CHAMP ELECTRONICS -" THE VALVE AMP HOSPITAL"

NOTTINGHAM, ENGLAND

 

REPAIR OF A LANEY VC30 GUITAR AMPLIFIER

 

 

 

 
Belonging to a gentleman named Andy who lives quite close, this Laney VC30 came in with the reasonably simple fault of randomly blowing the HT fuse; apparently when it was being driven hard. I actually didn’t think there would be much to do in sorting out this problem, but as I went further into it things became more complicated. 

On powering the amp up it actually came on OK though it was only doing about 21 watts. A little short of the claimed 30 watts! After removal from the case and back onto test it was very obvious that the set of Sovtek EL84’s were well down and under-par. I stuck in my faithful "test-set" quad set of used-but-good Mullard EL84’s to see what sort of a difference that would make, but to my surprise there was no improvement and all of a sudden one of my good ‘ole faithful Mullards gave me a firework display! Obviously the power was removed immediately, followed by a bit of "head scratching", and wondering what the hell was going off.

On close scrutiny, I realised what had happened. Someone had changed the four screen grid resistors at some point in time. Normally 100 ohms, these had been replaced with 82 ohm, 2 watt wire-wound. No problem or big deal, but as pin No.9 is the actual screen grid pin and pin No.1 on data sheets is "IC" (internal connection), these 82 ohm screen grid resistors were connected between pins No.1 and No.9. This means that pin No.1 was being used as an "anchorage point". Fine, but the problem with old Mullards and other old brands is that as pin No.1 is this internal connection and in reality connected to pin No.2, being the control grid. So, more than 350 volts was now being placed onto grid one, hence causing the firework display!

Most (but not all) brands of EL84’s nowadays do not class this pin No.1 as an "internal connection" anymore, but simply do not use the pin at all for any kind of connection. So, with most new branded tubes this problem wouldn’t exist. However it still would on some brands and most certainly on NOS Mullards!! My advise would be that if you have one of the Laney VC30’s, don’t put Mullards into it until you have had your amp checked out to see if the pin No.1's have been used as an anchorage point for the screen grid resistors!?

Moving on, I simply ripped out the pin No.1 connections and mounted all the screen (grid 2) resistors up in the air and rewired accordingly. (This can be seen in the photos.) This amp had seen other tech guys before and someone had fitted separate bias resistors and caps to the four EL84 output tubes! I have seen this a few times and often wondered why. This does not change anything and (or) improve performance in anyway. I therefore ripped this mod out and put all back to original as can also be seen in the photos. Personally, I think this pointless mod is simply a money spinning "OK, I’ve done this for you" yarn!!

Having done the above fore-mentioned changes I now fitted a complete new, matched set of JJ EL84’s. These are far better than the Russian crap, "well-down", Sovtek junk that were in it. (Only my opinion of course!).

After sorting a problem with the low-gain input jack socket, everything was now all complete. It was looking great on the test gear and was ready to put back into the case. By the way, she was doing a good 32 watts plus now at this stage.

After fitting the amp back into the case and one final test/check-up, I was surprised to see that the power had dropped significantly to about 24 watts, and the picture on the scope was looking rather strange to say the least. Wondering what was happening, I turned out all the lights to check inside the valves (tubes), and I could see that the EL84 second from the right (looking from the back) had it’s screen grid glowing very badly on full drive. Swapping two of the tubes around didn’t change things; the tube in this same socket was still doing the same thing. So.........out came the chassis again to see what the hell was going on, but back on the bench everything looked fine again! After putting the chassis back in the case for a second time, only to discover that this strange fault was back again, I suddenly realised what the problem was.

The four EL84 output tubes are mounted hideously close to the internal speaker’s magnet on this model. The EL84 second from the right is the closest one to the magnetic field from the internal speaker, and this was causing the drive to be crippled on one half, hence drastically changing the sound and performance of the amp. In addition, this particular EL84 had its grid two glowing so badly on full drive because of the situation that it is likely to be unreliable and fail prematurely because of this.

I am not even sure if Laney (BLT Industries LTD) are actually aware of this problem. The only one thing that anybody can do to rectify this problem is to install a shield between the output tubes and speaker magnet, using a special metal called "mu-metal". This type of metal shields "magnetic fields" and is the only answer to this stupid, hideous design problem!

Thanks for looking, John.

 

 

 

 

 

 

 

 

 

 

 

 

Somebody had modified the amp un-necessarily by  fitting separate bias resistors and caps to the four EL84 output tubes.

 

Things restored back to normal!

 

Those tubes are too close to that magnet !!!

 

UPDATE:

Since first doing this article I have done more of the VC30's and it looks like BLT Industies have done something about this magnet/tubes distance issue. It seems they are now fitting them with a 12" Celestion Model "Seventy 80". This has now given about an extra half an inch clearence from the magnet and the problem doesn't exist with this driver.......only the ones fitted with the HH Invader driver are the troublesome ones.
 
I have also been asked quite a few times about the "twin versions" of these and, the answer is that there is no problem, regardless of which drivers they have, as the output tubes are in the middle....between the drivers on the twin's.


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FURTHER UPDATE - DECEMBER 2008:

 
The following two photos are of an early Laney LC30 MkI. I believe this model preceded the VC30. (?) As can be seen, the front baffle is sloping, and although it still sports the HH Invader driver, because of the sloping front panel the driver magnet is nowhere near the output tubes.....No problem with these!




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