Mark III Preamp Tube Functions?

[bloodspoint]

Hi all,

Does anyone know what each of the 12AX7's in a Mk III does? I mean, obviously the last one is the phase inverter, but what does V1, V2, etc. do?
What I'm looking for really, I guess is, if I wanted to add gain to R2, which tube would that be? Or if I wanted a cleaner R1 sound, which tube would I want to swap for a cleaner one?

Thanks,

Sean

 

[phyrexia]

Put simply, V1 = all channels, R1
V2 = R2 (but lead goes back through V2)
V3 = lead

 

[edward]

Hi Sean,

V1: Input for both R1/R2 and Lead channels.
V2: F/X Loop buffer and either the R1/R2 OR the Lead channel, depending on which is selected.
V3: the "a" and "b" sides are in series for the Lead channel, and come before the V2 stage.
V4: PI

More gain in R2 with a cleaner R1 are mutually exclusive. The initial gain knob feeds both, as do V1 and V2. You could get the R2 Gain mod (not to be confused with the R2 volume pot mod) which will give you more control over R2's gain. I have the vol pot mod ...works great; no idea about the latter.

The "solution" here is to decide which is the more useful channel to you: R1 or R2. Then set the gain knob right where you get the sweetest tones for that channel, and compromise with guitar vol knob, or a pedal(s) for the other "less frequent" channel and tone you want. Hope this helps you a bit

Edward

 

[4Eyes]

I think it's impossible to do, by only swaping the tubes. Because Rh1 and Rh2 use the same tubes and also Lead channel uses the same tubes like Rh1 and Rh2. So swaping preamp tubes (V1 or V2 - because they are common for all three channels) will take effect on all channels. So u can cleaner Rh1 with tube with less gain, but it'll also mean, that Rh2 will have less gain and meybe lead channel, too.
and the same situation if u want to increase gain on Rh2.

better way seems to be (to me) to mod preamp schematic, if u have a person who can do it and who knows where to make changes to get an effect u want 8)

 

[bloodspoint]

Thanks, guys. I'm not actually trying to get a cleaner R1 and a gainier R2 at the same time. I'd actually give up some of R1's clean for a little more R2 gain. I don't usually need "true" clean, just a little less grungy for arpeggiating chords and so forth. Sounds like a higher gain R1 tube might get me in the right general direction?

 

[MrMarkIII]

Turning up Volume 1 and the Treble gets you more gain in all three modes. Merely switching one 12AX7 for another is not as dramatic a change.
Want dramatic? Switch out the 12AX7 in V1 for a 12AT7. 40% less gain. Play the amp like this for a week. Then reinstall the 12AX7.
Now that will be noticeable!

 

[rvschulz]

here is what i did to my Mark III last week - i like it ...

V1 - balanced Sovtek 12AX7WC - pretty high output, measured at 105/105 from Doug.
V2 - JAN 12AX7, nuff said.
V3 - Chinese 12AX7 - 9th gen, measured 95/95 ... or so it said.
V4 (PI) - Sovtek 12AX7LPS - spiral filaments.

I'm running "class A" with 6L6 ... it's putting dust on my LSC now ...