Re: The KISS AMP: a progress report

It would be useful to know the secondary voltage at that transformer
before the resistors are added, so as to get the 180V rectified voltage
you require. Yes, you have 500V ~500ma at the bottom, I assume that is
across the entire two coils (5-to-7) and not to the (virtual)
center-tap, but that too would be helpful to know.

Assuming that you are getting 250V/Coil at your stated input, the way I
read your output from the rectifier tube, that would come to some 415V
or so out of the 5AR4/GZ34, excepting the dropping resistors. So, how
did the value of 660 ohms of total AC resistance get chosen, especially
in light of the 4700 ohm bleeder resistor further down the line (at 25
watts) to get to the required 180V? And 'adding a small capacitor' to
adjust voltage... this is an iffy (albeit nearly heatless) expedient,
'cause if that cap should fail, *poof*. So, it would be useful to know
the required/expected voltages right down the line so that the correct
choices might be made.

Would it not be much simpler to drop the primary side to ~220V or so,
thereby almost eliminating the need for filament-voltage dropping
resistors excepting getting 5V on the rectifier? SWAG, assuming 230
single-phase hot/neutral at the primary, and about a 2A (allowing for
losses) total load shouldn't take much of a single dropping resistor to
get that 220V, or even slightly less. This way, individuals in other
countries with different standards could work to the required
_secondary_ voltages based on their wallplate voltage. If one is fond
of iron, either a small autotransformer or a small bucking transformer
could be used ILO a resistor for less heat.

This might also greatly simplify the requirements for your B+ side?

Two chokes? Why? The 5AR4 is a pretty good, quite rugged (if expensive)
rectifier. And it would seem simpler to add additional capacitance on
the line-side of the first choke, thereby eliminating the need for the
second? With all your prior losses, you are running well below the
saturation level of a single unit (400/575ma). 500V caps are cheap
these days, even 50uF or better, certainly cheaper than iron. And if
all you want to filter is 50/60Hz chop, one will do fine. As a small
aside, the 5U4 rectifier is much cheaper, albeit not quite as rugged
and without the slow-start characteristics that distinguish the 5AR4.
But in any case, a single choke with sufficient capacitance would drop
any chop way below other noise sources.

Are you familiar with Mad Man Muntz? He would go into his skunk-works
whenever his engineers decided they had a new product to develop, and
start cutting out parts (he was an excellent EE, so this was not done
at random) until whatever-it-was stopped working. *That* was the magic
point from which to design.

I question from the perspective of one who is usually presented with
smoking ruins and given the mandate to 'make it work'. Many times
without benefit of schematics, or if they are available, facing so many
whimsical modifications to make the schematics of dubious value. So,
"expected" and "actual" values are important things to know. At times,
I thank my lucky stars that this is a hobby.... so I do not have to
make a living at it. Otherwise, not 1 unit in 10 that I see would be
worth the time and trouble to fix.

Peter Wieck
Wyncote, PA

.

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