Hot Matchless amps

[e_c_mccollum]

Wow!!! I haven't been on this forum in a long time.

I know I will get honest answers here about this question. I currently have a customer that avoids to see the problem about putting a Matchless Chieftain combo amp inside a isolation cabinet with a microphone for recording purposes. No fan. No ventilation. Amp has been left on once damaging it, and again further damaging it. I rejected it stating send to Matchless.

Am I right by saying components would burn up, transformers possibly get damaged, sockets, and tubes would get damaged............??? I have been working on tube amps for quite a while and never seen such carelessness such as putting in a isolation cab with no ventilation. They used to mic them in closets to be used on the main FOH system.

I got a Matchless DC-30 from the guys brother that was left on overnight in an isolation cab. Fixed. Second time got operational, but noticed having soft clipping on the output and cant figure out why. New output tubes. No Oscillation that I can see. Blew one of the coupling caps for output stage. Was leaking voltage and blowing tubes. Replaced both coupling caps. I feel this amp needs to be sent to Matchless also. Never trusted overheated equipment.

Thanks for advice.........

[Dave H]

I've never seen a Matchless Chieftain but Google tells me that the chassis is at the top with the tubes hanging down meaning all that heat is going up into the chassis. It's in a box. You could turn it upside down so the heat rises away from the chassis.

[e_c_mccollum]

Yes, and it uses EL34's. When I got the amp, the tube bases were black!! Not good. Problem is an iso cab is a box that's padded with acoustic insulation. You close it up only hearing the amplifier using the microphone placed inside. Usually only a speaker is placed in there using an external amplifier.

Sounds like a fire hazard having foam insulation with hot tubes and no ventilation. Believe me, they have done it to two separate amps and both got damaged due to leaving left on overnight. The biggest issue the chieftain is power output. It not up to spec and clips early. Changed tubes with no luck. I think needs to be rebuilt. I do know heat kills parts, but what about transformers?? These are cathode biased amps and Matchless loves to run them HOT. I would certainly think could get runaway output tubes possible with all that heat damaging transformer. And the fuse didn't blow! Like the say, the circuit protects the fuse. Sometimes.

Curious what thoughts are of doing this and how it can damage the amp.

[Zozobra]

You can fix the amp but you can't fix stupid. Tell them to buy a reactive load box and to quit trying to burn their house down.

Perhaps the caps have taken a beating from the excess heat?

[Mick Bailey]

Even without enclosing Matchless amps, they run way too hot for my liking - especially the EL34 amps. One of the problems with these is their method of isolating the tube sockets. That liner and the mounting grommets means the phenolic tube bases get much hotter than in a regular amp. I used to get regular calls with these amps until I installed regular sockets and advised the owners to religiously switch them to standby when not actually being played.

With a cooked amp you're down to establishing that the DC voltages are correct and then scoping to find out where the problem lies. These are well-made amps with durable components so you don't want to be lifting or replacing anything you don't have to. I would begin by making sure the output tubes are biasing correctly. Sometimes the sockets get so hot they break down and start to leak voltage onto the grids and skew the bias. Then check what the signal looks like at the output tube grids to see if the early clipping is present there. A failed transformer would be way down on my list at this stage.

[alexradium]

yes they are very hot biased ,but inside an isobox any amp would cook ,that's where the head cabinet wins,so i would separate the chassis and make a headshell to place it outside.

[bob p]

Iso cabs are for speakers not tubes.

[The Dude]

...... Usually only a speaker is placed in there using an external amplifier......

There's your solution, right there. Use an external speaker with the amp and leave the amp outside of the iso-box.

[e_c_mccollum]

Exactly! Also suggested taking the head out of the cab and running outside the iso box. But what can you do when customers don't listen.

[e_c_mccollum]

Mick Bailey:

Yes these amps are way too hot. Push them to the max and then some. Second time I had the DC-30, I changed out the 62ohm bias resistor and installed a 72ohm instead to see if made a difference. It did calm down a little. These amps have a very interesting output stage that uses no feedback loop. I agree the transformers would be last on my mind and sockets should be replaced. I agree these amps are very well built, but did fry a 2watt 1.5kohm resistor on the output stage. It was also arcing to ground near that el84 tube socket chassis mounting bolt same side of screen resistor.

When I first got his DC-30, it did have a fire inside. One of the terminal strips had a hole burned in it. Was arcing plate voltage to chassis. I believe all this heat may also be breaking down the terminal strips. The original speaker was also damaged measuring open right at the leader wires going into the coil. Taking off the dust cap carefully scraping some varnish off the tiny copper wires going to coil also showed nothing. I thought just something simple broke in the speaker, but finally after autopsy of speaker revealed slight rubbing on the coil. May be speaker wasn't centered perfectly and with the current/heat the copper wire on coil expanded??? Your right about how tube sockets are mounted. The amp did have parasitic oscillation that was corrected by tightening the mounting screws a little.

What I don't understand is why someone who buys expensive amps would not want to spend the money to repair them. Customer wants cheapest way out. To be honest, getting rid of them was better than being married to them. Especially way they are being ran.

[bob p]

But what can you do when customers don't listen.

Don't fix their amp.

[alexradium]

Don't fix their amp.

or double the bill.

[Rhodesplyr]

I've heard of burning up guitar amps in isolation cabinets from several techs, so it's apparently a recent "innovation." I've heard of it in the context of contemporary worship churches. Apparently someone is going around selling them on the idea as a way to get the sound they want while keeping stage volumes down.

My brother used to see this in connection with SS power amps in dance clubs where, despite his warnings, they'd put the power amp in a box with no ventilation. Or they'd never clean them, so that they were completely clogged with dust and dirt.

Matchless amps are mostly based on Vox designs, and, as we know, the 60s Vox amps ran hot. That's the heritage.

[e_c_mccollum]

Warped plastic from all the heat. Says it all.

[J M Fahey]

I repair them and they hot bend plastic panels even *outside*, sitting on top of cabinets


notice the heat bent top panel, just above where the power tubes stand.

I always replace the 4 cathode coupling caps (the black ones straight in the center of the chassis) 1 or 2 times a year, because they dry up and sound becomes weak and mushy.
I use very good EPCOS ones, but they are 105C rated, i bet itīs hotter in there.

I installed a small 12V PC supply fan fed from rectified 6.3C , so it does not exactly *blow* air inside the cabinet (cooling tha chassis in the process) but gently "renovates air" a couple times a minute, enough to tame things a lot ... but after a couple rehearsals the Guitar Player cut the wires with a snipper ... he claims "it sound bad" which of course is ridiculous.

You canīt fix stupid.

At least he uses it "outside" , and driving a 4 x 12" Marshall cabinet.

My customers love to play LOUD!!!!! ... no cheesy attenuators or isolation boxes here.
Admittedly they usually play 48000 seat Soccer Stadiums.