Marshall Mod:

Yeah, I saw that mod too at the Headfirst Amps DIY Builders Group of Facebook. Definitely for those that want to max out the preamp gain control. The mods squeeze out a bunch more gain, reduces the lows and the ultra highs at the preamp stage and preamp gain control. Not shown on the schematic but described in the mod, is to boost the low end at the coupling caps between the phase inverter and power tubes.

I’m guessing these mods narrows the versatility range of a stock 2204/2203. To be used only at very high gain amp settings, and not much articulation when rolling down the guitar’s volume control, perhaps.

The mods wouldn’t be too hard, or too invasive to try though. Maybe if I get too bored one day… o_O
 
Build it and find out. I see a bunch of issues 4K7 high gain input pin 7 osculation city 22K is as low as you would want to go
4K7 .68 UF on V1 cathode that's drawing a lot of current pin 3. 2K2 /10K = 6.1K even worse pin 8 sudden death 12AX7
last the PI bridge .22uf with .1uf stupid just use .5uf and a 5K trimmer with 82K 100K to dial in 7.3 volt differential pin 1 & 6 for sustain

Marshall missed the boat on high gain even Leo Fender back in 1961 Brown face Deluxe 220K plate resistors pin 1 and 6 not 100K
one thing that Marshall did do well was the filter supply in the 1966 JTM/45 100 B+1 128UF B+2 16UF B+3 16UF B+4 16UF B+5 16UF
 
67plexi said:
Build it and find out. I see a bunch of issues 4K7 high gain input pin 7 osculation city 22K is as low as you would want to go
4K7 .68 UF on V1 cathode that's drawing a lot of current pin 3. 2K2 /10K = 6.1K even worse pin 8 sudden death 12AX7…
Click to expand...
Well, after all that, I’m not interested in trying the mods anymore… :rolf:

67plexi said:
… a 5K trimmer with 82K 100K to dial in 7.3 volt differential pin 1 & 6 for sustain
Click to expand...
This is a very cool and useful mod… (y)
Here’s my attempt of the PI balance control on one of my 2204’s:
7E781421-ECC4-4C05-9F37-002543E2D0A1.jpeg
 
Inspector #20 said:
Found this on the web....

View attachment 88167
Click to expand...
Not a well thought out modification, IMO of course. Would quite limit the frequency range of the amplifier. Aside from the higher frequencies (& harmonic content) being dumped by the 220pf to ground on the grid of V1 & the plate snubber caps on the first 3 stages, a lot of low end is being rolled off by replacing the 470k/470pf treble peaked with a 0.0047uf cap. Putting an 0.0047uf in series with the 0.022uf cap brings the total value to 0.003873uf. As Mr 67plexi said, 4k7 is too low a value for the V1a grid stopper.
Putting a 2k2 resistor in parallel with the V1b cathodes 10k (cold clipper) resistor brings the total value to 1k8, then adding a 0.22uf cathode bypass cap, well, instabilities (hence the band-aid approach of plate snubbers & 100k grid stopper on V2a). The secret to successfully modding for increased gain/overdrive is to add just a little to various stages, not a heap in one go. Even worse than doing a mod like this is putting it out there on the internet for others to do as well.
Mr 67plexi has suggested a couple of worthwhile mods. The V1a 220k plate load value is a popular one. Using the following calculator (that shows the typical Marshall V1a values), change the 100k plate resistor value to 220k, hit "recalculate" & watch the increase in gain figures. Cathode Bypass Capacitor Calculator
Adding a "harmonic balance" trimpot to a long tailed pair phase inverter's plate load resistor network is well worthwhile. Here's one on my latest build (that's being sold).IMG_20221227_172512.jpg
Another approach to balancing this type PI is to (temporarily) replace the inverting triode's 1 meg grid leak resistor with a trimpot (wired as a variable resistor) & dial it in on a 'scope & function generator. Don't be surprised if you find that a value of 330k ~ 390k works well.
Do some research before following modification suggestions found online,,, it may well be that the person suggesting the modifications doesn't know quite as much as they'd like people to think they do. Cheers
 
syscokid said:
Not shown on the schematic but described in the mod, is to boost the low end at the coupling caps between the phase inverter and power tubes.
Click to expand...
Once the lows have been rolled off early in the pre-amp (0.003873uf coupling cap), they can't be "brought back" so to speak by simply increasing the value of the output coupling caps, because, well, they've already been rolled off. Cheers
 
Inspector #20 said:
Found this on the web....

View attachment 88167
Click to expand...
This mod will kill all the highs.
The highs are necessary to obtain crunch.
And so this will turn out sounding somewhat like a mesa boogie. Overloaded low frequency.....
What you end up with is a bunch of hiss and buzzing noise.

