70s Marshall 50w - What Is Going On Here?

[Cap217]

I picked up this 74 50 watt last night. I knew it was a little bit of a mess but the sound was really good and the package deal I worked made it worth the trouble. Lots of weird things in this amp for sure. Looking for some input and help.

1. FX loop was added long ago and 2 new jacks and a tube were added. The old speaker jacks turned into the FX loop and the added jacks were the new speaker outputs. Tube and jacks are now disconnected.
2. It has an incredible amount of gain. All inputs work so I dont know if it is cascaded or not. Gain comes on the Volume 1 at 1 on the knob (not 1 oclock) and is full gain at 2 but the more you turn it up it just gets saturated.
3. Volume 2 and inputs 2 do not have gain and is a standard dark/loud clean.
4. All caps are now orange drops which I have never seen in a marshall
5. Wiring has a lot of messy changes that I cant make sense of
6. It has EL34s but I can only get it to 23ma on the bias pot
7. Plate voltage is 360
8. 1k 5w
9. Wiring on the impedance selector is strange. I dont know what the extra orange wire from the speaker jacks is doing.
   1. I see a dark green and grey original wiring here on the bottom
   2. I see new yellow, green, and white spiral added (white is to NFB 100K I think)

Any help would be appreciated. Im debating if I should put some 6550s in it or if I can without changing anything and running it. It does sound good but if there are things here that should be addressed by a tech, let me know and Ill take it in. I am not a tech and have a small understanding of what should be but not much. I am learning and thought this would be a good amp to mess with because my other old stuff is stock.

 

[_actual time_]

I'm definitely no expert, just an amateur tweaker, but it looks to me like there's an added cap in parallel across the cold clipper resistor, which is the resistor all the way to the right in your first pic, at the end of the circuit board. That's a very common mod for adding gain to the 2203/4 circuit, so it could be the reason or part of the reason why the amp has lots of gain.

The resistor (blue) looks to me the stock value, 10k, although definitely not the original resistor, but that little gray axial cap looks to be added across it. I can't tell the value, but high-gain mods, like the Friedman JEL, often add a 0.68 uF cap in parallel there, and/or lower the resistance of that 10k resistor. IIRC the JEL high-gain channel is 0.68 uF cap paralleled with a 2.7k resistor. Some amps lower the 10k to ~8k or 4.7k and no cap, for a moderate gain boost.

Good luck with the project!

 

[glpg80]

I hope you didn’t pay a lot of money for that. It’s not really a Marshall anymore as it stands except for the chassis and iron.

What you bought was someone’s tinker toy project. Same thing I started with on my 74 except mine also had a blown cap can - the demise and why it was put up for sale.

Good luck. You’d have better luck ordering a new board off of eBay than trying to source all NOS components.

I will say that it looks like someone put a 2204 board in or if it’s 70’s it’s much later 70’s.

 

[Cap217]

I hope you didn’t pay a lot of money for that. It’s not really a Marshall anymore as it stands except for the chassis and iron.

What you bought was someone’s tinker toy project. Same thing I started with on my 74 except mine also had a blown cap can - the demise and why it was put up for sale.

Good luck. You’d have better luck ordering a new board off of eBay than trying to source all NOS components.

I will say that it looks like someone put a 2204 board in or if it’s 70’s it’s much later 70’s.

Ive found 3 other 74 gut shots and they all have the same board. Mustards obviously but same board.

 

[glpg80]

Ive found 3 other 74 gut shots and they all have the same board. Mustards obviously but same board.

It’s not just the mustards, a lot of the original Phier resistors are gone. Carbon film resistors of today just don’t sound the same even if you tried to get closer to the original components. That’s why I said what i said.

What’s your gameplan? Return to stock or play it as is?

 

[Cap217]

It’s not just the mustards, a lot of the original Phier resistors are gone. Carbon film resistors of today just don’t sound the same even if you tried to get closer to the original components.

What’s your gameplan? Return to stock or play it as is?

Yes I agree. Basically all the components were changed. But the board is the same as the other 74s Ive seen.

