Proximity of output tubes to speaker magnet

[dynaman]

Reworking a small combo amp. I'd like to horizontally mount a quad of 6V6s and a couple of them will wind up having the bottom edge of their glass about 1" directly above the magnet ring. Too close??

Please and thanks.

[PCollen]

Reworking a small combo amp. I'd like to horizontally mount a quad of 6V6s and a couple of them will wind up having the bottom edge of their glass about 1" directly above the magnet ring. Too close??

Please and thanks.

a) http://music-electronics-forum.com/t18428/

b) https://tubeamparchive.com/viewtopic.php?
t=7045&view=previous&sid=27ab532438b03ff4733e68c3bb2a8a36

c) http://www.thegearpage.net/board/archiv ... 95850.html

Now I'll bet you're confused, hey ? Personally , I wouldn't worry about it.

[dynaman]

Yep, I am confused. I just held a little screwdriver at the position a tube would be and the magnet pulls pretty good. I don't know if that means squat. Normally, I'd just jump right in and start drilling holes, but the amp isn't mine and I need to get it right the first time.

I also read that there is no true shield, just stuff that will redirect. I tried an aluminum plate thinking I could "reroute" the field, but its permeable and does nothing. I tried a thin piece of sheet steel and that worked pretty good. The magnet pulled hard as heck on the plate, but my screwdriver on the opposite side was hardly affected by the magnet, as if the plate was" soaking up" the magnetism. Dang, mebbe I can cut some sheets of scrap and sell 'em as "High Fidelity Magnetic Sponge Plates".

Better yet, I think I'll just stick the amp in the box and worry about it later.

[paddy]

On one of the links that PCollen has provided, you'll read Tubeswell's post which tells of problems he encountered with a Musicman.
I had the same trouble with one of these. I had replaced the speaker with a larger one, which decreased the space between it and
the fat bottle 6CA7's. This caused the amp to sound fairly crappy. With the chassis out of the cab it sounded fine. The soulution was
to fit standard sized output tubes.

Cheers,
Paddy

[passfan]

I've had them a 1/4" off with no noticeable problems. I don't think the magnetic field is enough to affect the attraction of electrons inside the tube. Add more bias ;<) .

[passfan]

On one of the links that PCollen has provided, you'll read Tubeswell's post which tells of problems he encountered with a Musicman.
I had the same trouble with one of these. I had replaced the speaker with a larger one, which decreased the space between it and
the fat bottle 6CA7's. This caused the amp to sound fairly crappy. With the chassis out of the cab it sounded fine. The soulution was
to fit standard sized output tubes.

Cheers,
Paddy

Well maybe I need to go back and take a closer look at this then, thanks for the info.

[daydreamer]

I thought the other end of the tube, (round shiny bit!), was where most of the magic happens any way.

Besides, they would be parallel to the magnet polarity, and neither is in motion relative to the other. the speaker coil would be too far away to matter, and that motion would be in phase with the tubes, perhaps-maybe?. (Don't I sound all tech like )

[Philoffline]

The key consideration for me was: are the tubes (6V6) so close that you can't pull them out without bumping into the magnet?

I only realized this after the fact. Turns out they had enough room to fit, barely, and sound just fine.

[paddy]

Magnetic fields do effect streams of electrons (this is how scanning
is accomplished in CRT's, as used in TV's and CRO's).
If the internal electrodes of the tube such as the grids, screen etc. are
magnetic (I don't know if they are) then they may be affected by nearby
magnetic fields.

Cheers
Paddy

[passfan]

I believe the attraction inside the tube is voltage based and not magnetic. That being said, it's hard to argue with hands on experience:

On one of the links that PCollen has provided, you'll read Tubeswell's post which tells of problems he encountered with a Musicman.
I had the same trouble with one of these. I had replaced the speaker with a larger one, which decreased the space between it and
the fat bottle 6CA7's. This caused the amp to sound fairly crappy. With the chassis out of the cab it sounded fine. The soulution was
to fit standard sized output tubes.

[martin manning]

I believe the attraction inside the tube is voltage based and not magnetic.

True, but consider that a charged particle in a moving magnetic field experiences a force. If that charged particle is within a conductor, current is induced. Think guitar pickup. A charged particle moving within a static magnetic field experiences a similar force (it's the relative velocity that matters), and its path will be influenced by the field. Think speaker coil. The path of the electrons flowing from cathode to plate in a tube will be altered if the tube is placed in a magnetic field, and they will wind up somewhere other than the intended place.

[guitarmike2107]

I have had this problem on an amp before and ended up moving the power valve.

Apparently the magnetic field strength is inversely proportional to the distance squared from the magnet, so even moving a few millimetres can help if you have a problem

[dynaman]

I should have an inch clearance so I'm just gonna go for it. I'm squeezing the preamp for a Marshall 1959RR and 4X6V6 output into the cab and gutted chassis of a......Marshall Class 5. Tight fit!!

Thanks for all the help.