Replacement volume pot for Marantz 7C

I used a 500k dual gang PEC carbon pot for my Marantz 7C copy build but unfortunately, as I had heard others warn about, the tracking of the two tracks is off quite a bit and requires rebalancing every time I reduce volume any where near 1/4 turn or lower.

I noticed that Parts Express and some other suppliers I've used in the past does not carry anything over 250K in an Alps stereo pots which I wanted to try. Any suggestions for another brand that is good enough for the task in a reasonable price range. I guess the next question for anyone versed in old tube designs such as the Marantz 7C, would a 250K pot work for the task or would it cause any impedance mismatching enough to avoid?
 
Log. I looked at the Elliot Sound article and looks like a potential option for me.
I happened upon a thread where someone was needing to replace the volume pot on a Marantz 7T and in some of the responses, it was thought that as long as the tape head and microphone inputs were not used, that a 250K pot would be appropriate. Not sure why those input sources would change the input impedance to the pot at that stage in the circuit., Do you think a 250K log Alps might work? Or, do I need to stay with 500K?
 
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Joined 2011
The Marantz 7C has a phono stage output cathode follower, so the circuit is not highly sensitive to loading.
But lower than design loads on the phono stage (mainly the balance and volume controls)
will roll off the bass further, due to the 0.47uF output coupling capacitor.
Whether the source is phono, mic, or tape, in any event the source is not relevant, they are mistaken.

Beware of metric pots that have a different diameter bushing and shaft, or too long shaft length.
But the correct Alps pot will track better than your stock pot. Be sure to check the Alps spec for tracking.
 
I appreciate the feedback on the issue with bass roll off in phono.
I had already assumed I would need to figure out a thin shim to add shaft diameter for the 6mm shaft to mate up with my 1/4” knob bore.
What’s your thought on me using a 250k fixed resistor in series with the 250 k alps pot? I know it would limit max output but I never move beyond 1/2 rotation anyway.
 
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Joined 2011
It would be ok to just use the 250k control without an extra series resistor. The phono bass roll off will still be subsonic.

If the shaft is too long, don't clamp the pot body when cutting it off. That will damage the mechanism inside the pot,
from the force on the shaft. Instead, clamp the end of the shaft, leaving the pot body free while cutting.

If the pot body is not completely sealed, seal it in a small plastic bag with tape around the bushing,
to keep filings out of the inside of the pot.
 
Good to hear the 250k will only affect subsonic! I will give it a try. Fortunately, I've got lots of experience modifying potentiometer shafts over the years and yes, only way is to secure the shaft for cutting or grinding and good advice on preventing grinding debris from entering any potential opening.
 
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