Today I had a few friends around to share some musical moments.
The 845 SET amps i have running were fine.
We tried all sort of music and up to loud levels.
My voltemeter told me we rarely exceeded a couple of peak volts output.
The amps are set run 4 ohm speakers and give the amps maximum of 60
watts at 1.5% THD
and 50watts at 0.5%.
Next month the amps go to my customer with Piega floorstanding speakers
which are 4 ohms,
maybe with a dip below 4 ohms.
My speakers used today have 8 ohm bass, then D'Appolito pair of 6 ohm
mids in series
with resistive feed to the single dome tweeter so average Z is 10 ohms.
With 10 ohms, the amps make about 1/2 of the THD that occurs with 10
ohms,
and at most average levels THD is only 0.03%.
The max PO into 10 ohms is only 25 watts, but that was plenty for all we
played today.
I has occured to me that I could pull out one 845 of the pair and just
use one 845
if the load is 10 ohms. The load the tube sees is 14k, because the OPT
has a
1,440 : 1 Z ratio. There would only be maybe 15 watts available, but
maybe that's enough.
Anyway, anyone who thinks SET amps won't cut the mustard would be
surprised
by these amps.
Rout with the present OPT setting of 5.8k : 4 ohms
is 0.35 ohms with 7.5dB NFB which increases to about 8.5dB when
RL is higher, say above 8 ohms.
But without NFB the Rout = 1 ohm approximately including OPT winding
resistance.
The damping factor with 10 ohms connected would be fine.
Now I just have some metal work and finishing off
anmd active protection circuitry to do.
Patrick Turner.
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