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Old 1st Oct 2018, 7:43 pm   #21
astral highway
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Default Re: 120W PP EL34 amplifier re-born.

That's a satisfying outcome, Andy!

Nice one!

Have with with the woodwork,

Cheers
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Old 18th Oct 2018, 5:29 pm   #22
Diabolical Artificer
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Default Re: 120W PP EL34 amplifier re-born.

Making some progress. Monoblock #1 nearly finished, I just have to fix a couple of issues, EG the clipping indicator LED isn't working and I need to sort out the meter face. After struggling to rescale a VU meter face, I gave up, I havn't a decent printer anyway, and as the meter driver isn't 100% correct for proper VU meter ballistics, I had a brainwave to use runes and "mystic" sygils instead. So now the VU meter is an entropy meter, see pics.

Not sure I'm 100% happy with it, it was a first attempt. Have also been experimenting fitting a light to the meter. My spares box is a bit bare, no white LED's and no suitable bulbs. To be honest there isn't really the power anyway to run a lightbulb, so all I had was a dark blue LED. Waiting for it to get dark to have a proper butchers.

I finally found a finish for the woodwork I was happy with. I was after something to bring out the grain, and slightly colour the sycamore. I ordered some mellow brown Chestnut wax, but it was too dark. After watching some lads build boats on Youtube, I bought some raw linseed oil. So with sander/grain sealer and two coats of raw linseed oil, it looks good.

All I need to do now is drill four holes for some 4" brass handles that will fit on top, so as to make lifting out the amp "module" easier. That and find a volume control knob I'm happy with.

Monoblock #2 is underway, I've cut, planed and routed the wood and just need to plane the mitre joints. Getting them bang on 45 degrees is very tricky with just a Wilko saw and old plane that wobbles. I've part finished the PSU, bent, shaped and painted all the brackets etc.

TFL, Andy.
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Old Today, 7:05 am   #23
Diabolical Artificer
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Default Re: 120W PP EL34 amplifier re-born.

Here's the schematics if anyone's interested and some more pics. Not shown are PSU and aux circuits.

Yesterday was spent improving the clipping indicator by putting a diode and cap on the base of Q1 as recommended by Argus in another thread. One thing that's problematic is that the amp behaves differently when testing with a sinewave say to when playing music, EG the clipping indicator behaves differently with music. It's a minor issue, one easily solved.

One other phenomenon of notes is OP power. When testing with a sinewave OP is 33v before clipping, when playing music OP rarely goes higher than a few volts, so therefore higher than a watt or so. Obviously it will go higher instantaneously on peaks, but I haven't the test gear to capture this.

This is where measuring amp performance is tricky, and the standard nomenclature deceiving, using a SPL meter might be better. Folk like to see numbers on amp specs, but I bet most don't understand what they mean. So it's understandable when subjective terminology creeps in. You can hardly put "bl**dy loud without sounding cr*p" on the amp spec sheet.

The bottom plate was a pain. Marking and drilling the air holes wasn't too hard, but deburring said holes cleanly was a nightmare. In the end I couldn't do it, so had to use a file and sandpaper thus destroying the good finish so I had to paint it.

Andy.
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Old Today, 7:38 am   #24
mickyfinn
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Default Re: 120W PP EL34 amplifier re-born.

Really enjoying this Andy.When its finished and you have had a rest will you knock me one up please.
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