On Thu, 15 Apr 2004 14:21:46 -0400, Chuck Harris
wrote:

Gary Schafer wrote:

Hmmm...PP 807's @ same load Z as PP 811A triodes...interesting...



It doesn't matter what the actual impedance of the modulator or the
final is. It is the ratio that matters.

The ART 13 transformer has a primary rated at 15000 ohms and the
secondary at 7300 ohms. That is a ratio of about 2:1.

The Johnson viking 2 (pair of 1625s modulating a pair of 6146s) used a
transformer with a ratio of around 2.4:1 or so if I remember right.
That was a little high as the modulators would go into clipping before
100% modulation could be achieved on that rig. A little lower ratio
mod transformer would have allowed a little more modulation.

So the ART 13 transformer is just about ideal for that kind of setup.

73
Gary K4FMX

One small thing, don't go too ape s*** crazy with substitutions.
You must understand that for a given wattage transformer core, higher
impedance windings will have more turns than lower impedance windings.

It wouldn't be ok to sub a 2:1 transformer that was meant for 10K
for a 2:1 that was meant for 1K. Not at the design power for the
transformer.

Why does this matter? Subbing a much higher impedance transformer
for a low impedance transformer will result in greater losses due
to the higher resistance of the windings. Subbing a low impedance
transformer for a much higher impedance transformer will likely result
in core saturation and loss problems due to the higher fluxes the
core sees. There aren't enough turns on the core.

Try find a sub that is close to the design impedance of your desired
transformer.

-Chuck Harris
------------------------------------------------

While it is true that there is a limit as to how far you can "stretch"
the impedance of a mod transformer before getting into trouble, you
can go an amazingly long ways.

As an interesting example: The Stancor Poly-Pedance transformer that
has multiple taps to obtain various impedance ratios (15 to 120 watt
series) uses the same taps for 2000 ohms primary to 1150 ohms
secondary (1.74:1 ratio), as it does for 20000 ohms primary to 11500
ohms secondary. Also the same 1.74:1 ratio! That's a 10:1 range!

The link below has the ART 13 and other transformer info.

http://www.amwindow.org/tech/htm/modtran/modtran.htm

Select Miscellaneous to find the ART 13 transformer specs and
connections.

Other transformers (stancor etc.) are listed also.

73
Gary K4FMX