Amp gurus...do preamp tube covers change an amps sound/tone?

[JerryP]

No, vacuum tubes' function are based on the theory of electron flow. Electrons boiled off the cathode move toward the most positively charged space nearby. In a basic diode tube, this would be the anode. Grids are inserted in various different tubes for controlling this flow in some manner relative to how they are referenced in the circuit.

Electronic circuits and electrical circuits are two different things.

As with anything there are no absolutes, but even deviations from an ideal can be mathematically quantified and explained.

Here's a few things to look at.
Audio Cyclopedia pages 470-471

Merlin Blencowe's book on Designing Tube Preamps for Guitar and Bass page 32 on the Miller effect

Air gap capacitor

Wikipedia:
Capacitance.......
Any two adjacent conductors can be considered a capacitor, although the capacitance will be small unless the conductors are close together for long. This (often unwanted) effect is termed "stray capacitance". Stray capacitance can allow signals to leak between otherwise isolated circuits (an effect called crosstalk), and it can be a limiting factor for proper functioning of circuits at high frequency.

Stray capacitance is often encountered in amplifier circuits in the form of "feedthrough" capacitance that interconnects the input and output nodes (both defined relative to a common ground). It is often convenient for analytical purposes to replace this capacitance with a combination of one input-to-ground capacitance and one output-to-ground capacitance. (The original configuration — including the input-to-output capacitance — is often referred to as a pi-configuration.) Miller's theorem can be used to effect this replacement. Miller's theorem states that, if the gain ratio of two nodes is 1/K, then an impedance of Z connecting the two nodes can be replaced with a Z/(1-k) impedance between the first node and ground and a KZ/(K-1) impedance between the second node and ground. (Since impedance varies inversely with capacitance, the internode capacitance, C, will be seen to have been replaced by a capacitance of KC from input to ground and a capacitance of (K-1)C/K from output to ground.) When the input-to-output gain is very large, the equivalent input-to-ground impedance is very small while the output-to-ground impedance is essentially equal to the original (input-to-output) impedance.


I'll be the first to tell you I'm no engineer, but I'm no dummy either. There are still many things I don't understand. There are plenty of things in this world that are unexplained or not understood, doesn't mean they don't exist. There's no cure for cancer and many other diseases. Every book and theory that was ever written was written by someone. Where did that person learn it? There always has to be a first and they are usually looked at as kooks or crazy. How many times have theories changed? If the guy writing the book has certain beliefs he will impose them on the reader, does that make it correct? Most things are explained with measurement and testing, but not everything in this world. There are a lot of things that are someones opinion. The world was once flat and everyone believed it. Just because we can't see it, hear it, or have an explaination that meets what we've been told by other people doesn't mean it isn't or can't be true.
Jerry

 

[MississippiMetal]

An emitting cathode is referred to as "hot" quite credibly in many instances.

Heater current nor voltage changes to any notable degree when the valve is under signal. Cathode current is influenced by the amount of "pull" by the more positive element (grid, and then on to the plate). AC or DC applied to the filament makes no difference. Also, "AC Current" is kinda redundant. Alternating Current Current.

You are correct in saying they do not give you your signal alone. Your input voltage swing is what gives you your output signal. Why?

When the grid signal swings high, current flows through the plate load and creates a voltage drop across that resistance. Less voltage = more current. The larger the resistance, the larger the voltage drop across it, and the greater the current flow for given input signal. The tube is, in effect, opening up. Just like a valve. That's why I like to refer to tubes as valves, because it more accurately describes their function. This occurs because, in keeping with electron flow, even though the plate has a high voltage on it, it's not as high as the B+ rail. The plate load makes the plate less positive than the B+ rail, so electrons will flow toward the most positive charge. If there is no current flow, then there is no voltage drop across the resistance and plate voltage is equal to B+ voltage. The grid controls acceleration of the electrons to the anode by way of its electrical reference.

This is not me trying to "educate" you or thinking you don't understand this, but rather me demonstrating my fundamental understanding of the theory to clear things up.


Jerry, thank you for your constructive input. I'll consider it.

 

[glpg80]

An emitting cathode is referred to as "hot" quite credibly in many instances.

Heater current nor voltage changes to any notable degree when the valve is under signal. Cathode current is influenced by the amount of "pull" by the more positive element (grid, and then on to the plate). AC or DC applied to the filament makes no difference. Also, "AC Current" is kinda redundant. Alternating Current Current.

