Tube testing

[byoung]

I have a Fender Twin 'mid 80's' version that is having some small issues. Every so often when playing I get a slight drop in gain(drive) but not really in volume. So let's say I am playing with quite a bit of distortion, it may instantly seem like somebody turned down the gain knob a couple notches to a moderate overdrive.

I don't know exactly what the problem is, especially since it is so random and hard to recreate, but my first assumption is maybe a tube(in the preamp section), but I'd hate to buy all new tubes and have it not be a tube problem.

So I was wondering if there is a way to diagnose a bad or failing tube?

 

[pstamler]

Your Twin has two channels (plain and reverb); take the input tube from the channel you're not using and swap it with the one from the channel you're using. Problem solved? Then it was that tube.

Not solved? Look farther down the line; take a pencil and tap its eraser against various tubes. Does the volume jump happen? Or do you hear horrendous noises rather than a bit of a thud? That tube is bad.

Worst comes to worst, if you find that neither input tube is bad, and none of the other tubes misbehaves when you bounce the eraser off it, take the remaining tubes to an amp technician and have him (it's usually a him) check them on a tube tester.

A cheap alternative, after you swap preamp tubes between channels but before you try anything else: Replace the electrolytic bypass capacitors that parallel the cathode resistors on the input tubes. If one of them is going intermittently open, that would produce the symptoms of which you speak.

Peace,
Paul

 

[peterc]

Could also be old solder joints that get unstable when the amp heats up. It is a 20 year old amp, have a look at the soldering & see if it is matt grey with a thin layer of gunk over it. Try reflowing the solder joints with some new solder.

Peter

 

[byoung]

actually it has reverb on both channels, and has a clean and dirty channel, this is the mid 80's 'evil twin' as I've been told it is called ;-) there are too many 'twins' in my opinion.

I will see if I hear a difference on both channels before I do anything, then I will try those ideas and hopefully I will find the problem....if not it's off the the tech.

 

[Guest]

[quote author="peterc"]Could also be old solder joints that get unstable when the amp heats up. It is a 20 year old amp, have a look at the soldering & see if it is matt grey with a thin layer of gunk over it. Try reflowing the solder joints with some new solder.

Peter[/quote]

I second that; I've found it the main cause of troubles with similar amps. When you switch it on tubes heat up increasing tube socket contacts in size a little bit, when you switch it off they decrease in size, and so on, so over time of such machanical expansions/shrinkages solder around contact gets broken. Resoldering of tube sockets is always good thing, either to restore the functionality, or to prevent mailfunction in the future.

 

[safe as milk]

hi
quick thing to try,
spray out the fx loop send/return sockets,
if they are the switching type any gunk in there
could cause intermitant volume drops,

common on amps from the 80's, peavey's etc,

 

[Gus]

First

how old are the tubes in the amp and what brands?

Did you just buy this amp or have you had it for years?

Next this kind of problem can be a hard one to find. Sometimes it is faster to open it up and test any part that might be bad if you have the test gear.

Tubes are cheap buy a few known good 12ax7s and 12at7s at min. If you don't want to buy some tubes and sub GOOD LUCK with anything else

Alway try the tubes first if you don't have test gear. Test them in another good amps is something you can try if you don't want to buy any.

However as others have posted it could have a good chance of being bad solder if it is not the tubes and check and clean the send and return jacks if it has them

It could also be a coupling cap going bad. I have worked on a newer amp that a coupling cap would start to leak bad at a certain drive level. How I found it was removing the two caps to the output stage(the output tubes would red plate when to vol dropped) and using a sencore lc102 that has the voltage adjustable from 1 to 999v in the leakage test. The cap would start to leak well under the rated voltage.

This problem could be a number of things are there any ICs? sometimes the reverb or channel switching is IC and/or FET based in some tube amps so check the IC power supply

I have not worked on a mid 80 twin. Is this the one with red knobs?