Tube Preamp Question

kitjv -- Tue, 02/08/2011 - 11:37

I have a Audio Research SP16 tube preamp that is 4 years old w/ about 4,300 hours. As a result of considerable "hiss" emanating from the right main, I called Audio Research (ARC). I was advised that experience has shown that the tubes in this preamp have a life expectancy of about 3,000 - 4,000 hours. Although this is certainly just a rule-of-thumb, I suspect it's time for tube replacement.
ARC recommends using a single low-noise tube on the input side & regular tubes for the rest. (There are 6 tubes total). I also have the option of using 6 low-noise tubes. My question: Is there any discernable difference in sonic quality that is worth the extra $ for the low-noise tubes? BTW, I do not have a phono. My only input source is a Cambridge Audio CD player.
Thank you kindly.

Mullard EL34 -- Thu, 04/14/2011 - 14:23

Wow!  4300 hours out of a set of ARC small-signal tubes; you certainly got your money's worth!  As to the low-noise tubes, they're certainly to be preferred for keeping the noise floor low.  Of course, you need to consider the tube selection within the context of the rest of the music system.  Is the overall gain of your system such that you really can't turn-up the volume control beyond 25% of maximum?   Do you have high-sensitivity loudspeakers?  If so, the low-noise tubes could have benefits within your system.

Only three of the tubes are involved with the line-stage circuitry (see attached image of the linestage-only SP16L interior; note the unpopulated part of the PC board that normally houses the phono-stage circuitry)

If you only use the line-stage circuitry, then use a low-noise tube in the first position of the line-stage (i.e, the one nearest the rear panel); employing the standard tubes in the remaining two line-stage tube sockets should be OK without degrading the line-stage performance. If you don't intend to use the phono-stage, save a few dollars and just populate it with the standard tubes as well; note, you really shouldn't run the phono-stage with empty sockets... ;-)

Ded Frag -- Tue, 04/19/2011 - 19:14

 Excuse an ignorant question, but what do you call the plugs that short out an un-used input connection? 

Steven Stone -- Wed, 04/20/2011 - 11:07

Shorting plugs. I'm not trying to be a dip, that's what they are called...

Steven Stone
Contributor to The Absolute Sound,, Vintage Guitar Magazine, and other fine publications

Ded Frag -- Wed, 04/20/2011 - 17:48

I'm the 'dip' for not thinking of it. So, is there an advantage shorting the MM inputs on a phono preamp when only using the MC input?
Dip must be a US abbreviation for what we in Oz call a 'dip stick' or in the more colorful local language a 'dick head'.

Steven Stone -- Wed, 04/20/2011 - 21:34

It really depends on the phono pre as to whether installing shorting plugs will have a positive sonic affect.

Some phono-pre's have shared inputs, while others use unique and independent circuits.

I'd get a pair of inexpensive plugs and see...

Steven Stone
Contributor to The Absolute Sound,, Vintage Guitar Magazine, and other fine publications

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