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).

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.

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.

An added gain stage if needed, would let you use lighter tracking, lower output ceramic cartridges, and give you plenty of volume. A note 10" LP discs need the same 1 mil stylus to play back without damage too. On some post WW II phonos, an option.

Very interested to see tube schematics for this. I recently renovated a RC-45J - I replaced the cartridge with a 'new' stereo ceramic and wanted to boost the output a little. I used a 1:2 audio transformer which messed up the impedance matching, so I also added a JFET unity amplifier to provide the needed impedance. Maybe all a bit unnecessary, but a useful learning experience.

While I was doing this, I was wondering if I could build a tube version. One unit I came across in my googling was the Wurlitzer kit 87. I even found one on ebay, but I couldn't find a schematic for it, and anyway I discovered I had some JFETs lying around...

A single 6AV6 or 12AV6 resistance capacitance coupled stage preceding the output tube, will really kick the gain up a notch, and let you use a ceramic cartridge in place of the old crystal one. Attached is a basic circuit of a phono amp you can use as a guideline, from the GE tube manual. Note that if your output transformer does not have a hum bucking winding, it is fine, you can still use this basic schematic.

You may need to employ a bit of creativity to get the heaters wired up properly, but you can use the component values from this schematic for a good starting point.

Those tube manual example circuits are good to copy. The radio and tv manufacturers were likely to omit any component if it did not pay to include it but the tube makers saw some reason to include it.

As to circuits: if you have room, install a 12AV6.

If you don't have room, or don't want construction hassles, hold your nose
and use a 40 volt power supply op-amp and two 18 volt Zener diodes in series
to drop B+ and give a reference 18 volt above ground. A fixed gain
will be fine, say 5-10 depending on the cartridge output.

Reason why these are used in simple low tube count circuits, is to provide sufficient volume. The 3 volt crystal cartridge could drive a single tube amplifier very well. But the typical ceramic was 1 volt or under that's available today.