp tube goes little microphonic they make a wierd ringing sound through the amp.The cheesy aluminum ones are actually better because they do not resonate/ring . I pointed this out to Dr Z whan I got My Maz 38 senior ( long gone) back in 99 and I believe he switched types.The chrome ones look cooler though.They also serve to retain the tubes due to the spring on the top
 

dorianadams

Member
Messages
587
Ah! I'll try it without. Just can't get them to hold in place for some reason. Maybe I'll just put them on without the spring so they don't shoot out of place for no good reason.
There is a slight trick to installing them. You have to push down to compress the spring and then turn the shield so the nub on the tube base engages the slot in the retainer then release.
 

mojo2001

Member
Messages
119
There is a better option in shields. IERC made black heat-dissipating shields with little fingers inside to contact the tube. These were common in premium military and industrial electronics back in the 50s and 60s.

These types can actually help out with microphonics in some situations, due to the way they grip the tube. Other times they seem to make it worse.

The chrome or silver jobs restrict radiation of heat and are actually very bad for tube longevity. They should be painted or chemically finished FLAT BLACK to radiate heat away from the tube rather than reflect it back into the tube.

The IERC shields are anodized black inside and out for radiation cooling and also make good mechanical contact with the tube socket for conduction cooling.

I believe that a tube with an IERC shield runs cooler than a naked tube. A naked tube runs cooler than a tube in a chrome shield. Heat is the #1 killer of tubes, all else being equal, so do your hot snot Mullards a favor and get IERCs or at least paint your shields hi-temp flat black inside and out.

The IERC shields push on straight down and don't require the twist-on motion.

Here's a pic:

http://tinyurl.com/357tbk

They are a bit hard to order because they were made in many different sizes but well-worth hunting for. There is another kind of twist-on IERC shield that requires a special matching socket so beware.

A better mousetrap.

Joe Roberts
 

Guitar Dave T

Member
Messages
11,960
Could never tell the difference, sonically, with 'em on or off in my 20+ some odd old Fenders over the years.

I always figured they were their to keep the tubes from getting bumped lose in the bed of the truck on those unpaved back roads.

But you're right - they're a pain in the ass. Keep 'em stored safely someplace and otherwise go without.
 

LnTH

Member
Messages
63
mojo thanks for pointing those out.

As for now I took them out and I don't notice any noise issues so I guess I'll just store them until/incase I decide selling the amp.

Thanks for all the replies!
 

jspax7

Member
Messages
2,235
There is a better option in shields. IERC made black heat-dissipating shields with little fingers inside to contact the tube. These were common in premium military and industrial electronics back in the 50s and 60s.

These types can actually help out with microphonics in some situations, due to the way they grip the tube. Other times they seem to make it worse.

The chrome or silver jobs restrict radiation of heat and are actually very bad for tube longevity. They should be painted or chemically finished FLAT BLACK to radiate heat away from the tube rather than reflect it back into the tube.

The IERC shields are anodized black inside and out for radiation cooling and also make good mechanical contact with the tube socket for conduction cooling.

I believe that a tube with an IERC shield runs cooler than a naked tube. A naked tube runs cooler than a tube in a chrome shield. Heat is the #1 killer of tubes, all else being equal, so do your hot snot Mullards a favor and get IERCs or at least paint your shields hi-temp flat black inside and out.

The IERC shields push on straight down and don't require the twist-on motion.

Here's a pic:

http://tinyurl.com/357tbk

They are a bit hard to order because they were made in many different sizes but well-worth hunting for. There is another kind of twist-on IERC shield that requires a special matching socket so beware.

A better mousetrap.

Joe Roberts

How do you determine which one to order?
 

mojo2001

Member
Messages
119
>>re IERC: How do you determine which one to order?

Best is to get the seller to provide info on what they will fit.

By the time these devices were fashionable, there were many sizes of miniature tube bottles in use, some rather exotic.

I'll look around and see if I have one that fits 12AX7 for measurements.

I once had microphonics in a mic pre I built with super high transconductance triodes and I killed much of it by using an IERC shield with narrow strips of sheet lead epoxied on the outside of the shield.

I will also mention that many times corrugated black metal inserts were used inside the chrome shields to make physical contact with the glass envelope and conduct heat away from the tube. These usually get lost somewhere along the line. These inserts can also modify microphony characteristics of the tube and can help or hurt in particular circumstances.

All this being said, I usually take off the chrome shields unless I need them to deal with noise. The heat buildup factor is what concerns me. These things can get fairly hot to the touch and become a mini oven inside for the poor tube.

One other thing I should have mentioned above: If you decide to paint your shields black, leave the part that contacts the socket collar unpainted for better heat transfer and to ensure good electrical contact!

Joe
 




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