My ARC SP-6B modifications
Initial statement: I think ARC SP-6B is built, looks and sounds so good that there is nothing could be done to improve its sound quality. At least in my basement.
While doing it I've installed RF filtering capacitors between phono in ground and chasses.
Most common recommendations for any tube preamp
A. Tube replacement.
In the phono stage V1 is the most critical, V2 and
V3 not so much. When replacing V1 watch and test microphonics.
Different user's opinions posted in on the web:
- Use a good 12AX7 instead of the lower-gain 5751.
- RCA black plate 5751 (will decrease the overall gain)
- Amperex Bugle Boy 12ax7,
- Siemens ECC83, Telefunken ECC803s (gold pins) and Dutch Phillips ECC83 or E83CC (gold pins).
In the line stage V4 is the most critical. The same recommendations as for the phono stage could be used for the line stage.
Note: Noise in the input stages partially originated by plate resistors, grid-to-ground resistors, and grid divider attenuation resistors. In these locations should be used the lowest noise resistors, while followed stages with little gain can use noisier resistors without adding too much to the overall noise level of the amplifier.
Reference: wire wound resistors are the quietest, having only thermal noise, followed by metal film, metal oxide, carbon film, and lastly, carbon composition. http://www.aikenamps.com/ResistorNoise.htm