VAC Signature MK2a pre "phono tube questions"


Hi there I have a VAC pre with MC/MM, upon opening noticed there are two roles of 3 12AX7 tubes in each totaling 6.

Question;

1. Anyone have any experience tube rolling these and if so with what?

2. Are they three matched pairs or all matched?

3. They are a Chinese tube that came standard with the pre., anything special about them?
I have found on other gear which came with Chinese 12AX7 tubes they were nothing special and by substituting I found the sound was transformed, sounding allot better using other tubes.

4. Can you mix and match, say the first role Bugle Boys, second Tel for examaple or best to stay with one brand through out.

I have allot of really nice matched pairs but only that.

Any assistance would be appreciated.
dev

Showing 7 responses by wrm57

Hi Dev,

The phono circuit in your preamp is the same as in the Phi Beta, which I'll receive on Friday. I, too, asked Brent about rolling the 12AX7s a few weeks ago and received similar info. One thing I'll add is that he said the back row, the inputs, play the greatest roll in sonics--back to front, in order of decreasing influence. The front pair, under the most stress, have the least influence on sonics and will have the shortest lifespan. He said to place the tubes you care about the least in that position. I'll probably start the rolling process by leaving the Chinese tubes there and placing my fav 1964 USA Amperex 7308 into the back because they have such great focus, resolution, and balance. Then I'll see what middle pair is best for tone. But, of course, I'll ultimately experiment with all positions
D'oh! I meant medical grade Amperex 12AX7. I mentally misfired because I had VAC install a 6922 switch for the 8416 tubes (at their recommendation) in the linestage and was thinking the tubes I'll use to roll that position. No, I'd never put a 7308 into the 12AX7 socket. :) Thanks for catching it.
I asked VAC. VAC recommended tube rolling. In fact, Brent strongly recommended (three times!) installing the switch to accommodate the 6DJ8 family instead of just the stock 8416 tubes primarily because the latter was only made by Amperex and the former would give me a wide variety of "sonic flavors" (his phrase) to choose from.
Hi Lewm,

My preamp arrived and I checked out the VAC 12AX7s. Construction looks identical to my Psvane 12AX7s without the gold plating on the pins. So that means they're almost certainly made by Shugang, which also makes the Psvanes. They're really quite good!
Hello Dev,

You asked me to report back on tube rolling the phono section of my Phi Beta, which has the same circuit as your Sig 2a. I've had it for a week now and I'm ready to make a few preliminary observations after rolling a variety of high-quality 12AX7s, including 1964 Med Grade Amperex (very pricey), mid-'50s RCA long blackplates (one of my favorites), late-'50s Tele ribbed plates, and mid-'60s Lorenz (German-made Amperex). I have several pairs of each and I've tried them in different permutations in the first two positions. Because this circuit is so transparent, all changes in tubes affect the sound quite dramatically. Much to my surprise, however, so far the stock VAC tubes sound best to me. They are very balanced with excellent soundstage and tonality, weight and authority. They just sound "right," perhaps because VAC spends quite a bit of time tuning the sound by ear. I do find myself wanting a tiny bit more extension in the treble, and slightly faster transients, but this is perhaps because my tubes have quite a bit of time on them. I've ordered some Psvanes (I had a pair but wrecked one), which I expect to match this circuit very well since they appear to be of the same manufacture as the stock tubes (Shugang) but at a higher level of quality. I'll report back when I've tried them.

I'll add that I'm extremely impressed with the phono section itself. It's quite a bit better than I was expecting, and holds its own extremely well against my Steelhead II. I'm actually not sure which one I prefer. The Manley (which I've rolled extensively and optimized to my ear) is perhaps more dynamic, with more drive and extension. And it's more flexible in that it allows gain and capacitance adjustment. But the VAC is perhaps a bit more organic and natural. And it has plenty of gain even for a .2 mV cartridge. I could easily live with either, though I'm glad to keep both. Mine is MC/MC. At my request, Brent modified it to allow manual adjustment of impedance loading, unlike the stock Phi Betas, which have user-chosen fixed loading. He built these ingenious screw terminals that allow me to slide in resistors of my choice (I use nude Vishays) so I can really fine-tune the loading for any cartridge. For 100k loading, e.g., I use 10k resistors. It's both extremely flexible and extremely transparent. What a great company! I'm completely in love with this preamp. And you're right, it's a brilliant match with the Bryston 28B-SST2.
Follow up: I found the VAC 12AX7s to be better than the Psvanes in this phono stage--more open and balanced. The Psvanes are tonally richer but and not as extended in the highs. I just ordered another set from VAC as back ups.
My preamp is a dealer demo so it had a lot of time on it. The phono stage, however, had a cap replaced and the screw terminals for impedance loading installed, so those changes require some break-in, as do the nude Vishays I use for loading. In my hands, the unit's been on for three weeks with probably 130 hours of playing time (I work at home and listen while I work). So, you're right, the phono section is probably not quite fully burned in, but close.

I probably should have mentioned that the Psvane tubes are not fully burned in yet. Psvane recommends around 100 hours and I have probably half that on them, so their character might change. But the sound has stayed pretty consistent so I doubt they'll change much. Brent was pretty emphatic that the VAC 12AX7s are his favorites in the Sig phono circuit. He has quite a collection of NOS 12AX7s but prefers the stock ones in there. I have to agree.

I'm coming to the conclusion, subject to revision, that the Manley phono stage is a bit better. The VAC is excellent, don't get me wrong. I completely enjoy it and could easily be happy with it alone (if I only needed two MC inputs). But the Steelhead, right now, anyway, is more dynamic and throws a deeper, more articulate sound stage. My cartridges are all pretty low output, ranging from .2 to .28 mV. The VAC MC sections provide 60 db of gain, which is certainly enough for all of them to sound great. But the Steelhead may be selected for more gain yet, hence the increased dynamics. Running through the VAC linestage, the Manley sounds superb, better than I've ever heard it.

Since I'm running three arms (soon to be four) on two tables, it's good to be able to choose different phono sections for different arm/cartridge combos. But if I had to choose, at this point, anyway, I'd take the Manley. I haven't talked to VAC about the new stand-alone phono stage.