DC Mods - Revisited...........Sort of.............

[bendo]

Lead Channel Mods
More or Less Gain:
Between V1B & V2A there is a Voltage Divider (page 5 of the drawing).
You will see a 150K resistor going from the signal path to ground. Change this resistor.
Lower value = Lower gain
Higher value = More gain
I like 220K. 330K is really nice too, but starts to get a little bit mushy.
Any more than this and it gets really mushy but has gobs of gain.

What is the appropiate power rating of the resistor?

1/2watt are most of the resistors in a DC. Should be 1/2 watt and 1% tolerance.

 

[polaris20]

Monsta-Tone; do you still have that volume knob mod PDF on your computer? I botched up and deleted my site and moved it to a hosted Wordpress, but I forgot to save the PDF first. I want to upload it to my newer site, but can't find the PDF.

Let me know, I'd appreciate it. I actually now am just getting around to doing some of this stuff; three small kids has put a damper on Mesa/Boogie playing the last 5 years, and my guitar playing has mostly been going through a *gasp* POD HD500.

I'm planning on the volume knob mod, parallel to series loop mod, perhaps the lower gain lead channel mod, and lastly a Mercury Magnetics OT.

 

[Balint]

Hello,

I'm a happy owner of a DC-3 combo amp (REV B). I'm OK with the Clean Channel but I find the Lead Channel quite lifeless. I miss some highs that helps cut thru in band. I mean some extra brights or a little fuzz type high.

I've found many theoretical possible mods, but I want to implement only few to achieve what I want. The options I can see:
- increase the V1b grid resistor from 470k to 1M
- remove the .001 cap in front of V2a
- remove the .001 cap at the end of the Lead preamp after the Master pot
- play with the tonestacks (fixed and adjustable): slope resistors, etc.
- add a bright cap like in the Clean channel to the gain pot

What do you think about the above listed mods? Which one is the most promising for my needs?

Two more questions:
1. I found the REV A schematic of DC-3 on the net, but not the REV B. Does anybody have it?
2. In the DC-3 Lead channel's adjustable tonestack there is a 6k8 resistor between the mid and bass caps. This is not so common, I've never seen in other designs. What this resistor is doing?

Thanks,
Balint

 

[caseyb12088]

Hello! I've read these mods and they sound incredible. I own a DC-5 and have always wondered why the helicopter effect has occurred. I'm looking for the Series Effect Loop diagram. The website host seems to have dropped it since it was uploaded in 2009. Is there a new host site? If not, would some one mind emailing it to me at [email protected] ? ????

Thanks guys! Looking forward to it!

 

[Monsta-Tone]

Haven't been here in a long time, sorry guys!

balint:
Try jumpering the 6.8k resistor in your tone stack. That might be all you need. That resistor really dulls the tone of the lead channel.

Casey:
I have the drawing, but only on my home computer. I'll be on the Mainland all week, so I won't be able to email it to you till next week. Hopefully someone here will take care of it before I get back home.

I just re-bought my old DC-3 combo from Soundchaser1959. Can't wait! I might have a whole new set of mods coming up if I can ever find the time to work on amps again.....

 

[soundchaser59]

I just re-bought my old DC-3 combo from Soundchaser59.

And I was VERY VERY SORRY to have to let it go.......AWESOME AMP.......but it's in good hands! Lots of folks here in my town wanted to buy that thing from me! :mrgreen:

Now that I paid off my debts unexpectedly soon, I have another DC3 on the way. I'm quite curious to see how the bias adjustment mod was installed.

 

[hgus]

What an excellent thread! Thanks Monsta-Tone!

I have re-acquired a studio .22 to play with (electronically, not just with a guitar...) and it is very interesting to see how similar all the different Mesas are, with slight differences here and there that inspire mods to be tried. I'll try out your reverb and presence mods on that amp, I was going to start a thread on that model, but didn't get around to it yet. This is inspiring!

 

[soundchaser59]

On the second DC3 I got, there was a bias adjust mod installed. It also has 1645 Hammond OT and a 159P Hammond choke installed, with a "Fender" mod on the clean channel that actually works! It does sound amazingly like a Fender clean. Not quite as creamy as the Rivera clean I have sitting next to it, but it would work very nicely for rhythm guitar in a funk band! The only drawback to this mod is it has to be cranked up to about 3 or higher to really sound right!

