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Post subject: Twin Reverb Overheating
Posted: Thu Nov 10, 2011 6:26 am
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I have noticed that my "65 Twin Reverb gets pretty hot when I play for an hour or so.The volume has also gotten lower but will come back on its own.As far as I know this amp is original except the tubes have been changed over the years.The sound is still good when it comes back on.I realize that with an amp this old ,almost anything can cause the overheating.Does anyone have any suggestions as to the cause?
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Post subject: Re: Twin Reverb Overheating
Posted: Thu Nov 10, 2011 9:01 am
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There could a number of issues at play, given the amp's age and (presumably) a lack of regular maintenance over the years. I'd look to the power supply first -- specifically the diode stack, the filter caps, and the biasing circuit. Most thermal problems begin there.

HTH

Arjay

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Post subject: Re: Twin Reverb Overheating
Posted: Thu Nov 10, 2011 9:39 am
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" volume has also gotten lower but will come back on its own. "

This kinda sound like parasitic oscillation. A form of instability that causes lower and rising of volume and production of excess heat. Amp needs a good looking over. Often, it only take contact cleaning of sockets, jacks, plugs & checking wiring position and soldering points. Sometimes a scope and signal generator (good amp tech) is needed to narrow down the instability section.


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Post subject: Re: Twin Reverb Overheating
Posted: Thu Nov 10, 2011 1:32 pm
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What do you call " hot " ? Is it a "new " amp for you ? Or you have it since few weeks and you think is too hot but you never have Twin reverb before ?

With four 6L6 , heating is normal

I will first check tubes .

1- Power tubes check and check also their bias; hot bias = hot tubes = hot amp = shorter tubes life and sometimes sound problem
2- Cleaning socket tubes contacts . dirty = sound problem


Checking circuit for sound problem ;
1 -Checking for bad solder , many of those old amp have cold solder= sound problem

Speakers are originals and not lower impedance that needed ? lower impedance = tubes / transformer working hard = heating


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Post subject: Re: Twin Reverb Overheating
Posted: Thu Nov 10, 2011 2:16 pm
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Thanks for the replies.This is an old amp but it is new to me.I know how to change tubes and speakers and a few simple things but I know nothing about electronics.The overheating problem is occurring on the side where the 6L6 tubes, on-off switch and ground switch are located.The normal side remains cooler.Everything works and the sound is good.I'm just concerned that I might damage it by playing through it.
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Post subject: Re: Twin Reverb Overheating
Posted: Thu Nov 10, 2011 3:26 pm
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Ahhhhhhh !!!!

Ok now I know your story , all the story,

Heating ; as you say and I don't see ANY PROBLEM. Heating is normal with four 6L6 in a almost close box.

The sound problem ; Cleaning socket pin first. Remove all tubes (6L6 , 12AX7's ) and plug it many times . This will celan tubes pins and socket contacts. If you have contact cleaner to spray on tube pins before , the job will be better.
Tubes must be cold.

Good chance that your amp will play better now
If not ; check solder inside.


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Post subject: Re: Twin Reverb Overheating
Posted: Sat Dec 10, 2011 7:01 am
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I have cleaned the tubes and sockets with cleaner spray and also the jacks both input and speaker.The amp still has the volume problem.After I play through it for 20-30 min,the volume lowers and the sound changes.It becomes thinner.When I flip the standby switch, it will go back to normal and usually if I just keep playing, it goes back to normal on its on.This is the same on all channels.Could this be a tube problem?
Thanks


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Post subject: Re: Twin Reverb Overheating
Posted: Sat Dec 10, 2011 8:54 am
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The likely problem is an issue with the power supply or bias supply circuit.

Arjay

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Post subject: Re: Twin Reverb Overheating
Posted: Sat Dec 10, 2011 10:57 am
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I agree with Arjay. You have some dirfting going on with the amp. It could also be the grid stopper and/or screen resistors are not doing there job to help stabilize the 6L6GC power tubes. Giving you some kinda parasitic oscillation problems. In any event, may need a good amp tech to check over the amp.

Check out Gerald Weber blurb on the dreaded parasitic oscillation problem:

http://books.google.com/books?id=fvUVBP ... &q&f=false


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Post subject: Re: Twin Reverb Overheating
Posted: Tue Dec 13, 2011 10:19 am
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I also agree with Arjay and BMW. Make sure that the power tubes aren't red-plating or biased too hot. Which brings you back to the bias circuit. My Twin ran pretty hot. The quad of 6L6 power tubes as well as the power tranny can toss some decent heat. I also would let a good tech give it the once over. If it never had a cap job, it's seriously due.

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