Spencerleehorton

Re: Mesa boogie 3 channel dual rectifier tone mod
« Reply #20 on: March 13, 2018, 10:18:21 AM »
i will first check what component values i have in there, as it could be that the caps and the resistors are not what are on the schematic?
website: www.mohawkstudios.co.uk
email: [email protected]

If it hisses its probably the wrong impedance!!!


walter

Re: Mesa boogie 3 channel dual rectifier tone mod
« Reply #21 on: March 13, 2018, 11:41:17 AM »
What a mess! A lot of series resistors and caps to ground, they really had to tame that beast. Just looking at ch 1. The 47pF at V1b.7 and the 250pF across CH1 MSTR for example, eat high end, but may be necessary. You might try lowering the gain at each stage to reduce the need for these caps to ground, and maybe lower some of the series resistor values. V1a.3 and V2a.3 replace the 1k8 with 1k to lower the gain there, that may be enough to be able to remove the 20pF at V2a.2. The Two .047 between PI and power tubes, replace with .02 to block some low end, The .001 to ground after V1a.1, take it out if you can. The 220k after V1b.6, that won’t help high end. The tone stacks, replace the .1 and .047 going to the BASS and MID pots to .022 to clean up the mush.  I’m just making this up, but these are some things I would try. Looking at this reminds me how much I enjoy working on tube amps, it’s been too long, I wish I had more time to get on the bench.
Blown like a fuse

Spencerleehorton

Re: Mesa boogie 3 channel dual rectifier tone mod
« Reply #22 on: March 13, 2018, 12:39:31 PM »
Thank you Walter, best suggestions so far, very good directing!

Could you be a bit more specific on these two points please?

The .001 to ground after V1a.1, take it out if you can. is there just one of these or two to take out?

The 220k after V1b.6, that won’t help high end. take this out with link or reduce?
« Last Edit: March 13, 2018, 02:35:24 PM by Spencerleehorton »
website: www.mohawkstudios.co.uk
email: [email protected]

If it hisses its probably the wrong impedance!!!

john12ax7

Re: Mesa boogie 3 channel dual rectifier tone mod
« Reply #23 on: March 13, 2018, 09:30:48 PM »
Don't want to discourage you but have you considered getting another amp that is a better starting point and modding that? An amp where all controls at 6 oclock sounds  closer to what you want.  Trying to turn a mesa into a marshall would require a lot. Heavy mods on pcb amps also tends to involve a lot of expletives being uttered.

The  .001 cap after V1a is the one on the gain pot? That doesn't go to ground.  It is a treble bleed resistor,  effectively out of the circuit at full gain, at lower gain settings helps keep the signal bright. This can be removed or value lowered.  Sometimes it darkens the sound in a good way,   sometimes things get too muddy without it.

Before any of this have you tried a tube swap or bias adjustment? IIRC the tubes are biased rather cold in these,  good for lifespan, not the best for tone.

Edit : think they also use a fixed resistor to set the bias,  which makes adjusting it a pain.

Spencerleehorton

Re: Mesa boogie 3 channel dual rectifier tone mod
« Reply #24 on: March 14, 2018, 02:28:03 AM »
I’m defo up for a bias mod if there is one?

I’m not looking to sell it just mod it so it’s less bass heavy, clearer mids and not so fizzy on top end.
« Last Edit: March 14, 2018, 04:59:03 AM by Spencerleehorton »
website: www.mohawkstudios.co.uk
email: [email protected]

If it hisses its probably the wrong impedance!!!

john12ax7

Re: Mesa boogie 3 channel dual rectifier tone mod
« Reply #25 on: March 14, 2018, 05:53:45 AM »
On the schematic there is a "bias supply" section.  Most amps use a pot as part of a voltage divider to adjust the negative bias voltage.  Unfortunately mesa uses fixed resistors.  So you are stuck either changing resistors or rigging a pot in there.

Have you tried EL34s in the amp? I had forgotten about that being an easy option.

Some interesting reading
https://www.eurotubes.com/store/pc/mesa%20boogie%20amps.htm
« Last Edit: March 14, 2018, 06:10:21 AM by john12ax7 »

Spencerleehorton

Re: Mesa boogie 3 channel dual rectifier tone mod
« Reply #26 on: March 14, 2018, 07:49:00 AM »
Yes I have always gone for EL34 in there as I didn’t like the 6L6
website: www.mohawkstudios.co.uk
email: [email protected]

If it hisses its probably the wrong impedance!!!

Re: Mesa boogie 3 channel dual rectifier tone mod
« Reply #27 on: March 14, 2018, 08:00:08 AM »
From the sound of it, you want a good Marshall and a Rectifier is about as far away from a Marshall as you can get.  Modifying one to sound like a Marshall is, in my opinion, a bit of a fools errand.

Embrace it for what it is, and if that doesn't work for you, then you really should leave it alone and get a Marshall.

Some suggestions...I don't know what tubes you have in it, but Doug's Tubes has a tube "kit" for this amp that just kills.  That's alot of jack to drop on an amp that you already don't love the sound of, but it will definitely make the amp breathe more and get some more dynamics to it.  I picked up a 3 Channel Rectifier for myself a couple of years ago and was astonished at the difference the tubes made.  I suggested the same tube setup for a buddy of mine, and he really liked the difference the tubes made in his Rectifier as well.  Note...We both like the Rectifier.

