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The speaker field can be replaced with a 1500 ohm, 10 Watt wire
wound resistor. See the schematic for the S-40 or S-40A for details. I
am not sure what the plate load for the transformer should be but about
5K ohms is typical for single pentode power amps. Speaker side is
whatever matches the speaker you are using.
Your transformer sounds more like a line matching transformer than
an output transformer. Is the original transformer gone? Typically a
single pentode wants a plate load on the order of 5K. The DC resistance
of the speaker voice coil will be close to its impedance for replacement
type speakers, usually somewhere around 3 to 8 ohms, not very critical.
I wonder about the power rating, does it say that on the speaker? Sounds
to high to me. If this is a modern woofer or full range speaker it may
not be efficient enough for the low power amplifier in the receiver. The
single pentode amps will put out on the order of one watt or two watts
maximum with a lot of distortion. See a tube handbook or data sheet for
some idea of the impedance and power output. One watt into the kind of
speaker originally used in these receivers will make quite a loud sound.
However, the small, full range speakers used for quite some time now,
trade efficiency for low frequency response and may need a lot of power.
FWIW, in old tube receivers a typical single tube audio amp will
put out a maximum of maybe 2 watts while a push-pull amp will put out
anywhere from 10 to 20 watts. Distortion in either case will be
substantially greater than the sort of specs one sees on modern hi-fi amps.
The original speaker was mounted on soft rubber gromets to isolate
it mechanically. This is to prevent mechanical feedback to the
microphonic tuning capaictor which will cause howling at quite modest
volume levels.
Try your speaker. if it works that's fine but be aware of potential
problems.



On 6/25/2024 8:01 AM, John Watkins wrote:

I know I have asked about this before, but lost the return email.? I
have an 8 Ohm 5 inch 20 watt speaker and an audio transformer, primary
is 500, 1500, 2000, and 3000 Ohms.? Secondary is 4,8,and 16 Ohms.? What
should I replace the speaker field coil with, or just leave it as is?

Thanks,

John WD5ENU

--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998