farting 68 vibrolux

[melodymaker]

my vibrolux started farting the day after a loud gig.
she sounds fine at low volumes, but if I dig into the strings it sounds horrible... not like natural distortion...

The power tubes are both electro harmonix and glowing blue...one more so thatn the other. I suspect the tubes or speakers, but really have no clue. how can I test the bias of these tubes??

I tried toubleshooting the preamp tubes by switching like tubes wiht no change.

I live on a small island and getting the amp to town is a bit of an endeavor, so Im trying to do the best I can to figure out what the problem is....can anyone help???

[pdf64]

Don't worry about the blue glow, see
http://www.kcanostubes.com/content/n...p?ArticleID=10

You need good spare tubes for all tube types in your amps. If you can't get to a tech you'll need to find a way to check power tube bias, which needs to be done to allow the power tubes to work correctly.See
http://www.duncanamps.com/technical/lvbias.html
for various methods and safety precautions.

I'd guess that the vibration from transport and the gig have caused a connection which was on the verge of failure before to fail intermittantly ie when subject to vibration from playing loud. Or you may have a speaker problem or bad power tube.

Have you got a different speaker cab you can plug in to, to substitute for your vibrolux speaker? If another combo even better, as you could plug the amp from one into the speaker of the other?
The results from that would direct us where to go next.
Peter.

[Robert M. Martinelli]

Originally Posted by melodymaker

my vibrolux started farting the day after a loud gig.
she sounds fine at low volumes, but if I dig into the strings it sounds horrible... not like natural distortion...

I strongly suspect one of the output tubes went bad, your words make me think that because at lower volumes the quiescent bias could be enough to make only one tube work on the entire waveshape ( ala class A ), but, at greater volumes the bad tube could fail to amplify its half of the waveshape and you get that "horrible distortion" - try snapping in a couple of new power tubes, or try changing them one by one and see what happens.
Hope this helps
Best regards
Bob

[melodymaker]

thanks. can I unplug my speakers if I plug in another 2x10 cabinet, or do I have to run both? what about a 1x12?

I'm fairly clueless but eager to learn on amp tech , so thanks for the help.

here's another symptom I forgot to mention.. My amp will shut down once in a while when in full power and come back on when I turn the standby switch off.. It has done this before the farting and I don't know if it was an early symptom or completely unrelated.

thanks for the tube article. It was informing. I'll go to town and get some 6L6's and acouple 7025's.

thanks again

[Robert M. Martinelli]

Hi,
as to the speakers, it' s not a matter of "size", but rather a matter of "impedance" ( you can plug in whatever speaker system you might like, provided that you match the amp's output impedance, which is expressed in Ohms ), and the "minimum required power rating" ( connecting a 15 W speaker to a 50 W amp is the fastest way to kill your speaker and win an expensive trip to the closest speakers dealer ). Look at the rear panel, you should find some indications about the amp's output speakers' requirements, and, if the amp provides an "external speaker" socket, you should find an indication there as well about the external speaker(s) requirements . Connecting an external speaker is OK as long as the total impedance meets the amp' s requirements.

Should things stay the same putting in new power tubes, you might try to disconnect the original speaker and connect another speaker/speaker system with the same impedance and at least the same power rating, this will tell you whether your internal speaker is good or not.

As to the bias, I don' t know your amp' s model number, be aware that some Vibroluxs have a fixed bias network, others have an adjustable one, so, before thinking about biasing the amp you should identify the model number ( and, of course, have your problem fixed beforehand ).

A word of caution :
I don' t know how familiar you are with electronics and tube amps, but I' d like to spend some words for safety's sake - If you're a first timer, be aware that the voltages inside your amp ( or any other tube/valve amp ) can be truly lethal - and I mean LETHAL. If changing the tubes and testing the speaker doesn't address your issue, you' d better take the amp to a QUALIFIED tube amp tech, or wait until you have learned enough about tube amp servicing ( there' s a lot to be found on the internet ) before trying to service the amp yourself.
Hope this helps
Best regards
Bob