What Is The Purpose/Benefit of Preamp Tube Shields?
- Thread starter DennisM
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Reducing RF interference from the amp itself (transformers give off magnatic fields) and external sources like lights, LEDs, Neons, motors etc.
The modern world with dimmers, LED TVs and lots of electronics isn't nice to tube amps. Some amps have shielded inputs and grid stoppers on input wiring for the same reason.
You can sometimes hear your mobile through the amp or AM radio.
The modern world with dimmers, LED TVs and lots of electronics isn't nice to tube amps. Some amps have shielded inputs and grid stoppers on input wiring for the same reason.
You can sometimes hear your mobile through the amp or AM radio.
Timbresmith1
Poster Extraordinaire
I use them.
Tubes need heat. They grnerate it intentionally
The shields being metal dissipates heat. Regular operating temperatures don’t stress preamp tubes that much.It’s hard on capacitors, and transformer insulation. Also on transistors and chips.
Tim S
Friend of Leo's
In an amp like a Mesa Mark V, there are *6* gain stages. Any noise (Radio Frequency Interference) in a tube amp with multiple gain stages can be amplified loud enough to bleed into the beautiful sounds your guitar is making. The greater the gain, the more RFI needs to be considered.
Edit: Also some amp manufacturers are sadistic, forcing you to to pull the chassis out merely to replace a preamp tube because of the clearance required to install the shields.
IIRC, “heat is the enemy of tube amps” applies more to power tubes than preamp tubes (some say transformers are the most heat-sensitive components in a tube amp). But if I’m wrong, I’m sure I will be immediately corrected here.
Rather than thinking about tube shields holding heat in, an argument could be made that they work like heat sinks, pulling heat off the glass envelope of the tubes. I don’t think they’d make much of a difference to the glass temperature, but I suppose someone could measure it.
Edit: Also some amp manufacturers are sadistic, forcing you to to pull the chassis out merely to replace a preamp tube because of the clearance required to install the shields.
IIRC, “heat is the enemy of tube amps” applies more to power tubes than preamp tubes (some say transformers are the most heat-sensitive components in a tube amp). But if I’m wrong, I’m sure I will be immediately corrected here.
Rather than thinking about tube shields holding heat in, an argument could be made that they work like heat sinks, pulling heat off the glass envelope of the tubes. I don’t think they’d make much of a difference to the glass temperature, but I suppose someone could measure it.
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tubedude
Friend of Leo's
In combo amps the shields keep those upside down tubes from dislodging from shock during load in, load out.
All good info. The spring may also decrease vibration. But as noted, mostly about noise. That depends on your amp and electrical environment. On small simple amps, try with and without. You will notice many small upright heads go with exposed tubes standing up in space; some have a Faraday cage, many don’t.
Sea Devil
Friend of Leo's
They seem to reduce microphonics in the amps I have that use them.
stomped box
Tele-Meister
I have been under the belief this was the purpose of them. I guess it is that + RFI protection.
Thanks for the knowledge, y’all.
MuddyWolf
Tele-Holic
Keeping the tubes in. I doubt there was that much RF in Leo's day. He would have been doing the minimum in mfg to keep the price down for country musicians in the 50s.
Metal shields if they were designed differently could be used to dissipate more heat from the tubes.
Modern electronics usually have heat sinks. The Tone-Master amps have them for example to keep the CPU cool.
"Leo didn't do it" can be a silly reason these days. Those old amps weren't even grounded safely, is it any wonder they didn't have any thought process around heat dissipation or RF shielding???
Modern electronics usually have heat sinks. The Tone-Master amps have them for example to keep the CPU cool.
"Leo didn't do it" can be a silly reason these days. Those old amps weren't even grounded safely, is it any wonder they didn't have any thought process around heat dissipation or RF shielding???
Tim S
Friend of Leo's
I think you just defined a new product! A tube retainer/RFI shield/heat sink (fins and all). I’m kinda surprised no one has come out with these before.
It is not a good idea to heat sink a thermionic valve.
Vacuum tubes are built and engineered to run (internally) at a very specific temperature.
The glass envelope will heat to exactly the temperature it needs to be for proper operation, automatically.
The exception is when a tube is drawing too much current and red-plating. But that is a different issue altogether.
If you want to reduce heat in a tube amp, it is the transformers that you might want to put some mild air movement on, but do not blow air on the glass envelopes.
The will cool the glass more on one side than the other, and will likely contribute significantly to premature failure, or even spontaneous cracking.
The tube shields on preamp tubes help in many ways as pointed out in earlier posts:
1. RF shielding.
2. Protection from outside forces. Bumping, cables, etc.
3. They reduce the odds of burning your hand or fingers when you reach inside to tuck your junk in there.
4. The reduce the liklihood of ingniting the junk you stuff in there.
5. They provide a very stable temperature envelope for the valve to operate in. This is why you see 50-60 year old Fenders with original preamp tubes look new and still sound fantastic.
Heat dissipation is very important for semiconductors. Tubes take care of themselves.
" I doubt there was that much RF in Leo's day."
What? There were no radio stations, spark plugs, magnetos, generators, arc welders, lightning storms, or neon lights??
Must have been very dark and quiet.
-az-
Vacuum tubes are built and engineered to run (internally) at a very specific temperature.
The glass envelope will heat to exactly the temperature it needs to be for proper operation, automatically.
The exception is when a tube is drawing too much current and red-plating. But that is a different issue altogether.
If you want to reduce heat in a tube amp, it is the transformers that you might want to put some mild air movement on, but do not blow air on the glass envelopes.
The will cool the glass more on one side than the other, and will likely contribute significantly to premature failure, or even spontaneous cracking.
The tube shields on preamp tubes help in many ways as pointed out in earlier posts:
1. RF shielding.
2. Protection from outside forces. Bumping, cables, etc.
3. They reduce the odds of burning your hand or fingers when you reach inside to tuck your junk in there.
4. The reduce the liklihood of ingniting the junk you stuff in there.
5. They provide a very stable temperature envelope for the valve to operate in. This is why you see 50-60 year old Fenders with original preamp tubes look new and still sound fantastic.
Heat dissipation is very important for semiconductors. Tubes take care of themselves.
" I doubt there was that much RF in Leo's day."
What? There were no radio stations, spark plugs, magnetos, generators, arc welders, lightning storms, or neon lights??
Must have been very dark and quiet.
-az-
Greg70
Tele-Afflicted
I have a 1962 all-tube Sherwood stereo receiver. Some of the smaller tubes have shields and some do not. This receiver has been in my family it's entire life and to my knowledge none of them have been removed. My belief is that they are for RF shielding and not related to heat control. I also don't think a thin metal shield would effect temperature much one way or the other. It offers very little insulating property other than slowing boundary layer convection on the glass surface.
Also notice how the power tubes (four 7591's) do not have any retainers on them. Stereo receivers were not designed to be thrown around from gig to gig like a guitar amp.
Also notice how the power tubes (four 7591's) do not have any retainers on them. Stereo receivers were not designed to be thrown around from gig to gig like a guitar amp.
