Can anyone help me figure out why my '72 50 watt has so much more gain than my '76 50 watt?

RWMusic

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I own a '72 1987 and '76 1987. Both amps had a pre phase master installed before I got them, but since I use a Fryette PS100 I don't use them, I leave them on max at all times.

I know both amps are not stock circuit wise and have had a few changes but I am not 100% clear on what changed and their tonal effects.

I do know that the '76 has an added tube effects loop which is what the breadboard is for, but otherwise seems to be pretty original circuit wise. Although it looks like the bright cap is missing?.

The '72 also has a replaced power transformer. It was replaced with a early 80's Drake from a JCM 800.

Here is what I'm trying to figure out:

the '72 is a lot more aggressive and has tons more gain than the '76. There pretty much is no cleans on the '72, it starts to break up a lot even at 2-3 on the high treble volume. Both with single coils and humbuckers. The '72 even has more gain than my 2203 which is weird to me because I always assumed 2203's had more gain.

The '76 doesn't start to break up until about 5, and you need humbuckers to do it. Until then it is pretty clean, it doesn't really get super distorted until you get to about 8 on the volume.

I mean it when I say that the '72 on a volume of 3 has about equal gain to the '76 on 7-8, especially with humbuckers. This is with the master volumes on both amps on max.

I'm not really complaining as some of my favorite Marshall tones are 80's hard rock like EVH, Lynch, Dimartini.

the '72 on a volume of 4-5 or more is already at 80's hair metal gain to me, both on single coils and humbuckers.

I'm trying to figure out why this is, and what the '72 has in it's circuit that is different than the '76 to make this happen. Is it the bright cap?

I want to know for learning purposes. If I ever got another vintage marshall in the future and I wanted to replicate the gain of the '72.

I consider my '72 to be a "magic marshall" because of this behavior.

Here are the gut shots of both:

1976:

1972:
 
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TAZIN

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There is a ton of differences between the two amps. The '72 has a lot of modified component values due to paralleling. It would take me forever to write up all the differences between the two amps let alone explain the effects of the modifications done to the '72.
 

RWMusic

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There is a ton of differences between the two amps. The '72 has a lot of modified component values due to paralleling. It would take me forever to write up all the differences between the two amps let alone explain the effects of the modifications done to the '72.

Well that's why I'm here. Hopefully someone can help out.
 

Seanxk

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Well that's why I'm here. Hopefully someone can help out.
There's a reply from one of the top guys right there, perhaps while '' someone else '' is taking the time, why not do some research here, just pick a section of your amp that's modified there's plenty of discussions on component values and effect, you'll learn along the way.
 

RWMusic

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There's a reply from one of the top guys right there, perhaps while '' someone else '' is taking the time, why not do some research here, just pick a section of your amp that's modified there's plenty of discussions on component values and effect, you'll learn along the way.

I wouldn't even know where to begin. I know nothing about electronics.
 

paul-e-mann

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I own a '72 1987 and '76 1987. Both amps had a pre phase master installed before I got them, but since I use a Fryette PS100 I don't use them, I leave them on max at all times.

I know both amps are not stock circuit wise and have had a few changes but I am not 100% clear on what changed and their tonal effects.

I do know that the '76 has an added tube effects loop which is what the breadboard is for, but otherwise seems to be pretty original circuit wise. Although it looks like the bright cap is missing?.

The '72 also has a replaced power transformer. It was replaced with a early 80's Drake from a JCM 800.

Here is what I'm trying to figure out:

the '72 is a lot more aggressive and has tons more gain than the '76. There pretty much is no cleans on the '72, it starts to break up a lot even at 2-3 on the high treble volume. Both with single coils and humbuckers. The '72 even has more gain than my 2203 which is weird to me because I always assumed 2203's had more gain.

The '76 doesn't start to break up until about 5, and you need humbuckers to do it. Until then it is pretty clean, it doesn't really get super distorted until you get to about 8 on the volume.

