Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: hesamadman on March 06, 2015, 04:24:20 pm
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Im putting my custom amp into a 2x12 combo enclosure. I have it all laid out and even started the build. I noticed that once I get the amp inside the enclosure with the speakers, the tubes will be about 1 inch away from the speakers. (the distance from the back of the speaker to the tube surface)
I didnt know if somehow this would cause me any interference of any kind :w2:
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My gut tells me the speaker magnets could possibly attract electrons being that close?
Do you have another amp you could put some speakers up to and see what happens?
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I guess I could just do some tests with some speakers and see if it makes a difference.
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Ive only read one case so far where it is suspected to be an issue. 99% say its not really an issue. The one case was in a Laney VC30. To me, it didnt look like that amp was put together any different than any other combo Ive seen. Im going to go ahead and run with this and see what happens.
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My gut tells me the speaker magnets could possibly attract electrons being that close?
I don't know that will really be an issue. I'd be more concerned if it looks like it will be tight prior to assembly, often during assembly you find two things trying to be in the same space at once (i.e., "it don't fit").
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The 1st amp I built was a 5E3 and I had bought a 12" alnico speaker from Mojo for it. The speaker had no bottom end and flabbed out/distorted on the low E and A strings.
So I bought a Kendrick 12" ceramic black frame speaker put it in and the magnet was very close to 1 of the 6V6's.
Turned on the amp didn't sound right, looked in the back and the 1 6V6 was lit up white like a light bulb.
Had to make a new speaker baffle moving the speaker away from the tube, all was fine.
Brad :icon_biggrin:
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The 1st amp I built was a 5E3 and I had bought a 12" alnico speaker from Mojo for it. The speaker had no bottom end and flabbed out/distorted on the low E and A strings.
So I bought a Kendrick 12" ceramic black frame speaker put it in and the magnet was very close to 1 of the 6V6's.
Turned on the amp didn't sound right, looked in the back and the 1 6V6 was lit up white like a light bulb.
Had to make a new speaker baffle moving the speaker away from the tube, all was fine.
Brad :icon_biggrin:
How close would you say it was? I'm at about an inch right now but I redesigned the enclosure giving me about 2.5 inch clearance. Wondering if that's enough.
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It was back in the 90's, an 1'' or 2? :dontknow:
The Kendrick ceramic magnet was pretty big. I think the speaker was rated for 50w's?
You'll know if it's too close when you fire it up.
Brad :icon_biggrin:
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You can probably test the distance by taking a tube in your hand and moving it close to the speaker magnet.
You can feel the magnetic pull when the tube gets closer to the speaker. I think that this is the distance where you might experience some problems.
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There was a similar discussion at AGGH some time ago, may be you can find something of interesting reading there
http://www.guitargear.net.au/discussion/index.php?topic=36714.msg398597#msg398597 (http://www.guitargear.net.au/discussion/index.php?topic=36714.msg398597#msg398597)
K
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I would be more concerned by the heat than by the magnet. If a power tube by example would be one inch away from a paper cone, the paper would eventually dry out too much and start to tear off. A power tube next to a speaker magnet will never be affected, otherwise tube manufacturers would use magnets in their tubes instead of plates :l2: . Four hundred volts are way more strong than a magnet. More: a magnet would have to be oriented spacially most precisely to affect electrons in their way to the plate. Do the test: approach a magnet to a power tube of a working amplifier and see the result. A power tranny too close ( half inch ) to a preamp tube will definitely induce a hum and this is why we use shields on preamp tubes, never on power tubes.
Colas
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I always hoped to find a good reason for keeping these magnets around. I mean something besides pulling nails out of tires or just the WOW factor. I've been fascinated with magnets since before I ever started school.
So, I decided to see what effect a magnet might have on an amplifier. The test amp is my Plexi 6V6. The chassis is 12.5" x 6.5" x 2.5" (just for some kind of size scale). The magnet (I have a pair) came from a 5MW amplitron tube. This magnet causes an audible 60Hz hum to be present at the speaker when placed within 12" of the chassis. Moving closer and orientating the magnet in different positions causes the hum to become objectionable. The hum is being generated in the preamp only, not the power amp. I have a Marshall style pre PI master volume control that will kill the hum when turned to zero. The hum is very measurable across the speaker jack (that's how I know it is exactly 60Hz).
Anyhow, now I know that a strong magnetic field can have an adverse effect on an amplifier. My gut feeling theory for this is that in the presence of the strong magnetic field, the AC current flowing thru the filament wires and filament is inducing a 60Hz voltage into the sensitive grid structure of the tube. I'm not sold on this theory but it's the best I can come up with at the moment. You guys got any other theories?
Here's the test rig...
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Ciao Steve
to me, humm from a magnet, is a strange to happen chance and I'm not able to think why this can happen
have you a smaller magnet as to be able on focusing the source of the problem moving it around on the amp ?
I don't mean a very small magnet, but your is really an humongous one
I think one large as your DMM will be enough if not too big
Franco
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I used the biggest magnet I had because I didn't want to wonder if the magnetic field were a little bit stronger would it have more of an effect. I'm convinced.
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I'm convinced.
I believe you
Franco