• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

12ax7 preamp.

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Mashaffer's suggestion is a good place to start. The problem with the 12ax7 is lack of drive current. Don't expect it to drive anything below 40k input impendence very well. Attached is a good starting schematic thats pretty simple to build.
 

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kevinkr said:



Nope, :D My current faves are all dhts 01, 12, and 26 not necessarily in that order. I would also include the 6SN7 and 6J5 in my "second tier." And I use transformers...


Of course you're right.
I have received a pair of 26 from Netherland. Hope I could make them sings in the near time :)

But the first try would be without transformers :(
 
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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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. :D (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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kevinkr said:
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. :D (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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