How close should tubes get to Speaker Magnets?

[tubeswell]

Hi all - My first time posting on this forum.

I have done some of the Aloha mods to my C30, and am contemplating others - namely Master Vol and Pseudo Presence. There is no room on the top of the stock combo amp chassis to stick more pots, but there is room for mini pots on the back (near the Clean Ch Vol), or on the chassis 'underside'.

Because of relative ease of access during playing, I would prefer to stick pots on the back rather than underneath, but am concerned about the potential threat to their longevity on the back of the chassis the way the stock setup is. So I am contemplating cutting a deeper cutaway at the rear of the top board on the cabinet (along with new screw holes) to facilitate sneaking the chassis forward 1/2" to 3/4". This will give rearward mounted knobs some protection from being accidentally knocked off, but it will bring the tubes closer to the speaker magnet. Question is, apart from the obvious rattles (which I doubt will be any worse than they are now), I wonder what this may produce in terms of normal/abnormal tube function, having the speaker magnet real close?

Tubes of concern are EL84s, and as shown in teh attached .jpg, the tubes are currently 2and1/4" away from the magnet at their closest point (the tips), and with my hairbrained scheme they would end up being about 1and1/2" away. The speaker is a Carvin BR series 100W Eminence re-brand with a 5and1/4" diam. ceramic magnet. All suppositions, theories or other conflab welcome please.

Cheers

[Rob Mercure]

TW,

Well, unless you're using a flat panel display as you read this you're probably experiencing electrons being "guided" to the desired phosphor using an electromagnetic field. But in your specific case I suspect that there wouldn't be sufficient magnetic field strength to overcome the screen grid and plate's electrostatic attraction to those easily distracted little electrons (attention deficit hyperelectrostatic disorder). While it may seem "bass ackward" you could pull the chassis and the speaker and with the speaker correctly attached to the amp, the amp fired up, and with a signal move the speaker as close as your projected distance and see if there is any change. Like I've stated, I suspect that there wll be little if any change but you might want to also do this in a darkened room to look for a "distracted little blue glow."

Hope this helps.

Rob

[stokes]

I've got a couple of clones that have the power tubes 1/2" from the magnet of an EVM12L and there are no broblems at all.They didnt start that close with the original Eminence speakers but when I switched to the EV's,which have a much larger magnet structure they are now whithin 1/2" and all I noticed was improvement.I also built a 2x10 combo for my son and put a pair of GEC KT66's that are almost touching the magnet of one of the speakers,no problems at all.

[tubeswell]

Quoting RM

"While it may seem "bass ackward" you could pull the chassis and the speaker and with the speaker correctly attached to the amp, the amp fired up, and with a signal move the speaker as close as your projected distance and see if there is any change.

Thanks Rob - You have given me an idea. I will try 1/2" and 3/4" spacers behind the speaker in the cab to test the effect of closeness in this instance, and record any differences.

Cheers

[Enzo]

Hey, you made it! Great.

If you want to prove the tubes will ignore magnets, whack the magnet off a dead speaker and hold it up against a tube. I don't think you will notice any difference. And old speaker magnets are great for holding papers to a metal surface. (8 million refrigerators can't be wrong) And for magnetizing screwdriver blades. For that matter, it doesn't even have to be a dead speaker, just hold any speaker magnet next to a tube. I just enjoy collecting magnets. The sides of my tool cart are covered with them.

I have a suggestion. Instead of hacking up the cab and moving everything, why not mount a small metal project box on the side wall - slightly back from the rear edge to allow for knobs - and run shieled cables to the main chassis. Mount your controls in the box. Make suer a chassis ground grounds the box to the chassis. This in addition to any shielding.

In fact the knobs could point to the rear in a recessed box, or they could point towards the center of the cab, and the box need not be recessed. In any case, this leaves your cab with nothing more than a couple extra screw holes on the inside.

[daz]

I have a EV12L in my peavey 30 and the tubes are practically touching it. Amp sounds great and no problems.

[tubeswell]

Yep well the project box is another great suggestion, thanks. Might have to do that. I suppose the thicker the shielded cable in that case the better (or doesn't that make any difference)?

BTW if R56 is swapped for a pot to make a resonance control, should that be a log or linear pot and what value?, And do you think there should be any resistors or caps added (for frequency tweeks)?

[Enzo]

Thickness of the cable doesn't matter. Just get good quality 100% shielding. I like the foil shield stuff - remember, in this application it won't ne flexing like a guitar cord would. Belden 8451 or equivalent.

Get the schematic for one of the 5150 PV amps and compare the resonance and presence circuits.

[Rick Erickson]

How close should tubes get to Speaker Magnets?

I think they should be friends, but I would discourage them from dating each other.

[Satamax]

Hi everybody!

Enzo, instead of al foil shielded cable, is braided shield good enough compared to the former? Thanks.

Bye.

Max.

[Enzo]

Shielding is listed as a percentage of signal lead protected. Obviously 100% shielding is best, but not always needed. Unless there are gaps in the foil, foil shields are 100% But braid can be too. SOme have a loose braid over a conductive black plastic shield.

ANy shielded cable will work if it shields well enough and fits your needs. FOil is meant for permanent installation, it is not meant for flexing like stage cables.

Nothing wrong with braided shield other than I don't want to have to untangle the stuff. SOme flexible mic wire has a single twist shield rather than braid. That is relatively simple to deal with.

[matndona]

Hot Magnets

Magnets have what's known as a 'curie' temperature- it's generally 200F - 300F for hi perm materials. You could prematurely age (weaken) your magnet if you live in the danger zone. It might be charming or not......

There aren't that many watts involved, so the shield sounds like a great idea for the long run.

[tubeswell]

Well today I chucked out the Carvin and put in a recond. 35W Plessey Alnico. Now the magnet is a wee bit closer, but the tone is to die for. Speaker has a new everything including a fancy pancy high temp coil. I haven't even run it in yet, but it has very clean bass and makes the 2/5 and 4/5 p'up positions on my strat quackier than ever before. I must say it enhances the Aloha Tone Stack mods even more. Can't wait till it's properly run-in to see how it morphs. Total price $160NZ, so that's gotta be worth the experiment- (considering a new red fang is $450NZ and a celestion gold is beyond my humble mortal capability to own). The speaker is plugged to the 16 Ohm tap socket (another Aloha endorsed thingy) which I put beneath the sockets for the 8 Ohm Tap and footswitch - This is a handy position for quickly converting the combo to a head. I will get back onto the presence/resonance/MV mods now -post results soon.

[tubeswell]

... the Aloha Tone Stack mods ... (another Aloha endorsed thingy) .

Sorry again Steve, I mean't to say Ahola. Please accept my humble grovelling apologies.

[tubeswell]

Well I've just finished and tested the MV, presence and resonance mods and I used twin-cable with braided shield and it works just dandy.