I don't have a schematic handy at the moment, but......

(sorry I don't have a photo handy yet either)

On the main board the two wires from the adjust pot disappear just under a pair of orange caps, close to the big blue power caps. Next to the orange caps, next to the edge of the board, there is a diode and a 33k R. The bias pot has replaced the 33k R. Only thing is, I'm not sure which 33k resistor that is on the schematic. And I can't see the traces well enough to tell you how the resistor and the diode are connected.

There are two test points, each one has a 3w 1ohm 1% resistor to ground. I'm told this effectively lets you measure mV and since it is 1 ohm, it will directly reflect the idle current for one pair of tubes. So if the meter reads 40mV, that means there is 20mA of idle current to each tube in that pair. If each test point measures the same then (according to the tech who installed the mod) you know your quad is matched.

I put in a quad of new supposedly tested and matched Mullards and let it warm up, then I set the first test point to 40mV. Surprised to see that the second test point then measured 20mV, which I thought might be a significant mismatch. The tube seller responds by saying anything within 10% is considered "matched" depending on the circuit and the tester, etc. So I swapped the two inside tubes, one from each pair. When I remeasured this simple change brought the first test point down to about 33mV and the second test point up to about 30mV. I figured that was as close as I could get it given these tubes. I wanted it to run a bit on the cool side, and 30mA for a pair would calculate out to about 6 watts per tube, assuming 400v plate......right? So maybe about 25 watts output?

(Interesting to note that the quad of JJ's that came with the amp match EXACTLY on both sides no matter how I pair them up. The mod installer sent the JJ's with the amp)

So this particular DC3 is running nicely now, and is much quieter with the new preamp tubes. I also hit all the pots and sockets with Deoxit while I was in there.

BUT......

BUT...........it ain't even close to sounding half as good as the DC3 I had that was modded by Monsta-Tone! :shock: :mrgreen: :twisted:

 

[jargapix]

Hey guys. I've been futzing around with my DC5 amp for a few days now trying to refine my tone and would like to try the mods that Monsta has written about. Can someone share the drawings he is referencing with me please? A link to where it's stored online or if someone can email it, I would very much appreciate it.


Thanks!

 

[dodger916]

Hey guys. I've been futzing around with my DC5 amp for a few days now trying to refine my tone and would like to try the mods that Monsta has written about. Can someone share the drawings he is referencing with me please? A link to where it's stored online or if someone can email it, I would very much appreciate it.


Thanks!

Have you tried pm'ing Monsta?

 

[jargapix]

Hey folks. So, I cracked open my amp & see that there nothing to clip on the volume pot so I guess that's done.

I'm totally lost as to where to find the V6 for the blanket mod though. Any help here would be appreciated.

I'll report back if I figure it out.

 

[jargapix]

Thanks to Monsta-Tone, I found the culprits! I'm gonna take these guys out and see what goes down.

 

[jpistolas]

A computer back (drive crashed) I had the serial loop mod pdf and would like to get it again. All the links here for it are dead. Is there a new link or am I just simply missing it?

 

[jpistolas]

Better Reverb Mod:
1. Dwell or Speed of Reverb...To change the Dwell of the Reverb, experiment with the 1M resistor that ties Pin 2 of V5A to ground (Grid Resistor).
I like 680K or 750K a little better than the 1M. It slows the Reverb repeats down a little bit and makes it a little more Fender like.

2. Different response....There is a .002 cap from Pin 1 of V5A to ground. Move the side of the cap that goes to ground to Pin 3 of V5A.This should make the Reverb a little better sounding and change the feel of it a bit.

3. Stronger Reverb....On page 6 of the drawing, you will see a 22k resistor in series between the B+ voltage and the Red wire of the Reverb Transformer. This is placed here to prolong the life of the transformer.

You can experiment here with different values of resistors. I usually knock it down to a 5K 1 watt resistor. Removing it completely will probably put you into **** Dale territory.
I always use a 1 watt resistor here. I have had a few 1/2 watt resistors fail.

