Twin Reverb problems

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Twin Reverb problems

Postby Slim » Thu Oct 02, 2008 8:22 pm

Okay, sowhy would my Fender twin Reverb amp heat up ridiculously (the tubes) This amp has been in storage for years and just started when I put a new fuse in and turned it on. Got hot to the point I turned it off cuz I though it might light up! Any ideas?
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Re: Twin Reverb problems

Postby ricochet » Thu Oct 02, 2008 10:51 pm

Exactly what heated up excessively?

Power tubes and rectifiers can become gassy and overheat. Electrolytic filter capacitors break down over time without voltage on them, and often short out and overheat rectifiers. The filter caps themselves heat up when this is going on and sometimes explode violently. I keep a fingertip on them to feel for heating and watch rectifier plates for redness when powering up anything that's been off for a long time. That's really shadetree; it's better to start with about 70% voltage on a Variac and bring it up slowly. People have also rigged banks of serial light bulbs to drop the voltage initially and step it up incrementally. (Old "boat anchor" restorer's trick.)

Leaving a tube amp turned off for more than a month or so is a really bad idea, IMO.

Filter capacitors go bad and need replacing after a few years, anyway. Probably ought to go ahead and do that. Sometimes they'll re-form and work OK, if nothing burns up or blows up. Tubes that have become gassy may also "clean up" with an hour or so of operation, if nothing burns up. They may need replacing, though.

A resistor or capacitor in your bias circuit may have changed value over time and thrown the bias off so it will overheat with any tubes. You need to be able to test bias voltage, and ideally, idle current in the power tubes to know what's going on there.

If you're not a pretty knowledgeable tinkerer with at least basic electronic measuring equipment, you'd better head for the nearest amp tech.
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Re: Twin Reverb problems

Postby Slim » Thu Oct 02, 2008 11:48 pm

Well, back in college, I took intro to elect and ac/dc circuits....Can use a Fluke meter to test volts, Amps, Resistance, etc.... Havent messed with any electronics in a long time but well, I built my own house, wired it, plumbed it, ...I work on pumps, motors, turbine compressors, etc at work... I guess what I am getting at is that I aint scared to tinker! My only worry is making sure theres not any stored energy I get surprised with. So with a schematic and a fluke meter you think I could figure the problem out?? I have never messed with checking tube bias so will need a tutorial before I try any of that.

Nothing blew up when I powered it up but it got HOT....probably was on for 15 mintues about 5 minutes on standby and 10 powered up. Think I will order all new tubes and get some filter caps.... Hopefully wont blow anything up or electrocute myself! PS any tips on any of this will be appreciated...especially the not getting electrocuted part! Also where is a good place to order Tubes? Thinking some Svetlana (Sp.?) or the other Russian tube (What is that name?) I aint paying for groove tube to scratch one name off and repaint thiers! LOL! All will have to wait til I get home next week!
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Re: Twin Reverb problems

Postby ricochet » Fri Oct 03, 2008 12:56 am

Yep, go for it!

Some great places to look for general tube amp fixing info are the tube amp pages at: http://geofex.com/

Schematics can be found at: http://schematicheaven.com/

A couple of places I've bought tubes and parts:
http://tubesandmore.com/
http://triodeelectronics.com/
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Re: Twin Reverb problems

Postby Slim » Fri Oct 03, 2008 12:59 am

Oh.... The power tubes were what I saw that were hot...And glowing brighter than I rmemeber (Could have lit up a room! LOL) Also smelled a "burning" smell...that makes me think filter caps but the tubes were HOT maybe one cause the other?? How do you discharge caps? A resistor or just the old lightbulb rig? I dang sure aint popping them with a screwdriver...that makes me jump every time I ever did it!
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Re: Twin Reverb problems

Postby Slim » Fri Oct 03, 2008 1:04 am

Definitely getting some Filter caps ordered (actually we have a big electronics warehouse in Hattiesburg that probably has most of what I need cept the tubes maybe. Will try there first) Might as well get new tubes too. I mean they been in there since 71 or 72 or so....
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Re: Twin Reverb problems

Postby Slim » Fri Oct 03, 2008 1:09 am

Call me stupid but....if I unplug the amp and keep playing the guitar into it....shouldnt that discharge the caps to a great degree? Is this a stupid question?? Oh well, I got a meter so I will check this idea....
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Re: Twin Reverb problems

Postby leftyguitarman » Fri Oct 03, 2008 2:15 am

So I don't know a whole lot about tube amps, so when I was having problems with mine, I took it to my cousin who works on amps and guitars. Anyways, he took the back off and looks at me and said "Don't ever do what I am about to do."

