Author Topic: New tubes blew my Laney VC30 HT fuse? (Read 9596 times)

BadMonkey · « **on:** January 12, 2007, 06:56:31 PM »

Hi All,

Last night I noticed one of the power tubes in my Laney VC30-212 was glowing brigher than the others & some excess hum on the drive channel.

So I dutifully changed them for a matched quad of Tube Amp Doctor EL84's, the exact same make & type as my previous ones.

All four lit up fine & the amp played for a while before I noticed a burning smell & then the HT fuse blew. Unfortunately I didn't have spare fuse so I couldn't diagnose where the problem was (power tube short, pre-amp tube... though I haven't checked for arcing yet)

The fact that all 4 lit up makes me think the tubes are fine, but then these are the only thing I changed prior to the fuse blowing, so are the likely culprit.
Though memory serves that prior to it blowing all 4 were glowing very brightly; even their back plates were lit.

I contacted Watford Valves (the tube suppliers) about getting a replacement quad thinking that one of them had shorted.
They guy from WV reckoned that the tubes had been biased at too low a current. This caused them to glow very brightly & then blow the fuse.

Fine says I, & I'll have a go at rebiasing when I get a new fuse. The only thing is that Laney states that the VC30 is fixed bias??? If this is the case, why should I have to bias it, & will there be a simple screw or something which I can turn to do so?

Has anyone had any experience of VC30's, changing the power tubes & having this happen? Or biasing VC30's?

Many, many thanks!
BM

38thBeatle · « **Reply #1 on:** January 12, 2007, 07:10:13 PM »

I have a VC30 but haven't had occasion to change the valves but I would be interested to see what others come up with. I too thought that it was fixed bias and I am baffled as to why the HT fuse would go. Hopefully some of the good knowledgeable folk here will venture a suggestion.

HTH AMPS · « **Reply #2 on:** January 12, 2007, 10:48:30 PM »

the term 'fixed bias' is confusing if you're not familiar with amps.

by the sound of it the valves were biased way too hot (underbiased) and they eventually give up the ghost.

I would agree with WV on this one - the valves weren't faulty, you burnt them out yourself (from what you've said).

Try a new HT fuse and turn the amp on again with the new valves. If the HT fuse doesn't blow then you might have got lucky and the valves might still be ok. If this is the case, GET THE AMP BIASED!!!

:twisted:

BadMonkey · « **Reply #3 on:** January 13, 2007, 09:04:29 AM »

Thanks for the advice guys! I've got a pal with a multimeter who'll hopefully be doing the biasing next week.

I've had a look & at the Laney web-site & there is a new manual for the VC30 which replaces the one I've got.
It says that you don't have to re-bias if you replace the tubes with those of an identical type and grade.
The grade is apparently given by a number on the tube. The new tubes didn't have numbers. The old ones did. I'm guessing that if you buy these graded tubes direct from Laney/TAD they will be the same bias as previous ones.

Cheerio
BM

hunter · « **Reply #4 on:** January 13, 2007, 10:18:10 AM »

As far as i know VC30 is Class A and therefore doesn't need no bias adjustments if tubes are changed.

I think you just should make sure the quad is matched.

BadMonkey · « **Reply #5 on:** January 13, 2007, 01:15:18 PM »

That's what I thought, but this is from the VC30 manual:

The output tubes can be replaced singularly if you replace them with the exact same type & grade as factory fitted otherwise they should be fitted as a quad set. If the same type and grade are used then no rebiasing of the amplifier is required.
If you wish to use a different grade of tube then they should only ever be changed by a qualified service agent due to the grid bias voltage needing adjustment.

The "grade" is apparently a number written on the base of the tube. My new tubes didn't have this number. My old ones did.

I bought the quad as a matched set so I assume that they are.

hunter · « **Reply #6 on:** January 13, 2007, 04:40:50 PM »

But here it's different (and correct I would say):
http://www.laney.co.uk/manuals/VC30%20Manual%20-%202006%20-%20Issue%203.2.pdf

If you replace ONE tube on the factory set, it needs to be same grade. Otherwise it needs to be a matched Quad, that's what I read there. No BIAS nowhere. Maybe you are reading in the LC30 Manual (which is A/B amp).

Replace the fuse. If it blows again, one of your new EL84s is faulty. I would send back the whole quad and ask them to check and replace. Would expect that kind of service from TAD

msplines · « **Reply #7 on:** January 14, 2007, 12:07:10 AM »

One thing worth noting is that TAD changed the grading system a while ago and the grades are no longer the same numbers as the previous grading that they used with the old machinery. If the previous set of valves were more than a couple of years old, the grade numbers may no longer be valid.

BadMonkey · « **Reply #8 on:** January 14, 2007, 09:10:28 AM »

Quote from: hunter

But here it's different (and correct I would say):
http://www.laney.co.uk/manuals/VC30%20Manual%20-%202006%20-%20Issue%203.2.pdf

You are right...!
But some how I managed to download this manual:

http://www.laney.co.uk/manuals/VC30%20Manual%20-%202006%20-%20Issue%203.1.pdf

I've emailed Laney Technical support for clarification anyway.
I don't why they would have 2 different manuals.

Thanks alot
BM

HTH AMPS · « **Reply #9 on:** January 14, 2007, 04:14:19 PM »

even with cathode biased (what you're calling class A) amps, you should still check that the valves are not overdissipated.

for the most part you can get away with just plugging in a new matched set, but I prefer to 'know' whats happening in an amp and hotter bias doesn't always sound better.

:twisted: