Metropoulos Forum

This is my first post here, so try to be easy on me. I profess my lack of knowledge and experience right now.

I recently picked up a "Black" Matamp GTV - Twin Channel Amp. Love it to pieces but I absolutely need an effects loop.
After digging around I decided to go with Metro zero-loss. I have heard these live, in a metro amp and I know they sound great.
I do have some amp repair experience but not much. Mainly simple diagnosis and replacement of failed caps and resistors, tube biasing, minor repair stuff.
So I am hoping some of you savvy experienced folks out there may be able to help me along the way here.
Here is a link to the loops install instructions so you know what I'm working from
http://home.comcast.net/~jbjdav26/Loop% ... %201B4.pdf" onclick="window.open(this.href);return false;

1st I need help figuring out where to tap the signal.
The amp is a cascading gain style, two channel, 140 watt amp and there is a master volume for each channel. I'm guessing I would put the loop in the signal path between the "presence" and the two master volumes. Would this be right? After looking at the directions a million times and comparing it to mine... Would I take the 1st lug from my presence and route that to "in" on the loop, then run my "out" line to the right lug on my 1st master volume? I have included some pictures below that will show you what I'm saying.

Second, powering the loop.
I will definitely need help determining where exactly my B+ line is, where to test it to determine my decoupling resistor, and which one in the amp will need to be changed to compensate for the power the loop is stealing from the amp.
It also mentions "loop can be fed directly from screen node alternately in lower voltage amps, (overall B+ at or less than 430v). Would this be useful in my amp or no?

Unfortunately NO schematics of my amp exist. There were only 4 made, and each one is slightly different, and no schematics were ever drawn up.

Last, placement of the loop.
I was really hoping this would fit easily, vertically in my amp.. Unfortunately it wont. Which means I will have to take the jacks off the board, mount the board close by, and wire the jacks back to the board. I'm guessing I should use shielded lines for this. Is that going to hurt anything? It shouldn't have to be a very long run.
These Matamp chassis sometimes were used to build amps that had loops, which were located right next to the master volumes in front. As much as I would love to locate the loop there, it seems like the wrong choice as I really don"t believe there is enough room to mount the board anywhere near here.
So I believe I will have to locate it in the rear, in between the speaker jacks, and footswitch jack (and button). There are already extra holes in the chassis at this spot and plenty of room to mount the board... However I hate to run my "in" and "out" and my B+ lines so far, Do you think I could experience noise from this?? OR is there a way around it?

The pictures below will help show you guys what I mean.
1) is the inside as viewed from the front. The farthest left pot is Master Volume 2, then Master Volume 1, Presence, treble, mid, bass, gain 2, gain 1, voice. Also if you notice in the far upper right corner of the pic, this is my speaker jacks. There is a space in between the speaker jacks and the footswitch jack/button with pre drilled spots. I would like to put the jacks here and mount the board in the empty space right there and run cables to the jacks from the board. I will have to attach the board to the side of the chassis as I can not drill into the chassis here because my transformer is located in that empty area.
2) is the entire inside as viewed from the rear. The last three pots on the right are (in order from left to right): Presence, Master Volume 1, Master Volume 2. The phase inverter tube is located a few inches in front of the master volumes in between the two turret boards.
3) is a close up of the area I need to tap the signal. In the photo left to right its : Master Volume 2, Master Volume 1, Presence... Also This is a good shot of the phase inverter. Is the place to test the B+ line after the capacitor that is attached via the orange wire on pin 6 of the PI? I'm confused with figuring out the B+ line especially.

Sorry for Such a long confusing first post. I hope to really get involved in this forum as I have always wanted to start building amps and will definitely be trying out one of the metro kits in the near future.
Thanks again guys for any help. I am lost here.
-Aaron

UPDATE:

I have mounted all the components and I am ready to wire this up tonight.

I am pretty sure I figured out the B+ point but I'm not sure if it's screen node B+ or PI node B+. In my post's third picture of the phase inverter. The orange wire coming off pin 6 (anode #1) goes up to the turret and through a 330k resistor. On the turret point after the resistor I get a reading of around 360v. In my head, this would be screen node because it comes after the resistor. Is this correct? And if it is, would I just attach the 33k decoupling resistor here, and OMIT step 15 from the install directions?
Also I am going to be using a shielded wire for this as it's a long run, so would I run just one end of it to bus?

Also. In that same picture, you see a yellow cap running from the middle lug of my presence (farthest pot to the right) to the right lug of my Master Volume 1. I am thinking this is the spot to insert the loop. I was just going to attach right after the cap, then return from the loop right to the right lug of Master Volume 1. Any thoughts?

