Tube Too Close to Speaker Magnet??

[David Root]

Is this likely to be a problem? I have some steel tube shields.

[JamesHealey]

dunno I had my output transformer and two 6v6s touching the speaker magnet on a combo i made and gigged for months before dismantling it.

never caused a problem.

[iknowjohnny]

I always use EV12L's and i always use 1-12 combos. So you can imagine i am no stranger to this issue ! I have had to even make new baffleboards to fit the EV in some amps, and i have had tubes touch the magnet even. And in most cases they were at least close. Not once has it caused an issue i'm aware of. I wouldn't worry about it. I began the EV love affair in around '90, so i gigged like that for about 15 years before i gave up playing out. I still have a combo with the tubes very close to the EV in fact.

[dynaman]

That looks pretty close, but I bet you'll be OK. I went through this a couple months ago. I had some 6V6s that I was able to keep about 1" away from a big magnet with no ill effects. However, I did initially anticipate having probs so I experimented with using a plate between the magnet and tubes.

I wound up positioning an aluminum plate about 1/2" away from the magnet and held a small steel screwdriver directly on the other side of the plate. The plate did nothing and the magnet was able to give a strong pull on the screwdriver with ease.

I tried the same with a thin steel plate and it worked great. The plate itself had a strong attraction to the magnet, but there was nearly no pull on its other side. I guess the steel "absorbs" or at least redirects the pull.

I did a bunch of reading on magnetic shielding and came to the conclusion that there's no practical way to do it. Ya simply gotta redirect the pull. I did learn about some special tape for shielding. I don't what it cost but you may be able to wrap a tube shield with it.

Since using a steel plate prolly would have worked for me, maybe you can just try one of those old steel tube shields. I don't think an aluminum shield would do much. If you do have probs, I can send you an old steel noval shield.

[dynaman]

Does the magnet actually pull much through the cap?? I can't help but think that cap not only keeps dirt out, but also cuts down on the magnetic pull on external objects.

[David Root]

Thanx guys, this all sounds good. I have the steel shields, and I also dug around in my noval sockets and found three shield base ones with steel base material, so I think I have that covered.

There isn't a real strong pull thru the cap, it's actually quite weak, so I am hoping to get away with it without using shields, but if I have to, I have 'em in all steel construction.

That "special tape" is mu-metal tape, and it is expensive! But it is hands down the best material for magnetic shielding. It doesn't sound like I will need it, thankfully.

[67plexi]

David, This is what Fender did.

[paddy]

Check out this thread.

https://tubeamparchive.com/viewtopic.ph ... ht=#145639

Cheers
Paddy

[Structo]

Yep, we've talked about it before and I thought the consensus was it depends.

I think I would worry more about the heat being directed at the speaker and magnet, since I believe a lot of speakers (ceramic) have their magnets glued on.

How do they shield speakers that are near a CRT or other sensitive stuff?

Don't they use steel sheet metal?

[David Root]

Thanx again guys.

The '59 Bassman pic shows a plug magnet speaker opposite the preamp tubes. The back of the magnet is probably 2" away from the tubes and so the interaction would be relatively less I would think, even without a bell. The G12 I'm using is a much bigger magnet but it has the big steel bell on it, so......??

As far as heat from the tubes is concerned, the power tubes are far enough from both speakers in this combo arrangement that I don't think that's a problem. These are only four 6V6s in my case, not 6L6s.