This is a mutch overlooked device in audio amplifiers, yet it plays an important role in the overall quality, the power transformer.
This is the portfolio of power transfromers which I currently use in all of my projects. They are all quite versatile and allow different configurations. All of them have several features in common:
Grain oriented core laminations
This is a similar core material as is commonly used in output transfromers. It has higher permeability and produces less stray fields. It is also less prone to mechanical buzz compared to conventional cores of the same size. After assembly the complete transformers are vacuum varnished.
Dual screen windings
This is a very important feature of power transformers and it is surprising that it is not more commonly used.
Each power transfromer has parasitic capacitance between primary and secondary windings. This capacitance couples high frequency noise from the mains to the secondary side and thus into the circuit. A screen acts as a shield. With a screen there are parastic capacitances between primary and screen and secondary and screen, but not directly between the windings any more. This effectively shunts noise from the mains to earth. I even have my transformers made with two independent screens. The first one,which is closer to the primary is directy connected to the core, which should have an electrical connection to the chassis and thus to mains earth. The second screen is meant to be connected to signal ground. This provides an even improved isolation from mains noise. With this feature, amps which use such power transformers don't react much to polluted mains. No expensive external mains filters and isolators needed any more!
External flux band
The outer copper foil further reduces stray fields.
Felxible primaries and secondaries
All primaries have taps for 220, 230 and 240V. These can be used to adjust the voltages by +/- 5%. US versions of the power transformers come with 115, 120 and 125V taps. The secondaries are spec'ed such that they can be used in various configurations to obtain different voltages.
Pictured in the photo above are, starting in the front row from the left:
Filament transformer for AC heating:
This transformer has 3 secondaries:
1.25-0-1.25V / 5A
1.25-0-1.25V / 5A
2.5-0-2.5V / 5A
This allows many applications: Center tapped AC filament windings for 45, 2A3 or 300B amps. Either for monoblocks or Stereo. For stereo 300B the two 2.5V windings can be connected in series for the second 2.5-0-2.5V windings. The windings can also be used for rectifiers. The 5A are sufficient for 866As. Even a full wave Graetz bridge with 4 866As can be heated with one of these transformers. All windings are isolated against each other and against primary for up to 2kV.
Filament transformer for DC heating
This one has two separate secondaries with various taps which can be connected in series in different ways. Voltages from 1V up to 20V AC can be achieved in 1V increments. The current rating is 3A. Perfect for DC filament supplies even with choke input filters for tubes like 300B, 26, 10Y, 801A and many others. Of course it can also be used for DC heater supplies for indirectly heated tubes or other purpuses.
B+ transformer for small amps or preamps
This transformer has two secondary windings like the DC filament transformer which allows secondary voltages from 100 up to 600V AC in 50V increments. The current rating is 100mA. Perfect for preamps or small amps. It is meant to be used with a full wave Graetz bridge with 4 rectifier diodes. A heater winding 6.3V/5A is included. Sufficient to heat 4 6AX4s.
B+ transformer for medium current demand
The same as above but with a 200mA rating. It has two 6.3V/5A heater windings to supply a 6AX4 bridge and signal tubes separately. Ideal for a mono 6CB5A amp or a stereo 45 or 2A3 or other similar amps.
B+ transformer for high current demand
The same as above but with a 400mA rating. The secondaries are even more flexible to allow voltages from 100 to 625V AC in small 25V increments. The two heater windings have a 7.5A rating.
With Graetz bridge rectifiers and choke input filters, B+ voltages of up to about 500V DC can be generated with these transformers. With cap input filters even up to 750V DC. Higher voltages for tubes like 211, 845 or 6HS5 can be achieved with voltage doubler arrangements or by using two transformers in series.