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Old 15th April 2011, 04:43 AM   #1
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Question Idea for a 6n1p preamp

Well, I had some 6n1p's lying around a spare tranny, so i figured, heck, i'll build a preamp.

Then i decided, not only will i build one, i'll try my hand a designing one too. So this is what i came up with: Basically its a triode gain stage with a cathode follower. Nothing too fancy, but as a first design i don't think its too bad.

My only concern is:
1. Does it make sense to put the balance pot where it is in the schematic?
2. I'm pretty sure i have to float the heaters since triode in the CF passes to 100V cathode-heater voltage of the 6n1p, but I'm not 100% certain.

Schemy:
Click the image to open in full size.
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Old 15th April 2011, 06:06 AM   #2
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I like cathode grounded 6N1P battery biased grid the most.
Here you can find schematics http://www.artemislabs.com/docs/Arte...1_Manual_1.pdf
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Old 15th April 2011, 04:10 PM   #3
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It looks like each channel has its own volume control so there is no need of the balance control. If there is 100 volts on the cathode of the CF then you could probably direct couple the two stages (and elevate the heater to about 50V). If the bias on the first triode is about 2V then you can use an LED to bias.
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Old 15th April 2011, 04:33 PM   #4
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I intend to use a dual gang volume pot and i happen to have an alps balance control lying around so i intended to integrate some of the goodies i have, so thats why I asked. My logic in placing it where it is is that since the CF is elevated then it would be a bad idea referencing the balance control to ground.

As far as possibly avoiding elevated heaters, the other option I see is creating a separate 200V supply for the CF, but that would require finding a large value, high wattage resistor since i'd be burning 75V!

Palustris: Good catch with the LED. The first triode is biased at 2V@5ma, and to my knowledge there are no LEDs like that. However the second triode is biased at 2V@10ma so i will use one there.
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Old 31st January 2012, 04:15 PM   #5
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Hi Homik,

Did you make this finally? with a bias of 2v there should be 5ma flowing through the cathode, but that would indicate that almost all the voltage would be dropped through the load resistor - isnt 55K too much? Maybe half that is ok. and for the CF if the Ia is 10ma thats a bit too much as the max is 7.5ma.
Iam trying to design one too, so do let me know your experience and how was your power supply?
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Old 31st January 2012, 04:42 PM   #6
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I didn't recalc the values, but I noticed the following:
  • Rk=400R and Rl=55k seems strange. What operating point do you use ?
  • Load resistors for the tubes and thus currents are very different. This means uneven tube aging. Better use one 6N1P for each stage and both channels. Furthermore this makes it easier to use elevated heaters because there is no potential difference between the cathodes in the tubes.
  • This topology can be modified to a DC-coupled version pretty easy with Ohm's law.
  • No CCS, no feedback -> Distortion will be pretty high.
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Old 31st January 2012, 05:10 PM   #7
DF96 is offline DF96  England
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The bootstrapped balance control may work strangely. Best to keep grid current away from pot wipers, too.
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