Restoring the Cosmos Valve Amplifier

A few months ago I spotted a very interesting and attractive looking valve amplifier on gumtree that I couldn’t resist. The amplifier was being sold for repair or parts as one of the valves was red-plating leaving one channel dead. This really didn’t phase me as troubleshooting valve gear is pretty straight forward, and red plating is more often than not a sign of a faulty screen grid resistor or decoupling capacitor than an output transformer, so I felt it was worth the gamble. The guy selling the amp was insanely nice and also an enthusiast, so he threw in a lovely direct drive servo controlled turntable with the deal.

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As you can see it was in OK shape, there was a little bit of corrosion on the chassis from what I can imagine were years of sitting in storage but cosmetically it looked great. Unfortunately my SD card corrupted as I was transferring the pictures of the restoration, so all I have are sketchy reference photos from my phone. Luckily the previous owner had taken these pictures for the advertisement.

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The amp is running a 12AX7 as a preamp, into a pair of 6BM8/ECL82/6F3P in push pull for each channel. The output transformers are surprisingly small for an amp claiming to deliver 10 watts per channel, but somehow it really packs a punch.

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There is a little circuit board at the front of the amp with 4 transistors, this makes up the phono preamp for the turntable input. This amp gives you the option of selecting either magnetic or x-tal cartridges, where x-tal seems to have been a forerunner to ceramic cartridges. It seems that magnetic cartridges are velocity responding devices, while ceramic cartridges are amplitude responding devices, so it makes sense that there would be different gain and equalization requirements for both.

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The first thing I did when I got the amp home was take the back off and look for parts that were charred or physically damaged, and it didn’t take me long to find a whole heap of them. One of the main capacitors had been leaking electrolyte from around it’s legs which had caused shorting and some nasty melting.

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It looked like someone had previously replaced a few of the decoupling capacitors with modern foil types, but for some reason they had stopped two short of replacing them all, as luck would have it the remaining two had failed, splitting wide open.

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Finding replacements for the can type electrolytic capacitors was troublesome as each can contained 3 separate 20uf capacitors. So I decided to go ghetto and breadboard it up…

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I then proceeded to replace every other electrolytic capacitor in the whole thing… but why stop there, after measuring a few of the resistors I realised that a majority of them had drifted off value, some by rather alarming amounts. So all the resistors were replaced too!

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I can’t say that it wasn’t a tedious job, but it was strangely therapeutic soldering away into the early hours of the morning. Finally it was time to connect the speakers and fire it up, Creative Zen attached for the ride. And it sounded surprisingly good, far better than I had imagined, putting my old class A single ended EL84 amplifier to shame. I realised that if it sounded great with the old thrashed out valves on a set of test speakers, it was probably going to sound even better once my new Svetlana tubes arrived from the Ukraine.

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After the good results, and seeing that nothing was getting hot, and that none of my valves were red-plating I decided to finish the job by recapping the preamp as well. It was insanely fiddly and the board was rather well tethered to the chassis, so everything had to be done in situ.

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All set up with the new Svetlana valves and it sounds great, I use it not only for music and records, but I also run my media computer and projector into it. The bass response is really crisp for a valve amp while the highs are nice and clear. I also quite like how it is the exact same width as my speakers, a complete fluke.

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