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Old 16th August 2006, 10:57 AM   #21
phn is offline phn  Sweden
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pm, indeed. There should be a "real" 407A (and similar tubes) thread. Somebody else will have to start it, though.

Before that, I do appreciate the Aikido and the flexibility of it. But I like the simplicity of the 407A. The same goes for the 12B4, except I'm not a fan of the "required" regulated PSU. I think the 407A could be ideal for the chip and Class D crowds who want a tube pre, as well as output stage for TDA1543/5-based DACs. I do not doubt it can make for a great phono stage. But personally I would rather look into something like the Siemens C3g.

Arnold, the 01A, 26 and 112A should be fun. But unless you have a private stash somewhere, those prehistoric tubes shouldn't be that easy to locate.
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Old 16th August 2006, 02:47 PM   #22
arnoldc is offline arnoldc  Philippines
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phn, I only have pair of 01A which I got cheap from my friend who has a stash of 01A, 26, and 112

I'm going to build the 01A soon, but I'm still happy with the 407A (and try the RCA 407A!!) so I need a hard kick in the butt to get going with the 01A pre. However, I plan to run it with either 1k8 (for 2.5mA) or 3K (for 3mA) at the cathode with diyAudio CCS on the plate.

ps.

By the way, I had an opportunity to hookup my 407A pre to a pricey system with Mark Levinson (306??) amp + Dynaudio Confidence 3.1 (or 3.3 I can't remember) it replaced a known, expensive brand. The owner was shocked, and was shocked even more when the wife (who was also listening) said "how come that really ugly thing sounds a lot better than your (snip) that is so expensive?" (It was the wife who bought the high-end preamp, upon request of the husband)

After that, I decided to put in my ugly thing, in an even uglier (according to choky) chassis (See photo section)
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Old 19th August 2006, 10:24 AM   #23
phn is offline phn  Sweden
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Arnold, I just tested Gary Kaufman's circuit in LT Spice. Thanks again for the models, Robert. I have a gazillion LT Spice models so I can just as well learn the program. I had to remove the .47uF input cap or I got really weird readings. mach1 mentioned that already.

The bad thing is that I have learned next to nothing. The good thing is that I know what I need to learn, which should make reading Valve Amplifiers easier.

I'm not quite sure what your plate choke design is. It looks like a low-gain stage. I will test it in LT Spice.

The zip file is for the newbies. It includes my spice work file and 407A model.
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File Type: zip 407a-pre.zip (95.7 KB, 89 views)
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Old 21st August 2006, 09:07 PM   #24
phn is offline phn  Sweden
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I think I have my final design now.

I'm reading and re-reading The common cathode triode amplifier, chapter 2 of Valve Amplifiers, and scratching my head. Math has that effect on me. I have too many questions. I will settle with one.

Before getting to my question, one of the things I like about the Aikido line stage is that you can get anything from unity to virtually unlimited gain, at least more than what you will never need in a preamp, depending on tubes used.

The cathode somehow relates to the anode of the triode. There's further relation between the anode resistor and voltage. Anyway, by changing the cathode resistor to 2k2 the gain is reduced to just over 16 db. If I change the anode (and subsequent "balancing" cathode) to 18k, it's down to 14 db. That means 14-20 db, or 1:5 to 1:10. This is the main reason I try to understand tube amps--to be able to not just follow a schematic, but to customize it to fit my needs.

Save for the gain, what is the consequence of using a 2k2 cathode resistor instead of 1k, if any? It might be totally irrelevant. But I need to get this question mark out of the way.

I haven't been able to figure out what the 10uF cap is for. Looks like some sort of filter to me. I'll just leave it be.
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Old 22nd August 2006, 12:30 AM   #25
phn is offline phn  Sweden
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Doh! Forgot the perhaps most important question. What Watt rating should the mains transformer have? I figure one of those small 50-75 mA will do. But just getting one of those won't teach me anything.
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Old 22nd August 2006, 03:48 AM   #26
arnoldc is offline arnoldc  Philippines
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Quote:
Save for the gain, what is the consequence of using a 2k2 cathode resistor instead of 1k, if any? It might be totally irrelevant. But I need to get this question mark out of the way.
You'll be running the triode section at 2.8mA or so with 6.26V bias. With 1K about 5mA or so with 5V bias.

In my experience, running the 407A at 9-10mA is what I prefer
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Old 22nd August 2006, 10:08 AM   #27
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Now I begin to understand, I think. Still many question marks. Will know for sure when I go back to chapter 2.

Back to the drawing board.
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Old 22nd August 2006, 10:51 AM   #28
arnoldc is offline arnoldc  Philippines
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phn, actually I am lazy I'm using TubeCAD to give you the values.
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Old 22nd August 2006, 03:01 PM   #29
phn is offline phn  Sweden
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Aren't we all? But I want to get a grasp on the very fundamentals.

I have been looking into the TubeCAD. It's quite affordable. But I never knew quite what I would get--tube library, etc. I guess I'll buy it.
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Old 22nd August 2006, 07:32 PM   #30
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Perhaps no news to most people here, but the 407A is fundamentally different from the 2C51/396A in one regard. Minimum gain for the latter in common cathode configuration (which I think this is) is about 1:20. Higher gain is possible. 1:20 is more or less maximum for the 407A. At least in theory you can go as low as unity.

Upload LT Spice work file and 2C31/396A model for those that care.
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File Type: zip 2c51.zip (1.0 KB, 77 views)
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