2. amps-preamps
  3. 1028335851

Tube subtitutions 12AT7 for 12AX7

My old ARC SP6-B sounds swell with my phono,
but has way too much gain for my CD players.
To get normal volume levels,
I barely adjust the gain past 9 o'clock (3 clicks).
With the stepped gain, this leaves no room for
fine volume adjusments.

I would like to reduce the gain by substituting
the 12AX7 tubes (in the line stage portion only)
for 12AT7's. There is some discussion of this
at www.thetubestore.com, but more from a guitar amp
point of view, than audiophile amps.

Does anyone have any experience with this sort of thing?
Am I gonna wreck the preamp doing this?
tjbearman

Showing 2 responses by tjbearman

tjbearman OP
40 posts
 
Thanks for your advice, guys. Can I
ask a few more questions?

I'm using Sovtek 12AX7 WXT+ or EH or some marked CZ.
If I mix types, I always keep the same types together
in the phono section (3 tubes) or line section (3 tubes).
The original manual calls for ECC88 tubes, but these
really are equivalent to the 12AX7, aren't they?
Or would they maybe have less gain, like the 5751?

Am I happy with the SP6 sound overall? Hmmmm....I think another reason I was considering a tube swap was the tube
rush. I used to listen to tubes back in the 60's and 70's and forgot how "noisy" it can be. OK, it's minimal, but
noticeable compared to my passive pre setup.
The tube rush does NOT appear in the SP6 line out.
Perhaps the SP6 line stage has
some other problems with it that can't be corrected with a
tube swap?

For now, I'm using a passive pre for
the CDs and taking the line out (after the phono EQ)
from the SP6 for the phono and running it into the
passive pre, too. All that into an ARC Classic 30
(no tube rush there, to speak of) and into a
pair of Quad ESLs (original 57s).
tjbearman OP
40 posts
 
Oops, I meant to say the SP6 called for ECC83, not 88.
The ECC83 is, I believe, swappable with 12AX7.
Is the overall gain different for the ECC83 than 12AX7?

This is a stock SP6B, with some caps replaced/upgraded once upon a time and tubes, too, of course.
I guess a call to ARC is in order now to see about the
use of 12AX7s at all!

The root of my problem is a too-loud CD (all 3 in fact).
If I run these into a passive pre then the power amp,
it is clear that full output of the CD would be WAY too much volume. Maybe just a replacement of the gain control on the SP6, giving more room at the low end or getting away from a stepped attenuator is the way to go.

TJ
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