After all,what is a Boogie? It's a copy of a Marshall 2203 with too much bass.

If you want to know -------the secret is changing the layout.
Not adding a bunch of "gain." (its not adding any gain really)...it's just adding sensitivity.

The secret is taking parts out....not boosting the stages.
See?
if you took a Dumble and changed the layout, it would sound as dead as a door nail.
 
I modded a 4010 to get close to JMP tones. I lowered the value of the bright cap. Don't just clip the stock one out. It
will kill you sound with single coil guitar
Lifted a leg on the two caps in the treble peaking circuit. They run in series with two resisters.
I cant remember the values though.

Put a good 5751 tube in the V1. Another old trick is to swap out the 12ax7 PI for a 12AT7.
I found I like NOS Mullard CV 4024. They used to be 40 bucks

This is all very easy to reveres back to stock
 
First he boosts the gain way up....(Orange)
then he un boosts / sticks a bunch of resistors in to try to stop it from oscillating and then sticks in a bunch of snubber caps to kill the high frequency ...try to stop the hiss and noise...(Red)
2203mod too.png


He should have just copied the bogner schematic- it would have worked better.
That's what a bogner is, a modified 2203 !

I am not impressed by the head first mods.... they are reckless hacks.... but at least this time he drew a schematic.
 
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blues said:
I modded a 4010 to get close to JMP tones. I lowered the value of the bright cap. Don't just clip the stock one out. It
will kill you sound with single coil guitar
Lifted a leg on the two caps in the treble peaking circuit. They run in series with two resisters.
I cant remember the values though.

Put a good 5751 tube in the V1. Another old trick is to swap out the 12ax7 PI for a 12AT7.
I found I like NOS Mullard CV 4024. They used to be 40 bucks

This is all very easy to reveres back to stock
Click to expand...
See, the trick is
JMP is power amp distortion, with a clean pre amp.
4010 (2203, 2204) was made for pre amp distortion.

When the power amp overdrives, it's going to become loud.
If you want the JMP to sound good it must be turned up to at least "7" and therefore very loud.
Then as a consequence, you then need to control the power amp loudness. (speaker attenuator etc)

Don't try to play the JMP on "2," you miss the point entirely. You should have bought a Princeton Reverb.
Playing it on "2" is worthless, and a complete waste.

4010 is preamp distortion....the loudness is controlled by master volume.

See the difference between preamp and power amp distortion?
 
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Amp Mad Scientist said:
I am not impressed by the head first mods.... they are reckless hacks.... but at least this time he drew a schematic.
Click to expand...
Just to be clear, that schematic in the OP is not a Headfirst mod. It’s a 32 year old modded schematic drawn by someone with the initials of “WTN”, and then posted by a member (not Jason Tong) of Facebook’s “Headfirst Amplification DIY Builders Group”.
 
Amp Mad Scientist said:
See, the trick is
JMP is power amp distortion, with a clean pre amp.
4010 (2203, 2204) was made for pre amp distortion.

When the power amp overdrives, it's going to become loud.
If you want the JMP to sound good it must be turned up to at least "7" and therefore very loud.
Then as a consequence, you then need to control the power amp loudness. (speaker attenuator etc)

Don't try to play the JMP on "2," you miss the point entirely. You should have bought a Princeton Reverb.
Playing it on "2" is worthless, and a complete waste.

4010 is preamp distortion....the loudness is controlled by master volume.

See the difference between preamp and power amp distortion?
Click to expand...



I should have said not exactly like a JMP. It does not turn a JCM into a 4 hole JMP
Im not a dummy.

It just not as aggressive as a JCM 4010 can be.
It does knock the volume down. But not enough to matter when playing out.

I don't play metal so im good.

But it can still get loud. It still sounds great.
I had a 1974 50 watt metal panel head. So i know the difference.
I also used a 12at7 in the PI like a bunch of us did back in the day.
 
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syscokid said:
Just to be clear, that schematic in the OP is not a Headfirst mod. It’s a 32 year old modded schematic drawn by someone with the initials of “WTN”, and then posted by a member (not Jason Tong) of Facebook’s “Headfirst Amplification DIY Builders Group”.
Click to expand...
If I had a great mod for an amp...I don't suppose I would post it.
Because then everybody would copy it.
Especially mesa boogie. They would say they invented it. :pound-hand:
Then Behringer would design it in Germany...build it in Wuhan.

You don't see David posting the schematics to his mods.

The funny thing : adding gain ( and gain stages) really isn't the secret at all.
But almost everybody seems to think it is....almost.
Once you know the secret you're not going to talk about it.
And it is a secret so stupidly simple that almost nobody believes it's possible....thereby insuring that the secret is protected forever.
 
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