Linear power supplies are a very interesting subject. There are a few rules you can follow in order to size an un
regulated DC power supply.
Now to design a power supply system, you will need to provide some data starting design parameters;
Final DC Voltages, when fully loaded (B+)
Final AC Voltages, when fully loaded (Filaments for Rectifier and Tubes)
Choke or Capacitor Input Filter
As a simple rule of thumb you should size the HV+ current capacity of the power supply at 70%
of the published limit of the transformer.
You can increase or reduce this value a bit, and the ensuing consequences will show themselves.
Running a transformer with a very low current (very cool) draw will not provide the best 'performance'.
An under-loaded power transformer will not work well if the main voltage sags below a certain nominal value.
Some power supply designers prefer to under-dimension their transformers saves money. But others choose to put
the capacity at between 60% or 70% of total for better reliability.
For instance your un-regulated target DC voltage is 300 VDC at 100 ma.
You will need a power transformer capable of at least 330 to 380 VAC, and a current capacity of between 135 and 165 ma.
Or as close as possible to 30% to 35% more current than the load requires, for safety and better regulation, however small this
may be. This assumes that the power supply has a capacitor after the rectifier.
With a Capacitor as the Input filter the Dc voltage is 1.4x the AC value. For a Choke input filter, the DC value is less than unity.
So Choke input power supplies out perform capacitor input designs, yet are more expensive to build.
Choke input power supplies need far greater voltage and current capacity for the same equivalent capacitor input design.
But they out-perform Capacitor input power supplies for several interesting reasons, some having to to with a Flywheel effect vs. The Jackhammer diode effect.
The idea with un-regulated power supply design is to load the transformer between 60% and 70% or maximum so as to provide a stable DC voltage and current.
If the design is for a Regulated Power Supply, the high voltage section needs to be dimensioned to account for the extra current draw and voltage drops the regulator function will incur.
Therefore it's not unusual to have regulated DC power supply transformes with 50% or even 60% extra capacity.
Tube filament supplies should never be over-dimensioned to provide more voltage than at Full Load.
So If your tube monoblock that uses a quad of EL34's needs 6 amperes @ 6.3 VAC, shoot for that value.
Never use a transformer with less than 6 amperes here, you can use a 7 or 8 ampere with good results.
Over dimensioning the filament supplies, or using a 12 ampere transformer here, would probably end up putting 7.5 volts AC
on the EL34's and reduce their lifespan by 35%.
This over dimensioning issue in filament supplies has been the scene of heated exchanges. My rule, design for or greater than required, but not much greater, 20% extra current capacity on a filament supply is a good design goal.
Tube heaters break a sweat if they have more than 10% over the nominal value of filament voltage.
Tube rectifiers as well operate at of slightly above their nomimal 5 VAC RMS values. So a 5AR4/GZ34 like 2 ampere supplies, two of these tubes can run perfectly on 4 ampere supplies.
Now if you power a 5U4G with a 5V, 6 ampere supply, get ready for early rectifier failure. These tubes like between 3 and 4 amperes AC at 5 volts AC, keeps them warm and fuzzy.
"Un-Regulated" Power supply design is an excercise in learning how to 'park' a power supply so as to achieve the proper
balance of capacity and stability. Regulated power supply design is another thing. You now have the luxury of selecting the proper resistors and regulators to drop the voltage to what is needed. So we can tweak empirically, as long as the transformer and fuse are properly selected beforehand.
Enough DC voltage is required in an Un-Regulated design, after the last filter capacitor, to power your equipment.
After you drop a few to a few tens of volts over the series resistors, chokes and additional filter sections, the more filtering
you add, the greater the voltage required from the transformer secondary. So another strategy could be if you have a transformer that has a HV section greater than you need, this is a good candidate for regulated design as we have volts to lose and current to spare.
Additional current capacity is a de-facto requirement when you add a voltage regulation. On top of the extra volts needed to properly
feed a regulator and achieve the required voltage drop and current levels to the load, you have the extra loses through the tube rectifier and the series dropping resistance and resistors that feed the amplifier in question.
Achieving the desired voltage and currents conditions in a Power Supply is mostly an empirical exercise. You can model
much of this on a PC, but nothing beats putting the physical thing together.
As when we cut wood, too short and you need another piece, too long and we can always trim.
Power Supply design should err on the high side as it's always possible to reduce current or drop those extra volts.
But if the transformer is too small, it will never reach the proper design center and thus not work at all, or poorly.
You can readily determine the value of the required primary fuse.
Place a DC ammeter in series with the mains power, you may be able to
remove the existing fuse and alligator clip your meter as if it was a fuse.
Turn on the equipment and note down the value of RMS Amperes.
Multiply this value by 1.5 and this should be the minimum fuse value required.
A 1 ampere Preamp will require a 1.5 amp AGC fuse.
A 0.5 ampere Preamp will require 3/4 amp AGC fuse.
Now a 2.5 ampere power amplifier will need a 3.75 ampere (4 amp) fuse, you round-up.
A tube amp that draws 2 amperes RMS will need a 3 amp fuse (my KT88 Dynaclones for example)
Slo Blo fuses can de sized at x1.25 as they take a bit longer to react so can afford to be closer to the
fault current maximum value.
