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Post by mauro on Aug 1, 2018 0:05:35 GMT
So I unintentionally lied.
In my last post I stated that I wasn't going to work on anything for a while but, I just couldn't pass up a deal I uncovered! It's a very rough but restorable model 750 juke and my first 1940's jukebox (that I've actually owned.)
I'm poking around and evaluating the amplifier and found some some strange mods. I'm hoping someone can enlighten me, I believe it must have been done to accommodate the Jacob's tone arm, along with the very poorly home built preamp.
There is a 12K pot installed between pin 5 of the 6J5 input tube and the 4 pin input socket from the volume control.
There is also a very large 4.5K resistor that is placed between chassis ground and pin 7 of the speaker plug?
Why are these components there?
Mauro
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Post by Ron Rich on Aug 1, 2018 0:17:22 GMT
Hey Mauro, Guys here are good--but I don't think this is the site that has crystal balls--or even worse-Reverse, crystal balls !  Ron Rich |
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Post by mauro on Aug 1, 2018 1:05:45 GMT
Ron,
I was hoping, with the amount of Jacob's tone arms out there that someone has come across this before.
I'm sure the knowledge is out there.
I've seen many jukes with this tone arm but never worked on an amp with this mod.
If no one is familiar with this I will likely put it back to factory wiring but I'm willing to bet one of the amp guys have run across this previously.
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Post by Ron Rich on Aug 1, 2018 2:32:13 GMT
Mauro,
Check with Denny at Victory Glass--
Ron Rich
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Post by robnyc on Aug 1, 2018 11:06:36 GMT
Mauro, what you are seeing is likely the typical mods to accommodate both an external preamp and replacement speaker w/no field coil.
To confirm these assumptions I would look at the resistor from the G1 of the input tube to chassis ground. Since it is a pot, typically the arm would go to the grid pin and high side to input plug. Given no schematic or close-up pic, that is just a guess here.
On the speaker plug, is B+ going to the pin where the resistor is connected? This is how it is often done to prevent the B+ from going up when the normal load from the field coil is removed. FWIW: In the rare cases where I dealt with amps this old I preferred to add resistance - in series- with the B+ supply to attain the lowered voltage rather than just wasting current through a bleeder resistor.
RobNYC
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Post by mauro on Nov 15, 2018 2:58:25 GMT
Rob,
I believe you are correct about the pot, one side is connected to chassis ground and other side to pin 2 of 6J5 and the wiper connected to the input socket pin 2.
The jukebox tonearm has been converted to a Jacob's arm with crystal cartridge and yes there is a homebuilt preamp behind the mechboard. Very sloppy build!
So that I get now, if I go back to the original tonearm I will just undo the mod.
The added large 4.75K resistor is wired just as you guessed, one end to pin 7 of the speaker plug (high voltage) and other end to chassis ground.
There is also a remote rca speaker jack wired to the voice coil pins.
The internal speaker tests fine.
It seems that this jukebox was wired to run only a remote speaker with cabinet speaker unplugged??
Please help me out here, if I were to connect the cabinet speaker to this amp the field coil would be parallel to the large resistor and cut the value to about half and raise the high voltage, this would mean that I can't use the cabinet speaker without reversing the mod?
Is this correct?
Thanks,
Mauro
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Post by robnyc on Nov 15, 2018 5:33:18 GMT
Rob, I believe you are correct about the pot, one side is connected to chassis ground and other side to pin 2 of 6J5 and the wiper connected to the input socket pin 2. The jukebox tonearm has been converted to a Jacob's arm with crystal cartridge and yes there is a homebuilt preamp behind the mechboard. Very sloppy build! So that I get now, if I go back to the original tonearm I will just undo the mod. The added large 4.75K resistor is wired just as you guessed, one end to pin 7 of the speaker plug (high voltage) and other end to chassis ground. There is also a remote rca speaker jack wired to the voice coil pins. The internal speaker tests fine. It seems that this jukebox was wired to run only a remote speaker with cabinet speaker unplugged?? Please help me out here, if I were to connect the cabinet speaker to this amp the field coil would be parallel to the large resistor and cut the value to about half and raise the high voltage, this would mean that I can't use the cabinet speaker without reversing the mod? Is this correct? Thanks, Mauro Mauro, are you sure about the pot wired to Pin 2- The input will be pin 5 on a 6J5. The typical input for any tube amp is; Pot high side to source Arm to tube grid (pin 5 here). Low side of pot to signal ground. I am not too familiar with those pre-carousel Wurlitzers but looking at the schematic: www.verntisdale.com/schem/503.jpg you see the L&C tone shaping they used on all their early arms with magnetic pickups. Often when these boxes were modded for later carts the components in that box were removed and a high impedance tapped volume control was installed to better match the piezo cart. To reuse the cabinet speaker the dummy resistor must go as the field coil will serve to load the B+. Just reconnect as shown in the schematic. FWIW: this was why i often avoided buying jukes that'd been in homes. Tinkering by know-nothings often led to real damage that I might miss with just cursory exam. RobNYC |
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Post by mauro on Nov 15, 2018 21:22:28 GMT
Rob,
Correct, pin 5 of 6J5, I misread my notes.
Now I understand exactly what's going on inside this amp.
Thank you,
Mauro
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Ron Rich 