6H30PI/6922 Preamp Circuit
I have attached a schematic of my latest preamp design. I based it on the 6H30PI for it's linearity, high transconductance (for low noise) and medium mu (for a reasonable gain). My goal was to design a low noise and accurate preamplifier.
The input stage is biased at 25 mA (12.5 ma/tube), and the output to 10mA nominally. The input 6H30PI tube has a plate voltage of approximnately 85 volts. The 170 volt and -5 volt supplies are both regulated.
I built a breadboard of this circuit and measured a gain of 5 (14dB), a bandwidth of 800 kHz, and a THD of .005% at 10kHz and 2 volts RMS. I used a 100K dual potentiometer (not shown in the schematic) on the input to adjust the volume level. The sound is neutral with no background noise but a faint mains hum. I plan to switch the heaters to DC to see if that eliminates the hum.
I'd welcome any comments on this design.
Here is the schematic. I did not preview this time.
AC or DC on pre tubes
I have a photo album of some of the gear I have fusse with here.
I have some line stages there.
I have gone to DC on the filiments for anything tube as you can run a little bit lower voltage for the same heat, and no hum.
I started using a 12VDC wall wart and a full wave bridge, three 1000mfd caps, and a TO-220 TIP31c and a 12V or 13V 1 watt zener diode as filter reg. I tried other methods, but this "Dumb" circuit workd great and if you use a 2200mfd/12v cap across the bass zener, a slow turn on.
Some tubes like 12ax7 were made so when run with AC, no hum.
A lot of other tubes either need two 100 ohm resistors, or some forgotten ground method. Use lots of copper plate around the tubes and I always put the PS in a box on the floor, run a cable.
You can see if the filiment AC is the problem by turning off the filiment power, and see if the hum fades away. The tubes will run for a few moments before fading out.
I have Russian paper oil caps that I used in this line stage.
I still have enough of them to build four more.
I sthink they are the best, love the sound.
Oil caps are just smooth but still have lots of mid.
Important not to use electroylitics.
I would also add a 120 ohm 1-watt carbon in series from the u3a cathode to c3 as one trick I have found is to use slow smooth carbons in series to buffer caps overshoot and also slow down fast transits for smoother highs.
You can use them after the gain stages to smooth sonics.
I do not like having a current source on cathodes as the current flow now is controlled by Q2 and Q1???
I would use a high idle current tube like 6sn7 or 6fq7 or 6gu7 in the output as having a stage idling high gives a "Big Soundstage".
I found the best combo is a smmoth tube in front, and a big dynamic tube in the out stage. I started usin 6fm7 tubes with the ten watt triode idling at 7.6watts for the out stage.
These gave a huge sound, bigger triode, bigger sound.
I used the 6gu7 tube as line driver in a stereo tube preamp I built last year for a customer in New York. This tube kicks big time and has more power with low impedance loads than a 6sn7!
It is the same wiring as 12ax7, at7, au7 etc. but 6 volt fill
Best of luck
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