As far as the amp itself.  If you want Marshall, and assuming a 3 channel version, Ch2, Vintage is where you want to be.  Keep the presence down (9 o'clock), tone controls at 5, and on the back turn the loop off (the master volume and solo functions will stop working, use the CH volume for volume control), set the rectifier type to "tube," and power to "spongy."  Adjust tone controls to taste, watch the presence control. 

If you can bear it (which I doubt), the amp will get less bassy/fizzy and more mid heavy when the volume gets up around noon, but that's definitely NOT a bedroom setting. 

This (in my opinion) is about as Marshall as a Rectifier can get.  If that does not work for you...Please...Don't mod the amp and go get a Marshall.  Even a JCM 900 will be miles closer to the sound you want than a Rectifier will ever get.

I personally do not like EL34's in Recto's, but those tubes will have less bass and more mids to them.  When I bought the Doug's Tubes Recto kit, I bought both the EL34's and 6L6's.  I took the EL34's for a spin...But that took the amp in a direction I didn't want it to go.  You might really like that option though.



Spencerleehorton

Re: Mesa boogie 3 channel dual rectifier tone mod
« Reply #28 on: March 14, 2018, 08:33:29 AM »
Yes I totally agree with everything you have said, I have had this amp sounding great before but with changing tubes and cabs etc I didn’t document what I was using when it sounded great!
By the way I don’t want a Marshall sound I would just like less bass and more mid, I have liked the clarity and tone before from this amp as I’ve said.
I have lots of 6L6 and EL34 and GZ34 tubes along with a fair amount of EcC83s so will experiment with some combinations.
Just so I get my head around the bias switch: 6L6 = lower voltage bias?
EL34 = higher voltage bias?

Might even try the KT88 and KT66 option before messing around with soldiering anything.

I also have the modded FX Loop which means I can adjust the overall level of the amp from the pot at the back, so maybe running the channels and master higher and the FX Loop master down might give better tone bias?
« Last Edit: March 14, 2018, 03:24:05 PM by Spencerleehorton »
website: www.mohawkstudios.co.uk
email: [email protected]

If it hisses its probably the wrong impedance!!!

Spencerleehorton

Re: Mesa boogie 3 channel dual rectifier tone mod
« Reply #29 on: March 14, 2018, 03:02:19 PM »
I just realised i'm using 5Y3S rectifier tubes, im going to try some others
website: www.mohawkstudios.co.uk
email: [email protected]

If it hisses its probably the wrong impedance!!!


john12ax7

Re: Mesa boogie 3 channel dual rectifier tone mod
« Reply #30 on: March 14, 2018, 07:33:07 PM »
For the bias switch,
6L6 position makes the bias  voltage more negative (-52 V)
EL34 position makes it less negative (-39 V)

Also since you have the loop you could use it to try out tone changes. Get an eq, pedal or other,  back off the lows and highs,  crank up the mids. See if it's taking the sound where you want it.

You can also use eq before the amp. Doing both of these steps would help isolate where it would be best to make changes for what you want.

Spencerleehorton

Re: Mesa boogie 3 channel dual rectifier tone mod
« Reply #31 on: March 15, 2018, 08:54:40 AM »
Ive put back in the 6L6's and changed a few preamp valves and gone back to the the 5U4 rectifier, bass is still a problem but ive got a better sound on the clean channel now, bass off!!! mid up full along with treble, presence on about a 1/4.
putting the series up nearly full gets more power, then i set master about 1/2 way.

Will gig it and see how it fairs but i think when i get some dosh i'll try the KT88/KT66 with the GZ34 rectifiers as everyone seems to say this works much better, just can’t justify shelling out £215.00 on new valves!!!
« Last Edit: March 16, 2018, 02:03:44 AM by Spencerleehorton »
website: www.mohawkstudios.co.uk
email: [email protected]

If it hisses its probably the wrong impedance!!!

Spencerleehorton

Re: Mesa boogie 3 channel dual rectifier tone mod
« Reply #32 on: March 17, 2018, 07:49:05 PM »
Well I’ve just finished a gig with using 6L6 and 5U4 and with the series loop up high the amp sounds awful, way too much bass and no character to the amp at all, just soulless.
Bringing series loop to half way then turning channel masters up over half works better but still lacks a bit of something and there is still way too much bass!!!
I’m going to put a dummy load on the amp and do some frequency sweeps to find the frequencies and then do some mods and then frequency sweep again to see the result.
If everything then looks good I will try it out again.
Will report back.
website: www.mohawkstudios.co.uk
email: [email protected]

If it hisses its probably the wrong impedance!!!

john12ax7

Re: Mesa boogie 3 channel dual rectifier tone mod
« Reply #33 on: March 17, 2018, 10:07:04 PM »
I used to have a single rectifier 50w amp. Liked it a good bit for home practice and recording.  Sounded good at lower volumes,  but it could never do the classic tube amp thing where it sounds better the more you crank it.  Might just be something inherent to the way the amps are designed.

