Fender Twin not working - fixed!

[Francer]

Hi all

I wonder if anyone can advise me.

My twin has stopped working, just makes a loud hum when powered up, no guitar sound from any input or channel, no response to volume controls, hum remains constant whatever I do. I notice that two of the power valves are glowing blue, the other two look like they do on standby, just a low orange glow. - when the amp actually is on standby all valves appear to glow orange the same amount. Looking at the rear of the the amp it's the two valves on the right hand side that are glowing with the blue colour.

Any thoughts/advice greatly appreciated, i'm moderately ok with the tools and basic troubleshooting, i own a meter and a soldering iron, so maybe I could try some basic checks myself before resorting to putting it in the shop.

[salty]

If my memory serves me correct, I would verify it though, the Twin can actually run on a single pair of tubes. The trick is that it has to be either the 2 outside or two inside ones that are pulled. The only reason I am bringing this up is that if you think 2 of the tubes may be the problem then you could temporarily arrange them to work as only two tubes and see if there are in fact a couple bad ones. I would not do it for more time than it takes to see if the sound comes out, because really you will nee to rebias to leave them that way...again, I think.

Now some blue is normal with tubes. Are all your preamp tubes lighting up orange? Do you get sound on one channel and not the other? Normal or Vibrato?

[Francer]

Thanks for the response, i can't see the pre-amp tubes because each one has a metal (heat shield?) cover over it. I'll perhaps take them off later just in case something obvious jumps out.

I've been doing quite a bit of reading since I posted this, but there seems to be many potential issues, maybe caps or rectifier circuit etc. and the most common advice is take it to a tech as it's really hard to troubleshoot from a distance.

To be honest, I'm nervous of messing with it powered up as the hum is really loud and unpleasant, so i'm kind of jumpy whilst I'm looking at it, just in case it really coughs out loud or something. There is a local guy who serviced it a few years ago who seems pretty good, I think I'll give him a call tomorrow, this amp is long overdue some TLC.

[Racing]

That a powertube has a slight blue-ish glow to it CAN be all normal.

You´re correct in that those aluminium thingys are indeed shields. They´re on there with a bayonet fitting. Just push upwards and twist gently and they should come right off.
Take them all of and check that the top of each of the smaller tubes still look like they have "chrome" inside the glass. If one or several looks white,replace those.

[surfingonacid]

I would avoid running the amp if you don't know what's up. Could cause serious damage if left on and you don't know where the issue is.. I learned my lesson over the past year when I had an issue with my Dual Showman Reverb. On low notes there was quite a bit of distortion, then it blew a fuse and I saw smoke come out the back. I replaced the fuse and a couple of the power tubes were getting VERY hot and now there are some blackened areas on the chassis.. I had a couple spare power tubes and did some swapping and then had thermal runaway to one of the preamp tubes (the glass actually breaks when the amp cools off).

If you have any knowledge of electronics I suggest not running the amp anymore and taking a look inside. I wish I had opened mine earlier instead of trying to swap tubes for a while because when I did I saw two of my screen grid resistors had blown. I replaced those. No more thermal runaway it seems but still some low note distortion at gigging volumes which I suspect is caused by a capacitor somewhere (coupling cap maybe).

It's best to take it to a tech if you can't look into it yourself. Your problem reminds me of when I once had a Super Reverb that had original power tubes and one of the power tubes was missing the plastic guide and got put in wrong = loud hum! No guitar sound. My guess is you've blown a power tube, but something could have caused the power tube to blow. You could grab a spare 6L6 and start swapping to see if the noise goes away.

[Francer]

Yep, it's going to a tech, i'm not going to meddle with it.

I guess i could open it up, and maybe get lucky and spot something that looks fried, but unless i can work why something failed i'll be taking a chance on it happening again.

Thanks for the responses, i'll update this thread when it's fixed.

[NoiseNoiseNoise]

I leave things like this to a Tech because a) If it goes wrong they're the ones to blame , and b) Tube amps have those "Ooh dearie me, these are LETHAL!" voltages flying around inside!

[surfingonacid]

Yeah I should have mentioned before poking around you need to discharge the filter capacitors first. Stay safe.

[surfingonacid]

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[boppy]

I had the same problem recently with a Fender Champ 12--constant loud hum whether or not anything was plugged in, and regardless of how the knobs were set.

I swapped out the filter-caps and everything was fine after that.

If you have reasonable skills with a soldering-iron, and take the necessary safety precautions, it's an easy DIY job,

[Racing]

I had the same problem recently with a Fender Champ 12--constant loud hum whether or not anything was plugged in, and regardless of how the knobs were set.

I swapped out the filter-caps and everything was fine after that.

If you have reasonable skills with a soldering-iron, and take the necessary safety precautions, it's an easy DIY job,

Agreed.

[Francer]

Well I finally got my amp back from the tech this weekend after 6 long months of being away, whilst he busied himself with his day job and other stuff that these guys tend to have going on the side.

The problem was a short circuit that occured in one of the power amp tubes which was not protected by a fuse so it also burned out one of the resistors (and maybe some other stuff ... )

The tech installed a new set of tubes and replaced the damaged components and she seems to be as good as new now. He also mentioned that the old tubes were of a slightly different design where they push in to the holder, they had a longer spigot which was then pushing against a resistor that sits across the valve base, so the valve never sat fully home in the socket.