Mesa boogie 3 channel dual rectifier tone mod

[Spencerleehorton]

Going to put in 10k 0.5W resistor in replacement of the 15k resistor and 50k pot will replace the switch.
I will then dial in -39v and see how the EL34 sounds.
Need to figure out how to measure the current though!!
Any ideas?

 

[john12ax7]

Get a bias probe to measure current,  something like this

https://www.eurotubes.com/store/pc/bias%20probes.htm

 

[Tubetec]

In my experience halving the value of a coupling cap is barely audible , but id say your on the right track incrementally bringing the bass under control over a few stages . Its funny how guitar amps have gone more towards treble and bass with a lot of mid suck nowadays , the modern chinesse version Ac30 bears very little resemblance to the lovely chunky tone of a proper JMI , I think its become a bit of a race and many manufacturers are getting caught up in it . Of course the kind of guitar and pickups you use makes a huge difference too. There is a few interesting mods for the efx loop on boogies too ,worth a check around on.

 

[Spencerleehorton]

Thanks john I’ll make myself one of them!

 

[Spencerleehorton]

Could you post up any details of FX loop mods please?

Also what has been said about this amp is it doesn’t use any neg feedback to keep it linear.

 

[john12ax7]

Thanks john I’ll make myself one of them!

I would say do it with the cathode so you are not dealing with B+ voltages.  The two options are measure cathode current directly with mA meter.  Or some people put 1 ohm resistors in series with the cathode,  then measure mV across the resistor (which would equal mA).

 

[Tubetec]

I did mod the efx loop for a customer a while back ,unfortuneatley I recently binned the documentation I had on the mod ,I think it was a serial efx loop he wanted ,in the end I put in a small toggle switch so it was easy to go from series to parallel.
If your not using the efx loop ,bypassing the buffer stages alltogether is a good one ,but always bear in mind you might hurt the resale value ,I generally prefer non destructive mods where you can easily switch back to standard mode at the click of a button .I'll have another scout around tomorrow and see if I can find something for you .

 

[Spencerleehorton]

Just found a lead that has come off the bias switch!!! as it was all strapped down you couldnt see it, on closer inspection it has come off!!!!
So it seems i have been running this amp not biased properly!!! no wonder it sounds crap!!!

attach lead, test valves, install valves and hopefully i don't need to modify anything, will probably put back the 0.047uf on the power amp as well.

 

[CJ]

drop the B+ voltage to the first preamp tube, 

if that does not work, trade for JCM800

 

[Spencerleehorton]

someone suggested to put in an ECC82/12AU7 in V1 and it sounds terrible so i've gone back to ECC83.

Haven't had time yet to do bias plug but will try and do over the next few days and test the bias, then will put in the 10k and the 50k trim and have a play with the bias.

I have read somewhere around the 40mA-44mA gets good bias?

 

[john12ax7]

The typical rule of thumb for class ab guitar amps is to bias at 50-70 % of plate dissipation at idle.

 

[valveandsound]

This amp is essentially a clone of a Soldano SLO100. modern 3 channel versions have 1 clean channel and channels 2/3 are switchable gain and masters and EQs between the OD mode. I personally think making it sound more like an SLO is the better improvement (lol). Trimming out a bunch of gain in the front and increasing the gain later in the signal path seems to make it "better" IMO.

If you look at the cold clipping stage before the cathode follower i believe it has a 39K cathode resistor. Mike Soldano chose this value because he was trying to keep the clipping in the amp more symmetrical. I personally like to drop this down to around 10-14K. It tends to make the clipping less "buzzsaw" sounding.

Also, If you look at the tone stack for the OD channels you'll see that the "presence" control is bootstrapped after the main EQ. in Vintage mode this control is actually wired like a presence control on your typical marshall. in Modern and Raw modes it disconnects the global negative feed back and wires the control in post EQ - I personally think it sounds terrible when wired that way. So i disconnect the wiring that allows the presence control to connect to the post EQ.

First gain stage has a 220K plate load. I change this to 100K (or even 80K) to help eliminate the buzzsaw tone. It also improves the clean channel too (they share the first gain stage) IMO.

As far as clean channel improvements, I do away with the presence control on the clean channel entirely - its a simple tweed style tone control placed in circuit and is pointless IMO. I also change the slope resistor value in the clean channel tone stack to 100K (essentially making it more fender-esque)

tweaking the plate load on the cathode follower sounds good too. I want to say I used a 110K/150K/220K here...(one of these values is the stock value. Im going off memory here so forgive me)

Adjustable bias is absolutely necessary IMO. There are lots of mods on how to do this online so i won't be a broken record.

Clipping out the mix knob on the Effects loop and wiring the unit to work in serial effects only is a big (and common) improvement as well.

A cool mod is to put 100K/56K/33K slope  resistors values on a 3 way toggle and sub them in for one of the OD channel tone stack slope values. slope resistor value is a really powerful way to shape the OD tone in one of these things.


I had a bunch of other tweaks for this amp written down; its somewhere around the shop. Anyways, there is a lot to digest here. Hope it helps!

 

[Bo Deadly]

Really i would like to go more Marshall sound

Honestly I don't think the preamp / tone stack matters as much as people think. The output stage is more important because of how the power tubes clip, how the supply sags a little in reaction to punchy notes / sharp chords, how the OT is over driven to add bit of a "gong" sound, how the presence control tightens up the response, ... Changing a few caps in the preamp isn't going to do spit.

 

[valveandsound]

I don't know how much the output section really gets pushed in this amp. Unless you have the master up fairly high. The factory bias setup in this amp is set very cold - it takes a lot to push the power amp to clip. Even more for the OT to saturate. Much of what this amp does is in the preamp IMO.

 

[Bo Deadly]

I don't know how much the output section really gets pushed in this amp. Unless you have the master up fairly high. The factory bias setup in this amp is set very cold - it takes a lot to push the power amp to clip. Even more for the OT to saturate. Much of what this amp does is in the preamp IMO.

Actually I was sort of thinking of a Marshall. If the OP is looking for a Marshall sound, I think the output stage is important.

 

[CJ]

another bias method -  measure the DCR of the output transformer,

hook a clip lead to the center tap,

measure each leg from center tap to brn or blu or whatever color they use,

lets say you have 40 ohms from red to brown  (leg 1) and 38 from red to blu (leg 2)

lets say you are running EL34s at 65% power ,

EL34 plate disp is 25 watts, but they don't roll those like metal base Mullards anymore,

so make it 24 watts.

65% of 24 =  .65 x 24 = 15.6 watts idle disp

how many plate volts, 500?  watts is EI,  15.6 watts/E = I,  15.6/500 = 31.2 ma

back to the transformer and Ohms Law

38 ohms x  I-ma = volts across transformer leg #1
40 ohms x I-ma = volts across transformer leg #2

38 x .0312 = 1.1856 volts
40 x .0312 = 1.248 volts

so clip volt meter across leg 1 and turn the pot

check leg 2 and compromise your readings

EL34s sound better biased hot,  6L6 likes 60% for clean punch, EL34 likes 65% for that Marshall sound,

don't be afraid of high voltage, you are not going to live forever,  get in there and mix it up, get some excitement going,  you cant enjoy a good shock after you make the transformation into the spiritual world, be a man, so what if you die? you get a brand new body, girls will seem exciting again,  ;D

 

[Spencerleehorton]

Love it CJ, LOL!!!!! im going to go with the adapter first, i had 310v on my elbow a few months ago from a little fender champ amp!!! didnt really enjoy it that much!!!