You are correct in saying they do not give you your signal alone. Your input voltage swing is what gives you your output signal. Why?

When the grid signal swings high, current flows through the plate load and creates a voltage drop across that resistance. Less voltage = more current. The larger the resistance, the larger the voltage drop across it, and the greater the current flow for given input signal. The tube is, in effect, opening up. Just like a valve. That's why I like to refer to tubes as valves, because it more accurately describes their function. This occurs because, in keeping with electron flow, even though the plate has a high voltage on it, it's not as high as the B+ rail. The plate load makes the plate less positive than the B+ rail, so electrons will flow toward the most positive charge. If there is no current flow, then there is no voltage drop across the resistance and plate voltage is equal to B+ voltage. The grid controls acceleration of the electrons to the anode by way of its electrical reference.

This is not me trying to "educate" you or thinking you don't understand this, but rather me demonstrating my fundamental understanding of the theory to clear things up.


Jerry, thank you for your constructive input. I'll consider it.

never heard of a cathode termed hot in that manner, the hot references to the needs that have to be met for electrons to flow - the cathode in general will in fact be low voltage and not necessarily "hot", although the surrounding heater pins will be. i would not suggest touching any of it to find out

there is always a voltage drop across the plate resistor when no signal or a signal is present - how much depends on the bias of the valve and like you mentioned, the alteration of the input sin wave.

also dont forget that the output is not in phase using class A cathode bias and tapping off of the anode (collector). when a sin wave input goes high you will get an inversion at the output with an amplitude gain depending on the tube itself. never is the B+ = to the anode of the tube if a plate resistor is used. the current through the resistor referencing a voltage across the resistor supplies the limitations and operating voltage that the tube sees at any given point in time and references accordingly the inverted sine wave from this point.

 

[Jimmie]

Here's a few things to look at.
Audio Cyclopedia pages 470-471

Merlin Blencowe's book on Designing Tube Preamps for Guitar and Bass page 32 on the Miller effect snip

You can consider Miller's Effect, or perhaps it would be easier to understand by looking at the Field Electron Emission of a 12AX7. Am only guessing here, but I suspect somewhere back in history, someone came up with the idea to control the high amount of Miller's Capacitance associated with a 12AX7 by putting a metal canister or barrier around the preamp stage tubes, and in doing so, slightly altered the tube's flow characteristics and tone. While the barrier may isolate the stray fields emanating from the tubes thought to be potentially effecting other components within the circuit, the barrier also becomes a capacitor in itself once its anchored to the chassis.

But its late on a Friday night after a long and grueling week, and thats all I've got for now...

 

[RoidRage]

There is another reason that tube sheilds on/off make a difference in sound... VIBRATION!

This is also why some people get inconsistant results, and hear different things or - nothing at all.

It depends on the particular tube, the sonic charactoristics of the AMP, and the Sheild...

Case in point. There are the sheilds with FELT under the spring, and ones without... NOW if the tube is vibrating or ringing, and you put on one of these with felt it will DAMP the tube and stop the ringing
Making it smoother, and a little more polite... Now that could be a good thing, but it also depends on the amp and the players taste!

See if the amp was a little lame to begin with.. THEN this would make it WORSE... If the amp was messy sounding, it would make it better, and it also depends on what the player wants...

I have been a Audiophile for years and years... (Expensive Stereo), and Tube At That. And we listen to everything INCLUDING ELECTRICAL SOCKETS IN THE WALL! I was one of the first to do that
There was a time when people thought wires are wires, and they all sound the same! Of Course almost anyone knows this is not true now... But the audiophiles knew it 30 years ago.

Bottom line is "SEASON TO TASTE" ! Try Things, and TUNE TO YOUR LIKING..... Try different Sheilds, on/off, and everything else... ELECTRICAL CORDS ARE HUGE!!! AND I DON'T WANT TO TELL YOU
HOW MUCH I SPENT ON THOSE FOR MY HI-FI!!!!

Almost Anything MAKES A DIFFERENCE!!!! AND YES DAVE FRIEDMAN!! EVEN THE DIRECTION OF THE FUSE.. AUDIOPHILES KNEW THIS BUT ALMOST NO ONE ELSE...

