Mark V and VII - high gain mode gain stage questions

[mace]

Hi all,

So, in comparing the V:90 to the VII I'm noticing the VII is a more "gainier" in IIC+ and IV modes than the V:90 is. Then I remember that my V:90 has an AT7 tube in the V4 position, so that may explain the gain difference (although CRUNCH seems a tad more gainy and brighter in the VII than the V:90). (At least, I think the AT7 is still in my V:90.... I don't remember ever removing it ).

So, I got to thinking... is there an equivalent AT7 mod for the VII to bring down the bite a bit? Looking in the Mark V:90 manual tube task chart it shows that CH3 has six gain stages and the Mark VII manual shows only five gain stages for IIC and IV modes.

Anyway, the Mark VII manual tube task chart only lists five gain stages for IIC and IV modes.

Questions:

1) How can the "same" modes in V:90 and VII have a different number of gain stages?

2) In the VII, where should an AT7 be placed to achieve the same (similar) thing as the "AT7 V4 mod in the V:90"? I'm thinking also V4 based on it being 4th gain stage for IIC and IV modes (V4 in the Mark V is also 4th gain stage).

At any rate, I'll have to give it a whirl to see what it sounds like.

If you guys have any thoughts I'd be interested in hearing them!

 

[bandit2013]

I would not be too concerned about it. Mark V90 may have a few more gain stages in the path but does not mean more gain overall. That lead drive circuit feeds into V3A, there are a few attenuator voltage dividers before it runs into V6A so the signal level is reduced and will not overdrive the next stage. V6A is more of a voicing stage for the three modes. The IIC+ and IV modes are not as robust as they are with the Mark VII version (this with the 12AX7 in all tube locations in the Mark V90). The same could be said about the JP2C. Both of which only have 5 preamp tubes in total. That design dates back to the IIC+, III and IV. The Mark V was the odd ball in the series. The added gain stages I believe was more of a compromise in the design so that the three modes could be realized on CH3. Overkill perhaps? Mesa did it much better with the Mark VII and got rid of the excess that was not needed to accomplish the end goal.

I tried a JAN/philips 12AT7 in the JP2C, that did not work out very well. If you need to cut some gain from the amp, I would try a JAN 5157 instead of the 12AT7 as that will have more of a similar characteristic to the 12AX7 but with less gain, not as dramatic as the 12AT7. If you needed to drop some of that gain out, I believe the JAN 5751 would have a better fit in the V1 position. Still the crunch and VII modes will place the lead drive circuit in between V1A and V1B. Not sure what would make more sense. IIB uses V4A so a 12AT7 in that position would probably not be all that great for that mode or voicing. I personally would not use the 12AT7 in the Mark VII.

 

[mace]

Thanks for the insights and 5751 recommendation. Since I have some JAN AT7's laying around I'm gonna give it a go!

And yeah, I saw that other modes would be affected in the Mark VII V4 position (VII mode and IIB mode). But, I'm curious.

 

[bandit2013]

I would try the AT7 in the phase inverter. I have thought about that for some time. Most of the JAN AT7 I have are in the bass amp. Those actually sounded much better than the Tung Sol AT7.

I have two JAN/GE 5751 tubes to play with. Will see what happens with them. I have a hunch that V1 would be the ideal spot for that tube. Now I have to give it a go since I recommended it.

 

[mace]

I went ahead and ordered some JJ 5751’s the other day to play with. The JAN one were too rich for my blood.

 

[rarebitusa]

The JAN one were too rich for my blood.

my thought exactly

 

[mace]

Got the 5751’s today. Went down, plugged into the VII and was playing around to calibrate my ears to the ‘as shipped’ tone. Eventually looked at the time and 2 hours had passed. So, I guess the 5751’s will have to wait! LOL. Dang Mark VII…

 

[bandit2013]

If you lost 2 hours of time with the Mark VII with the stock tubes, is that a good thing or a bad thing?

 

[mace]

Depends how you look at it. Got in some good practice time but spent money on these new tubes and they are still sitting untouched in the box! Someday I'll have to do some tube rolling, but it's been too much fun just playing guitar for now.

 

[bandit2013]

I get it. Why change if things sound good?

I have rolled in a few 12AX7 in the one Mark VII out of curiosity. Tried the JJ E83CC tubes (frame grid type), those actually sounded better than the stocker Mesa branded JJ ECC83s. More notably in the noise floor, the E83CC were almost dead silent on the higher gain modes. The other tubes were the old Mesa 1990 Beijing Square Foil Getter tubes, Chinese Miliary 6N4-J which were the stock tubes for the Mark III when I bought it. Just added a hint of more gain, and a bit less noise than the stock tubes.