tube testing ot

[doug deeper]

is there an easy way to tell what tubes i need to replace???
(for my traynor custom reverb   )

[Peter Snowberg]

If they're well used, ideally you'll want to replace all of them since they go dull over use.

If it hasn't been recapped, you should change the caps before the tubes. That might be your problem and the tubes might be okay.

Make sure all tubes have the filaments glowing. If they are, check each plate connection for high voltage and be careful. You will be messing with 400+ volts when you're in there which can be quite fatal.

You can trace the signal through the circuit with an audio probe and a second little amp. Something like a Little Gem is ideal. Just attach a .1uF 600volt cap to the end of a cable and touch the GRID of each tube down the line until you find where the signal stops. Hit the strings and then touch the cap, but for safety, don't touch the guitar and the amp at the same time :!:  

I hope that helps. Oh, and I think there is a tube amp debugging page over at GEO.

-Peter

[The Tone God]

is there an easy way to tell what tubes i need to replace???

That question is too vauge to give an accurate answer. If you could be alittle more specific then a better answer can be given.

Andrew

[fretbuzz2003]

Why do you want to replace the tubes? Are you trying to get better tone or are you getting some sort of noise from the amp? And are you talking about the preamp or power amp tubes?

[doug deeper]

it just breaks up to quick...and way to over the top...
so i was thinking one of the preamp tubes was shot....
resulting in the for mentioned "metal tone"...
doug

[Peter Snowberg]

I suspect you have a capacitor problem. Electrolytics only last so long and they get abused in hot tube amps. If they're over 15 years old, I would replace all of them.

Make sure your tubes are all the right type and make sure your power tubes are biased properly.

When tubes go they usually just stop working or get too microphonic to use. I've never had one amplify more as it died.

-Peter

[fretbuzz2003]

Look at your preamp tubes with the amp on. If there is a frosted color to the glass or they glow blue, then the tube is bad.

Also try tapping (lightly) on them with a pencil or chopstick. If one of them makes a lot of noise than it is bad.

The only other way I can think of to test the tubes is to swap them out with ones you know are good and see if the problem goes away.

[Ammscray]

The easiest way is if you can find a schematic with the working voltages of the amp that you can test at all stages...

If the voltages are right, then the amp is probably supposed to sound that way...most people like amps that break up quick but there are ways to lessen the gain...

 If you do in fact have all the right tubes in the amp that are supposed to be there, if you have a 12AX7 tube in your first gain stage position, try swapping it out for a 12AY7 for less gain and later break-up...sometimes a 12AT7 or 12AU7 can also work there, depending on the circuit...

You could also remove the bypass cap on the cathode resistor of the first gain stage to decrease gain quite a bit, and/or also increase the size of the cathode resistor itself, from let's say the common value of 1.5K, to 2.7K, 3.3K, 4.7K, etc...til you find what you like...and this can often be repeated in the second gain stage (if there is one)

Either way, it's a good idea to determine that the amp is working correctly, filter caps are OK, etc,  first before you start modding...

Lastly, it may be your OUTPUT tubes and not the preamp...your power tubes could be shot, resulting in premature distortion, or the bias could set too hot so they're breaking early...