replacing tubes, AR sp6A preamp

To tell if the tubes are bad....turn it on, hear any noise? How far do you have to rotate the volume control before you have noise? Connect (or turn on a source) how does it sound. Dull, rolled off. Perhaps your tubes are over the hill and could use replacement. Bright - glary? Perhaps you might like a warmer tube. You should have working tubes in all sockets including the phono sockets, however the phono socket tubes can be in any condition so long as they are drawing current. If they are working the only tubes you would need to replace are the line stage tubes. As to which brand of tube, read Job's tube lore in AudioAsylum FAQ's for a description of the tonal quality of the various brands available as NOS and current production tubes. Consider the sound you would like to hear and then post here and in Audio Asylum and ask what other ARC owners have used to produce the sound you are looking for. Hope this helps a bit. Oh, also do a search in the tube forum in AudioAsylum, this pre-amp is discussed quite often.

Hello Phil. The arc sp6a is a very good preamp even by todays standards. I used one for a 2 or 3 year period before acquiring the SP-10 mk2 back in the mid 80's. As Newbee correctly pointed out, as long as a tube is in the sockets of the phono stage they do not have to be great sonically as your using the line stage only. You didn't mention the manufacturer of the tubes that are in it now. When it was first introduced, I believe ARC was delivering them either with amperex holland 12ax7's or seimens. Quality tubes like the Amperex, mullard, seimen and telefunken 12ax7's were not so scarce in those days. My all time favorite tube for the any of the ARC sp series preamps, from the sp3a right up to the sp-8 was the seimens 12ax7, although the mullards,telies, and amperexes were all very good as well. The preamp was just so suave and more articulate with the seimens 12ax7's. You may want to test the tubes that are in it as most nos 12ax7s will traditionally last a good 3000 hours unless they turn noisey or microphonic. The 3 tubes in the phono stage could be rolled into the line stage if they still test strong. Hope this helps. All the best. cheers

I had a -6B a LONG time ago. The first tube in the fono stage (V1?) will be the best one graded by noise and microphonics, then probably V2, then V4, then V5. The cathode followers (V3 and 6) are the least-important tubes in the preamp. If I were using it only as a line stage, I'd simply switch the tubes between the 2 sections, maintaining the same relative positions.

BTW, when 'testing' for audible noise, be sure to short the input you're using or at least have something not turned on plugged into those jacks.

Lucky, the SP-6 series responded VERY nicely to coupling-cap upgrades, and the 'A' version came with pretty mediocre-sounding versions. If I remember correctly, the 'B' used a pair of 0.47/400 interstage couplers and a pair of 8/200 section-output couplers. Know anyone who could install new caps for you?
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Thanks to all three of you for the info. you've given me. I've done a ton of research on most things you've said and now have whole sets of letter and number combinations that get me viscerally excited without having any direct experience whatsoever (12ax7;12ad7;12dt7;5751, gold brand,jhs;caa;jan, etc.)
I'm about to begin very carefully opening my preamp and start gaining some experience.
Do I understand you correctly, that because I am using only a CD player as a source in my preamp (AR SP6A), the phono stage slots ( I think V1,2) and the cathode following tubes, V3 and V6, can have any tube as long as they draw current, and I can experiment with the V4 and V5 line stage tubes to affect sound quality? So far every question I have researched becomes much more complex as soon as I get into it, so I'm imagining this may also.

Also, without owning a tube tester, are there places to get tubes tested?
Thanks again for the info you shared.
Phil Del Prince