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
You might want to look into 5751 tubes instead which offer less gain than the 12ax7. Never heard of subbing a 12at7. Run the 5751 by AR.
I agree with Dekay. Although it is not recommended to substitute a 12AT7 for a 12AX7 a 5751 is simply a special (industrial) type of 12AX7 with lower gain and a slightly higher heater current. Amplification factor of the 12AX7 is 100 where the 5751 is 70. I have both NOS 12AT7 and 5751 tubes in stock if you need them but would not recommend the 12AT7 in this case.
Hi TJ: A couple thoughts. It’s late at night here or is it early in the morning?

With guitar amps one of the reasons for the switch is to change tone/sound of the amp too isn't it? I think i remember it, at least in some guitar amps, being called crunchy and/or dirty which is actually sought out by some players for certain kinds of music. Distortion in the circuit is actually considered a good thing. Anyway, to my knowledge the 12AT7 has lower plate resistance and higher transconductance than the 12AX7. I took a quick look at on-line tube data and they are not mentioned as substitutes on the couple sites I visited. You can look here for the plate curves of two makes of the two tubes. They are quite different.

plate curves click on the left hand column to find the the info for the 12AX7.

In any event, I think you would be changing most of the operating points and likely the sound of your pre in order to change the gain for one source. This might cause more trouble than it solves.

Do you like the sound of the pre now?

Is the gain about right for your vinyl now? If you decrease the pre’s overall gain will it still be ok? (You generally want as little gain as possible and still be adequate)

I do not know anything about your pre. If you like the sound of it now and the gain in your phono is about right I would maybe attack the gain problem for the cdp alone instead of introducing all sorts of unnecessary changes to the entire system.

My guess is that the most direct way to solve a gain problem with a cdp would be to add some resistance in that individual circuit. Maybe as simple as adding a resistance to the interconnect or the OP stage of the cdp. You can install different pads (a “pad” is just a pair of resistors) on each input jack if you have sources of different sensitivity. Usually simple and inexpensive. Any local DIYer or guitar amp repair place can do it or you. (Depending on the circuit there is a little art in how best to do this because you don’t want the added resistance to combine with capacitance/reactance and create a filter.)

You might check over at the Audio Asylum forums and see if someone has experience with your pre’s circuit. I can’t tell you what size resistors to use w/o more info and even with more I might still be, well....clueless. Or call the manufacturer and explain the problem. This must be a common problem because the output of phonos and cdps is so wide these days. For most cdps these days lots of people choose passive or nothing at all.

I remain,
You didn't say what type of 12AX7 you are running but European types, Amperex, Mullard, Tele, tend to have much less gain than the modern Asian and Slavic equivilents. The 12AT7, although having the same pin out as 12AX7, is not electrically equal and should not be substituted although it is unlikely that damage will result.
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).
TJ: ECC88/6dj8 is not a sub for a 12ax7. What model/version of the SP6 do you own and is it in stock form? Earlier versions used 12ax7's and later ones may have used both ECC88's and 12ax7s (can't remember for sure, but think that ECC88's might have been used in the phono section on some later versions).

Anyway, first start by using the proper tubes that it was designed for and/or if you still require less gain for a 12ax7 position then check with AR to see if a 5751 is OK.

There is no direct sub for the ECC88 family except perhaps a PCC88/7dj8 (you would not want this unless the amp runs the tubes very hard).

The ECC88/6dj8 family consists of ECC88/6dj8, E88CC/6922, 6h23 (Russian E88CC/6922 type) and E188CC/7308 tubes. Again check with AR in regard to using the three latter types, just in case that there is a design quirk that prefers ECC88/6dj8 tubes (can't imagine this to be the case as they are all so close in spec, but better safe than sorry).
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
TJ: ECC83 is the same as 12ax7, so again this leads to checking if 5751's will work. This should lower the gain, and hopefully you will prefer their sound.

Who knows, perhaps the gain will end up being user adjustable when you check with AR?

I would also run this through the Audioasylum forums as someone there may have already gone through this issue with an SP6.
The Sovteks that you are using are very high gain and are not suited to the circuit from that standpoint. Tele, Mullard and even Amperex will have much less gain. The 5751, along with the stock GE 12AX7 would also have less gain but may sound a tad dark (not a bad idea in many systems). Things will get better, you pretty much have the worst possible choice in right now.