Some Notes on Gary Pimm?s Phono Preamp
- From: John Stewart <jh.stewart@xxxxxxxxxxxx>
- Date: Sun, 09 Apr 2006 21:20:42 -0400
First of all, I had always understood active loading to be in respect to
the phono cartridge on the input circuit. You can do that with an Anode
Follower as the input amp. I did one many years ago using ½ of a 12AX7
working back into a Ronette Xtal Cartridge. I guess it was OK since the
guy who got that amp was very happy. Various FB networks could be
switched in to get the correct playback curve. In those days there were
several.
The 47 K input resistor forms part of the playback correction curve in
combination with the inductance of the pickup. Did anyone mention that?
Someone forgot to bias the output 6GK5. The 22M contact potential R7 is
OK for an AC/DC radio. But not here in my opinion! That gets about a
volt on a good day. As the signal from the previous stage gets larger
the 6GK5 grid will conduct, increasing the bias. A quick look at the
plate curves looks like the Q point would end up at about 75 volts. That
leaves (258 ? 75) volts??.183 volts across that BBMCCS thing. Don?t have
any data on it. Is it that good? Looks like a failure waiting to happen!
What about that output coupling circuit to the OPT? The 3 microfarad cap
is series resonant at 53 Hz with the 3 H in the primary. Depending on
secondary loading there should be quite a good peak in the response!
If this thing is to drive a tone circuit the output will need to be
about one volt anyway. With all their losses many tone circuits have
only a gain of one or so. If it is to go strait into an amp sans tone
circuits there will still need to be at least a volt. Is that possible?
Pimm sez he has set the transformers for a 4:1 step down ratio. That
means 4 volts min at the plate of the 6GK5, working into 4K.
The total plate current is set at 2.5 mA, so the signal at that point
can go from zero to 5 mA, max. Into 4 K that translates to zero to 20
volts, or 7 volts rms. Looks like it can make that requirement OK. But
what about fidelity? The 6GK5 curves don?t look very promising. OK for
RF but perhaps not here.
The power supply is a great example of over design. A 6X5GT or 6X4 would
have done fine. And with care on the same chassis as the preamp.
Did I leave anything out?
My thoughts, anyway!
.
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