Metropoulos Forum

I am working on a 100 watt Metro Plexi Kit amp circa 2007 for a friend of mine. I have built and worked on all sorts of Marshalls, Fenders, and you name it for years so I am not a newby at this but this one has an issue that is a stumper at the moment.

He brought it to me initially because someone else had put in a MV that was improperly done and he wanted it fixed. Basically what had been done was the dude tried to do the Dual 250K MV post PI type of MV but had done it all wrong with a single gang 250K pot and it was causing him all sorts of grief. He had done what I had seen 100s of times and put the MV pot in place of one of the channel I input jacks. My buddy had a new set of Mullards for it, so I removed the faulty MV wiring and pot, took it back to stock, reset the bias for the new Mullards and tested it out all with him there watching. No problems at all to this point.

My friend wanted a working MV in the amp and had a 1 Meg single gang pot so I proceeded to do what I had done many times before and put in a 2203 style MV pot after the Treble control and before the .022 mfd cap just as in a 2203. Treble control center wiper to the pot input lug, center MV pot lug to the cap connection, other connection of the MV pot to the shield, using shielded wire with the shield connected to the preamp ground only at the treble pot side. I have done it many many times before without issue. I fire up the amp, recheck the bias, start bringing it up on a test load watching the scope and at about 2/3 the way up on the bright channel with the MV on full I get parasitic oscillations from hell on the scope. Deep channel works just peachy all the way to clipping putting out about 120 watts. I start checking wiring, solder connections, lead dress, all the usual things and nothing, still oscillations from the bright channel.

At this point, as a part of what I was going to do anyway to the amp, I started doing a little reworking. I put shielded wire on the inputs and moved the 68k resistors to the tube socket, reworked the preamp, presence control, and power amp grounding per the "Larry" style "star" grounding technique, updated the grid resistors' wiring to using 5.6k resistors on each OP tube, triple checked all of my connections and made another "close eyed" once over the entire amp again. I STILL wound up with the same oscillation problems yet again. Now, I pull the MV pot wiring completely loose and direct wire the connection again just like stock from the treble pot to the cap and "woila", no oscillation whatsoever.

I am really stumped with this one. I have done tons of MV mods like this over the years and not once have I run into this issue. Anyone have an idea?

~Avguy49

avguy49 wrote:I am working on a 100 watt Metro Plexi Kit amp circa 2007 for a friend of mine. I have built and worked on all sorts of Marshalls, Fenders, and you name it for years so I am not a newby at this but this one has an issue that is a stumper at the moment.

He brought it to me initially because someone else had put in a MV that was improperly done and he wanted it fixed. Basically what had been done was the dude tried to do the Dual 250K MV post PI type of MV but had done it all wrong with a single gang 250K pot and it was causing him all sorts of grief. He had done what I had seen 100s of times and put the MV pot in place of one of the channel I input jacks. My buddy had a new set of Mullards for it, so I removed the faulty MV wiring and pot, took it back to stock, reset the bias for the new Mullards and tested it out all with him there watching. No problems at all to this point.

My friend wanted a working MV in the amp and had a 1 Meg single gang pot so I proceeded to do what I had done many times before and put in a 2203 style MV pot after the Treble control and before the .022 mfd cap just as in a 2203. Treble control center wiper to the pot input lug, center MV pot lug to the cap connection, other connection of the MV pot to the shield, using shielded wire with the shield connected to the preamp ground only at the treble pot side. I have done it many many times before without issue. I fire up the amp, recheck the bias, start bringing it up on a test load watching the scope and at about 2/3 the way up on the bright channel with the MV on full I get parasitic oscillations from hell on the scope. Deep channel works just peachy all the way to clipping putting out about 120 watts. I start checking wiring, solder connections, lead dress, all the usual things and nothing, still oscillations from the bright channel.

At this point, as a part of what I was going to do anyway to the amp, I started doing a little reworking. I put shielded wire on the inputs and moved the 68k resistors to the tube socket, reworked the preamp, presence control, and power amp grounding per the "Larry" style "star" grounding technique, updated the grid resistors' wiring to using 5.6k resistors on each OP tube, triple checked all of my connections and made another "close eyed" once over the entire amp again. I STILL wound up with the same oscillation problems yet again. Now, I pull the MV pot wiring completely loose and direct wire the connection again just like stock from the treble pot to the cap and "woila", no oscillation whatsoever.

I am really stumped with this one. I have done tons of MV mods like this over the years and not once have I run into this issue. Anyone have an idea?

~Avguy49

can post a chassis shot?

Yeah, probably a little later this evening. I have a couple other things I am going to give a whirl here shortly.
~Philly

Here's a few pics of the chassis. This was built by my friend and is not as neat wiring wise as my builds but everything seems intact and the deep chan works fine. Cranking the presence makes it oscillate also. I am wondering if that pot making a 1 meg connection to the ground at that point might be affecting things but I don't think so because a 2203 looks just like that.