Just trying desperately to figure this out. Thinking about giving it a go and wiring it up tonight.

Wait now that I'm looking at it more. I'm thinking that yellow cap off my presence is attached to ground/bus.. And that the straight wire running from far right lug of my presence to master volume 1 and 2 (left lugs) is my signal. So I'm thinking I would go from right lug of my presence, to the loop, and back to the far left lug of my master volume 1.

Shit I hope that's right!

I am not a tech, but have installed one of these loops in one of my metro's. It started as a 1959 and I converted the front end to a 2203 style amp.

Presence control is part of the negative feedback circuit, not where you would put the loop. In a 2203 style amp (from memory), you would want to put the loop in between the treble pot and the input to the phase inverter.

Any techs out there please correct me if I am wrong.

With all due respect, this amp is nothing like a Marshall though. Presence is just a name at the end of the day, and could be anything. Without seeing a schematic I couldn't even begin to get my head around this.

Well, I went ahead and installed it last night. Put the loop between presence and Master 1. Everything seems fine except the overall b+ power dropped quite a bit and the loop itself is only seeing 260v (30v shy of 290 minimum).
I think I'll just need to change the dropper resistor and maybe use a different decoupling. I'm not quite sure what to change it to though, this kind of math isn't something I understand completely.
My original dropping resistor is 330k and pre install my B+ (post 330k dropping resistor) was 352v so I used the 33k decoupler, now my B+ is something like 60 or more volts lower and my plate voltage dropped significantly as well. Can't remember off the top of my head what the exact drops were, but they were significant. I'll post those up in a bit.
Other than that it seems to be working fine and passing the signal through the loop correctly, and all knobs function as before. My overall volume is lower right now and the feel is not quite right, but I think that's just a matter of getting the B+ situated. I won't be using it until I get that figured out. Then hopefully all will be swell.
I'll post picture's soon.
Thanks for the comments guys.
-Aaron

Good call Demonufo, I made the assumption that presence control meant the nfb loop. But with this amp who knows how it's laid out or how the front controls are marked. Lesson learned - never assume.

Well here is the photo's. You can see I had to attach the decoupling resistor to a long run of shielded wire, but it is completely sturdy, heat shrinked, not touching anything, and grounded to the bus. Also, as the loop would not fit anywhere as a direct mount I had to mount it inside the chassis and run new jacks to it. This actually worked out just fine, although its not the prettiest. The only altercations I had to make were the two small holes in the side of the chassis used to hold the bracket I made to hold the loop.

So as far as my voltage goes, here's my readouts. And I was wrong the plate voltage didn't drop at all.

Previous to installing effects loop:
B+: 352v
Plate Voltage: 475v

After install:
B+: 329v
Plate Voltage: 475v
Voltage loop is seeing from B+ line: 267v (recommended range is 290-310v)
The Dropping resistor is 330k and the decoupling resistor I used was the 33k. Can someone out there whose good at this math tell me what I would need to bring my B+ back up to around 352? I thought maybe a 300k? But I'm not entirely sure. And, would that send a touch more voltage to the loop as well or should I change out the decoupling resistor too?

Actually I tested this again through a different cab (the one I initially tested with was a cheap piece of junk I grabbed where I work just to test the voltages, etc.) and through a different cab it sounded pretty much exactly as before. At least I could really hear much difference
However I would still like to get the voltage back to where it should be.
If anyone out there can help me figure out the B+ situation I'd be grateful!

Zombie thread. You can ignore all the old posts as they are more concerned with the b+

This is in regards to how to grab the signal post master volume(s).

I haven’t used this amp in ages but I’m trying to get it in my setup again. I’m having issues with the zero loss.

As I left it, it actually works as it should when tapping the signal between presence and the first master volume.
However, I absolutely need to move my effects loop insertion point to post master volumes for delay trails and loops.

*Side note, I also have a Slo clone I built which I installed a zero loss in and I had a similar issue with it, but eventually figured it out after a chat with Mike. This seemingly should be the same way I tap the matamps signal, but it just isn’t working correctly.

So... the matamp master volumes each run into their own little solid state switches and then that runs directly into the PI pin 2. This by all means should be where I take the signal. This should be super simple. However when I do this, I get awful noise/hum/buzz, my signal sounds weird, and my pedals make terrible clicks etc. it’s entirely unusable.

I’ve checked everything I can think with no luck. My zero loss is properly fed b+, properly grounded, my lines are shielded, etc.

Tonight I tried running straight jacks to a pedal at this same insertion spot (post master volume solid state switching mechanisms and into the pi) and it worked with no hum or buzz, but I lost a ton of volume running a passive loop there.

Thoughts on what I can check it if I’m doing something wrong?