YOU KNOW WHAT THAT TELLS ME... DAVE FRIEDMAN HAS HIMSELF SOME SET OF EARS... NOW I'M REALLY GLAD I SENT HIM AN AMP!!!

Carry On...

Roid RAGE

 

[Nigel Tufnel]

absolutly not any capacitence difference ,"electron flow" whatever is so minor it cant be heard by the human ear,it is all voodoo ,the sheilds are there to protect the tubes and most importantly....to hold them in so they dont fall out ,..thats it ,if your amp is made so the tubes hang upside down,..leave them on,..if not take'm off ,but if your head is on top of your speakers like everyone elses i would say leave them on so you dont shake your tubes loose

Maybe you don't have sensitive ears... I can hear it with my Marshalls. I do agree that if an amp has tubes upside down, leave the covers on. In my 28 years of playing I have never seen a tube pop loose

i have seen several in the shop,in the past 2 years alone, where the tubes are laying on the bottom and a guy saying"i dont know what happen it just stoped",my ears are sensitive and i tested more models than you'd beleive every amp is different and any difference is so minor its not worth leaving them off,if you need that minor difference in the the amp,...your playing the wrong amp, i've been playing 30+ years and have seen it happen even to my own amp let alone the hundreds of repairs i've done

I'm really not trying to be jerk here so please don't take it the wrong way but if the tubes are falling out the sockets should be re-tensioned or replaced if needed. Surely you wouldn't rely on such a crappy connection as a spring loaded tube shield pushing the tube down anywhere else in the amp. I would bet that tightening up those tube pins would make a more noticeable difference in tone or noise than a tube shield ever would. Now if the guy is playing a dimed 2x12 Twin every gig then the tube shields would make for some good extra insurance but there probably isn't anyone left within ear shot of the damned thing to hear it when it finally shakes a tube loose. Lots of guys here are playing heads with the tubes up, so the falling out thing is no issue and if the sockets are good they aren't gonna rattle loose.

 

[Nigel Tufnel]

capacitance

capacitance

capacitance

I swear if this board got any broader it wouldn't just smack you upside the head it would smack you from head to toe. Seriously, this is WHY. Can you you hear it or do you give a shit that is fully debatable. I get that some of you guys really need some hard scientific white paper kinda proof to get behind something and that's cool. I like science very much and while I appreciate it and use it on a daily basis I also believe in the human experience. Not everything that we perceive can be quantified by science, then of course everyone perceives things differently. The human ear is a very strange thing, I'll pretty much leave it there.

 

[Meathead]

I had a few friends over today and we took the preamp tube covers off of a JCM800. All of us immediately noticed a difference.

How would you describe the difference? What exactly did everyone hear?
I've never compared having the preamp tube covers on/off... and several of my amps have preamp tube covers that I leave on.

 

[Meathead]

I had a few friends over today and we took the preamp tube covers off of a JCM800. All of us immediately noticed a difference.

How would you describe the difference? What exactly did everyone hear?
I've never compared having the preamp tube covers on/off... and several of my amps have preamp tube covers that I leave on.

With them off, some of the high end is slightly cut and the amp got a little more chewy. We actually heard a change as each tube cover came off.

That's interesting. I haven't tried it, but I don't doubt that it could make a bit of difference. Something is actually touching/dampening the tubes, then the tubes are free.

I have an EF86 preamp tube in a Vox (which is more prone to microphonics and vibration compared to a 12ax7) and with/without the teflon tube dampeners I can hear some difference quite easily.

 

[Heritage Softail]

I am leaning toward having one or having two beers will make a bigger tonal diff... just sayin...

 

[Jimmie]

I am leaning toward having one or having two beers will make a bigger tonal diff... just sayin...

Yep, or the hard stuff, even more so

 

[parntz145]

Hey Guys, I was at Daves house today with Joe (Immeresed). I bought that mint and I mean mint 83 JCM800 off of Joe and since he is local we all met at Daves so I could pick it up. Joe thanks for a great deal. So we were all playing and decided to do a little test on my new Marshall with the tube covers. First off I have been removing them for years and always liked the tone better. Put it this way. Someone here said that it was a mouse fart of a difference. I dont think so, if you dont hear a difference you have got to be deaf. You notice a difference right away. It more like a bear farting in your face of a difference.

