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Old 11th September 2008, 12:49 AM   #11
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Yes Jerluwoo, that is the kind of thing I had in mind. I have heard that the AX7 does better with higher B+ and larger plate load but I have not actually tested that theory. I think that SY mentioned something on the order of 200K+ on the plates for best linearity but I think you might want to go for a larger HT in that case. Maybe SY will chime in with some definitive guidance in that regard.
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Old 11th September 2008, 12:53 AM   #12
kevinkr is offline kevinkr  United States
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Here is a circuit that I designed long ago that you might find interesting - it's a good performer and very simple.

You need 300V of well filtered dc to make this design work optimally.

Please note this design is for personal use only. No commercial use permitted.. Legal boiler plate. (The design comes out of my old pre-amp project book which is still available.)

Gain is about 20dB..
Input impedance of amplifier used with this design or any design based on a single 12AX7A for that matter should be >70K. Output Z is ~1.5K.

An unregulated but well filtered supply is required. A 6X4 or 6V4 rectifier tube should be adequate for the 5mA required. The filament supply for this design should be dc to minimize hum and buzz coupling to the cathode circuit as this is not a low impedance node.

The transformer could be a 500VCT @ 20mA or so, with a single filament supply of 6.3V@1A for the rectifier.

I recommend a separate 12.6Vdc floating filament supply. You can use a 16V@1A transformer driving a bridge and a 4700uF/16V cap. This feeds a heatsinked 7812 with a 1N4148 diode in series with its ground lead to give 12.6V, a single 10uF cap across the output should be sufficient.

Note that I said the filament supply is floating - this is because the negative side of the supply is tied to a voltage divider between B+ and ground.

The voltage divider consists of 3 components, a 249K resistor connected to B+ on one side and to a 10uF/100V Cap (+) and 49.9K resistor in parallel. The other end of both the cap and 49.9K resistor are grounded. Connect the negative side of the filament supply to the point common to all three parts. Use only pins 4, and 5 for 12.6V operation with the 12AX7A. This will bias the filament supply to 50V.

Several former clients built this pre-amp and with care it can sound very good indeed.
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Old 11th September 2008, 03:21 PM   #13
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Quote:
Originally posted by kevinkr
Here is a circuit that I designed long ago that you might find interesting - it's a good performer and very simple.

You need 300V of well filtered dc to make this design work optimally.

Please note this design is for personal use only. No commercial use permitted.. Legal boiler plate. (The design comes out of my old pre-amp project book which is still available.)

Gain is about 20dB..
Input impedance of amplifier used with this design or any design based on a single 12AX7A for that matter should be >70K. Output Z is ~1.5K.

An unregulated but well filtered supply is required. A 6X4 or 6V4 rectifier tube should be adequate for the 5mA required. The filament supply for this design should be dc to minimize hum and buzz coupling to the cathode circuit as this is not a low impedance node.

The transformer could be a 500VCT @ 20mA or so, with a single filament supply of 6.3V@1A for the rectifier.

I recommend a separate 12.6Vdc floating filament supply. You can use a 16V@1A transformer driving a bridge and a 4700uF/16V cap. This feeds a heatsinked 7812 with a 1N4148 diode in series with its ground lead to give 12.6V, a single 10uF cap across the output should be sufficient.

Note that I said the filament supply is floating - this is because the negative side of the supply is tied to a voltage divider between B+ and ground.

The voltage divider consists of 3 components, a 249K resistor connected to B+ on one side and to a 10uF/100V Cap (+) and 49.9K resistor in parallel. The other end of both the cap and 49.9K resistor are grounded. Connect the negative side of the filament supply to the point common to all three parts. Use only pins 4, and 5 for 12.6V operation with the 12AX7A. This will bias the filament supply to 50V.

Several former clients built this pre-amp and with care it can sound very good indeed.
thanks Kevinkr. the PT you said that is 500VDC at 20ma. , Does it support one side or both side....... I prefer to put them all in one Chassis. ........
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Old 11th September 2008, 03:21 PM   #14
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Quote:
Originally posted by MQracing
check out the following url

http://www.shine7.com/audio/12ax7_pre.htm
what's happening on it ?//
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Old 11th September 2008, 03:31 PM   #15
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by longji98

thanks Kevinkr. the PT you said that is 500VDC at 20ma. , Does it support one side or both side....... I prefer to put them all in one Chassis. ........

Yep, supports both channels..
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Old 11th September 2008, 04:55 PM   #16
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Quote:
Originally posted by kevinkr



Yep, supports both channels..
Thanks Kevin. , , it is very helpful for me. thanks a lol
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Old 11th September 2008, 06:40 PM   #17
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Quote:
Originally posted by longji98


what's happening on it ?//

Hello. Not sure what your asking or commenting on....

I hope that's not tears I'm seeing


It's a relatively straightforward circuit.... with published test results...
and the author also provides two alternative strategies (tube types) in the same basic configuration.

MSL
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Old 12th September 2008, 02:20 PM   #18
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It is sorry about that I was making a mistake on it and misunderstood. actually it is a great web ,I got more idea from there , thanks a lot..
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Old 12th September 2008, 02:24 PM   #19
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by the way, I am also looking for one 24 Step plated attenuator kit. ... any suggestion about it? thanks.
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Old 12th September 2008, 07:48 PM   #20
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Quote:
Originally posted by longji98
by the way, I am also looking for one 24 Step plated attenuator kit. ... any suggestion about it? thanks.
do a websearch for DACT and Goldpoint.

If cost is a consideration--- you might also want to look at the PEC-- though not a stepped attenuator--- many people report favorably on it's sound quality.

MSL
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