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.

 

Nevertheless, simple stuff could be done to improve its functionality.

  1. Installation of the power cord receptacle to be enable usage of different power cables.

Before:

And after:

  1. Replacement RCA connectors with better quality gold plated

Before:

And after:

While doing it I've installed RF filtering capacitors between phono in ground and chasses.

  1. Modify one or two inputs to increase resistance and kill some gain for better match with modern high output CD players.

  1. Add rubber rings on tubes to lower microphonics

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.

B. Other typical proposals that I passed.

Typical recommendations for people who canít sleep at night and have problems to stay in one position for more than one minute, or concentrate on one task for more than five minutes (called Attention deficit disorder):

-          replace the film capacitors with WonderCaps,

-          change the resistors to metal film,

-          replaced pots on the tube voltage bias circuits,

-          install new Litz wiring throughout

-          see my Counterpoint modification page for the list of all "smart" ideas

I trust ARC had the time and resources to find the best suited for the job components. THD of 0.0002% is the perfect overall characteristic of the proper amplifier design.

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