mark III clean MOD

[Nick_cor]

hey, I was wondering if anyone knows anything about this mod that could be done to a mark III, i believe it has to do with the reverb knob and it affects the volume input of the rhythm 2 channel, does anyone know if this is possible ? and if it is, is it possible to do it to a non reverb mark III?

 

[Aart]

Sorry for the OT, but could you please post a rig pic in the post your rig topic?:wink:

For GAS sake :lol:

I'm totally ready for a second mark in the rack but still don't have the money for a III or IV

 

[phyrexia]

It is the R2 Volume knob, but it acts as a R2 output volume, not R2 input volume, so you cannot adjust the gain of the channel. You can, however, balance the levels of R2 with R1 and Lead.

Usually the new potentiometer is installed where the Reverb knob was originally. I do not have reverb on my amp, for instance, so there was only a plastic plug to keep the hole for the reverb pot closed.

There are threads available that will give you the step by step method.

 

[seajay]

It is the R2 Volume knob, but it acts as a R2 output volume, not R2 input volume, so you cannot adjust the gain of the channel. You can, however, balance the levels of R2 with R1 and Lead.

Usually the new potentiometer is installed where the Reverb knob was originally. I do not have reverb on my amp, for instance, so there was only a plastic plug to keep the hole for the reverb pot closed.

There are threads available that will give you the step by step method.

Or if you have a long chassis head w/reverb, the reverb control is mounted on the front and there is an unused "hole" for the reverb control on the back filled with a plastic plug. I think this is because the long & short heads used the same template for the controls on the back.

Anyway, the extra R2 volume can be mounted here too, without disrupting the reverb.

 

[Boogiebabies]

The R2 master volume would install the same way if the amp has reverb or not. The only difference is the V4 is missing. Since you don't have reverb, you can mount the pot in the reverb spot. There is a great post by someone who did the mod and showed how he hooked the pot up after removing R130 and hooked the other wiper to R131.

 

[edward]

The R2 master volume would install the same way if the amp has reverb or not. The only difference is the V4 is missing. Since you don't have reverb, you can mount the pot in the reverb spot. There is a great post by someone who did the mod and showed how he hooked the pot up after removing R130 and hooked the other wiper to R131.

May I ask what R131 does, and also what effect hooking it to the R2 vol pot's other leg does? Thanks!

Edward

 

[timv]

Please read through the other posts on this subject. Search for R2 mod. That point on R131 is connected through a trace on the circuit board to the other side of R130. Mesa installs one of the wires to the point on R131, my guess is because it gives a little more to solder onto so is a bit more stable.

 

[Restless Rocks]

If your amp does not have reverb or if you intend to disable the reverb you may simply insert your R2 master pot in place of R130. If you wish to keep your reverb functional it will be necessary to attack one of your connections at R131 (not replace it) while removing R130 and attaching the other lead from your R2 pot at the side near the LDR and relay. Other wise you will get a serious drop in your reverb when in R2. If you are simply replacing R130 you may use the 10K pot already mounted as the direct out control (provided you never use it) and it will work fine. If you are doing the "keep your reverb R2 mod" you will need a 250K linear taper pot. Like BB says one of the fellows posted some excellant pics of the mod if you want to do a little searching.

 

[timv]

The connection on R131 and the one side of R130 are the exact same point in the circuit so it doesn't matter where the pot is connected, however you want to do it is fine. It doesn't matter if you have reverb or not. I've done this to 3 different mk 3's now and have done it both ways and the pictures on the other thread are mine.

 

[Restless Rocks]

On amps without reverb you are correct however of the many R2 mods I have installed the first two who wished to keep their reverb functioning returned complaining of the drop in reverb level when engaging R2. The R2 pot is grounded by the action of LDR3 and the R2 relay and the unseen track on the underside of the board from the other side of R130 goes to R131 POST Reverb and will show continuity at the FX send jack . I have found that to maintain balanced reverb level in all three modes the R2 master must act on the reverb SEND side of R131 as does the lead master. R131 buffers the send and return of the reverb circuit. On the "official schematic" the placement of R130 is also innaccurate in relation to LDR3.

 

[edward]

OK, I'm learning here, guys, thanks!

So since my head has no reverb, the simple placement of a pot in lieu of R130 is all that I need, which is what I had read in past threads. But now Restless, you mention the 10K pot from the Direct Out ...I thought what I needed, IIRC, was a 250K pot, and in audio taper, not linear. Which value should I get, and in which taper? Thanks guys! ...forgive the ignorance

Edward

 

[Restless Rocks]

Since you are replacing a 6.8K resistor (R130) the 10K pot will work in a pinch. The difference in an audio taper and a linear taper pot will is that the audio pot will sound louder at an earlier setting. Many people find a setting for the R2 master and then rarely change it anyway. There are some more slightly complicated reasons why you need the 250K pot when attaching at the R131 (reverb send side) having to do with signal impedance and such. The 250K will work in either version but the 10K will not.

