December 16th, 2011, 11:38 AM
Sorry if this isn't the right place to post this, but i'm in need of some advice and thought it might be worth a try on here.
I have a laney VC30 amp. It seems to work fine but a couple of weeks ago it blew a power tube unexpectedly.
now i've replaced them (a whole set of new ones) the tube in the same socket as the one that failed seems to glow very brightly (yellowish colour) for a few seconds the moment i switch it on and then slowly fade to normal - it's inside the tube near to where the pins are connected to the tube socket. it does it with any tube, as long as it's in that socket.
aside from that the amp seems to function normally but this is a little bit worrying - does anyone here know what could be causing this to happen and/or how i could fix it, or is it nothing to worry about?
December 16th, 2011, 02:55 PM
Hey Blomp, sounds like you hade quite a scare there!
Always impressive to see a tube burn up like that.
Did you re-bias the amp after plugging in the new tubes? Usually they're not 'plug-n-play', unless you got tubes branded & marked specifically for your amp.
From your description my best guess is that either something fried along with the tube (screen resistor?), or the tubesocket got dirty or warped from the heat of the burning tube.
I don't know how much you're into electronics, otherwise it might be a good time to take the amp to an experienced amp tech.
December 16th, 2011, 03:14 PM
yep defenitly needs to be biased
December 16th, 2011, 03:38 PM
Don't listen to him, he's biased!
December 16th, 2011, 03:50 PM
Absolute bias problem.
December 16th, 2011, 06:03 PM
That's just one of those things you need to do (or get done) if you own a tube amp. Unfortuantely, even the amps that are supposed to have a fixed bias, or "auto" bias simply need the adjustment.
The good news: It will sound better once it's been Biassed by a professional...
December 17th, 2011, 07:48 AM
Agree with the above about having it checked by a tech. If one of the el84s is acting differently at power up, you may have a fried resistor. If you don't get that sorted, best case, you could lose more tubes or the HT fuse, worst case, it can take out a transformer, which is an expensive fix.
I've read some of the Laneys use pin 1 as a tie point for the screen grid resistors, some older el84 tubes have pin 1 connected internally, and this will melt them. When it's at the tech, ask him to make a note of that when he's in there, best to know what you have, so you know what you can and cannot put in there (or if it is, have it changed so you don't have to worry about it).
found the link again: http://www.chambonino.com/work/miscguitar/misc6.html
though that amp had been fiddled with a bit otherwise as well..
December 17th, 2011, 08:51 AM
thanks for all the advice so far, but it seems i've not really explained the problem well.
the tube didn't burn up, it was just glowing much brighter than the others for a few seconds when i switched the amp on - the amp appeared to work fine. i suspected this was the same thing that caused the last tube to go bad.
the socket is the 2nd power tube socket from the right.
the amp is cathode biased which i thought meant the amp didn't need to be re-biased. but yep this definitely sounds like it needs to be taken to a tech.
although i've been reading the link HHudson has posted and it seems like this may also be something to do with how close the tubes are to the speaker, particularly the one that is causing me trouble.
December 17th, 2011, 09:19 AM
December 17th, 2011, 12:15 PM
the link was more for info on the pin issue on some of the later Laneys, just in case you ever want to try out some NOS tubes in there, and save you from a nasty surprise.
Seeing that you have lost a tube - which could just be the tube, but may also have been caused by a bad resistor, or caused one to become so - , and that one tube ( I assume all of the same type and a matched set?) is lighting up notably more than the others on power on may be indicating a problem, which should be checked at least, so you don't have to worry about losing more tubes or worse.
Yup, cathode biased are less troublesome with exchanges, and usually ok, but when you swap tubes for a different new set, it's best to check that they are not running too hot (Laney has that in the manual I believe as well)... there is a lot of variance and the factory matched/rated sets are not always matched/rated very well. Don't know about the Laneys, but a Vox tends to run the el84s very hot, and the wrong tubes in there can suffer.
> Not guilty!