Spencerleehorton

Re: Mesa boogie 3 channel dual rectifier tone mod
« Reply #34 on: March 19, 2018, 07:27:31 AM »
Hi All you guitar heads!!!

right i've setup my experiment this morning and have some interesting results.

I have setup a dummy load 8ohm 25W resistor and put this on the amps 8 ohm out put, i have then setup a scope to look at these terminals set to AC.
I have then fed into the input of the amp a frequency generator and i'm monitoring the output and adjusting the frequencies and looking for differences in amplitude.

Looking at the frequency spectrum, nothing really happens below about 80hz, starts about 100hz, I’ve set the frequency gen at 0.775v at kHz, and measured the AC amplitude at the other frequency and it fluctuates a  bit, lower voltage at 20hz which slowly builds up to 0.775v at 1khz then goes lower when going up to 20khz.
I have setup the EQ of the clean channel and Bass is off, mid up full and treble up full, channel master is half way, along with gain half way, i have then set master to about a 1/4 volume and series loop at the back is set to half way.

At 100hz at 0.2 per div and delay time set to 1 I’m seeing a nice waveform and its taking up 2 segments above and below.
This goes up a segment to 3 segments top and bottom at about 150hz-160hz, 2.5 segments at 250hz, back down again to 2 segments at 500hz and at 1kHz its still at 2 segments top and bottom, higher frequencies to me are not that important as they are not a issue to this test.

So to my eyes this proves there is a bass boost in the low frequencies, i have also put all the EQ to the middle and the Low frequencies are almost double everything else.

So i will look at the 0.047uf caps before the power amp and change them for 0.022uf caps and repeat the experiment.
Hopefully will see less of a bass boost.
« Last Edit: March 19, 2018, 07:34:01 AM by Spencerleehorton »
website: www.mohawkstudios.co.uk
email: [email protected]

If it hisses its probably the wrong impedance!!!

Spencerleehorton

Re: Mesa boogie 3 channel dual rectifier tone mod
« Reply #35 on: March 19, 2018, 08:20:24 AM »
putting in .022uf and testing.

Just tested and looks good on scope!!! now to plug into speakers and have a listen!!!

Plugged in and sounds much better, ch2 and Ch3 sounds pretty good at all volumes, bass up a little from off now and on about a 1/4 on all channels.
Ch1 still sounds lifeless so will continue and have a look at the tone stack as i seem to be heading in the right direction now.
« Last Edit: March 19, 2018, 10:28:41 AM by Spencerleehorton »
website: www.mohawkstudios.co.uk
email: [email protected]

If it hisses its probably the wrong impedance!!!

Spencerleehorton

Re: Mesa boogie 3 channel dual rectifier tone mod
« Reply #36 on: March 23, 2018, 09:27:05 PM »
well i've made some progress but not quite where i want to be yet.
I have a slightly better sounding amp with the changes ive made but its still got too much bottom end,250-300hz.
I put a multi FX pedal through the FX loop and just had an EQ and cut the bottom end at 300hz by 5db, sounded so much better!
I then raised at 2k by 6db sounded even better, i then cut the top end at 10k by 5db, better still.
My question now is, how can i do what ive done with the EQ to the amp so i don't have to use the EQ?

On another note i acquired a really old beat up  AC10 and the rectifier was faulty so just put in a couple of diodes.
Works like a charm and when i put my fuzz face through it, its tone city!!! back off on the volume and you can have a rich clean sound thats clear and punchy, wind it up and thick rich distorted fuzz o rama!!
If this Mesa could get anywhere close to being more Vox i would very much like to try it, if not i think im gonna build a AC30 version and make a nice tube screamer!!!
website: www.mohawkstudios.co.uk
email: [email protected]

If it hisses its probably the wrong impedance!!!

Re: Mesa boogie 3 channel dual rectifier tone mod
« Reply #37 on: March 23, 2018, 11:23:28 PM »
Whip the low end into line ,you might end up not having to f**k with mids n highs  ,bass weight sets the story .....my motto

Spencerleehorton

Re: Mesa boogie 3 channel dual rectifier tone mod
« Reply #38 on: March 24, 2018, 03:56:03 AM »
I’m quite surprised that the low end isn’t better as I’ve changed both 0.047uf to 0.022uf just before the power amp.
What other changes can I now make?
website: www.mohawkstudios.co.uk
email: [email protected]

If it hisses its probably the wrong impedance!!!

Spencerleehorton

Re: Mesa boogie 3 channel dual rectifier tone mod
« Reply #39 on: March 24, 2018, 04:01:42 AM »
Would changing this 0.22uf in the FX loop help cut the bottom end?
Could I put an adjustable bass cut here?

Answer to this is yes HPF it is, I worked out if I put a 0.02uf here that would be 300hz roll off.

Before doing this I’m going to mod the bias and check the current on all valves, this I think is my main problem
« Last Edit: March 24, 2018, 10:12:31 AM by Spencerleehorton »
website: www.mohawkstudios.co.uk
email: [email protected]

If it hisses its probably the wrong impedance!!!