etnier, My unit is a Steelhead. I am not sure that it is configured exactly like a Chinook, so you would need to confirm that first. I was also unable to obtain a schematic of the Steelhead. Even my professional tech who runs a thriving repair business was unable to obtain it. My insight comes from a verbal description of the circuit provided by Evanna Manley in the context of a review of the Steelhead published years ago in 6 Moons. According to EM (and confirmed by my investigating the circuit with an ohmmeter), the 6922 is the only tube involved in the phono equalization and gain circuitry.
The two 5687s are used as cathode followers (CF), as follows: Output from the phono section goes through the first CF, which is actually a complex type called a "White Cathode Follower", requiring both halves of one 5687. This drastically lowers output impedance and drives the volume control via a 47 ohm resistor in series with a 30uF capacitor in the original design, if you use the Steelhead as a full-function preamplifier, as I do. (If you use it only as a phono stage, bypassing the volume control and other switches, then the 47-ohm resistor/30uF cap would be driving the input of your linestage.) According to my meter, the volume control has a 5K ohm input resistance, which is very low compared to most. However, 30uF is very high value for a coupling cap, even one that has to drive a 5K load. And it's way higher than needed to drive any reasonable linestage. Manley used a (mediocre in my opinion) metallized polypropylene type here. By my calculation, all you need is 12uF into 5K, to get a hi-pass filter at 2Hz. The closest to that value and best I had in my stash is a 10uF/200V MIT polypropylene film and tin foil type, far superior to the original cap in SQ. (Last time I looked, Michael Percy sells the MIT caps.) So I used that in place of the 30uF cap. (The difference between the calculated 12uF and 10uF is academic at best.) The 30uF cap and the 10uF MIT cap are about exactly the same in size, so no problem with fitting it in. I also bypassed the 47-ohm resistor; this serves no needed function unless you are driving VERY high capacitance cables to a linestage, and I don't. It's totally superfluous for driving the volume control.
The volume control drives another WCF using both halves of the second 5687, which drives the input of an amplifier, in the case where you use the Steelhead as a preamplifier. Most amplifiers have an input impedance north of 10K ohms, if solid state, or north of 50K ohms, if tube type. The Steelhead has another 47-ohm resistor in series with another 30uF capacitor at its output here. The 30uF capacitor is once again way overkill for driving even 10K ohms. I deleted the 47-ohm resistor and replaced the 30uF capacitor with a 4uF/250V polystyrene and tin foil coupling capacitor. (I had these in my stash; you cannot buy them any more, but you would use the best possible 2 to 4uF capacitor here, if you are driving anything above 20K ohms.)
The difference is "startling".
