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Old 08-20-2018, 03:56 PM
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DavGoodlin DavGoodlin is offline
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Phono - cartridge amp refit

I have post war radio/phono consoles that use crystal cartridges, which are troublesome compared to common ceramic replacements.

I have several of the latter in my parts, i.e. Varco CN65 and other mono carts not suitable for stereo records but fine for 10" LP and 78's using their .3 mil sapphire stylus.

Is there an old thread some where about building in a X10 gain pre-amp to use the ceramic cart? I plan to make one from a 6AV6, 6J5, 6SQ7 or other triode with only 0.3 amp heater rating (equal to two #47 pilot lamps).
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Old 08-20-2018, 04:51 PM
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Electronic M Electronic M is offline
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I use these cheap ($1.40 each) Chuo Denshi carts: https://www.ebay.com/sch/i.html?_sop...0&_stpos=23518

They work fine for pre and post WWII AA5 table radio/phonos. The output is approximately the same as the original crystal cart.
On a single tube (IIRC 25L6) mono 45 RPM changer that originally used a roughly 3V cart I just looked up (google) the schematic for my RCA 45-EY-1 and copied its preamp stage...Granted I had to grind some ohms law (to add a parallel heater resistor) to make it work in series with the other tube and the motor.

I've also more recently tried putting the two stereo channels in series and gotten increased output.

The only thing annoying about those ebay carts is that they lack a 1/2" mount...My solution is to take some metal shears and slice a band of thin steel (from a discarded tuner shield from a VCR or BPC set) and bend a strap (actually two, one upper one lower) halfway around the cart and drill two screw holes in the bands...It is modeled on the needle guard of the original carts in the RP-190 changers...I had more changers than carts with needle guards so I started fabricating new ones to make up for that. You can see here what that guard looks like:https://www.youtube.com/watch?v=5mPOWaTEq-E
I've also made a couple of 1/2" based on the plastic adapters I've seen out of thin spring steel (one strip goes front back another goes across the screw holes in the arm then the straps are soldered together perpendicular to each other)...They are a bit trickier to make, but another option.

Over the weekend I've been considering growing new Rochelle salt crystals and trying to rebuild some original 78rpm carts (I've tried rebuilding with piezo buzzer elements and the output was too low for even AA5s ). There are tons of youtube vids on how to make the crystals. If I could grow them the right thickness and aprox right size I could probably make it work...I'd also want to encapsulate them to protect them from moisture damage.

Sorry for rambling I've been on a bit of a phono kick lately. I've had R/P sets with dead carts sitting 20 years that those Chuo Denshi carts are finally making it cheap enough for me to fix.
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Last edited by Electronic M; 08-20-2018 at 05:20 PM.
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Old 08-22-2018, 04:25 PM
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Tom, That sounds like a good plan. I was trying to avoid crystal entirely because of assumed high mortality due to temp and humidity. I was also looking at using a tube versus a transistor for closer impedance matches. Nice to know there is an alternative for a high output level.

A guy in Ohio rebuilt the Astatic crystal cart for my 1934 Philco console with an automatic changer but have not done the rest of the Philco's restoration, not even knowing if it will work. I have lots of records to try, 78's can be very interesting and do not sound nearly as bad as one might assume.
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