Hal,
See below;
*********** REPLY SEPARATOR ***********
On 3/2/06 at 10:37 AM HAROLD B MANDEL wrote:
>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?
If there's not as much core area in square inches as the old reansformer, and
used the same type steel, it is undersized from the old. The core area is what
determines the power output of the transformer (along with the window area
somewhat).
What they did was run the core at a lot higher flux density than the EI core.
Actually though, Hipersil is the same as M-6 steel in EI lams, and should be
ran at a max of 15 kilogauss. They say it will run up to 17 kilogauss, but
there is huge losses after 15 kg, the permeability really starts to drop off,
and you get a hot running transformer because the magnetizing current really
starts to raise too. They designed that though around the 17 kilogauss figure I
believe. 15 kilogauss is actually runing either type out of the linear region
above the knee on a BH curve (past maximum permeability) so you get somewhat
distorted waveforms out of the transformer too. Linear is around 12-13
kilogauss, below the knee on the curve, and below maximum permeability.
>
>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
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Best,
Will
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