300B SE Amp
Hum reduction:
Use an aluminum chassis.
Magnetic steel conducts magnetic fields from the power transformer and B+ choke to the output transformer.
If you must use a magnetic steel chassis, elevate the SE output transformer off of the chassis with spacers to reduce the magnetic fields to the output transformer. A 1/4 inch of space should make some improvement of hum levels.
Some kinds of stainless steel are not magnetic, or only mildly magnetic.
Put the power transformer(s) and B+ choke at one end of the chassis, and put the output transformer at the other end of the chassis.
Orient the power transformer coil and B+ choke coil in one direction. Then orient the output transformer coil at right angles to that of both the power transformer and choke coils.
Your schematic does not show a filament winding for the 300B. Do Not use the same 5V filament winding that the rectifier tube filament uses. That will burn out a lot of parts and circuits.
You could use the 6.3V winding, and two series resistors, 0.5 Ohm 3 or 5 Watt each.
Wire from one end of the 6.3V winding to one end of the 0.5 Ohm, and the other end of the 0.5 Ohm to pin 1 of 300B. Wire from pin4 of 300B to one end of the other 0.5 Ohm, then the other end of that 0.5 Ohm to the other end of the 6.3V winding.
That series circuit is: start of 6.3V, 0.5 Ohm, 300B, 0.5 Ohm, return of 6.3V.
Or, you are going to need another transformer to power the 300B 5V filament.
The hum pot on the filament will allow you to null out some of the hum. That gets you up and running.
But you probably will eventually want to use a floating 5VDC supply for the 300B filament. That will reduce the Intermodulation of 2 x the line frequency on the test sine waves, and on the music notes. 50Hz power x 2 = 100 Hz intermodulation; 60Hz power x 2 = 120Hz.
You could start with a floating 5VDC supply, but that does complicate your fairly simple and straightforward schematic (does not change anything on your schematic, but adds the circuitry of a filament transformer, solid state rectifier, filter caps, resistor, and another filter cap (CRC).
Now, lets prevent a major cause of hum: The B+ ground loop.
Connect the B+ high voltage secondary Center Tap directly to the negative lead of C6.
Connect a wire from negative lead of C6 to both negative leads of C7.
The point is that you do not directly connect the center tap nor the negative lead of C6 to the central ground point of the amplifier. But after you connect the negative of C6 to negatives of C7, only then do you connect the negatives of C7 to a central ground point.
The central ground point is where Most the grounds and returns connect.
You also connect this central ground point to the chassis. That shields the other circuitry.
Connect the following to a central ground point:
The negative leads of C7; bottom of R3 C2; R6 C4; the negative lead of C5; the Chassis; the Common of the output transformer secondary; and the bottom of R1 C1.
Use insulating shoulder washers to isolate the RCA phono input jack from the chassis. Then connect the outside of that RCA jack to the bottom of R1 C1.
The reduces the input ground loop.
Safety:
We have accounted for all the grounds, except possibly one:
In the US we have 3 wire power, Hot, Neutral, and Ground.
I use a 3-wire IEC socket on the amplifier, and a 3-wire plug to IEC cord.
On my power amplifiers, I connect the Ground of that IEC input socket to the chassis.
That provides safety for those who might come in contact the chassis. (even if there is a short of B+, or output transformer primary to the chassis, there is no shock danger.
Safety First!
Your power mains may or may not require that, I do not know what your power mains are like in your country.
All my other audio equipment: CD player, Phono Preamp, Turntable, and AM FM Tuner have 2 wire power cords. But those are UL and CSA safety rated to be used with 2 wire power cords. They do not have an exposed chassis that I could touch, and the only other points on them are the RCA phono jacks, and they end up connecting to each other which means they are ultimately connected to the power amp, and the power amp inputs are grounded to the input bias circuit, and from there to the central ground point of the amp, and to the 3-wire power mains ground.
Use a chassis that allows the tubes to breathe, and so you can space your tubes away from any heat sensitive parts, like an electrolytic that is on the top of the chassis.
Safety First!
I use both a fast blow fuse, and a slow blow fuse in series. That protects my amp from different kinds of failures.
For example, I have an amp that uses a 1.25 Amp fast blow, in series with a 600mA slow blow.
That amp has solid state diodes in the B+ supply. And filaments are very low resistance when cold; both of those creates an inrush transient current at turn on, the 1.25A protects against that. The slow blow is not affected by the transient, but if one of the tubes starts to fail and draws too much current, or if an electrolytic begins to fail, the 600mA slow blow will open.
Safety First!
Shhh, don't tell anyone, but I am building a 300B SE amp again, but this time I will use 5VDC not AC for the 300B filament.
And I plan on building a 2A3 SE amp again, but this time I will use 2.5VDC not AC for the 2A3 filament.
Happy building, and happy listening.