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Old 14th February 2007, 05:21 PM   #1
Elkaid is offline Elkaid  Canada
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Default Aikido with dual personality

Hi everyone

I'm currently building an Aikido preamp (4X 6SN7) but I would like to have the possibility to switch between 6SN7 and 6AS7 output tubes.

Since 6AS7 is voltage limited compared to 6SN7, would it be a bad idea to use a 200VDC power supply for both topologies ?

Mr. Broskie suggests running 6SN7 @ 220-300V but I'd like to know if running 6SN7 at 200V would be catastrophic.

I plan to go on the "cheap" route at first with a simple RC power supply. (a 263CX 180-0-180, 250mA hammond transformer using only one side of the center-tap and 220ohms/220uF RC filter).

Anyone has tried ? Your ideas would be appreciated !

Thanks !
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Old 14th February 2007, 05:34 PM   #2
Duo is offline Duo  Canada
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A 6AS7 should probably be alright on 200V if you're careful with it.

I've had other tubes over 150-200% their specified range without problems.

The worst it'll do is arc inside. Not great, but not going to blow anything besides a resistor up either.
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Old 14th February 2007, 06:25 PM   #3
Elkaid is offline Elkaid  Canada
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Thanks !

But is there any issues running a tube under the recommended value ?
It is simply a matter of a lower amplification factor ?

Also, is there anyone who used 6AS7 in their aikido ? I cannot find any recommended component values when using 6AS7 in M. Broskie's doc.

Thank you !
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Old 14th February 2007, 06:26 PM   #4
Duo is offline Duo  Canada
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I've never worked with the Aikido.

As for running tubes out of spec, one can generally get away with quite a bit.
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Old 14th February 2007, 07:51 PM   #5
pchw is offline pchw  United States
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Another consideration, 6AS&'s heater draws 2.5A va 0.6A of 6SN7.
This implies almost another 8A demand to the filament PS....
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Old 14th February 2007, 08:24 PM   #6
Elkaid is offline Elkaid  Canada
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Quote:
Another consideration, 6AS7's heater draws 2.5A va 0.6A of 6SN7.
You're right !

However, I'll recover a 8A, 6.3V transformer from a previous stereo double push-pull project which had 10 tubes to supply.
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Old 14th February 2007, 08:46 PM   #7
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John Broskie has actually shown how to use a 6AS7.

http://www.tubecad.com/2005/July/blog0051.htm

From the link

The formula for the optimal ratio of power supply for the bottom tube is

Ratio = 1/mu + ½

As you ca readily see, the greater the mu, the closer the ratio gets to 50%. But with a tube like the 6AS7, with a mu of 2, the needed ratio becomes ½ + ½ = 1 or 100%. Thus an Aikido amplifier with a 6AS7-based output stage should look like this:



Notice the missing voltage divider; all of the power supply is presented to the bottom triode’s grid.
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Old 14th February 2007, 08:47 PM   #8
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Hi Elkaid

If I undersood well you plan to use 200VDC for the B+. If yes... In an Aikido that implies in 100VDC (half the B+) across each tube. If you go for a B+ of 300VDC you will have 150VDC across each tube, a voltage that is better suited for the 6SN7. The 6AS7 is rated for V Anode max 250VDC, so you can safely run it with B+ of 300VDC, as there will be 150VDC across the tube.

Note that the 6AS7 will draw much more current than a 6SN7. Using CRC filters will make the voltage drop in the power supply increase, resulting in a different working point for the first 6SN7. It will work, but it will be different.

I hope this helps you

Erik
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Old 15th February 2007, 01:07 AM   #9
Elkaid is offline Elkaid  Canada
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Thank you guys

Quote:
If I undersood well you plan to use 200VDC for the B+.
You're right, I was referring to the voltage at the output of the power supply. Since the voltage seems to be "cut" in half, I'll be nowhere near tube's maximum voltage using 300VDC.

Quote:
Notice the missing voltage divider; all of the power supply is presented to the bottom triode’s grid.
In that case, I presume the B+ isn't halved right ? If that's right, should I conclude that it would be a little more complicated than previously thought to be able to switch output tube without modifications ?

Thank you very much !
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Old 15th February 2007, 08:30 AM   #10
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Hi Elkaid

The voltage divider Bas refers to is only passing AC noise present at the power supply. See that it 'starts' with a cap, which blocks DC, but allows flow of any AC. This AC (the B+ noise) is than reduced thorugh the voltage divider...and fed to the grid of the triode.

You can make a voltage divider that is ideal for the 6SN7, and place a jumper or swith across the upper resistor. With jumper/ switch closed the circuit works for the 6AS7, with it open it works for the 6SN7. Should not be much of a hassle, as you can change the setting while you are changing the tube

Hope this helps

Erik
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