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    Building, troubleshooting and testing of these amplifiers should only be
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1939 Rockola amp

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I am in the process of restoring a 1939 Standard Jukebox. I have replaced all the caps and tube and the amp powers up and work except I have a 60 Hz hum. If I pull the 76 tube the hum go away. I have checked the DC voltage and it looks ok. Have two 76 tubes and they both have the same problem. Anyone have any ideas ?
I am attaching the schematic. Thanks.
 

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Short the input and see if it is still a problem with the 76 tube in place. It could be a pickup problem if the hum goes away. Also make sure the ground on the cathode resistor is good.
Have all ready done both of those. The hum level is low and once music starts you do not notice it. The schematic shows the plate voltage is suppose to be 165 V and I am reading around 88 V. I have the 245 on the high side of the 30 K resistor and the resistor ohms out ok.
 
Have you got the electrolytic capacitors the right way round?

Some caps have the negative marked some have the positive marked.
NB make sure you discharge the supply before you touch anything!

Regards
M. Gregg
Checked the cap polarity and all the resistor values. Shorting the input has no effect. The only thing I have found is the plate is suppose to be 165 VDC but I only have 88 VDC. I have the correct voltage on the high side of the 30K plate resistor.
 
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Checked the cap polarity and all the resistor values. Shorting the input has no effect. The only thing I have found is the plate is suppose to be 165 VDC but I only have 88 VDC. I have the correct voltage on the high side of the 30K plate resistor.

What voltage do you have on the last supply cap feeding the anode of the 76?

Its fed via a 30K.

Regards
M. Gregg
 
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Slightly off topic,

Does anyone know as shown in the diagram a modern substitute for the PH3055?

Sorry back to the thread..:D
Interesting circuit..

Regards
M. Gregg

Check with Mike LeFevre at Magnequest.. He has made this sort of phase splitting transformer/choke in the past. Possibly also Electra-Print. Not sure someone in the EU could not replicate this at a reasonable cost.
 
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This may be suitable. However there is a conflict between specified inductance and secondary impedance.

http://www.hammondmfg.com/pdf/560N.pdf

That looks interesting,

I have never seen this done before, have you ever had any dealings with this type of splitter? How good/bad its is..I'm just surprised they didn't drive the primary of an interstage and split with the secondary can you see any reason for the design being done like this?

Regards
M. Gregg
 
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