03-12-2019, 12:58 PM
(03-12-2019, 07:01 AM)Diabolical Artificer Wrote: 15w is pushing it a bit for two EL84's ...
Didn't seem to be a problem for Vox in the AC15, Andy.
The four EL84s in all the AC30s that I saw got a real thrashing!
In fact, of all the AC30s I encountered in the 60s, I was only ever asked to repair one - and that was because part of the earth bus bar was missing! This was a story that began a few days earlier ...
I answered a frantic knock at my front door. Could I come and help? They were setting up at London Road Club and were blowing 13A fuses! I got there in time (as I thought) to stop the time honoured silver paper fuse coming into play!
Check Live to Neutral - no problem. Live to Earth? Dead short! So, continually isolate kit until the faulty item revealed itself. It turned out to be a Klempt Echolette, an excellent echo chamber made in Germany.
As soon as I took the top of the plug it was immediately obvious what had happened but not why. Could someone tell me what had happened? It transpired that a friend had asked to borrow it for a couple of days. Now, in the 60s, the 13A plug was by no means used universally in the UK so the plug had been changed but, when the unit was returned, the 13A plug had not been refitted so there was a plug and a mains lead!
The wires in the lead were white, black and a very bright red. Now UK colours in the 60s were red and black for live and neutral and green for earth so logic said the red and black were the same and the white wire must be the earth - wrong! Prior to the colours we know today being adopted as the standard throughout Europe in ~1969/70, every country had its own choice of colours and, in Germany, white was live and red was earth!
One of the Trader magazines published a table showing the different colours by country which I'd kept and religiously transferred to the back of my pocket diary every year.
Plugged correctly refitted and a new 13A fuse and they were back in business.
Fast forward to Sunday morning. Knock at the door and the lead guitarist was asking for help. He was trying to practice but all he could get out of his amp was a loud hum.
It took a bit of head scratching to find the problem because there were no visible signs. The earth bus bar vanished under one of the tag boards but was no longer connected to the bit that emerged at the other end! It had obviously been the weakest link in the fault current path between live and earth earlier in the week and it was only because the Echolette was not connected this time that the fault was evident.
I found a suitable length of wire, passed it under the tag board and soldered it to the bus bar at each end. The AC30 now performed normally.
Which leaves a puzzle that can never be resolved. I assumed that the silver paper trick had been tried that night before I arrived - but what was the rating of the fuse protecting the 13A sockets on that stage? A lot more than the normal 30A! A 30a fuse has a fusing current of ~60A but the fusing current of a 16swg bus bar is 166A!
Even if it was only 18swg, that's still 106A! Surely Vox didn't link the two halves together with a bit of very thin wire?
Anybody else got any ideas? (Table of fusing currents here.)
