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Old 7th Jul 2009, 5:59 pm   #1
Pamphonica
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Default 10 Watts from a skip (Linear 5-10 copy)

To those who have followed this interesting restoration, heartfelt thanks.
This thread started it all off, and shows what state it was in originally: https://www.vintage-radio.net/forum/...ad.php?t=37470.
Anyway, it has now taken its turn on on the operating table, and my little Linear monobloc (non tone-control) amp is looking spick and span again and sounding very sweet.
I first noticed the amp in a neighbour's shed 10 years ago. The house was then sold, and I assumed it had been junked. But a few months ago I spotted it in a skip as my new neighbour started some building work. It could not be left there looking sad, so was duly scrounged.
It was very rusty indeed, but at least had a base plate which kept the worst out of the gubbins. No valves of course.
No circuit diagram meant a lot of delving and forum advice. But finally I had it sorted out and started to replace all capacitors (Wima and dubilier) except the big can. I used decent 1000V polypropylenes - surely well-deserved after surviving a damp shed for 10 years! The resistors were not all quite right but close enough. The main cap can reformed nicely using my slowly-does-it constant current approach.
Next I had to source some valves (not having a nice stock at hand). Harpenden proved a source for everything I needed. The output pair were selected for reasonable balance on My VCM Mk4. The line-up is EF86, ECC83, EL84 x2, EZ80 operating in ultra-linear. Basically yet another clone of the original Mullard 5-10.
Next came some careful stage by stage testing, with a nice final result as I saw 10W clean power just before symmetrical clipping into 15 ohms from 125mV of drive (all RMS) at 5KHz.
Now the grunt-work started. The chassis was rubbed down, the valve holders, input/output sockets and main cap masked and both transformers unbolted so I could spray paint the chassis.
I am not a dab hand at spray painting, but managed a quite nice golden Hamerite finish (a tad brighter than Leak) over a zinc primer coat.
All this painting took a few days, with false starts along the way (watch out for very long drying times). I resisted the temptation to spray the capacitor can like a Leak - I've kept the original look.
But I think the end result looks authentic for an amp of this age (1964?). The flash photo makes the gold finish look grainy - it is in fact qite smooth.
It will will be partnered with a CD player until I decide it needs to be passed on to a deserving new owner.
Thanks again to everyone who gave me such good advice throughout this small but worthwhile restoration.
All the best
Jeremy
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Old 7th Jul 2009, 6:35 pm   #2
Colin
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Default Re: 10 Watts from a skip (Linear 5-10 copy)

Nicely done Jeremy ... turning base metal into gold!
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Old 7th Jul 2009, 7:18 pm   #3
Hunts smoothing bomb
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Default Re: 10 Watts from a skip (Linear 5-10 copy)

Well done, that looks just fantastic!!

One thing though, are you sure it should be an EZ80 and not an 81? it's just that the EZ80 can only manage 90mA and the 81 150mA.
Superb effort and glad the trannys survived.

Cheers.
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Old 7th Jul 2009, 7:28 pm   #4
Pamphonica
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Default Re: 10 Watts from a skip (Linear 5-10 copy)

Thanks. Good point re EZ80 - it might be better to put in an EZ81 for now. I will check the drop across the 100 ohm "smoothing" resistor in the PSU section under load and see what the amp is drawing. And yes, it's amazing that all the innards survived. (It did go in the airing cupboard for a week before I even started work on it!)
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