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Tubes in the Audio Research SP8





PHOTO: SP8 Revision 4 circuit board- the "red board" from ARC database

MarkSel,


Tubes in the Audio Research SP8 / Revision 4 through 7:

2- 12AX7 low noise
2- 12AX7
2- 6922
1- 12AT7
1- 12BH7

The gain tubes- the ones that increase the signal from the source in the SP8/ 4 are the 12AX7s, two low noise specified for the phono stage and two for the line stage amplification. The 2-6922s are cathode followers which I've not found to have an effect on the sound, but for best results should be quiet and have good section matching. The 12AT7 and 12BH7, which I think are power supply tubes, also have little or no effect, but to be on the safe also should be quiet, rugged, and have matching sections.

As you mentioned having the "red board" version, you might know that that model SP8 is somewhat special- I have a red board SP8 also and was told by the Audio Research dealer when I bought it that only 50 were made at the transition from what is now called "MKI" to "MKII". The SP8 was officially known by the revision numbers: what is now called MKI's are Revisions 1 through 3 and MKIIs are Revisions 4 through 7.

I was told that when the SP8 was being discontinued, a group in Asia (they wouldn't say, but probably Korea) commissioned ARC to build a special after production run of 50.

Tube choices: My SP8 still has all it's original 1985 tubes with no signs of needing replacement- remarkably they test over 90% of new, but I've bought replacements and tried various combinations. With the 12AX7s in the SP8, I strongly prefer Siemens, which I find are clean, open, detailed, punchy, quiet, and dynamic. The ordinary early to late 60's Siemens ECC83 is just fine, but particularily, I like the premium 10,000 hour Siemens triple mica E83CC in the phono section- just fantastic in my view. My second choice for phono is the Amperex Bugle Boy, but really the ordinary early to late 60's Philips / Amperex /Valvo work really well- a bit less energetic than the Siemens, a little sweeter. If you're not using the phono stage, the phono tubes can be about anything just to act to maintain the current draw. For the line stage, I have high expectations for a pair of Sylvania 5751 triple mica, black plate square getter- a little lower gain, with a beautiful, refined sound. As line sources seem to be such hihg output, the SP8 never goes past a 9:00-10:00 setting, the lower gain for use with a CD player would be welcome. For the 12AT7, I'll eventually use either a Siemens triple mica E81CC or a Valvo, Hamburg 6201. The 12BH7 will be a 1949 RCA black plate, and the 6922s, a matched pair of mid -70's Tungsram red label E88CC, which is said to be based on the Siemens. I'll also be using the Tungsram in secondary 6922 positions in an SP10 (1987).

The SP8 in my view is still one of the best phono preamplifiers ever made. In several aspects, the SP10 is better sounding than the SP8, I find the SP8 more "friendly". I have a lot of LPs that are poor recordings- like 60s-70's CBS, worn or budget reissues of 40'and 50's recordings that were the only way to hear certain artists, and the SP8 somehow "forgives" them and makes them delightfully listenable. This is unlike the SP10 which relentlessly reminds me of all the defects in the recordings and system- an $800 cartridge and $225 interconnects are on the low end of medium quality for example. I had been somewhat unhappy with the slightly dry sound of a Cambridge Audio 640C with a solid state ARC LS3, but with the SP8, the 640C sprang into life. The problem today with having an SP10 is that it uses twelve of the 6DJ8 family,and I feel the new production are not yet as good as the best NOS. And, NOS are becoming stupidly expensive- $100+ each for phono tubes- and simultaneously pushes them along so that only certain ones will survive. The use of 12AX7s in the SP8 makes great NOS choices much, much easier. With all the respect I have for the SP10, I've had thoughts over the years of selling it, but it's never occurred to me to sell the SP8. The SP8 is also the best build quality- with care these should last nearly forever. - It's one of the great ones!

If you change tubes in your SP8, let us know what happens.

Cheers,

Bambi B

[ARC: SP8, SP10, LS3, D115, D130]


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