~Philly

If you relocate the MV pot to another place in the chassis ( far away from the input) does it make a difference?

Problem solved!! I reworked the channel I (bright) input switching jacks such that the 68k grid stoppers were mounted right at the tube socket like the Channel II (deep) channel was wired. Then I made sure that even the crossover wire on the switching jacks was a shielded wire. Then re-dressed out the shielded wiring to and from the new MV so that it was up and away from the rest of the wiring at the inputs and tone pots and the oscillation went away. The amp puts out a rockin' solid 30+ volts into 8 ohms at clipping and is quiet as can be with the new grounding scheme.

And I DO love this obsolete technology. Nothing sounds quite like my PRS with soapbars through a cranked 100 watt Plexi, and NOTHING ever will!

Ciao Kids,
~Philly

I got a message from a guy dealing with a similar issue who wanted to see the final pics. It's definitely a rockin' bugger now man. Roughly 115 watts at clipping.

~Philly

Here are the remaining pics. Like I said before, I didn't do the original wiring job and the poor amp had been hacked on by a crazy man but I managed to save it I think. It SURE rocks now baby. We get a chance to hear it tonight, so we'll see. Hope this helps someone.
~Avguy49
~Philly

Hey everyone, just as a follow up, my buddy Jesse wrung out the renovated 115 watt Plexi "Moresnarl" Friday night and that puppy was rock solid and rockin' kids. It was singing sweet on leads and when he went straight with no distortion petal it had that killer 100 watt punchy bottom that you could feel out in front and with the new grounding scheme it was quiet as a mouse, no noise at all. Posted a pic of Jesse too. You can see the "Purple People Eater" behind him onstage.

Ciao Kids,
~Avguy49
~Philly

Nice for your buddy, but regarding the horrible sloppy wiring & soldering work in this amp, the question is: how long will it last
So do him the favour and rewire this thing completely to the "Metro amp" standard

Regards

Robert

Yes, without wanting to seem mean, that heater wiring alone is plain scary. Even just using straight runs in parallel (not twisted) like the later JCM800 style will almost guaranteed have quieter results than that random mess. The important thing is keeping the leads EQUIDISTANT to each other, but twisted is better since then in theory they are;
a) tightly coupled and
b) pass each other at right angles - thus limiting the effects of back EMF.

+1!

Ok guys, enough is enough here. I plainly stated the fact that I did not build this amp right from the start. The other fact that I have not mentioned to this point is that this piece is still a work in progress. This particular MV config we implemented #1 - To make the amp useable at all. The whacko that he let put the MV in originally not only screwed up that, but screwed with lots of other wiring too on the inputs and elsewhere in the amp making it virtually unusable. #2 - At this point we got the amp back up and running for him to evaluate over a month or so how he liked this version of MV as we had discussed the "Larry et al Post PI MV" also. We are not finished yet. This is still a work in progress at this point and as I go along I will rework other aspects of the design too not just heater wiring.

Look, I appreciate everyone's input and it is glaringly obvious that there is still more work to be done on the amp. That much is clear as a bell, I just ask that you all bear in mind that we're still working on this amp and while I would just LOVE to rip into it and rewire the entire shebang time and circumstances do not permit that very often with working musicians. This guy plays anywhere from 4-7 nights a week and has to have this stuff running, besides that I have a full time job myself which limits the time I can spend at any one sitting working on this.

I should perhaps also mention the fact that I am now 57 years old and I have been working on and grew up around electronics and tube equipment since I was old enough to hold a soldering iron. My dad was a radio man in the Navy and I grew up around tube type shortwave and Ham radios since I was about 7 years old. In addition to that I have worked in the music business as a Production Manager, FOH Engineer, and Systems Designer since I was 18 years old. I have a list of credits that includes ZZ, SRV, Reba, Restless Heart, JD Souther and tons of others too numerous to mention here including live show credits that include things like Farm Aid II, etc. My point is simple. I know what I am doing and what the risks are in working on and building amps like this. I have literally re-built blown amps in a dressing room during sound check and had them onstage for the show that night. I thought this was a forum for sharing things and that is what I am doing just bear in mind the situation/s involved here.

Ya'll have a great day kids,
~Philly
~Avguy49

We figured as much dude. No-one was pointing the finger at you anyways. We're just amazed that you got it working sweetly without having to rip into it deeper than you did.
Which proves that looks aren't everything.

No problems, I was just making a few things clear is all. I hadn't gone into the deal about it was still a work in progress really. I just had to open it up and pour the CORRECT voodoo into the box is all. After it is all said and done the player is the one who has to be happy with the result. If it doesn't make him smile then my time was wasted no matter what the inside looks like. All other calculations, designs, theories, and other things aside most of the time it comes down to having enough experience and judgment with these beasts to know HOW to put the correct voodoo in the box and have it all hold together while a player like my buddy whips the crap out of it for 4 hours a night you know. Learned judgment calls and voodoo make these beasts tick....



Take care,
~Philly
~Avguy49