As a matter of fact we took the V1 cover off and heard a difference. Then we took the V2 cover off and heard more of a difference. Im not saying its better or worse because everyone doesnt hear things the same, but you cant say that you dont hear a difference between the two. I happen to like the tone with the covers on this amp. Sounded tighter to my ears. If you dont believe it give it a try.

 

[Jay Strange]

I had a few friends over today

male? picts?

Clipzz?

 

[70strathead]

I don't have covers on mine because i dumped them years ago in my Marshalls from going through the period of swapping preamps tubes out with others. Thank God I'm over that ! Im not gonna even touch this one. if it aint broke dont fix it..

 

[Meathead]

I would like to attend your next all male preamp tube cover removal party.

<removes cover from v1 tube> ...giggles and thumbs up signals
<removes cover from v2 tube> ... high fives and butt slaps
<removes cover from v3 tube> ...everyone begins rubbing lotion on their bodies

 

[glpg80]

I would like to attend your next all male preamp tube cover removal party.

<removes cover from v1 tube> ...giggles and thumbs up signals
<removes cover from v2 tube> ... high fives and butt slaps
<removes cover from v3 tube> ...everyone begins rubbing lotion on their bodies

 

[kirk]

absolutly not any capacitence difference ,"electron flow" whatever is so minor it cant be heard by the human ear,it is all voodoo ,the sheilds are there to protect the tubes and most importantly....to hold them in so they dont fall out ,..thats it ,if your amp is made so the tubes hang upside down,..leave them on,..if not take'm off ,but if your head is on top of your speakers like everyone elses i would say leave them on so you dont shake your tubes loose

Maybe you don't have sensitive ears... I can hear it with my Marshalls. I do agree that if an amp has tubes upside down, leave the covers on. In my 28 years of playing I have never seen a tube pop loose

i have seen several in the shop,in the past 2 years alone, where the tubes are laying on the bottom and a guy saying"i dont know what happen it just stoped",my ears are sensitive and i tested more models than you'd beleive every amp is different and any difference is so minor its not worth leaving them off,if you need that minor difference in the the amp,...your playing the wrong amp, i've been playing 30+ years and have seen it happen even to my own amp let alone the hundreds of repairs i've done

I'm really not trying to be jerk here so please don't take it the wrong way but if the tubes are falling out the sockets should be re-tensioned or replaced if needed. Surely you wouldn't rely on such a crappy connection as a spring loaded tube shield pushing the tube down anywhere else in the amp. I would bet that tightening up those tube pins would make a more noticeable difference in tone or noise than a tube shield ever would. Now if the guy is playing a dimed 2x12 Twin every gig then the tube shields would make for some good extra insurance but there probably isn't anyone left within ear shot of the damned thing to hear it when it finally shakes a tube loose. Lots of guys here are playing heads with the tubes up, so the falling out thing is no issue and if the sockets are good they aren't gonna rattle loose.

right thats why i get them to change the sockets cause they are worn and tubes are falling out because they "think" it sounds better without them but i get guys who wont let me change sockets because then the amp isnt"original" yes thats what they say so i tighten up and tell them,"it doesnt change the sound,use the covers" those twins are loud and playing them at hi volumes can shake a tube right out of the socket beleive me or not,i've lost count of how many i've seen ,...no offense taken everyone has opinions thats why we come here,to hear everybody's thoughts,..i don't claim to know everything so i'm always leaning something new,i once read lee jackson say he used to replace alot of bias pots in fenders ,i've seen hundreds of fenders and usually they have the original bias pots in them,and i've never had a bad one ,..ever,not to say lee is wrong maybe its the climate where he was living or all his repairs were guys who did somthing to them,just that it hasn't happen to me doesn't mean it didnt happen, like the marshall up right tubes,ya i've gotten amps in where the customer says he didn't do anything and the cover and the tube are rolling around in there ,stick the tube back in and the cover on and it plays fine,...go figure

 

[johnnyjellybean]

Clips! We want clips!

 

[steve_k]

I would like to attend your next all male preamp tube cover removal party.

<removes cover from v1 tube> ...giggles and thumbs up signals
<removes cover from v2 tube> ... high fives and butt slaps
<removes cover from v3 tube> ...everyone begins rubbing lotion on their bodies

 

[kirk]

ya clips,...all you guys say it makes a difference?! show us,put up some clips