[Amps] Alpha 78 voltage sag

[HAROLD B MANDEL]

Dear Martin,

My current project is a renovation of an Alpha 76a.

The original transformer, made for two 8874 triodes, would not
support a set of two 3CPX800A7 triodes, both in the 
filament and in the anode class, so an outboard supply
was started a-building.

In the interim I purchased the PWD recommended power
transformer for a conversion to 800A7's with the correct
filament voltage.

However, the replacement power transformer, touted as an upgrade,
was physically 30% smaller than the OEM E-I style and how can
that be any sort of upgrade?

The handbook first says that capacitor input power supplies 
should provide better than 3% ripple voltage to anodes. There is
some feeling on this that AB1 or AB2 amps can get by withat
ten percent or more ripple voltage. I disagree because the
capacitance in the filter provides the energy when the a.c.
input goes through the zero-voltage crossing and R.F.
is much faster than 50 or 60 Hertz line voltage.

The manufacturer of three different  transformers in this
project, Tom Hand in Georgia, was receptive to a bench
test at the hotel here where the first filament transformer,
when loaded with sets of 250 watt resistors, sagged
below the acceptable level the winding specification
requested. Tom trashed those completely and came up
with new models that do not sag under the specified load.

The anode transformer he wound for me was specified
as a two amp, CCS with a dual primary so I can move
from 117 to 234 volt environments. Likewise, the transformer
was tested by Tom before he shipped, and shown to not
sag appreciably, nor overheat when connected to a
bridge rectifier and filter capacitor with 100K as a bleeder
and running at two amperes through resistance.

However, in my outboard design, it was necessary to use
a set of four oil-filled capacitors to achieve a total
capacitance of 136uF, which will effect a less than two
percent ripple.

The current Handbook has an excellent section on power
supply filter design. The formulae are spread out a bit,
and need to be assembled in reverse order than listed
in the chapter, but the end result will be an understanding of
how to achieve anode supply regulation with a filter cap
supply.

Respectfully,

Hal 
W4HBM