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Old 3rd October 2012, 02:20 PM   #31
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Dust has pretty well zero effect inside a TV
I was a TV repair tech in Miami from 1968 to 1971. Many houses did not have air conditioning and the humidity ranges from 50 to 100%. The 25 KV second anode HT in a color set does attract dust which collects moisture and turns to a brownish slime in the presence of ozone. At 25 KV it is conductive! GE was the first manufacturer to make the high voltage cage from a plastic material. HV + brown slime + plastic + heat and humidity = FIRE! I saw several cases. I never saw a cabinet combust though. Usually there was a burnt and melted high voltage box and severe soot damage inside the cabinet. Some of these TV's were completely replaced by GE.

From 1971 to 1973 I was the service tech at an Olson Electronics store next to the University of Miami. I was at an outdoor rock concert and witnessed one of the many Ampeg SVT flame fests that occurred in the early version amps. The band was running the amp full blast into a pair Traynor of 8 X 10" cabinets that were likely not rated for 300 watts. I heard the sound of voice coils melting and becoming unwound (rattling distortion) followed by loss of speakers. It took less than a minute before all the remaining speakers were blown followed by flames and intense smoke shooting from the amp. In a rare twist the amp wound up in my service department for repair because the local big name music shop (ACE Music) would not fix it. OPT, Power transformer, 1 or 2 6550's and several resistors were fried. The wood cabinet was not involved and just needed a little cleaning.

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sometimes tube amps (MusicMan in this case) really DO catch fire from within:
The flames seem to be centered around one of the speakers. I have seen (and caused) speakers to catch fire. It usually involved plugging them into a wall outlet though! There are plenty of examples on Youtube.
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Old 3rd October 2012, 07:11 PM   #32
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Originally Posted by tubelab.com View Post
I have seen (and caused) speakers to catch fire. It usually involved plugging them into a wall outlet though!
I to have done that

Not for a number of decades though - it's the sort of thing you do when you're young and foolish.

Better here of course, with 240V mains

IN4148 diodes go BANG really nicely directly across the mains as well.
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Old 3rd October 2012, 07:17 PM   #33
shanx is offline shanx  Canada
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Paper speaker cones right in front of a mains supplied coil short: I smell BBQ!
Now there's a fire hazard issue
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Old 3rd October 2012, 10:26 PM   #34
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It usually involved plugging them into a wall outlet though!..... Not for a number of decades though - it's the sort of thing you do when you're young and foolish.
Young, yeah I was about 19. Olson HQ had a bad habit of finding nothing wrong with some of the speakers we returned under warantee. I made sure everything we sent back was indeed quite defective.

Most Olson branded raw drivers were made by Utah or Eminence. We had a 12 inch coaxial that sold for about $15 in 1972. It was a decent sounding speaker but wasn't up to UM frat party power handling. Upon "testing" prior to return the woofer would emit one loud pop, but the tweeter would faithfully reproduce all the HF noise on the power lines until the crossover cap exploded, often several minutes.
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Old 3rd October 2012, 11:24 PM   #35
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I have built a few amplifiers inside wooden cases and not really found any problems.
So long as the input signal is in screened wire then ot works fine.
I have left the back of the cabinets open which is a shock risk but I am the only person using them so its not a problem.
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Old 4th October 2012, 02:25 AM   #36
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I have been building tube amps with wood cases for over 40 years, no problems. The only material that I have had problems with is Plexiglass. It melts when near hot tubes. Lexan and most Polycarbonate materials are OK.

I have seen some wood cabinet based guitar amps that were just plain dangerous. Granted they were old, but I have seen designs where it was posible to reach in the back and touch live parts.....no power transformer either.

I have a Park (Marshall) 100 watt amp with only one small vent across the top to let the heat out. No place for cool air to enter to displace the hot air. The wood cabinet gets hot on top, but the amp still works after nearly 40 years and still has all the original parts, except tubes.

Build it right and there will be no problem. It should be impossible to touch hot (electrical or temperature) parts. All exposed metal must be grounded. There should be ample ventillation and the IR energy radiated by the output tubes should be allowed to escape the box wherever possible.
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Old 10th October 2012, 09:25 AM   #37
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Really? It didn't shorten the life of any caps in that valuable vintage Park?

