• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

M65 stereo preamp question


2007-12-30 9:39 pm
jn2630 said:
Oh, ok, the tubes. So the line between the 9 and the 4,5 is the tube also?


The 12AX7 has a center tapped heater. Pins 4 and 5 connect to the heater's ends, while pin 9 connects to the center tap. The arrangement allows for the heater supply to be either 6.3 V. or 12.6 V.

While we can debate the merits (or lack thereof) of Shure's signal circuitry, it's clear that the PSU shown is not acceptable. Energizing phono section heaters with AC is a NO-NO. Half wave B+ rectification puts a "standing" DC voltage on the power trafo, which is not good.

In your previous thread, I told you how to build a regulated 12 VDC heater supply. That advice still holds.

Full wave bridge rectify the O/P of an Allied 6K27VF power trafo with 2X UF4007 SS diodes and a 6AL5 vacuum rectifier. The 6K27VF will provide the 6.3 VAC heater power the 6AL5 needs. CRC filter and finish up by using a LR8 TO92 case adjustable 3 terminal regulator.

BTW, don't try to mate Shure's circuitry to SS equipment without adding a buffer. 12AX7 based circuitry, even with loop NFB, simply has too high an O/P impedance.


Paid Member
2005-01-09 2:32 am
On a whim I purchased a Shure M-65 tube pre from ebay. Was in pristine condition, but was not too impressed with the sound - I was not using is as a phono pre, just as line stage.

The Fourplay III I had was so much better in every way, but that should not be too surprising as all of the M65 components were original (circa 1960's?).

Anyway, sold it back on ebay for almost double what I paid for it, so I wasn't too unhappy for long...