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Old 5th November 2006, 03:34 PM   #11
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Default 12AX7s in an Aikido

12AX7s are quite useful in an Aikido circuit if you are building a phono preamp (e.g., two Aikidos with a RIAA eq circuit between the two). There, you will certainly need the gain. If you need to support a MC cartridge, you will probably need a MC step-up transformer as well.
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Old 7th November 2006, 03:06 AM   #12
Bengali is offline Bengali  United States
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Hi Erik,

Is an expensive Hammond transformer necessary for the best audio? I read on this forum some one used a low cost isolation transformer and reversed it for 220V out.

I know the Hammond transformers are especially designed for tubes so do they produce superior sound vs. trying to use a cheap alternative?

Do you know if having seperate transformers(small one for the heater and large one for power) vs. one transformer for all voltages reduces the ac noise level?

thx.
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Old 7th November 2006, 03:42 AM   #13
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Hi Bengali.

I ended up using a dedicated transformer for B+ (300vdc) as well as a seperate transformer for the heater (12.6vdc). The reason why I chose a seperate filament transformer is that it allows me to use one of those octal delay tubes - the filaments are on for 60 seconds prior to the B+ transformer kicking on. You could use two switches and a single transformer for soft starts.

As for Hammond, they are reasonably good and relatively cheap (e.g., $35-40) compared to some of the other available mains transformers. Given that I needed 300vdc, the isolation transformer route doesn't work for me (though I suppose the voltage doubler route could work).
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Old 7th November 2006, 06:20 AM   #14
Bengali is offline Bengali  United States
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if you don't mind me asking, currently what tubes are you using for the 300V? I'm going to use the 6GC7/6Dj8 tubes and per the aikido table requires 200V. I guess I could get away with using the isolation transformer. I just don't know if there is any audible difference with one transformer vs. another(noise issues).

Hopefully that valve amp book you suggested has some tube rectifier voltage regulator circuits. I'm sure I will get my fill of answers from those books.

I never heard the 6GC7/6DJ8 tubes before, they sound pretty good? I'm only familiar with the sound of 12at7/12ax7's.


thx.
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Old 7th November 2006, 02:23 PM   #15
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I am building an Aikido-based phono preamp for a very low output MC cartridge (Linn Troika - 0.1mV). I am using the following configuration: Peerless 4722 MC step-up/12AX7/12AU7/RIAA/12AX7/12AU7. B+ will be 300vdc.

I am not sure that you need to go the voltage regulator path - it should be pretty easy to build a PS that gives you 200vdc +- a volt or two.

6DJ8/6922s are commonly used in phono preamps. They should sound fine.

BTW - I am using a 115N060 delay relay with the two PS transformers.
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Old 8th November 2006, 09:01 PM   #16
Bengali is offline Bengali  United States
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do you know of any online sources that carry torodial transformer covers?
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Old 11th November 2006, 12:10 PM   #17
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How would a 6ES8 work for an input tube?
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Old 11th November 2006, 01:22 PM   #18
SY is offline SY  United States
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Relabelled 6ES8 is what gave ECC88/6DJ8 a bad rep in audio for a long time. It's a variable mu tube, designed to be nonlinear.
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Old 11th November 2006, 03:44 PM   #19
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I guess I will throw them against the wall and listen to them pop as they hit. Add to list tube is junk.
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Old 11th November 2006, 08:24 PM   #20
jayme is offline jayme  United States
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For the PSU, I too have a very simple design, CLCLC:

250V mains transformer
5ar4 rectifier
3.3uF cap (shunt)
15H choke
120uF cap (shunt)
15H choke x2(one for each channel)
20uF cap (shunt) x2(one for each channel)

The chokes can be replaced by resistors, altho the regulation is better with the chokes. I used PSUD2 to model the whole supply. That first choke is the most important, tho, and I really wouldn't skip it. And, PSUD2 is awesome for determining what values you need to hit a specific B+. I get 315V out of the above configuration, but if you add some power resistors or a smaller first cap you can lower the effective B+.

And, I had a big thump on turn-on/turn-off when I had too little capacitance in the PSU...bumping up that middle cap to 120uF from 20uF eliminated the thumps completely.

Morgan Jones has some darn good stuff in his book for PSU designs, but they are more for power amps that having varying current draws. The Aikido design has a stable current draw, so you don't need regulation or anything special in terms of hitting a specific voltage.

Heck, the best part of the design I have above is that I can sub in various rectifiers and operate the preamp at a different B+ (if I use a 5u3, I get about 280V) and a slightly different sound (tho the Aikido design is relatively immune to different rectifiers due to the cancellation properties inherent in the design...I think the small differences I hear are more due to the change in B+ than the actual rectifier type).
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