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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
Edits: 02/08/16 02/08/16Follow Ups:
In all fairness, and for proper disclosure, that 2A3 op point is NOT my original idea, but rather, it was taught to me by Dennis Fraker, of Serious Stereo.Jeff Medwin
Edits: 02/08/16
Dennis specs a 2500 ohm primary impedance airgapped to support the plate current that he runs.
MSL
Builder of MagneQuest & Peerless transformers since 1989
Thanks Jeff - yes of course you are correct.
Cheers,
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
I like the tradeoffs!!
I'm actually using a 5K in my 2A3-40 amp, custom wound by Intact audio.
I would have liked a 5k load, but it would have left me with pretty marginal power in my situation. Still 4k2 will be nice match with EML tubes that already sound pretty clean.
Dave winds some mighty fine iron! The transformer will use is a custom J&K Audio nano-crystalline C-Core. Almost 6Kg for an output transformer in an amp that produces ~2W - seems excessive, but hey...
Cheers,
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
I am also considering a 6C6 like the JE Labs.Would the 6C6 be good for driving tubes like KT88's and EL34's??
I am not sure, but can't see why not. I like them, but as Steve mentioned below there are other options and the EL84 is supposed to be excellent and is more readily available.
I have seen some designs that run pentode driving a pentode. I think T-K may have suggested one. Would be interesting!
Cheers,
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
Sounds interesting 91. I am wondering if your 91 moniker comes from owning or ever having listened to a WE91 clone? Joe Roberts version in the first issue of SP magazine is a great amp and hard to beat for SE.
Hi vinnie - I currently run a 6C6 - 2A3 pretty much per the design on the JE Labs site. I like it, but would love to try some all DHT amp at some point.
Cheers,
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
I just got through bread boarding a 300B amp using a pair of 26's as input and driver tubes. It sounded good, but on comparing it to the SP 91A, it seemed pretty much a draw so I decided against building it. The all dht 26/26/45 however is a different story. As I posted back awhile ago I really like it's sound and currently am gathering parts to build it.
I have built several 2A3 amps over the years, including the JE labs "simple" amp and JC Morrison's "micro". Both are good amps, but the 2A3 just doesn't float my boat. Just my taste in tubes I guess. Do keep us posted on your project.
Edits: 02/08/16
Hi vinnie, I year you re the 2A3! They can sound a touch zingy or "strained" as Jeff would say. I reckon the differences between operating points and manufacturer are more important than the differences of tube type: an EML 2A3 will sound a lot like an EML 300B; a WE 300B will sound rather different to an EML 300B. But of course, if you have a fave manufacturer that produces a nice 300B, well, there you have it!
I have thought about going for a 300B and will visit this next... unless I get distracted by the 50 which seems like it could be my ideal tube.
cheers,
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
It similar to the 300b but has 2.5v filament so Ac heating is doable
I have a pair and like them, but prefer the EML solid plates for a similar but more even, slightly more resolved (especially in the bass), and honest sound... in my system with my music. I mean no disrespect the mighty JJ though. It is a fine tube: the most dynamic and propulsive 2A3 I have tried.
I suspect my system is simply more suited to the EML than JJ. That may change in the near future.
Cheers,
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
I would like to try a 50 too, but I think the prices are crazy. Maybe If I trip over a pair at a garage sale some day ...... : )
My favorite variation is based on Thorten's work with an EL84 driving the 300B. Very dynamic, great bass definition. If you do some searches you can find his old articles describing this. I've not built it with the regulated PS, always used a more conventional PS. These have been my go-to 300Bs for a few years now, no plans to change. And yes, I've gone the 6C6 route, as well as other pentode drivers but the EL84 brings something to the party that the others miss IMHO.
Hi Steve,
Good to know about the El84 - sounds like it is worth giving them a shot in the near future. Bonus - no need for messy grid caps etc.
Cheers,
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
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