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Old 17th Sep 2021, 7:21 am   #1
Diabolical Artificer
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Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,639
Default Class AB2 Valve amp build.

Built a new amp, a Class AB2 PP amp using a 2C34 double triode. Originally I was going to use a HY615, 2C22 and 6BL7,the HY615 as the first gain stage, the 2C22 as a cathodyne phase splitter and the 6BL7 as cathode followers driving the 2C34 grids positive. However the HY615 just doesn't have enough gain, it's the only valve in the circuit providing gain, the cathodyne and cathode followers all having less than unity gain, as we need 100v P-P on the grids of the 2C34 double triode, the HY615 with a gain of 20 ish just doesn't cut the mustard.

So, a re-think was needed, I swopped the 2C22 for a Russian 6SN7 (6H8C) configured as a LTP run off a CCS off a -80v rail, this gave more gain,enough to get 10w out of the 2C34. While I was at it I swopped the 6BL7's also for a 6H8C,the 6BL7's are capable of nearly 10w themselves,they were wasted here, the 6H8C also running off constant current sources at 5mA each triode, this with an anode voltage of 170v (250v HT) gives us about 800mW, enough to drive the 2C34 grids positive.

The power supply consists of a homebrew handwound toroid, HV secondary, bridge rectifier and at first two 820u caps in parallel trying to do away for the need of a choke, but the HV sec winding wasn't beefy enough, miscalculation on my part, I was getting heavy ripple, over 10v RMS because the winding couldn't provide enough current to charge the big caps. I pulled the 820u caps, swopped em for two 220u and used a 25H choke, ripple now 2mV, much better.

The chassis is an old Grundy ali roasting tin found in a neighbours dustbin,it makes a grand chassis, a few coats of etching primer and matt black and presto, job done.

It all worked bar a few minor faults, the usual teething troubles,only one continuing issue is one HY615 has a partial heater cathode short causing hum when first powered up,a tap with a stick and it goes quiet. Usual video's here - https://www.youtube.com/watch?v=BE7U7nMDI24

TFL, Andy.
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Old 3rd Oct 2021, 9:31 am   #2
tri-comp
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Join Date: Oct 2008
Location: Aalborg, Denmark
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Default Re: Class AB2 Valve amp build.

Hi Andy,

Nice to see you got there.

Best,


/Torben
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Old 10th Oct 2021, 12:17 am   #3
retailer
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Location: Adelaide, South Australia
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Default Re: Class AB2 Valve amp build.

I checked out your youtube videos - your amp projects are wide and varied and look interesting.
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Old 12th Oct 2021, 12:42 pm   #4
Gabe001
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Join Date: Oct 2019
Location: Stockport, Greater Manchester, UK.
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Default Re: Class AB2 Valve amp build.

Yes I agree, they're also very instructive, going through the circuit and troubleshooting the issues. I may borrow your idea of a roasting tin as a chassis - genius!
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