Magnatone 480A Bias + Plate issue

[FPALB]

The reverb recovery circuit starts with the reverb tank output. Have you tried to short or disconnect it?

Ah, understood. Disconnecting it changes the tone slightly to have more buzz than hum when mixed over to the reverb side. The hum is very very consistent when the reverb depth is at zero (grounded reverb signal).

The reverb foot switch jack is a TRS jack, but im a bit confused on what the circuit is doing - can someone explain? I actually found someone selling what they said is the 480 reverb footswitch and it has a TS jack, which makes zero sense to me as the sleeve (Ground) and the ring (reverb) would be immediately grounded. I dont quite understand what the tip of that jack is doing.

I went around with a chopstick and no changes when pushing on grounds, but I may still go back and reflow everything.



The hunt goes on.

 

[abbey road d enfer]

Grounding the tip attenuates the direct signal by about 15dB. Maybe it's some kind of boost for soloing...?

 

[trobbins]

You indicate the hum stopped when you cut right after the 3.3k resistors and before the 5uF. But also said "Grounding the 3k3-3k3-5uF node to the gnd node of 2N306 Foot increased the 120hz dramatically".

Perhaps worthwhile checking if anything related to that 2N305 circuitry is a cause/contributor first, and then go backwards if it isn't.

That 2N305 circuit feeds the 12AU7 - perhaps run a link between those two gnd points, and include the gnd point used by the 4k7, and the 'B' feed negative gnd point, in case there are some grounding issues.

Is the 2N305 centre biased at idle using the 100k trimpot?

Going towards the reverb circuitry, perhaps ground the incoming end of each 3k3 mixer (ie. their pot wipers) and check if that gnd point is a concern.

 

[FPALB]

Is the 2N305 centre biased at idle using the 100k trimpot?

Correct.
Ill give the recommended checks a go upon return from the thanksgiving holiday.

Thanks for your input!


Another question: why would the 2n305 circuit be housed outside of the main chassis? If visualization is needed view images from post #8. I was thinking it was to adjust bias on the germanium transistor as time would require recalibration, but there is no way to access the 100K bias pot from the back amp cavity - you have to first remove a nut to rotate the plate the circuit is mounted on and to do that you have to pull the chassis.