Sounds great as is. I want to get the bias right so if I need to get some 6550s and all the components are right and it will bias properly then I will just play it as is. Im not sure if I can drop 6550s in or if someone attempted to update it for EL34s but did a terrible job. Stock was 6550s on this.

1. Will 6550s go in and bias with no changes? - current el34 only bias to 23ma at 360 plate voltage
2. If changes are needed, should I do 34s or 6550? I dont have any 6550 amps around
3. Id like to figure out why this has so much gain and if there are any mods or just a mess of stock values
4. I want to know if the impedance selector is wired properly. It doesnt make sense to me right now.
5. Id like to know where the NFB comes off, which tap
6. Adjust NFB tap if needed
7. Maybe lower the 5,000 pf bright cap if that will do anything for me. I assume this is why it has no range of gain/volume. Id like a little bit of room to adjust the gain besides full gain at 1 and just more saturation.

I have a bunch of stock or pro modded marshalls but nothing Id ever go in and mess with. So this would be the one for that. And Im not sure what a fair price is on this vs what I paid. The cab is in good condition, no back panel.



Here it is at 23ma bias right when I got home through a power station. Its brighter in person. My 73 cab is darker as well. Really tames the high end and has great mids. Nothing is dialed in really, it can be better.

Whats unique about this amp is the gain and how it comes on so fast but also the bass isnt flubby. I never had a 50w so that may be a reason of the super cold bias but all of my Marshalls from 68-70-72-72-79-80-83 get flubby in the bass and most super harsh in the highs.

 

[kurtsstuff2003]

Don't touch it...I think it sounds great!

 

[Exo-metal]

You might want to have a fire extinguisher handy when u run that up Chris. Those pcb traces are notoriously thin when they moved away from ptp assembly my friend.

 

[Cap217]

Don't touch it...I think it sounds great!

Well I have to get it to correct bias so that needs to happen.

 

[glpg80]

I know 2203 bias like the back of my hand but 2204 is different and I think taps off of the B+. I’d wait for one of the others who are more 2204 savvy reply to your list of questions regarding grid stopper resistors and whether the bias circuit is for EL34 or 6550.

 

[glpg80]

Can you take a good detailed picture of this area? And another next to it? The majority of your gain mods are located in the yellow area here.

It looks like they played with voltage dividers and cathode resistors. Looks like a something in the phase inverter has been changed. The slope resistor also isn’t stock.

I’ll be able to know more with close up detailed pics

 

[panhead]

Who cares, if sounds good itis good. That dick rick puts orange drops in 5000 amp lol

 

[panhead]

38 ma dont matter if it 100 or 50

 

[Cap217]

38 ma dont matter if it 100 or 50

Even with 6550? Do I need to do anything to this to put 6550s in?

 

[Cap217]

Can you take a good detailed picture of this area? And another next to it? The majority of your gain mods are located in the yellow area here.

It looks like they played with voltage dividers and cathode resistors. Looks like a something in the phase inverter has been changed. The slope resistor also isn’t stock.

I’ll be able to know more with close up detailed pics

View attachment 108338

 

[RedPlated]

It’ll bias up with 6550.

That big metal oxide 47k needs to be lowered for el34. 33k should do it. I’ve never had an issue doing it this way.

 

[panhead]

Even with 6550? Do I need to do anything to this to put 6550s in?

Didnt know it was 6550`s 48-55ma i would think

 

[panhead]

If it only goes to 23ma with el 34`s try the 6550`s and see if theyll bias up

 

[gtrwun]

If it only goes to 23ma with el 34`s try the 6550`s and see if theyll bias up

I biased a 2204 with similar voltage recently. With 6550’s in there it should be fine. I set mine around 42-43 I think?

 

[gtrwun]

Don't touch it...I think it sounds great!

It does sound good. I wouldn’t worry about anything once you bias it.
Kurt….. remember that “STOCK” 2204 I sold you years ago and you opened it up and every cap in there was an orange drop? Jesus…. What an idiot I am ………???