Mark III - R2 volume pot mod

[Tuna141]

I just picked-up a Mark III blue stripe combo. The amp sounds great as is, but would prefer to have a separate volume control for R2. I know there's a mod for this and scoured the board to find more info and all I could find out was that the mod basically concerns replacing R130 with an audio taper pot. Is this true? What value pot? Is a refernece designator for R130 silkscreened on the PCB? If not, can you tell me where it is located (photo?)

I'm going to go over the amp this weekend - clean it up, pull the chasiss for inspection, vacuum dust, fix reverb (not presently working - assume it's just oxidation on the ground contact), etc. I'll also probably perform the reverb mod (add 3.3K resistor in series) once i get it working.

Any help you experts could provide regarding the R2 volume pot would be greatly appreciated.

Thanks!

 

[timv]

Look at this thread I started. I put pictures to help out on my yahoo pages and the link is in that thread. Good luck.
http://forum.grailtone.com/viewtopic.php?t=1028&highlight=

 

[Tuna141]

Excellent. I must have missed that thread when I did my search.

Thanks TimV !

 

[Tuna141]

Yup, that's the one. I had to chase the guy a little to finally get the chance to check out the amp. He did seem a little "grumpy" in his emails, but I figured it would still be worth checking the amp out since he was located only about 10 miles away.

He was much friendlier when I got there - had huge Rottweitller pup (intimidating when I saw him, but just playful). The amp was in his studio. It was kind of dirty/dusty and was missing two of the back rubber feet, and the reverb wasn't working. I powered it up and it worked perfectly other than the reverb being out - everything seemed to work, no scratchy pots, etc. It also had the EV and one of the Rhythm/Lead footswitches. The grill cloth was perfect and the tolex looked very good too - just a couple small scratches/marks - no rips or visible dings. It had all 6L6's that he claimed had only about 10 hours on them -he had the two different types, but had them grouped wrong (alternated types).

When cleaned-up, I think the amp will look like new. I'm going to go over it this weekend, and also try to get the reverb working. As you know it was a good price, but I was kind of kicking myself afterwards for not trying to get him to go even lower, which I think he would have because he really didn't seem to know a real lot about the amp (probably explains why he ggot agitated when you asked him "tough" questions). The amp sounds great and I got it for a good price, so I'm very happy with the purchase. It was worth the aggravation I had to endure to buy it :lol:

 

[Tuna141]

I have another question: what type/brand of 250K audio pot did you use - shaft size & length?

Did you order a pot and knob from Mesa (do they even sell them?), so that it looks like the Mark IV pots?

 

[MaxBoogie]

Look at this thread I started. I put pictures to help out on my yahoo pages and the link is in that thread. Good luck.
http://forum.grailtone.com/viewtopic.php?t=1028&highlight=

None of the pics are there anymore.. can you repost, or otherwise make them available? TIA

 

[timv]

Try here, follow the yahoo link and click on R2 mod or whatever I named the folder....
http://new.photos.yahoo.com/tvogelin/album/576460762338090853

 

[MaxBoogie]

Got it... Thanks.

 

[Tuna141]

Thanks for all that helped
Thanks for your help TimV.

I performed the R2 mod and it works excellent.

I removed the Direct Out pot and moved-up the direct out jack to it's place. I then installed the 250K audio taper pot where the direct out jack had been.

I also added a 10K trim-pot in line where the direct out pot was in the signal path. This way I can still use my direct out. If I need to adjust the direct out gain I can simply slide the chassis out a little and tweak the trim-pot.

BTW - for reference, here's the Mesa part number for the 250K audio taper pot required for the R2 volume mod: p/n: 591048. I think it costs about $3.

 

[psychodave]

Does the R2 volume mod change the tone of the lead channel or any part of the amp?

 

[guitarsawicki]

I haven't noticed any difference in any channels. Go for it!


David S

 

[Tuna141]

I didn't notice any tone change either.

The mod lets you balance the channels out really well.

 

[phyrexia]

How do you wire the pot? Like a 'variable resistor', R130 to the outside lug and r131 to the middle lug?

 

[Tuna141]

Yes, you basically just replace the resistor with the pot. One wire goes to the center lug of the pot and the other wire to one of the outer lugs.

It does matter which of the outer lugs you wire to. Running the wire to the "wrong" outer lug will simply make the R2 volume control wprk in reverse (i.e. the volume will be loudest when the pot knob is at 0 and softest when at 10). I guessed wrong when I installed mine, and had to swap the wire to the other lug to get the volume pot to work properly.

Sorry, I don't remember exactly which of the two outer lugs the wire needs to go to. If I had to guess, I'd say the bottom lug when the pot is installed, but I could be wrong... .

 

[psychodave]

I think I am gonna do this mod this weekend.

 

[psychodave]

With the 250k pot, how do you wire it up? Do you attach one of the side lugs to the middle and use that contact point and the remaining lug, or do you simply add the wires to each outside lug? This mod seems very easy, yet I am unsure about the pot wiring :?

 

[phyrexia]

Edited!

I did this today. Took me about thirty very precise minutes. I have soldered before and I'm relatively confident about my ability but I've never removed and added components to anything this complex. I had a friend hold the solder while I held the wire and the iron.

R130 to lug 1.
R131 to lug 2.

I don't have reverb so I installed it where the reverb pot would be. I installed my pot so that it was oriented just like the presence pot. When you are looking at it from this angle, R130 is connected to the left lug.

Also I will add it is functionally the same as the "Lead Master" knob, not like the "Volume" knob. So it doesn't control the gain on R2, vol still controls that. We'll have to do sawicki's relay mod (III version IV?) to get away with that.

Victor

 

[psychodave]

Cool. Thanks. I was just going to cut away R130 and connect the post inbetween.... If there isnt enough of a solder contact, I will goto R131. I will try to take pics to make it easier for others to do.

 

[psychodave]

Well I had a few minutes tonight, so I did the volume mod. It was pretty easy. I wish I had time to take pics, but I was kinda rushed to get it done. I did use R131 as a contact point. It was a lot easier to get to than the other side of R130. Thanks for the help guys. 8)