 

[edward]

Thanks for the specifics, Restless. Probably won't be using the Direct Out, I may just go that route. I appreciate the clarity and help.

Edward

 

[Mercury25]

Should there be any difference in the R2 output between the 10K and 250K?

I did the mod last night and "re-used" the 10K pot and it works beautifully, but I do notice that I have to turn it all the way up to match the R1 volume.

Also on my dB meter there is still a 3-4 dB difference between R1 and R2--R1 being louder...

Does this sound correct or would I gain anything by using a 250K pot instead?

FWIW I used the R130 connections only--so I don't have any reverb signal drop.

Thanks!

 

[Restless Rocks]

Using the 250K pot will allow your R2 volume range to be louder or softer than R1 and you will then be able to do the R130 to R131 connection which is prefered. Please carefully reread the previous posts in this thread. And good Luck!!!

 

[Mercury25]

If you are simply replacing R130 you may use the 10K pot already mounted as the direct out control (provided you never use it) and it will work fine.

Sorry for the questions, but I did read the entire thread several times before posting.

Maybe I misread, but your post says that you can use the 10K pot if you are just using the R130 connections--no?

So my question remains: What would I gain by using the R131 connection and a 250K pot at this point? I thought the reason for the R131 connection was it was easier to access than the one R130 leg.

If there is much better volume control i.e. more loudness for R2, then I will happily take the amp apart and do the other mod version.
I was just wondering from all the "experts" that have done more of these than I have (none :? )

Thanks again for the reply...

 

[edward]

FWIW, I did the R2 volume pot mod with the 10K pot. My MKIII has no reverb, and from what I understand from the previous threads, you can only use the 10K pot (from the DO) if you have NO reverb; those with verb need to do the R131 ...I think I got that right

Anyway, my R2 mod works perfectly, if that helps you any. Hope this helps

Edward

 

[Restless Rocks]

If you have NO reverb simply replace R130. If you DO Have reverb it is prefered (Like the R2 mod would be done by Mike B.) to attach one lead from your 250K pot to the R130 pad (after removing R130) nearest the LDR and the other lead to the pad at R131 nearest the group of orange drop capacitors. Here's the catch I've just found with using the 10K pot: If you like to run your gain control above 7-8 (or if you have not removed R130)the 10K pot will attenuate R2 just a couple db too much. In other words you can turn it all the way up and R1 may still be a bit louder than R2. If you set your gain below 6 then 10K will give you the ability to adjust R2 to be louder or softer than R1. I suspect that when designing the gain structure of the MkIII preamp an average gain setting of 6 was used as reference for the R2 attenuating resistor (R130) the value of which varies among different stripe versions. Soooo when doing the NON reverb R2 mod if you are able to decipher the value of R130 and get a linear taper pot which is at least twice it's resistance (let's say that 25K is easy to find) then your nominal setting for the R2 control would be somewhere in the middle (around 5) instead of between 1 and 2 as is the 250K pot. Hope I haven't caused anymore confusion and remember guys be very careful working inside your amp! And by the way credit is due our freind "timv" who posted some very nice photos of the actual mod if you can hunt them down.

 

[Mercury25]

OK--

I replaced the 10K pot with a 250K and it NOW has BALLS-A-PLENTY!!

So just to be clear for any others that travel down this road in the future:

If you have reverb, you MUST use a 250K to get any additional volume. The 10K at it's highest setting is the same as the pre-mod volume.

Thanks to all those that tried to help me--I'm new to all this.

EDIT: Thanks RR--our posts crossed paths!

Here is a link to the timv pics:
http://forum.grailtone.com/viewtopic.php?p=111579&sid=78a26306c2c74308777fb56567481b94

 

[edward]

Yeah, I noticed the volume drop, too, with using the 10K pot. But the way I set my MkIII, with the Vol1 gain set right at 7, and I can get the R1 and R2 volumes dead even. But I did thik it was a bit odd that the pot is turned up pretty high, around 80% or so if I recall correctly. But since it works the way I need it and the R2 vol is where it should be with respect to the R1, I don't see any need to swap it for a 25K (or 250K) pot. I'll try it with the gain set higher, though, and see if the 10K pot becomes a hindrance. Thanks, Restless, for that insight ...very good to know!!

Edward