Metro zero loss loop questions

[SpiderWars]

I finally got around to installing a Metro loop in my Jose build. It had a pre-tone stack Jose Master Volume. When I installed the loop from Treble wiper to PI input it was pretty noisy when engaged and the bypass switch popped. I even tried adding a 1M resistor to ground from the PI input but that had either no or minimal impact.

So I changed it to a regular Marshall Master (Treble wiper to MV input, MV wiper to PI input) except now the loop is between the Treble wiper and the MV input. That got it working right. Apparently this loop needs a hot signal as-is.
I actually left the Jose 10k/470n combo because I really shoehorned it in there and the 100k cathode resistor is buried. No 1M to simulate the old MV pot because I have no clipping diodes so I don’t think it’s needed.

Is there a mod needed to make it not noisy when going straight into the PI?

Any other improvements? This one is a V4 from 2017.

 

[harddriver]

I dunno about the Metro Loop , if you installed a standard Pre-PI JCM800 master the loop should work flawless with that master layout, I know with the Jose master you have to tweak the Metro to get it to work right.

Sorry I couldn't be more help......

I did notice you still have some early gen 1 Sozo's in your amp, I still had one 2203 5watt single ended 1x12 combo amp that was running gen 1 sozo's circa 2005 in all the coupling/cathode caps and I always thought the amp sounded kind of smeared in a way, so out of curiosity I replaced them all with IC MPW polypropylene caps and bango, better clarity and bite the amp just souned all around better after the swap. It's probably not a big deal where you still have them though but ya never know. Those Polyester's you have on V1A and V1B sound great with nice bite and definition.



I do like the new generation Synergy Royal Mustards though, I think Suhr is still using the Mallory 150's in his SL67 and 68's while Friedman is using the Synergy stuff the last I knew.

Good luck with getting the loop tweaked.

 

[SpiderWars]

Yeah I think it’s only one of the early Sozo in there (unless I have the history wrong) and it’s at the PI second grid. The rest I think are either Next Gen Sozo or Synergy. And since that one is to ground I always think in my mind it’s no big deal there. That second triode of the PI gets signal thru the cathode and the grid is at AC ground. So I always figure that cap is ‘non-tonal’.

And that’s why I’m trying to use them up there and only there!

 

[RedPlated]

I sent you a PM

 

[OUTLAW1969]

I know with the Jose master you have to tweak the Metro to get it to work right.

When you say "tweak the Metro when using with the Jose Master", are you referring to dialing in the op-amp, or "value" changes, to match it the B+ ?

 

[SpiderWars]

When you say "tweak the Metro when using with the Jose Master", are you referring to dialing in the op-amp, or "value" changes, to match it the B+ ?

Not the opamps, those should be dialed in if the B+ is right. And the B+ is pretty easy but I think it’s about the levels.

 

[Code001]

Time stamped at around 12:43

 

[SpiderWars]

Time stamped at around 12:43

Thanks for posting that! As it turns out, I had changed it over to the Hybrid circuit he does here except I don't have the clippers or the 1M resistor. So I don't think it matters in my case because he showed that the signal is the same at both points if the SAT switch was OFF.

I did think the Hybrid sounded a little toothier and more aggressive in the tone test at the end.

 

[psychodave]

The one thing that stands out is the loop is upside down and components are closer to the PI tube socket. It might be fine, but just wanted to mention.

 

[SpiderWars]

The one thing that stands out is the loop is upside down and components are closer to the PI tube socket. It might be fine, but just wanted to mention.

Since the board is mounted underneath the pots, the components are about the same distance either way if not better this way. When mounted the other way that board is right over the socket. I wanted the Send closer to the preamp end and Return closer to the power amp end...to me it just makes more sense. I remember this was a thing a long time ago, lots of folks mounting them upside down while keeping in mind there is a level adjustment pot and a rack/pedal switch on top. You could probably pull them and mount them on the bottom.

EDIT: I'm pretty sure the loop Jason @burger does and sells at Headfirst addresses the Send/Return backwardness and probably other issues. If I didn't already have this one from years ago I would have bought the Headfirst loop.

 

[glpg80]

You are correct to move the metro loop. I mounted mine post treble pre master. If you don’t mount it there, you’ll have master volume dependent send level variability. Trying to install it post master pre phase inverter also kills your noise floor.

 

[shar-vell Dan]

Since the board is mounted underneath the pots, the components are about the same distance either way if not better this way. When mounted the other way that board is right over the socket. I wanted the Send closer to the preamp end and Return closer to the power amp end...to me it just makes more sense. I remember this was a thing a long time ago, lots of folks mounting them upside down while keeping in mind there is a level adjustment pot and a rack/pedal switch on top. You could probably pull them and mount them on the bottom.

EDIT: I'm pretty sure the loop Jason @burger does and sells at Headfirst addresses the Send/Return backwardness and probably other issues. If I didn't already have this one from years ago I would have bought the Headfirst loop.

My friend & MAGNATONE Lead Engineer Obeid Khan [great guy & GP] mounted my Metro loop the same way. He felt the ML pcb was too close for comfort hovering over V5 power socket. Ive never had a problem with this amp since 2011 cept when i tried lowering the line voltage w/a
variac to 90v. The Metro loop lost control voltage and shut down the amp.
I had to troubleshoot with Obeid to figure it out.
That said i had used the amp with the loop no problem for several years before the variac stunt.
Obeid's #1 Rule of amps : if its working fine dont Fuck wit it. Rule # 2 : dont forget rule #1.