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Old 22nd Jun 2021, 6:26 am   #21
Diabolical Artificer
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Default Re: New valve amp build - 30w 6146 AB2 PP.

Getting there but also encountered yet more problems. As you see from the pics it's all painted and pots/knobs fitted. I no longer have oscillation but do have leakage of signal between left & right channels. I suspect this is a ground issue or a bogup of wiring on my part. Grounding of stereo amps is tricky, in my case I've gone for each IP goes to each channels gain stage ground. This is slightly complicated by the cathodes followers on each IP, these are two triodes in one ECC82. Each CF is grounded separately instead of both triodes sharing a common ground as would usually be the case; this may the seat of the issue. Anyhoo, no biggy.

What is more of a problem is self induced PSU issues. The amp uses two tfmrs, one supplies HT & heater supply for both OP stages/CF-drivers & phase splitters. The other tfmr supplies a negative supply/bias, screen grid supply and IP/tone stage heater supply. I wound both tfmrs myself but have bogged up on the latter - the negative rail is sagging, should be -100v say -80v loaded,it's dropped to -62v. The regulated screen grid supply is also going into SC protection mode, probably a mistake on the PSU board.

I tested all these supplies but have obviously made a mistake or mistakes somewhere, ce la vie, I'll sort it but am suffering from a lack of enthusiasm at present so I've parked the amp out of the way for now and will drop back onto when I have a few other jobs/projects sorted. Anyroad,here's a few pics.

Andy.
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Old 22nd Jun 2021, 6:31 am   #22
joebog1
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Default Re: New valve amp build - 30w 6146 AB2 PP.

Its OK to park up a design that gets upya nose Andy.

Do summit else for a few days and it will be clear to you when you un-park it
Best of luck

Joe
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Old 22nd Jun 2021, 9:05 am   #23
vinrads
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Default Re: New valve amp build - 30w 6146 AB2 PP.

It's looking better now for a coat of paint ,love the colour , you do right have a rest from it for a while , then start a fresh with a clear mind ,good luck with it . Mick.
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Old 25th Jun 2021, 7:56 am   #24
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Default Re: New valve amp build - 30w 6146 AB2 PP.

Interesting following your travails. Looks very nice!

Could it be worth trying a motor run auxilliary cap in the PS?

Is the schematic the one in post 12?
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Old 27th Jun 2021, 8:46 am   #25
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Default Re: New valve amp build - 30w 6146 AB2 PP.

Cheers Joe. The paint is cheap stuff from a Woolworths clone Mick, a lot cheaper than proper British Racing Green.

Quote:
Could it be worth trying a motor run auxilliary cap in the PS?
That problem is fixed at present also AFAIK to address the HT PSU resonance requires a very big cap. There's no room to fit an electrolytic of the required capacitance, motor run caps being bigger than electrolytics also rules this out.

Yes the schematic is the one post # 12, essentially; there are several changes though.

Andy.
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Old 28th Jun 2021, 10:47 am   #26
Al (astral highway)
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Default Re: New valve amp build - 30w 6146 AB2 PP.

Neat work once again Andy! Sorry it's bugging you but I'm sure with your hallmark tenacity and perfectionism you will find a way!


Meantime, I hope you enjoy a few days' R & R from it.


Cheers
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Old 30th Jun 2021, 6:26 am   #27
Diabolical Artificer
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Default Re: New valve amp build - 30w 6146 AB2 PP.

Thanks Al. A few days working on other stuff has the juices flowing again. The workshop's tidy, I know where most things are and it's also raining (again) so can't go out to play; excellent conditions for tackling the b******.

Andy.
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