Magnatone 280 restoration

[LarBal]

I've just started looking at this stereo amp someone brought me. Chassis is intact, all OT secondaries cut at the speakers, and crossover tweeters disconected.
Will be a pretty big project. I've got all the original tubes in a bag. Apparently this has been passed around a couple techs over the years

Thusfar the only thing that I'm confused about is the selenium rectifier , that's not on the schematic.
You can clearly see the bias circuit : yellow/black bias tap> reversed diode>cap>voltage divider . You can also see that someone added a GIANT bias cap, which is essentially in parrallel with the smaller old metal bias cap.
They both are oriented correctly, + to ground, but this is the only modification I can find in the chassis, and a red flag.

Anyways, should I replace the selenium rectifier? Also, I'm just not really clear on why it's there? Isn't the little black diode doing the job to create neg voltage?

Schematic and pics attached

[LarBal]

All I can think is that the small diode was a mod as well, and originally the amp didn't have it, just the selenium rec, but I'm not sure this was implemented correctly

[Helmholtz]

I guess the selenium rectifier was the original bias diode.
Now it's out of circuit and replaced by the SS diode and can be removed.

[LarBal]

I guess the selenium rectifier was the original bias diode.
Now it's out of circuit and replaced by the SS diode and can be removed.

Ah I see, I didn't take enough time to see the neg terminal of the selenium rec is disconnected . Thank you.

I'm gonna snip out the strange large replacement, replace the stock bias cap and diode mod with fresh ones and move on.

[LarBal]

Why is there a .1 blue molded cap as the second filter in the bias supply? Would another 25u/50v electrolytic be more effective?

I happen to be working on another magnatone of my own, and the last filtercap in the main Power supply it 47n.

What is the purpose of non-polarized caps in these applications?

[LarBal]

Also of interest, the NFB was disconnected from both poweramps you can see the green wires wrapped round some other wires after being snipped

[Pixel]

Why is there a .1 blue molded cap as the second filter in the bias supply? Would another 25u/50v electrolytic be more effective?

I happen to be working on another magnatone of my own, and the last filtercap in the main Power supply it 47n.

What is the purpose of non-polarized caps in these applications?

The impedance of a capacitor is a function of capacitance and frequency. So perhaps they were trying to deal with a specific problem in the bias supply. Or trying to lower the esr.

Low value capacitors are typically non-polar. I think ideally all capacitors would be non-polar but we use electrolytics because they are much cheaper when capacitances are higher.

[Helmholtz]

Why is there a .1 blue molded cap as the second filter in the bias supply? Would another 25u/50v electrolytic be more effective?

I happen to be working on another magnatone of my own, and the last filtercap in the main Power supply it 47n.

What is the purpose of non-polarized caps in these applications?

Non-polarized caps are not required here.
I can't think of a technical reason why they used such low capacitance.

But before doing any component changes I would get the amp working.
You could always add larger value ecaps later if there's a hum problem.

[The Dude]

RFI maybe?