Metropoulos Forum

Hi Guys,

I am new to this forum, But "not" new to amp building in general, Just no real Marshall experience of than a scratch build 18W lite.


Anyway, here is the story,
I picked up over the weekend a Genuine Lead 100 with the build date tag of 1973.

I also picked up a Lead 100RI I beleive is a 2000.

It was being conveted with a Metro board, and populated with NOS mustard caps, Allen Bradley resistors and Mercury Magnetics Tone Clones.

When I aquired these items, I did not get any of the Metro documentation with the purchase, and since the original owner is no longer with us, those docs are lost forever.

When I got the amp, it appeared that it was nearly completed with only 2 wire not attached. The Grid wires to the power tubes(1 orange, 1 green)

I attached them, and checked/compared everything as best as possible to the 1973 side by side. Everything looked great so I fired her up.

Well, when luck is a factor, I got really lucky!! This conversion really works, Loud, No Bias issues, and everything appears functional, Well sort of.

Here is a link to a page I set up with photos of both the conversion and the original 1973 Lead 100 he was apparently using for reference.

Picture Page

Well, not really knowing anything about the lead 100, I am pretty sure I have a couple of issues that need addressed.

The Bright channel appears to have a couple problems.

#1 It is so overly bright, that even on a Strat neck pickup it is bright as a Tele in the brightest setting.

#2 I get an ultra high frequency squeal when the presence and treble are turned up on the bright channel.

#3 The normal channel seems fine, and I can dime the amp without issue, " BUT" only if the bright channel volume knob is at zero. The normal channel is IMO rather dull, and it requires the treble and presence knobs to be set at or near max to sound decent, But since the bright channel squeals when the treble and presence are up, It basically requires the bright channel volume to be set at 0 at all times.

Maybe, someone here might direct this thread to the correct location for this discussion. And Possibly, If there is any way I could get some of the documentation, It might really help.

Ed

Welcome aboard Edmax...

About Metro documentation, check this link out: Click me

Hey,, Thanks A Million for the Docs link, That pretty much confirms The general common items in the build looks correct.

Now I just need to beat this squeal thing.
It looks like he used the 1968 /69 parts layout as far as most of the cap values.
I do see at least 2 incorrect resistor values.
A 68K instead of an 82K and a 27K feedback resistor instead of the 47K

I best check more layouts before I commit to which one, The 1968 is not exactly the same.
Or he may have not followed the layouts exactly and used a few published tweaks.

Ed

Welcome Edmax.

It looks like parts of the board are '67 specs like the 56k slope resistor (and probably 250pF cap) and the 27k NFB resistor. Those aren't necessarily "wrong" but they're different than a '68 which usually has a 33k/500pF slope and cap and a 47k NFB.

The current values will have less gain and be a little smoother or "Fender-ish" sounding which you may or may not want. If you're going for high gain or EVH tones, you'll probably want to switch those to '68 specs.

As for channel 1 being too bright, you can remove the bright cap on the volume pot or lower it to 100pF or so.

The squealing may be caused by the purple NFB wire routing. On the presence cap side, try moving it away from the tone pots and OT wires coming up through the chassis hole. On the impedance switch side, get the purple wire as far away from the output tube grid wires (green and orange) as possible. You may have to remove it from the bundle of wires that are cable-tied together.

toner wrote:Welcome Edmax.

It looks like parts of the board are '67 specs like the 56k slope resistor (and probably 250pF cap) and the 27k NFB resistor. Those aren't necessarily "wrong" but they're different than a '68 which usually has a 33k/500pF slope and cap and a 47k NFB.

The current values will have less gain and be a little smoother or "Fender-ish" sounding which you may or may not want. If you're going for high gain or EVH tones, you'll probably want to switch those to '68 specs.

As for channel 1 being too bright, you can remove the bright cap on the volume pot or lower it to 100pF or so.

The squealing may be caused by the purple NFB wire routing. On the presence cap side, try moving it away from the tone pots and OT wires coming up through the chassis hole. On the impedance switch side, get the purple wire as far away from the output tube grid wires (green and orange) as possible. You may have to remove it from the bundle of wires that are cable-tied together.

I think you have me headed in the correct direction. The issue apparently is on the presence cap side. It actually acts microphonic in that area when tapping on localized parts.

It seems like its not grounded correctly. I used a insulated soldering probe
and found that shorting the buss wire to the chassis right at the pot clears up about 90% of the problem.
The buss wire is clearly well soldered but the additional ground point did seem to make a major improvement.

Also, in doing this, It seems that the normal channels overall voice improves. Perhaps due to the fact that I am not getting as much parasitic junk from the presence/ground setup.

I have another question. I was looking closer at the 1973 Lead 100 and noticed the presence pot has an additional 2700 ohm resistor and is wired differently? It is visible in the pictures #2 Vs #3 original 73
Any insight on that?

Now I have to soul search a bit, 1967 spec or 1968 spec? decissions decissions.
I have never owned any high powered Marshalls before, Now Fender Tweed stuff, I have lots of.

I tend to play a bit more Blues, and Classic rock stuff.

Ed

It sounds like the buss ground on the back of the presence pot may not be very solid. If touching that up doesn't help, you could ground the presence to a ground lug somewhere.

