A Little Knowledge
Member: Sideband
This little incident happened on a Saturday at my first job.
Harry was already working when I got in but there was a third person in the workshop today. It was the manager’s son Reece who rather fancied himself as a budding engineer and liked to think he knew it all. He was one of those people where you couldn’t tell him anything. He was studying something at college but we had no idea what. Whatever, he rather fancied himself.
Harry and I were getting on with our jobs for the day while Reece was looking at an ancient PA type amplifier which looked like it hadn’t been used for years. He told us that he’s found it at college and because he ‘knew all about electronics’ had been asked to repair it so it could be used for their amateur dramatics courses. True he had a little knowledge but never mastered the finer arts of serious fault-finding. If a valve change got something going, he was happy.
It was typical of these sorts of amps with a couple of EF37A’s, 6SN7’s and push-pull KT66 with a 5Z4 rectifier. One of the KT66’s was missing. When he enquired about a replacement valve, I had a look in our stock and found a good second-hand one, at least it measured good on our valve tester. I gave him the good KT66. ‘I hope you’re not just going to plug that amp into the mains are you’? I said. ‘Yeah, why not he replied. ’It only wants the replacement valve. I’ve checked everything else on the Avo, there are no shorts so it should work’. ‘Um…. it looks like it’s not been on for ages so the smoothing caps may be duff’, I said. ‘Oh it’ll be all right’ he said. ‘Really good these are, really loud. Should go like a bomb……’! I thought it was rather the wrong choice of words but left him to get on with it…..always best where the boss’s son was concerned.
Anyway some half an hour later there was a sudden burst of music from the amp accompanied by a hum. There was a lot of crackling and then silence. Another few moments and another burst of music, slightly better this time with no crackling but still that hum which didn’t seem to be getting any better….
Harry looked at me and I looked at him. The hum was getting louder and instinct (and there was a faint sizzling noise) told us that something was about to ‘give’! There were two doors in the workshop and we decided to discretely leave by the far door, which was the opposite end to where Reece was working on the old amp. We went down the front stairs and into the shop to join Reg for a mid-morning cuppa.
Whilst we were supping our various brews a few moments later, Reece came down with a big grin on his face and said ‘See I told you it would be OK’, found his ‘Sgt Pepper’ LP and went back upstairs. Just as The Beatles where tuning up in the opening bars of that famous album there was a loud bang……!
‘That was it!’ said Harry. ‘What a beauty’! The silence that followed suggested something nasty had happened so amid some chuckling and spilt tea, we went back up to take a look. Through the smoke, we could see that Reece was sitting somewhat white-faced looking at the smoking amp while bits of foil and flotsam where floating down. The thing that really caught my eye was the sight of the failed capacitor sitting atop a neat spiral of foil that had evidently blown out through the seal at the bottom. Oh how I laughed….’Told you not to plug it straight in. The hum should have told you something was wrong’!
As it was the boss’s son we made him a cup of tea and saw he was OK. His hands were still shaking whilst drinking his tea. Later, rather quietly he picked up his coat and departed. The manager rang later enquiring about the ‘blown up’ amplifier and we put him on the speaker-phone and explained what had happened. I’m not sure if there was a suppressed chuckle as he put the phone down but on the following Monday he informed us that Reece had no intention of going anywhere near it and would one of us try and sort it out…..
Guess who had to fix it? It hadn’t done the 5Z4 reccy any good either which flashed and sparked when the new capacitors had been fitted. We didn’t have a 5Z4 and although it would be frowned on now, I fitted a couple of BY207’s and surge limiters in place of it. After all it was being done ‘on the house’ so to speak and BY207’s were cheap. Those and two coupling caps on the KT66’s sorted it out and it gave reasonable hum-free quality.
Reece picked it up the following Saturday and when we demonstrated it, I noticed he kept a respectful distance for the first few moments….
We kept the failed capacitor sitting atop the spiral of foil and mounted it on the shelf above Harry’s bench as a sort of trophy
