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.

 

[trobbins]

Perhaps no room for those parts under the chassis, as could arise if the trimpot etc was a revision mod. Looking at a photo of the underside indicates that could be the situation.

 

[FPALB]

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.

Grounding either or both increases the hum drastically. Unsure what this means - can you fill me in?

 

[abbey road d enfer]

Grounding either or both increases the hum drastically. Unsure what this means - can you fill me in?

Weird. Where do you ground it? Grounding it at the foot of the 2N306 stage should decrease hum. If it doesn't there's something wrong with grounding.

 

[FPALB]

Weird. Where do you ground it? Grounding it at the foot of the 2N306 stage should decrease hum. If it doesn't there's something wrong with grounding.

Ah interesting - this is where I can really use a lesson. If I ground either incoming end of the 3k3 mixer resistors as trobbins recommended to the foot GND of the 2N306 stage I get reduced hum, but If I ground the same spot to the foot of the power filter section, I get increased hum. I always default to the power section GND because its usually easy to find.

 

[abbey road d enfer]

Ah interesting - this is where I can really use a lesson. If I ground either incoming end of the 3k3 mixer resistors as trobbins recommended to the foot GND of the 2N306 stage I get reduced hum, but If I ground the same spot to the foot of the power filter section, I get increased hum. I always default to the power section GND because its usually easy to find.

But remember, the PSU ground is usually the most polluted (noisy).
So it looks like it's not as weird as it seemed.
Which 3k3 reduces hum the most when grounded to the foot of the 2N306?

Or, can you modify grounding as per drawing?
In order to do that, you need a tank where the output is not grounding to the tank's chassis?

 

[trobbins]

You may well have some 'floating' sections/parts that should be grounded. It can be as simple as a poor electrical connection which looks to be connected but isn't. As before - perhaps run a temporary link between the 2N306 ground node, and the 12AU7 ground node, and include the gnd point used by the 4k7, and the 'B' feed negative gnd point, to fault find if there are some disconnected ground points.

Perhaps also show a photo of how the HV CT connects to ground, and how the first 40uF/450 filter cap connects to ground, as that should ideally from a single negative point. Is the chassis used for multiple ground connections, or is it a single link from 0V to chassis as in modern amps?

Perhaps use an ohmmeter to check if the trimmer pot and the small chassis section are solidly connected to main chassis, as the photo just shows a single screw connection.

 

[FPALB]

Thanks for your inputs! Will follow up in a couple of days when its back on the bench.