I just picked up a nice old running Super Twin chassis: 180 watts with 6 6L6s etc. and healthy-sized transformers that run pretty warm. The way Fender hangs the chassis upside-down it can get REALLY hot. This is the biggest upside-down amp I know of except perhaps the reverb version. This has no reverb or tremolo or effects loop or balanced direct out, one channel, really nice tone controls, then a 5-band rotary graphic EQ (with 5 inductors) but little usable guitar distortion. People are discovering they are better bass guitar heads than combo amps! But in the original big deep open-back combo case they got lots of ventilation. When people cram them into a standard Fender head case I can't imagine they get enough cooling air. I really don't want a fan. So I'm planning a taller cabinet, I imagine about like a Fender Super Showman with Reverb (which I don't have the exact dimensions for). And it will be a little deeper, like the combo, so the air flow out the upper vent in the cabinet's back plate will get around the chassis a bit better, and I'll make that upper cutout in the back plate slighty oversize for better airflow and for easier access to the speaker jacks etc. which the deeper cabinet will cause to be deeper recessed, and I'll relocate the lower two oval air intakes in the back plate a bit lower and enlarge them slightly, and I'll add some intake vents to the bottom of the cabinet, slightly taller feet to permit airflow from beneath, and the front of the cabinet will have two rectangular cutouts behind the grille cloth so that some air might actually come thru the grille. I'd replace that grille cloth with an open-mesh metal grate if it didn't look so industrial. I might even add some vents and grates to the sides of the cabinet. I think just making the cabinet a little taller will improve the chimney effect and improve air circulation in & out. Any other good ideas, short of a 'vertical' cabinet? Can I ventilate the INSIDE of the chassis via some vox or marshall style vents in the top of the cabinet, or will it accumulate too much dust or smoke & grease inside, or make the pots get dirt sooner?
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Old 10th October 2012, 02:08 PM   #38
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Really? It didn't shorten the life of any caps in that valuable vintage Park?
I don't know how much use the amp really saw. The story that I got is that it was found in a storage area inside an Atlantic City casino that was being renovated. It was found sitting on top of a pair of Altec A7's and a pair of the matching bass bins (B?).

The entire contents of the casino wound up at a scrap/surplus dealer in rural Georgia. The stuff was sitting in a large warehouse with broken windows, resulting in a thick coating of pidgeon crap all over everything. That is where I first saw it. I bought some tubes from the guy who ran the surplus yard. He didn't want to sell the amp because he was going to fix it someday.

Ten years later a surplus dealer friend bought the entire estate after the owner passed on. He plugged in the amp and it blew a fuse, so ge gave it to me. It had an extremely gassy (bright purple) 6CA7. I plugged in 4 EH EL34's and let it rip. I have put about 2 hours on it, with at least 1 hour at full volume. No signs of leaky or bad caps, so I boxed it up. I will eventually sell it and it is probably worth more as an unmolested original.

I could get my user name revoked if I told you what I am using for a guitar amp right now.

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Can I ventilate the INSIDE of the chassis via some vox or marshall style vents in the top of the cabinet, or will it accumulate too much dust or smoke & grease inside, or make the pots get dirt sooner?
It depends on where you want to use the amp. Ventillation will improve the life of the components, but may not be the best idea if the amp will be used for gigging, especially in smoke filled bars. A brown sticky slime results.

My big amps use a PC fan that was intended for 12 volts, but run on rectified heater which is about 8 volts with a Hammond power transformer. The fan intake is filtered with the foam filter that is found in the plastic drive bay covers on cheap PC cases. Air goes in under the deck, up through the chassis around the tubes, and out through the top (no filter).
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Old 10th October 2012, 11:14 PM   #39
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Originally Posted by ingenieus View Post
Hi all

It surprises me that guitar amps use wooden cabinets to house the electronics. Wood is great for a speaker cabinet, but when heat is applied to it the results can be disastrous if care is not taken. Then there is the issue of grounding.
The way it has been done for 50 years is this:

1) the electronics is build inside a metal chassis. The chassis is grounded. Then the amplifier as a unit is mounted into a wood cabinet.

2) The back of the cabinet is left open (or mostly open) so the tubes can cool.Remember that tubes cool by radiation. For that to work the tube needs to "see" a clear path into the rest of the room.

Some boxes have small vents, but remember again tubes cool by radiation. There is no other way to transport heat across a vacuum. The glass is almost transparent to the radiation to it just shines onto the inside of the wood box. The box has a large surface area and re-radiates the heat.

Don't think for a minute that a tube amp is cool led like a cheap tower PC where heat sinks heat the air and then a fan blows out the hot air. It is not like that. The tubes are MUCH hotter than a CPU heat sink so it is radiation, not convection that cools a tube amp. (I said "cheap pc" because the better ones will us air ducting or conduction.)


About wood and heat: Some early Chinese spacecraft where designed to drop parts back to Earth. They actually used solid oak heat shields. The wood would turn to charcoal and slowly burn but it only had to last a few minutes. millions of years of forest fire and Darwinism combines to make oak reasonably heat resistant.

Last edited by ChrisA; 10th October 2012 at 11:30 PM.
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Old 11th October 2012, 01:48 PM   #40
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Some boxes have small vents, but remember again tubes cool by radiation. There is no other way to transport heat across a vacuum....Don't think for a minute that a tube amp is cool led like a cheap tower PC
Tubes do indeed radiate most of their excess heat in the form of IR radiation. The glass is not perfectly transparent to IR so it will absorb some IR and get hot. Much of the IR will impinge on metal components inside the amp causing them to get hot. Some of the heat will be conducted out of the tube via the base.

The OPT in a tube amp dissipates a few watts at most due to resistive and magnetic losses, yet it gets hot. Black ones get hotter that silver ones. This is absorbed IR radiation and it will raise the internal box temperature considerably, especially in a poorly ventillated box.

All of the conducted and reasborbed heat in a tube amp can be cooled by air flow. Heat generated by transformers and resistors can too. It is not necessary to establish a full cooling system in a tube amp like found in a good PC, but a little air flow will make a big difference in chassis temp.

I have built a few tube amps in Lexan cabinets. A small fan can be the difference between a melted amp and a cool runner.

Note: Don't use Plexiglass! It melts even with a fan.
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