Magnatone, Starlet Model 107 circuit query.

[larhinds]

Recently a friend brought me a Starlet Model 107 that he had recently got back from a tech in Asheville. The amp still hummed pretty good and started to squeal [oscillate] beginning about a 3rd of the way up in volume. The first thing I noticed comparing the wiring to the schematic was the addition of a jumper from ground to what is affectively the path that eventually feeds the plates of the pre amp [12AX7] tube. [See supplied schematic and photo.] Seemed odd, so I removed it. I acquired and installed a new 1meg pot and switch combo which seemed funky. Amp still hummed and now the squeal was even more drastic. Long story short, I tested for value, just about every component in the thing. Experimented with a star ground concept. Nothing would change the behavior. Eventually, in frustration, I replaced the said jumper. What??? The squeal was no where to be seen. Still a bit of hum, but sounds nice. I can turn it up to 10 with no squeal. Can someone please explain how the addition of this jumper, not of original design, can correct this amps wows? Thank you.

 

[ruffrecords]

This is a lethal unit in its original form because it directly rectifies ac mains. You really need to feed ac to it though an isolation transformers which will then allow you to connect the chassis to safely earth (the third prong on the mains cable).

Cheers

Ian