I mean it when I say that the '72 on a volume of 3 has about equal gain to the '76 on 7-8, especially with humbuckers. This is with the master volumes on both amps on max.

I'm not really complaining as some of my favorite Marshall tones are 80's hard rock like EVH, Lynch, Dimartini.

the '72 on a volume of 4-5 or more is already at 80's hair metal gain to me, both on single coils and humbuckers.

I'm trying to figure out why this is, and what the '72 has in it's circuit that is different than the '76 to make this happen. Is it the bright cap?

I want to know for learning purposes. If I ever got another vintage marshall in the future and I wanted to replicate the gain of the '72.

I consider my '72 to be a "magic marshall" because of this behavior.

Here are the gut shots of both:

1976:

1972:
Do you have a use for the 76 as is? If not and you LOVE the 72 take both amps to a competent tech and have them see what the differences are and change the 76 to match.
 

neikeel

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The 72 still has the big bypass cap on Vol1 pot (brings lots of signal in early on in vol pot sweep) so v little clean headroom.
The paralleled output couplers will increase low signal passed by the output stage when pushed.
The NFB has what appears to be paralleled 100k on 8ohm tap so typical plexi 50k/8.
The mf on input grids will increase front end sensitivity a little. There is a 0.68uF on V2a. Cathode (boosts upper Mids).
The 76 has 100k/8ohm (should be 100k/4 so more NFB than stock for that year)
You probably find with series loop it is likely lower than unity gain.
Easy to work out with couple of hours and a scope😉
 

TAZIN

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Plus the '72 has 22nF coupler for the V1b plate (as opposed to the normal 2n2) so you'll be passing more low frequency signal.
 

stickyfinger

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Also, look at the 72 presence pot. Looks like the typical 5k/.1uf plexi presence circuit. However a .68uf is also wired from wiper to end (like a MK2 JMP) but no 4k7 resistor. I am actually confused as what this does.
 

neikeel

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The 76 is stock 25k/4k7/0.68 presence so more zing at higher pot settings. Someone has tried to replicate that with the 72 by parachuting a 0.68 over the stock plexi 5k/0.1uF arrangement. As Tazin said - a lot of parellelled bits.
 

steveb63

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Deleted...... sorry wasn't being very "nice"

@neikeel is an expert in this stuff, you'd do well to follow his advice.

Rather than run us down at a different forum. Real class.
 

stickyfinger

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The 76 is stock 25k/4k7/0.68 presence so more zing at higher pot settings. Someone has tried to replicate that with the 72 by parachuting a 0.68 over the stock plexi 5k/0.1uF arrangement. As Tazin said - a lot of parellelled bits.
But in this mod the .68uf is missing the 4k7 resistor and is using the 5k pot as ground in the 72. I have no idea what this arrangement is supposed to sound like or what the frequency response is doing.
 

neikeel

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The 4k7 only has a role on the 25k as shunt to get nominal pot value to 5k and take D.C. off the pot.
The big cap will effectively change the pi tail at selected frequencies (act as another upper Mids boost at the sacrifice of some lows (maybe why pi output couplers increased to compensate)
 

neikeel

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Oh, I see:

RWMusic said:
Want to thank everyone for their analysis. Looks like it’s the bright cap that is having the most effect. Much nicer group here than at the MarshallForum

Funny 'cos the person advising you is a forum member here:thumb:
 

Seanxk

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I wouldn't even know where to begin. I know nothing about electronics.
I wasn't being funny in my post as you do state in your first post you'd like to learn and you mention the bright cap, that's a good start........ to learn for next time
 

Maxbrothman

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If you got hold of a row of '76 50 watts and compared them all, would they have varying amounts of gain compared to each other if stock?
 
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TAZIN

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If you got hold of a row of '76 50 watts and compared them all, would they have varying amounts of gain compared to each other if stock?
No not really, providing your using the same set of tubes for each amp and they're all biased to the same dissipation. What you'd probably hear would be a slight difference in character between some of the amps due to PT and OT wind tolerances.
 


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