tube position in the mini recto

[dmraco]

I am looking at re-tubbing my mini recto pre amp tubes.

the positions are V1-V4. I was wondering what position really effects channel 2. It looks like V2. What I am un sure of V2a vs V2b? V@a said is the third gain stage for ch 2 and 2nd gain stage for 1 and 2. What does this mean? I would like to add some gain to channel 2 without changing the cleans in channel 1.

Thanks

 

[domct203]

I am looking at re-tubbing my mini recto pre amp tubes.

the positions are V1-V4. I was wondering what position really effects channel 2. It looks like V2. What I am un sure of V2a vs V2b? V@a said is the third gain stage for ch 2 and 2nd gain stage for 1 and 2. What does this mean? I would like to add some gain to channel 2 without changing the cleans in channel 1.

Thanks

A 12AX7 tube is really two tubes (triodes) in one bottle, refered to as Triode-A & Triode-B.

The Recto's use cascading gain stages where one stage feeds into another.

When you are in channel one, the (simple) gain stage flow is:
INPUT==>V1B==>V2B==>V1A

When you are in channel two it becomes:
INPUT==>V1B==>V2B==>V2A

What do you mean by "add some gain"? There is plenty of signal gain in the recto as is. If you are referring to using a "high-gain" tube, you'll be disappointed as the 12AX7 is the highest gain of the 12A*7 series, and going from the standard gain rating of 100 to a "high-gain" rating of 110 is not a night and day difference in the level of overdrive.

If you want a hotter signal (and more tube overdrive) try a clean boost or OD pedal in front of the amp.

By all means, experiment with preamp tubes, while subtle, there are differences in tone and dynamics between brands. V1 will have the most effect, V3 the least.

Be careful of what you try in V3, it's a cathode follower position and New Sensor brand tubes (Sovtek, Electro-Harmonix, Tung-Sol reissue & Mullard reissue) have been known to fail there.

I hope this helps,
Dom

 

[Addictedtokaos]

I just want to echo everything that Dom said about the gain stages and 12ax7s in general. Ive spent plenty of money on different 12ax7s and will agree that, to my ear, the differences are very minimal.

The only thing is that unlike the bigger mesa recitifers, the mini does NOT have a cathode follower tube. so you can use which ever 12ax7s you please for V1 -V3.

 

[domct203]

........ The only thing is that unlike the bigger mesa recitifers, the mini does NOT have a cathode follower tube. so you can use which ever 12ax7s you please for V1 -V3.

Wow, that's somewhat disappointing to the Recto purist in me LOL.

I wonder how Randall chose to match impedance between stages to drive the tone stack?

It seems odd for him to stray from the "all tube signal path" mantra. I wonder if he is using an opamp/transistor setup?

Did you see this on a schematic?

Dom

 

[Shemham]

The only thing is that unlike the bigger mesa recitifers, the mini does NOT have a cathode follower tube. so you can use which ever 12ax7s you please for V1 -V3.

Really? I was on the impression that the driver tube on the manual tube chart was synonymous to cathode follower.

Wish I had a schematic for the Mini Recto...

 

[elvis]

I don't remember any amp with a cathode follower driving the tone stack. Generally just a CC stage. CF gets used to drive the FX send, if at all.

Admittedly I haven't ever looked at a recto schematic.

 

[Addictedtokaos]

No, Ive never seen a schematic, nor would I be successfully be able to understand it . I was concerned myself when I first got my mini a couple years ago and I contacted Mesa. The mesa tech assured me there was no cathode follower in the Mini Rectifier. Its always possible the tech had no idea what he was talking about...

 

[domct203]

I don't remember any amp with a cathode follower driving the tone stack. Generally just a CC stage. CF gets used to drive the FX send, if at all.

Admittedly I haven't ever looked at a recto schematic.

It's probably just my limited exposure, I'm not an EE or certified tech, but I've seen & discussed CF used to drive the tone stack and/or FX Loop in Marshall JMPs & DSLs, Fender Bassmans and Mesa Rectos & Stilettos so it seemed common place for me.

Dom

 

[elvis]

Just checked a rectifier schematic online. Sure enough, CF driving the tone stack. I find it interesting that there are 4 gain stages before the tone stack. Sort of the opposite of most amps where the tone stack comes in front after just one gain stage.

It wouldn't bother me if they used an opamp in place of a CF (or better, a JFET) since that is pretty far downstream and I use the FX loop, so I am throwing opamps in the signal path anyway.

What does bother me is that Mesa advertised that the circuit is identical to the bigger recs.

 

[Shemham]

Yeah, they don't seem to share the exact same signal path in preamp, although tonally they're quite similar.

The difference could be heard easily if someone with both amps would actually plug mini rec's preamp into bigger recto's power section and vice versa.