daveshel wrote:Thanks for the input on the issue. I'm still wondering about the unexpected wiring of the front panel sockets (not so much) and the 1k resistors (in particular).
The assembly manual says the front panel socket cannot be used for bias set is it is wired for preamp power. So I figure that most buyers in the 60s would be using preamps with their own power supply so the socket was wired for easy bias set instead pf preamp power.
The wiring of the 1k resistors is still puzzling me. I remember reading something along these lines at some point - but I've been reading everything I could find on the subject for 10 years! So whether this is an official circuit revision is later models or a modification published by Van Alstine or somebody...
From the Stereo 70 assembly manual with italics added for emphasis: "Note that the use of pin #8 as directed in the wiring instructions means that this pin
cannot be used as a connection point for preamplifiers if the preamplifiers take power from the power takeoff sockets of the front panel. Most preamplifiers do not require the use of this pin.
However, for those which do, some other pin must be used as a bias check point."
You could power preamps from the Stereo 70’s front sockets while they were also wired for the Biaset check on pin 8, as long as the preamp power connection does not use pin 8. In fact, if you look at the pictorial diagram for the Stereo 70, this is exactly how the front panel sockets are wired. In use, you’d check the bias on pin 8 first, then plug in the umbilicals to provide power to the preamps. But, if the preamp was wired so that it needed pin 8 as part of its power connection, then you'd just pick a different, unused pin to use for checking the bias while still being able to power the preamp. The only way you wouldn't be able to use the front sockets to both power a preamp and to check the bias would be if the preamp required all eight contacts for its power needs, which was an unlikely scenario.
For the 1K resistors, if you have a look at a datasheet for the EL34, you’ll see that pin 6 is not connected to anything inside the tube. Therefore, socket lug #6 is just hanging in space and does not require that anything be connected to it for the amplifier to operate. You'd end up with exactly the same circuit
whether you 1) connect the 1K resistor directly from the PCB to lug #5, or if you 2) connect a wire from the PCB to lug 6 and then the 1K resistor from lug 6 to lug 5 as instructed in the assembly manual and as shown in the pictorials. For the latter, socket lug #6 is used as nothing more than a convenient wiring connection point because, again, it has no connection to anything inside the tube. The lack of solder on lugs #6 is evidence that this portion of your wiring was not modified after it originally left the Dynaco factory.
Based upon what you have thus far described, it sounds to me like you have an original, unmodified Stereo 70 that is just exhibiting original production run variations of no particular consequence to its sound quality.