Note 1) Good
care should be taken when making the design of any DH rectifier. It
must be prevented to have strong current peaks, through the first
capacitor, because consequently this flows through the tube anode, and
transformer windings as well. The transformer will produce mechanical
hum by this, most specially if windings symmetry fails. Also the tube
will suffer. For this reason, the first capacitor (C1) should never
be larger than stated in the data sheet. The mechanical
transformer hum, and also tube current peaks are greatly reduced by
smaller capacitor values, and use higher choke values instead. Although
higher capacitor values are at lower cost, using lower capacitors and
higher chokes values instead, is always more satisfactory in the end.
The result will be: Lower transformer hum, less electrical field
radiation into the pre-amp, and more lifetime from the rectifier tube.
This is why we recommend using largest chokes. From Lundahl, high value
chokes are available at the same price as HiFi capacitors, like from a
Mundorf or Black Gate. So we have to go back to the roots, and use high
quality, large value chokes, like in the old days of radio design. For
best ripple suppression, increase the choke to any value you need, or
even use a C-L-C-L-C circuit, as also advised in the historical RCA
data sheet.
Note 2)
Higher capacitor values up to 20uF can be used, if you work at lower
voltage and lower current than maximum. This will be the case for
instance with smaller amplifiers, such as using the 45 tube.
Note 3) As
a rule of thumb, high voltage power supplies are best built with large
size chokes, especially at high output current, whereas low voltage power
supplies can be built more easily with large capacitors.
Note 4)
Winding symmetry is needed with HV transformers to prevent hum. A HV
winding with center tap, requires FOUR separated HV windings inside the
transformer, which are arranged for the end user as TWO windings which
are in series. It is remarkable to see that this is widely unknown today by
many transformer manufacturers. Also this is more expensive. So often,
they build tube rectifier windings only from two internal windings, and
not FOUR windings as needed for tube rectifiers. However we have a tube
data sheet here, no transformer construction manual, so we can not
explain this in more detail.
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20A /
20B $450 
45 Globe - $630