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Old 7th April 2004, 07:54 AM   #1
s2kov is offline s2kov  Canada
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Default First Try on Tube Preamp

Ok this is my first try for tube preamp, your opinion on the circuit will be highly appreciated.

Below is my operating point for 12AU7 twin triode connected in parallel:

Plate Voltage = 185V
Plate Current = 7.75mA
Grid Voltage = - 6V


Thanks in advance....
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Old 7th April 2004, 09:00 AM   #2
s2kov is offline s2kov  Canada
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BTW, i need your expert opinion with the circuit if i'm getting right with the values indicated and getting the right track.

Am i really on the right track?
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Old 7th April 2004, 10:44 AM   #3
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hi

How load resistance is expected for that
stage?

Federico
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Old 7th April 2004, 10:53 AM   #4
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You might get better performance by using a 180 ohm resistor and a 47ufd capacitor on each cathode rather than connecting both cathodes together.
That will eliminate the "current hogging" that can occur if the triode sections aren't matched.
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Old 7th April 2004, 11:05 AM   #5
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Kabayan s2kov!

I see you are using 12AU7, the foreplay preamp of bottlehead uses 2 of those. Unfortunately, I think it is copyrighted so no DIY!

Regards,
JojoD
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Old 7th April 2004, 11:25 AM   #6
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Bottlehead is a simple common cathode stage followed by a cathode follower. Direct coupled between stages.
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Old 7th April 2004, 11:51 AM   #7
s2kov is offline s2kov  Canada
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Federico,

I'm using Dynaco ST70 and if i'm not mistaken input impedance is 470K.

Regards.


Andy


Quote:
Originally posted by fscarpa58
hi

How load resistance is expected for that
stage?

Federico
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Old 7th April 2004, 04:07 PM   #8
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Hi s2kov,

Yes, your working point is o.k.

if you want less distortion try 15k at anode
and 200 at cathode. You still have 5V peak to peak before
saturation. Gain will increase of 1.8 db. But output res will rise from 2.5k to 3.2k OHm.
Speaking about output res it seems quite high.
I will consider the use of an output buffer or of an SRPP stage.

Quote:
...by using a 180 ohm resistor and a 47ufd capacitor on each cathode...
if res and caps as to be put at each cathode (advisable thing) the value of res has to be doubled and the value of caps halved to retain the original characteristic.

Bye
Federico
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Old 7th April 2004, 04:25 PM   #9
s2kov is offline s2kov  Canada
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Hi Federico,

Thanks for the help! Actually, operating point for each section of triode tube, Rp is 15K and Rk is 744 ohm. Since the design is connected both halves in parallel, Plate current will become 15.5mA (7.75mA + 7.75mA) thus, Rp = 7.5K and Rk = 372 ohm

Is it right?

How did you get 200 ohm for cathode resistor?

Regards,

Andy
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Old 7th April 2004, 06:28 PM   #10
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Quote:
Originally posted by Frank Berry
Bottlehead is a simple common cathode stage followed by a cathode follower. Direct coupled between stages.

How I wish I have a schematic
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