What the world needs is another WE91-inspired amp!
101.183.54.40 |
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| Posted on February 8, 2016 at 01:49:44 | ||
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Posts: 1101
Joined: December 25, 2014 |
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:
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.
Ludo is our cat, not my girlfriend. Cheers,
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| RE: Have you tried the JJ2A3-40, posted on February 9, 2016 at 17:37:36 | |
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Posts: 1067
Location: Florida Joined: October 17, 2003 |
It similar to the 300b but has 2.5v filament so Ac heating is doable
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