12au7 preamp tubes in a Mark IV

[bloodbath666]

Have any of you guys tried putting a couple 12au7s in the mark IV's v1 and v2 preamp slots? I'm doing it right now and it sounds pretty amazing for a more laid back tone. The volume decreased significantly though. I had to pump the volume a little more. Is this normal?

 

[Russ]

I haven't tried the 12au7 in the Mark IV yet but I have used 12at7's in the reverb position on my A and in the PI of my JMP. Knowing that the gain level compared to a 12ax7 is considerably lower with the 12au7, I would only assume that you would have to turn your volume up considerably as well if you were to use the 12au7 in V1. The 12au7 compares to a 12ax7 as you could think of about 20:100 or about 1/5 the gain of a 12ax7.

 

[t0aj15]

Here's a handy chart for determining preamp tube gain factors:

Relative tube gain on scale of 0-100, with 0 being the weakest and 100 the strongest.

100--12AX7 (aka) ECC83, 7025, ECC803, E83CC, 6681

70--5751

60--12AT7 (aka) ECC81, 6201, 6679

45--12AY7 (aka) 6072

41--12AV7 (aka) 5965

19--12AU7 (aka) ECC82, 5963, 5814, 6189

 

[jamme61]

I really liked the 5751 in the V2 I still changed back to stk 12ax7 but I might try it again. The 5751 left the sparkle.

 

[jamme61]

Just switched back to 12 at7 mullards in v1 and v2. thanks for reminding me.

 

[jamme61]

just checked out a 12au7 in V1 and got about half the volume and gain from the amp. It was a big change, this may be the way to go for a lot of people, that aren't into metal. I settled on a 12at7 in V1 and a 5751 in V2 sounds great to my ears. You can usually p/u N.O.S. 12au7's and at7's for a good price. I p/u mullard 12 at7's new old stk for $19 each.

 

[marklyn]

I had a 5751 in v2 for a while in my Mark IV A. I just changed it back to a Telefunken 12ax7. With the 5751 I felt that I lost some of the singing distortion in the lead channel and some of the chunk on Rhy2. The sounds weren't bad, and had some cool character, just not quite as much gain. There were some cool blues tones in Rhy2 with the 5751 in the just past clean range. I really like the smooth lead gain tone and I'm digging it with the 12ax7 back in v2. I didn't really like the 5751 in v1 for what I'm doing.
It's cool that you can get such a difference with a tube change, it brought a whole different level of control.