Audio Research SP-10 6922 or 6DJ8

The electrical characteristics are close enough that the 6DJ8, 6922 and 7308 will all three work in the ARC circuit. There is one other possibility, the 7316, which is very low noise military version, also electrically similar to these other three. If you want an ultra clean and linear sound, try the USA Amperex 7308, especially the CEP version. Somewhat less linear and more warm would be the Dutch Bugle Boy 6DJ8, or the Mullard 6922. The Siemens 6922 CCA is a very detailed and low distortion tube, but is less friendly to bright recordings with the ARC. You did not say what you were listening to right now, and if you were happy with the tonal balance and dynamic range provided. I and others here can probably advise you, given that information, and what you like and dislike about the sound right now.

I owned an SP-10 for 8 years in the 80s and found the RAM tubes to work very well. In the later years these were 6922. When I switched from the older 6DJ8, the sound was less noise and more resolution but I did not know if this due to newer tubes or the new tube type. I typically replaced the tubes in the phono stage every year and ALWAYS went with the lowest noise tubes as possible. RAM Labs had a special series of low noise tubes. I then rotated the existing phono tubes into the line stage. It was just too costly and I think unnecessary to replace the entire tube set very often. But the phono stage was very sensitive to tube noise. ARC has come a long long way since then to make lower noise preamps, but only a few models since then have had the musical involvement and magic of the SP-10. Hope this helps a lot.

Thanks to both of you for your advice. My SP-10 MK 2 posessed the most perfect midrange phono reproduction I had ever heard.....until my phono (4) tubes went. They were big bottle 6dj8s marked with no name, except "made in USSR" The sound was big, lush, 3D bigtime! and no brightness. The plates on these tubes were extra wide. Tried the ARC preferred Sovtek 6922 and was astonished at how bad the sound had become. The line section tubes were also a bigger bottle tube marked AMPEREX, made in Poland, Anyone ever seen an Amperex made in Poland? Also wonderful sound. The Telefunken ECC88 is wonderful, but somewhat hyper/detailed and missing the ultimate vocal warmth of the old tube