I'm not sure if the '73 presence pot with a resistor is stock or not. Someone else can advise you on that. The JCM 800's have a 4.7k resistor wired like that but they also use a different presence pot value.

As for '67 vs. '68, you may want to leave it as is for blues/classic rock. With the exception of the 27k NFB, I basically have '67 specs and prefer it for those music styles. Each amp is a little different though.

toner wrote: I'm not sure if the '73 presence pot with a resistor is stock or not. Someone else can advise you on that. The JCM 800's have a 4.7k resistor wired like that but they also use a different presence pot value.

As for '67 vs. '68, you may want to leave it as is for blues/classic rock. With the exception of the 27k NFB, I basically have '67 specs and prefer it for those music styles. Each amp is a little different though.

I really checked out that 73 very closely, and every solder joint has the bit/dab of Nail polish? or whatever on it.

That resistor is for sure part of the original build. One thing for sure, after spending 3 days reading about these amp, there are sure a ton of variances.

As soon as I get this conversion bugged out, I want to start re-capping the 73. I also need to locate a new original fuse holder for it which I broke today.

I went to remove the fuse, When I tried to turn the screw, the body broke loose internally.
Whoever put that thing in must have torked it with a breaker bar.
The real bummer was, once I got it out, The fuse was the original and STILL GOOD!.

I hate when that happens.

Ed

Edmax wrote:
The Bright channel appears to have a couple problems.

#1 It is so overly bright, that even on a Strat neck pickup it is bright as a Tele in the brightest setting.

Sounds like a normal late superlead to me!

Edmax wrote:#2 I get an ultra high frequency squeal when the presence and treble are turned up on the bright channel.

This is normal with the head out of its box - it needs aluminium alloy sheet under the chassis touching the chassis to stop RF interference and feedback

Edmax wrote:#3 The normal channel seems fine, and I can dime the amp without issue, " BUT" only if the bright channel volume knob is at zero. The normal channel is IMO rather dull, and it requires the treble and presence knobs to be set at or near max to sound decent, But since the bright channel squeals when the treble and presence are up, It basically requires the bright channel volume to be set at 0 at all times.

Again pretty normal, that is why most people plug into hi bright channel and bridge the low bright to high normal with a patchlead. I then tweak each channel to taste - try 4 on bright and 3 on normal as a starting point with the head in a shielded headbox.

Edmax wrote:Maybe, someone here might direct this thread to the correct location for this discussion. And Possibly, If there is any way I could get some of the documentation, It might really help.

Ed

This seems as good a place as any. Suggest you earth the presence end of the bus bar to one of the filter cap bolts with a short black wire.

Good luck

Suggest you earth the presence end of the bus bar to one of the filter cap bolts with a short black wire.

I added a ground point from the pot to the chassis at the nearest power supply cap, Instant cure.

Thanks guys, That was really bugging me. Now I do not get any squeal even out of the cab, upside down on the bench. Perfect.

However, I must ask or mention, on the normal channel I expected a fatter bottom end. It is very tight sounding, but I expected a deeper kind of punch.

Right now, the Bright channel is way over the top bright, I am talking Byrds Rickenbacker Turn Turn Turn Bright X5. No volume issue, But I do not dare expose myself to that much brightness at over 3 on the volume know LOL Maybe drop that 500pf to 380 or 250Pf?

It was mentioned above by another poster that the cap and resistor values appeared to be 1967 spec, Is there a better combination?
Or is it best to just jumper it and leave it alone?

I have Mercury Magnetics Tone Clone Transformers and Choke.

Ed

Edmax wrote:...Right now, the Bright channel is way over the top bright, I am talking Byrds Rickenbacker Turn Turn Turn Bright X5. No volume issue, But I do not dare expose myself to that much brightness at over 3 on the volume know LOL Maybe drop that 500pf to 380 or 250Pf?

It was mentioned above by another poster that the cap and resistor values appeared to be 1967 spec, Is there a better combination?...

Strats and Tele's will be bright through that amp. That's normal. What speakers are you using? They can make a huge difference in brightness.

Lowering or removing the bright cap will help a lot. You may already know this, but the bright cap allows the highs to pass at full volume even when you turn the volume down. As you turn it down, you're cutting bass and mids so it can sound thin and bright at low volumes.

The differences between the 56k/250pF, which you appear to have, and 33k/500pF are mainly gain and mids. The latter will have more gain and upper mids. One isn't really brighter than the other.

toner wrote: Strats and Tele's will be bright through that amp. That's normal. What speakers are you using? They can make a huge difference in brightness.

Lowering or removing the bright cap will help a lot. You may already know this, but the bright cap allows the highs to pass at full volume even when you turn the volume down. As you turn it down, you're cutting bass and mids so it can sound thin and bright at low volumes.

The differences between the 56k/250pF, which you appear to have, and 33k/500pF are mainly gain and mids. The latter will have more gain and upper mids. One isn't really brighter than the other.

Hey, Thank you for all the help!!, I think I will just keep it as is. I am going to try out a couple humbucker equipped guitars and see how it goes.

My brother Has a Hamer Echotone that has the dullest hummers I think I have ever heard, I bet it will sound amazing on here!!

Now the big project starts, Getting this 1973 up and running and in top shape.

Once again, Thanks for all the help!!

Ed