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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. 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!
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 ohms, these had been replaced with 82 ohm, 2 watt wire-wound. No problem or big deal, but as pin No. This means that pin No. Fine, but the problem with old Mullards and other old brands is that as pin No.
So, more than volts was now being placed onto grid one, hence causing the firework display! However it still would on some brands and most certainly on NOS Mullards!!
Moving on, I simply ripped out the pin No. 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.
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 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 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. 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!
Things restored back to normal! Those tubes are too close to that magnet!!! 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
REPAIR OF A LANEY VC30 GUITAR AMPLIFIER