What the world needs is another WE91-inspired amp!

Posted by 91derlust (A) on February 8, 2016 at 01:49:44

No, it doesn't. But I'm going to do it any way. My first build was a 6C6-2A3 and I'm not ready to move on yet. I like my first build, now that it does not screech at me. I'd also like to evolve the basic design a little and maybe give back. I hope I can give back something better than an expensive mess. We'll see.

Objectives

Well, I'd like to (un)learn from this build - to add to my build notes and probably tear others up. As for the amp itself:

Ideally, it won't buzz, fart, squeal, hum, burn through tubes, or burn through the floor.
It will serve as a platform for the occasional experiment - elegance be banished!
Would be nice if it could help tidy up the system. Disclosure: my partner is not a fan of the "ears"; I prefer to call them RF antennae masquerading as grid wires.
Most importantly, it will bring me enjoyment listening to music, night after night.

Topology

Please bear with me. The topology is a 6C6 directly coupled to a 2A3, running at low dissipation. The circuit was drawn by Jeff Medwin - he did a really nice job and included the important voltages, current flows, wattages, calculations etc. I will not be building it as drawn though. Why? Because it would not meet my objectives, of course. While I am happy with the operating points etc., I want to build with more conventional capacitor values, loads, hum nulling, and power supply. Rest easy my friends.

Details? I agreed not to publish Jeff's circuit, even though I am only using parts of it. But, a decent description will get you 90% of the way there, which may be 85% too much for some. These are designed as monoblocks.

The 2A3 runs at Jeff's favourite ~42mA/250V/-48V bias (grid to cathode) operating point. That is also consistent with my belief that 2A3s and 300Bs like being run at roughly 1:6 plate mA:V and at less than 75% of max dissipation. Being directly coupled, the grid sits at ~150 VDC to grnd. The tube is loaded with a nice 4k2 output transformer - just a little higher than optimum. Hum is nulled with my usual combo of 22Rs and a 10R Vishay 26-trun bulk foil trimmer. It works.

~460V B+ is supplied to this stage by a good ol' pi filter (CLC) done pretty well. The filter has been calculated to provide low impedance at audio frequencies and have a recovery time <= 40mS - fast enough to accurately reproduce my home recording of Ludo meows Burial's Street Halo. Modelling in PSUD shows a PS with no ringing and low ripple. Details? A nice Super Hi-B power transformer juicing 5R4 --> 10 uF film C --> 7 Hy/10 DCR L --> 130 uF film C. A CL-80 thermistor (with C bypass) will be used for slow start up. I hope it all works.

The 6C6 input/ driver runs at the same points as per the final JE Labs Radiotron design. 295V B+ for this stage is just a RC node (~50 uF C) after the CLC, using a voltage divider to shunt about 10mA to ground. This reduces the variation of V across the dropping R (i.e smaller R) and and drains the power supply caps at switch off. Some adjustment of current is possible in the Cathode via another Vishay 26-trun bulk foil trimmer, this one making up about 25% of the total cathode resistance.

So, that is it. Now you know where I am coming from with this build. Actually, now you probably know more than you wanted. I'll keep it brief in future. Next? I have no idea - maybe transformers or chassis design.

Ludo is our cat, not my girlfriend.

Cheers,
91.

"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein

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Topic - What the world needs is another WE91-inspired amp! - 91derlust 01:49:44 02/08/16 (16)

RE: What the world needs is another WE91-inspired amp! - drlowmu 14:54:41 02/08/16 (2)

RE: What the world needs is another WE91-inspired amp! - mqracing 21:20:29 02/08/16 (0)

RE: What the world needs is another WE91-inspired amp! - 91derlust 16:55:46 02/08/16 (0)

RE: 4K Opt is a good choice - deafbykhorns 10:44:52 02/08/16 (1)

RE: 4K Opt is a good choice - 91derlust 11:15:51 02/08/16 (0)

RE: What the world needs is another WE91-inspired amp! - lugnut1 06:20:00 02/08/16 (1)

RE: What the world needs is another WE91-inspired amp! - 91derlust 11:34:17 02/08/16 (0)

RE: What the world needs is another WE91-inspired amp! - vinnie2 04:19:56 02/08/16 (8)

RE: What the world needs is another WE91-inspired amp! - 91derlust 11:07:45 02/08/16 (5)

RE: What the world needs is another WE91-inspired amp! - vinnie2 12:00:24 02/08/16 (4)

RE: What the world needs is another WE91-inspired amp! - 91derlust 13:46:52 02/08/16 (3)

RE: Have you tried the JJ2A3-40 - deafbykhorns 17:37:36 02/09/16 (1)

RE: Have you tried the JJ2A3-40 - 91derlust 03:57:44 02/10/16 (0)

RE: What the world needs is another WE91-inspired amp! - vinnie2 18:35:51 02/08/16 (0)

RE: What the world needs is another WE91-inspired amp! - SteveBrown 08:59:25 02/08/16 (1)

RE: What the world needs is another WE91-inspired amp! - 91derlust 11:04:22 02/08/16 (0)

What the world needs is another WE91-inspired amp!

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