[Gvt1911]

I just purchased a VTL Maximal preamp which uses 2-12AX7s for the phono and 2-12AT7s for the linestage. Nice sounding unit with the right tubes. My question is how to adjust the 2 bias pots on the back of the unit? I now have them set to the lowest gain. Anyone have one of these with some instructions? No paperwork came with unit and VTL never answers emails and I could not get anywhere on the phone.
Thanks ....

[steve k]

According to the VTL book, those pots are not for bias but to adjust the gain on the phono stage to accommodate different output carts. They increase or decrease up to 5dB gain per channel. They also allow you to center the image if your tubes aren't matched.

Hope this helps.
steve

[Gvt1911]

Steve,
Thanks for the info on the preamp. You are correct that the pots are "gain" pots and not bias pots as I said. Per chance does the VTL book state that I maybe able to substitute 12AU7s for the 12AT7s ? Every 12AT I tried seems to have a lot of gain and is microphonic when tapped on with finger. I know that the tubes are low noise and the microphonics are low also,I checked them all with a GK low noise tester. I may try a 12AU7 just for the heck of it, lower MU than the AT tubes...Any comments would be appreciated...
Regards,
Joe

[steve k]

Joe,
While I didn't see any specific reference to swapping those tubes in the Maximal in the VTL Book, I know from dealing with Luke Manley and my VTL power amps, he doesn't recommend even rolling in other brands from the stock tubes in his amps.

OTOH, I've tried lots of different 12AT7's in the input and driver tubes of my amps and found the best sounding IMO is the Sylvania 6201 Gold Brands. They have a nice blend of smoothness and detail. Others close in quality are TungSols and RCA 6679's.
Good luck,
steve

[Gvt1911]

Steve,
Thanks again. You are indeed correct about the 6201s. I found a few and had NO problems at all. The sound was quite nice with no harshness anywhere. All the exotic Euro vintage brands were not cutting it one way or another. Going to dig around and see how many more 6201s I have lying around. Many Thanks for your time !
Regards,
Joe