So he starts poking around the inside with a screwdriver and tapping the tubes and stuff. I didn't think it was a great idea. Your comment made me remember that, Slim.
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Re: Twin Reverb problems

Postby Slim » Fri Oct 03, 2008 2:51 am

Well, capacitors have stored energy (sometimes quite alot). You have to discharge that power to make them safe. One method is to take a screwdriver and basically "short them out" from one end to the other. When you do that they spark, and pop really loudly. SOmetimes enough energy to burn the circuit board they are mounted to. Not really dangerous I guess but will make you shat your britches by surpise from the sound !! This is not the preferred way to discharge them. Another way is to rig a lightbulb and put one wire to each side of the cap (The light is basically nothing but a resistor)

The preferred way is to take a resistor and dsicharge them through it. I cant remember but I think like a 20k ohm 2 watt wire wound resistor.
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Re: Twin Reverb problems

Postby ricochet » Fri Oct 03, 2008 3:50 am

The resistor value isn't critical, it's just to reduce arcing. About 1K works fine. Definitely do discharge the caps, and it's best to then clamp a grounding wire across the high side of the caps and ground (that you MUST remember to remove when you power the amp back up, don't ask how I know), because electrolytic caps can actually redevelop part of their charge after discharge, due to ion diffusion.

I burned a nice crater in the end of a screwdriver by discharging a larger than usual filter cap without the resistor. Pretty nearly caused fecal incontinence on my part, too.

Hot power tubes aren't a filter cap problem, that suggests loss of negative bias voltage, gassy tubes, or both.
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Re: Twin Reverb problems

Postby Slim » Fri Oct 03, 2008 3:58 am

Okay, so the gassy tubes would be fixed with new tubes. How do I change the negative bias voltage? I am guessing I would check the voltage through pins on the tube socket? How do i go about this and what do I do with this info... LOL? Is there a pot or something to change the negative bias voltage? forgive my ignorance.


Caps have several times almost caused me to discharge (rectally) upon discharging. My brother used to get a kick out scaring me in this way.


Also will prolly replace filter caps anyway due to the age of them. While I am doing surgery might as well remove the appendix....

Hell if I get this thing working I am gonna tackle my old Grundig radio that quit working 20 years ago.
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Re: Twin Reverb problems

Postby Slim » Fri Oct 03, 2008 4:05 am

Same brother works in live electrical service boxes...all his kleins and screwdrivers have notches in them. He is kinda an idiot...Too many years working in SCR rooms on drilling rigs. Also seen him stick his finger in a light socket to check if it had power.....smart guy I tell you! LOL! He does usually work with one hand behind him though. Gotta be safe, you know.
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Re: Twin Reverb problems

Postby NEONMOONY » Fri Oct 03, 2008 12:46 pm

Actually, as crazy as it sounds, there quite a few guys that work regularly with electricity that will check for 110 by touching the sockets or terminals.
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Re: Twin Reverb problems

Postby ricochet » Fri Oct 03, 2008 10:18 pm

I've done that. I've also accidentally discovered 450V in similar manner. It'll teach you to be careful.
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Re: Twin Reverb problems

Postby Troubleman » Mon Oct 06, 2008 1:32 pm

Slim wrote:Well, capacitors have stored energy (sometimes quite alot). You have to discharge that power to make them safe. One method is to take a screwdriver and basically "short them out" from one end to the other. When you do that they spark, and pop really loudly. SOmetimes enough energy to burn the circuit board they are mounted to. Not really dangerous I guess but will make you shat your britches by surpise from the sound !! This is not the preferred way to discharge them. Another way is to rig a lightbulb and put one wire to each side of the cap (The light is basically nothing but a resistor)

The preferred way is to take a resistor and dsicharge them through it. I cant remember but I think like a 20k ohm 2 watt wire wound resistor.


Hmmm.... Depends upon the amp and the capacitor. Those giant bottle-caps used in 100 watt tube-powered Marshalls...? I've seen them hold a charge for a couple years. They can pack enough of a wallop to do human damage.
As for the OP's Twin Reverb - get it serviced. It's been sitting. You may have dried-up caps, cracked resistors, or have been visited by some small wire-gnawing critter (that happened to a Fender Reverb unit of mine, kept in a self-storage place for a year).

pEAcE
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