Does anyone have experience with Diego's Monophono preamp for 78rmp discs, published in Sound Practices Issue 16? Gain is from the triodes of a 6SL7, maybe 30dB at the start from the input triode, then minus 7dB insertion loss for the cross-over frequency network, another 30dB gain from from the second triode, then maybe a dB or two insertion loss for the treble de-emphasis network (though not so much at-mid-band), plus perhaps another dB lost in the cathode follower output. So maybe 50dB gain all up?
Any builders out there with mid-band measurements? Any thoughts on estimating gain?
Any builders out there with mid-band measurements? Any thoughts on estimating gain?
Hmmmm .... checked the hard copy of Sound Practices and the digital archive and it looks a lot more like 120k than 620k. And I'm not the first to ask (https://www.diyaudio.com/community/threads/monophono-preamplifier-by-enjoythemusic-it.285165/). Artosolo, in your sim which value (620k or 120k) best implements the 1590μs bass cut when paralleled with the 270k resistor?
Looking at the valvesI have to hand, I could build the Monophono with a 5751 and one half of a 6CG7.
But perhaps sub-miniature valves will do. The 6112 is much like the 6SL7 and 5751. I have some 6021s that could serve for the cathode follower output. However, the 6021 is not much like (!!) Diego's suggested 6J5 and so the value of its cathode resistor will need adjusting to achieve direct coupling to the previous stage. Artosolo, did your sim indicate quiescent voltages on each of the anodes of the triodes in the 6SL7?
But perhaps sub-miniature valves will do. The 6112 is much like the 6SL7 and 5751. I have some 6021s that could serve for the cathode follower output. However, the 6021 is not much like (!!) Diego's suggested 6J5 and so the value of its cathode resistor will need adjusting to achieve direct coupling to the previous stage. Artosolo, did your sim indicate quiescent voltages on each of the anodes of the triodes in the 6SL7?
After estimating load lines and operating conditions for various substitute valves (6112, 6021, 5755, 6P5G, 6J7 as triode), I have come full circle back to Diego's original choices. They strike a good balance. So I will plan my build for 5751 (instead of a 6SL7) and instead of a 6J5 half of an 8SN7GTB (which needs an elevated heater voltage and its own transformer so the "odd" heater voltage is not much of a complication)
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The power supply and heater reg boards have gone into a salvaged chassis (from a Sansui AU2900) with valves mounted horizontally.
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The wee power transformer can't deliver enough heater current for both a 5751 and an 8SN7GTB so the 'SN7 went back into the stash and was replaced with a 6J7 pentode wired as triode. Not quite as gutsy as a 6J5 and I will need to adjust its cathode resistor to ensure proper (direct) coupling to the preceding section of the 5751. I have added a switch to the front panel to enable playback of "hill and dale" 78s like Edison Diamonds. Next week I hope to have time to test voltages and currents .....
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Is the copper bar on the left bottom corner tied to the chassis and connected to the ground wire?
IMO it's not a good idea, can be an inexhaustible source of hum.