• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
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I was so intrigued by the Mu Follower idea that I stayed up way too late last night playing with Spice. Probably the most exciting aspect to me is that I would no longer need to attempt to cram another tube socket in the already crowded chassis.

I started using the 12AY7 as I wanted to take advantage of the low microphonics but the bandwidth was not good. I suspect Herr Miller was getting in the way. Using the lowly 12AU7 provided much better results extending f3 from 15k out to over 35k. PSRR was dramatically better than my first circuit and even better when I used a 6N1P as the top tube.

I think this is well worth pursuing.
 
JMO, put JFET source followers as close to the arm pillar as is possible. HF performance of piezoelectric carts. is poor enough, without cable capacitance exacerbating things.

Once again you are right on the money Eli. :) My issues with getting a Mu Follower gain stage to work well were entirely due to the large source impedance created by the balance and volume control networks which were tailored for the Ceramic Cart. Much better to toss the original pots in favor of smaller values and use a separate follower at the tonearm to buffer the cart. Good frequency response and gain for the 6N1P gain stage and less concern for cabling issues. All for just the cost of a few jfets, a handful of pots and a low voltage power supply. It also makes it quite easy to add a phono preamp and semi-modern TT at a later date if desired.
 

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With semiconductors going obsolete in what seems like hours it is quite frustrating to find currently available suitable devices. I search mouser or digikey with every suggested part number only to get "0 results found". I probably have suitable devices on various boards in my junk box but unlike our fire bottles they are nigh on impossible to identify visualy. :(
 
Getting closer to a final design and doing some real physical work. Cleaned out remnants of old circuitry and began chassis prep adding a tag strip.

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I changed it around to use the octal socket after all so that both the initial gain stage and the tone control use a mu follower. This gives plenty of gain to make up for the balance control and provides enough drive to handle the power amp and subwoofer amp connections without a separate buffer. I also tweaked the caps in the tone controls to move them a touch farther out from the midrange.

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I drove the passive tone stack in SX780 with 6922 triode and 2N5401 CCS. The volume pot is driven by the other half gain of 12. The 100k Volume / loudness/ balance stack sees a signal of 20v p-p with a max overload above 100v p-p. The SS amp has a gain of 15 to keep s/n high. sound quality is outstanding while frequency response is flat at center positions to .2db from 20 to 20khz. I also moved the boost / cut curves further from the 1 khz point. lower bass - higher treble
 
Took a break from the electronics side to fiddle with the speakers. The 32 ohm woofs and 16 ohm tweets seem like they will work well together. I played around with some crossover components and so far it looks like2nd order on the woofer and 1st order (a bit higher up) on the tweeter will tame the shout. Final tuning will have to wait until I get the cabinet modified and braced as it is contributing a lot of resonance.

In addition to gluing some mdf to the walls cross bracing I hope to be able to make an enclosure angling down under the tuner and preamp chassis and to the back ending in an aperiodic type port. This should improve bass and help damp resonances. The front to side interfaces are radiused so will have to glue in some 1/4 round or fill in the joints with duct seal or something.

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It seems worthwhile to do some cutting on the chassis so that I can use all 9 Pin tubes so I have a modified schematic with 12AU7 for both tubes in the tone control section. I also added the voltage divider for the elevated heater (need to put a cap across the 22k resistor). Using the resulting currents I iterated with Power Supply Designer to get hopefully a fairly accurate lock on the voltages and currents to expect.

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