3. Stronger Reverb....
Could I substitue an approiate pot (1M?) to be able to manually adjust the dwell. What type would I need?
Maybe a switch for the the different reverb strength resistors. 22k/5k/none. Be nice to hear that **** Dale drip. LOL!

 

[Archeopteryx]

Just curious if anyone has the bias mod schematic/diagram available.

To confirm its:


"Quick fix -Piggyback a 470K on the 120k in the bias circuit. This will bring the bias voltage down to a more tube friendly -14vdc instead of the insane -11 with 400vdc on the plates."

Is there a non-quick fix? Any help is greatly appreciated.

 

[simonc83]

Hi there... I've been reading all this post but.. in the end is the blanket mod recommended for a DC5?
Does it worth the effort of making the change? How many did it and are happy with?
Thanks,
Stefano

 

[jpistolas]

Lead Channel Mods
More or Less Gain:
Between V1B & V2A there is a Voltage Divider (page 5 of the drawing).
You will see a 150K resistor going from the signal path to ground. Change this resistor.
Lower value = Lower gain
Higher value = More gain
I like 220K. 330K is really nice too, but starts to get a little bit mushy.
Any more than this and it gets really mushy but has gobs of gain.

I'd like to do this mod (original resistor 150k/220k/330k but on a switch. What would be the easiest way to do this? Sorry about the dumb question in advance.

 

[jpvargasj45]

Is there a DC5 mod out there to make the push pole in the rhythm channel foot switchable I remember about 20 years ago. There was one on the Internet, but I can’t find it anymore. Someone recommended just wiring the pot to a foot switch, but I’m not sure how to do that.

 

[JustPassingThrough]

Looking at the schematic for the dirty or 'lead' channel, there's room for some mods to make it better I think.

I'm curious about the lack of cold clipper on this circuit, and maybe that's why it feels a bit bland and stiff on the drive channel? There's also a lack of gain to my ears. OK for Iron Maiden or classic rock, but not modern metal.

I'm wondering if, on V1b, the resistor network of pin 8 can be replaced with a 39k resistor to GND - the 68k on the plate might need to be upped to 100k to compensate, or maybe not as there's still 3 gain stages to come.

V3a- make the 100k a 220k, so it would be a 220k plate, 1k5 with a 1uf bypass cap off the cathode. That would beef it up a smidge, the cold-clipper would make it feel a bit more interesting and less boring to play through.

This might require a plate bypass cap, something like a 1nf/1kv to remove any high-end futz that comes with more gain.

I'll run this by my amp tech friend and give it a go.

 

[JustPassingThrough]

Alright guys. It's 2024. Let's update this with some actual pics.

This isn't the 'blanket mod' but I think it achieves the same thing just earlier on in the circuit.

HIGH END RESTORATION:

After the treble pot in the dirty channel there's a network to ground right after it comprising of a 22k and that .001 cap to ground. This is a low pass filter knocking off some high end. Lifting one side of the 22k removes this filter from the circuit and restores some much needed high end.

The 22k lives here just behind the reverb pot. You can see I only lift one end, this breaks the circuit but also means it's easily reversible.

MOD 2:

VOLUME MOD. This is the 220k resistor that sits across the input and output of the LEVEL pot. Removing it restores a normal/predictable taper of the pot.

And it lives here right by the 2 big blue filter caps. Again, I just lifted one end.

GAIN MOD:

This is changing the 150k to ground after the treble peaker off V1b, between v1b and v2a.

Schematic:

And it lives here sort of 'on' the tube. It's a bit difficult to see but in this pic it's the one I've switched out for a 220k beige one. Interestingly the stock one on mine had drifted to around 120k. Use the blue 3x 30uf filter caps as your compass and you'll easily find it.

OBLIGATORY WARNING: Don't touch random ****, especially around the big blue filter caps, or anything connected to the power supply. Luckily Mesa do usually install bleeder circuits that drain the residual current when the amps are unplugged, buf it that bleeder fails you're going to be in for a nasty surprise.

Of course don't touch anything when the amp is plugged in and running unless you know what you're doing, and learning how to discharge caps safely, and also how to use a multimeter to test for voltages would be prudent (people should learn this anyway, it's a handy skill).

This isn't my original work. Thanks to Monsta-Tone, and some other guys from Rig Talk. Each forum has their own set of mods, these are just the ones I've done so far.