1973 Marshall Super Lead 100w Handwired - Questions on V2 cap, NFB and Bright cap

[cap217]

I am trying to find some info on 73 marshall super leads. I assume this will go for 72s as well.


1 - Is there a stock bright cap? If so, what value? I have seen some with caps and without that had the dye so they come different from the factory. But is there a standard? I would think 4700pf would be a standard value there.

2 - Is there a V2a cap over the 820ohm resistor? Again, I have seen some have it and others not. And if there is, is it .68 or 220uf?

3 - What is the NFB resistor value and what ohm tap? I think 100k/4ohm is standard. This may tie into #2 question because I read somewhere that when they switched to 100k/4ohm they may have removed the v2a cap. Not sure if this is true or not.



I have a 73 that has no bright cap, 100k/4ohm, no v2 cap. It lacks brightness and gain. I have a 72 that has all of these things and it has a lot more gain.

 

[neikeel]

For 73
bright cap 5000pF
1k cathode resistor V2a unbypassed
NFB 100k/4ohm
0.68uF/25k/4k7 on presence.

 

[cap217]

For 73
bright cap 5000pF
1k cathode resistor V2a unbypassed
NFB 100k/4ohm
0.68uF/25k/4k7 on presence.

The presence is another weird on then because me 73 hw had a 5k/.1.

Is there v2a capacitor because of the 100k/4ohm nfb? Why no cap there?

And what bright cap was used? Like the actual look and brand of it?

 

[neikeel]

Often RS circular ceramic disc with RS 0.005 or 5nF on it (can’t remember off top of my head)
73 was Year of change to pcb and early turret board amps will be 0.1/5k.
Check with @TAZIN re precise swap of 0.1 to 0.68
Yes the move to 100k/4ohm was same time as dropping the V2a bypass cap

 

[cap217]

Often RS circular ceramic disc with RS 0.005 or 5nF on it (can’t remember off top of my head)
73 was Year of change to pcb and early turret board amps will be 0.1/5k.
Check with @TAZIN re precise swap of 0.1 to 0.68
Yes the move to 100k/4ohm was same time as dropping the V2a bypass cap

If I added v2a bypass cap on a 73 100k/4 ohm NFB, would it be worth trying?

 

[Matthews Guitars]

My '73 1959 has the .005 RS bright cap. As for the rest, just look:

 

[cap217]

My '73 1959 has the .005 RS bright cap. As for the rest, just look:

Thanks. Does it look like this one?

 

[Matthews Guitars]

Yes. Far as I know there's only one flavor of R.S. .005 bright cap.

 

[Jeff West]

I think the stock .005 disc on my ‘73 1987 is a Lemco

 

[Seanxk]

Those look a bit early, 60's as opposed to 70's

 

[neikeel]

They were light brown earlier and made a reappearance early 70s

 

[cap217]

Well I have the red or the brown you see in the pics. Then I have a 4700 tan that has a little gloss. If anyone has a link to the exact cap needed Id appreciate it. But I think I have seen a few versions, even lemco barrel on the earlier handwired 73s.

 

[TAZIN]

The stock bright cap is the tan colored Lemco 5K; labeled as such.
Position V2a should be a 1K resistor with no bypass cap.
NFB is a 100K resistor from the 4-ohm tap.
Early '73 amps used the 0.1uF/5K Presence setup and by mid year the 0.68uF/25K/4K7 seems to have been introduced. This later Presence setup may have started with the USA export amps since the ' ENG' tend to use the earlier values when Marshall introduces circuit changes.

 

[purpleplexi]

Are you confident that every amp was identical at that point? I know that the earlier amps could be a bit varied depending on what was available that day.

For 73
bright cap 5000pF
1k cathode resistor V2a unbypassed
NFB 100k/4ohm
0.68uF/25k/4k7 on presence.

 

[neikeel]

I don’t think anyone can be totally robust about these things! Friday pm amps with random assortments of parts do crop up particularly this era. eg clusters of ceramic caps to make up correct value or bass amps with all 0.1uF caps across the board!
All with original dye markings.

 

[Ned B]

I can add a bit on the V2 bypass cap as I have been sorting out a friends '74 1987 lately. I believe that amps from '73-'75 or later had low voltage with a lot of distortion. This particular amp is about 385V. It has 2 10K dropping resistors. voltage at V1, pin 1 was 105V which is low. Adding the .68 bypass cap put the amp into oscillation and added harsh trebliness to the distortion. What worked for this amp was going with one 10K dropping resistor and a 10uf bypass. This is an area you can experiment with to fine tune to your personal taste.

 

[neikeel]

Interesting on the 10uF fat cathode cap. Was that with 820R or 1k resistor?
I’ve always found a big cap there a bit too much affecting how the amp cleans up.

 

[Jeff West]

I have Oct ‘73 1987 w/first pcb . Has the later presence with big RS cap, no bypass on V2, B+ is about 435 with EL34s which we’re definitely stock. USA export model

 

[Ned B]

Interesting on the 10uF fat cathode cap. Was that with 820R or 1k resistor?
I’ve always found a big cap there a bit too much affecting how the amp cleans up.

This '74 has an 820. I think the fat cap use really relates to the level of voltage fed to the preamp. You may remember the 12XXX series I posted pictures of with a 250uf modern Phillips can across the 820. Voltage at pin 1 is about 185 volts on that amp and it does clean up great. The fat cap does have issues with low voltage for sure.

 

[TAZIN]

I don’t think anyone can be totally robust about these things! Friday pm amps with random assortments of parts do crop up particularly this era. eg clusters of ceramic caps to make up correct value or bass amps with all 0.1uF caps across the board!
All with original dye markings.

I remember those examples. Very odd indeed.