wurlitzer 1900 (cobra upgrade)

Thanks to charly49 for helping me get this beast up and running. I'm now turning my attention to upgrading the cobra cartridge. I've followed everything that Harold Hagen described in his Wurlitzer Volume #2 book.
Page 21 "Replacing Cobra with ceramic cartridge".
He goes on to say that, You don't have to remove all the components around the cobra oscillator section. Just cut the wire to pin 6 (the plate) of the 12AU7. This disables the oscillator.Then mainly you have to remove the coupling capacitor (C6) from the 12AU7 plate and connect the cartridge through a voltage divider network to the grid of the 6AN8. R9 must also be removed, it connects to the wire extending downward from the grid. He also recommends removing C7 (you will lose treble with it in place). Disconnect the wiring to the input plug (RCA) and connect a 2.2 meg resistor from the plug to the junction of R4 and R5. Replace R4 with a 1 meg resistor and jumper R5.
Now it becomes a matter of trial and error to get the level and tone compensation right. At this point I'm assuming you've done the capacitor job, and the AVC capacitors (C30 and C31 - the 1 and 2 MFD caps) are new. Set the level to get an AVC voltage around 12 volts across the 2 MFD capacitor. He goes on to say that he usually has too much gain and has had to reduce the value of R4. If you are lacking in treble you may have to bridge the 2.2 meg resistor with a small ceramic disk. Try values around 100 picofarad to start. This won't affect the operation of the AVC shunt.
I am also going to use the Pfanstiehl P132D cartridge. I also have a late model stereo tonearm to take the standard 1/2" cartridge.This amplifier is a 532, and not the original 530.
My question would be. Is this still the preferred method of doing this ? Is there a better sounding cartridge available ?
I' m sure this subject is old news to a lot of restorers, but it's all new to me. I can only seem to find bits and pieces about this procedure. Also, what sort of tracking pressure are we shooting for 6 grams? 4-5 grams? I've read something about, if you don't have enough tracking pressure that you can have problems with the trip switch (end of record playing).
On the retroaudiolab web site they use a .1 capacitor instead of a 2.2 meg resistor. They also recommend using a Audio-Technica AT-70 cartridge.
Does anyone have this conversion in a .doc or pdf format that could be posted somewhere ?

Thanks in advance...
Tony

Hello Tony

I know Harold Hagens books and also the contributions about cobras, but I have another opinion. I think, it's a sin to replace the Cobra cartridge with a ceramic or magnetic cartridge. Maybe it sounds better and there is less trouble with stereo records, but it sounds no more like a Wurlitzer. The famous Wurlitzer sound war one reason for the enormous success. Wurlitzer was market leader for many years with the Cobra system. Success faded, when a ceramic cartridge was used (Cobras are only for mono) and the box sounded like any different one.

Of course you can make the conversion if you want. The 532 amp is nearly identical to the 530 and Harold Hagens instructions should be valid for it. I think, there is no problem with the cut-off switch, if you reduce the tracking pressure th 4 - 5 grams.

I don't know the article on the retroaudiolab website and I can't find it. However, it is very strange, to use a capacitor instead of a resistor. The AT-70 is a magnetic cartridge and you will need a preamp for it.

Greetings - charly49