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In the Forum: Audio Discussions
In the Thread: How to get a LOT of SET power.
Post Subject: GU81M with a filterPosted by N-set on: 8/23/2011
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I had some free time which I'd decided to waste with this "project".
For some unknown to me reason (I'm an idiot when it comes to filters)
the Sallen Key topology needed one more 12AX7 stage, unlike
I happily drew some posts ago. The resulting SPICE'd proposal is below:
I'd be happy to provide LT SPICE files should anybody want to play
with the model.
I've SPICE-settled for -86V @GU81M already few versions ago,
which I'd hapilly split between the coupling
choke RDC and some explicit -30V Romy bias supply to adjust power/operating point.
It all turns to be quite complicated when it comes to power supplies:
+/-200V for the input
negative bias for the input
300V driver
1530V plate
500V screen
auxilary negative bias GU81M
6! phew!
The power is a bit down according to SPICE to 150W@1R with 1.85Vrms input (@20Hz).
THis input is just about to touch 0V bias at GU81M.
Looking at the simulated spectra,
the two 12AX7 seem to add mainly higher order harmonics (4th, 5th, ...) to the
6E5P-DC-GU81M "engine" at full power. The spectrum looks a bit more even (the
monotonic decrease) with a healthy load of harmonics (I don't bother to evaluate THD,
I suspect some of the harmonic "richness" is a numerical artefact)
The frequency response (the filters set at 50Hz), simulated at the load, is a mystery to me:
Why the corner freq. 50Hz is 7dB down wrt 20Hz???
It is at 12db (20Hz at 19dB, 100Hz at -9dB).
The roll-off towards 0Hz is due to the finite coupling inductances.
Apart from this issue, which perhaps somebody with knowledge will
explain/correct, it does seem to "work" in SPICE at least.
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