Hohner Kingstar 20 (1964)

 

Schematic
Note: The schematic shows two 24M resistors in the vibrato section. These are not resistors, but VDRs (150V/10mA/2W) made by Philips. Whoever made that schematic must have mistaken them for the color code (red-yellow-blue), but they look more like ceramic caps: VDR picture.

 

 

Description:

This is a 2 x EL84 Amp with tremolo (called "vibrato"), reverb and a Vox style preamp. It had been "repaired" before (spell: it didn't work). The entire reverb section was missing, there was a goofy standby switch (miniature toggle switch with 350V on it!) and one tube socket was held in place by a nail with a big solder blob on it. There were several other issues (tacked-on capacitors, pilot light, open speaker cable and so on) not really worth to mention.
Needless to say that I did not power it up in this condition. You can see the restoration process in the pictures below. I also rebuilt the reverb from the schematic, with an old reverb tank I had lying around which seemed appropriate for its age.
After I got everything up and running, it sounded really sweet with its old Rola Celestion 10W (2 x 10") speakers, but had a bad hum with it, not really loud but annoying. After trying literally everything (new tubes, filter caps, DC heating and many other things) I came to the conclusion that the hum must have been there from the beginning - crappy design. So I let it rest for a few weeks and then decided to rip it apart and rebuild it with terminal strips and point-to-point wiring. I also changed the parallel filtering on the B+ to a serial arrangement which gave me slightly lower voltages in the preamp but much better filtering.
This was it! No hum at all, even with my ears close to the speaker. I measured clean output power of 14 Watts ("Kingstar 20" seems a bit optimistic) - still loud enough for my garage...

 

 

Pictures: