Metropoulos Forum

Bad Kitty wrote:It could be my inexperience talking here but I'm hearing a buzz on everything. Before anything is played I'm hearing it and it's like on top of every note played.

you've got parasitic oscilation!

i know i maybe making this difficult - i actually hear this problem with alot of amps - i hear it all my 68 spec clips - with now gained knowledge/awareness i look forward to going back to plexi spec to see if i can tame the parasitic beast.

Mark

You mean the little pick flub?
PO can sound many different ways, depending on what frequency or band of frequencies it's centered on. I think the PO sound you're referring to is where there's an annoying high freq transient that attaches itself to notes. Where the amp feels like it's going to run off into feedback constantly.

It can also make an amp lifeless and saturated at any setting.

And can go so far as motorboating.


George

VelvetGeorge wrote:I think the PO sound you're referring to is where there's an annoying high freq transient that attaches itself to notes.

George

yes - the weird high freq that sounds like a damaged speaker coil( coil drag) - its as close i can get to decribing it.

bigger grid resistors on the output tubes is perhaps needed here

motor boating i thought was more of filtering out dc ripple.

which on a side note Gerald weber says the LCR cans do not do a good job of this - SORRY LARRY!

I talked t the owner and I will be installing an OT. I'll try to do it tonight and make a new clip. It might not happen until tomorrow night.



Motorboating is caused when the filtering is low enough that the signal modulates the supply line. Or when the circuit is unstable and cvan occillate at a low frequency.


george

interesting George. thanks for the posts!

VelvetGeorge wrote:I talked t the owner and I will be installing an OT. I'll try to do it tonight and make a new clip. It might not happen until tomorrow night.



Motorboating is caused when the filtering is low enough that the signal modulates the supply line. Or when the circuit is unstable and cvan occillate at a low frequency.


george

So George, does that mean the new OT is going to be like the one in the 12000's series amps, or will it be period correct to 1969?

I got very good results with a MM O100JM connected to the HW, but I assume, that George will swap in a Dagnall 1998 clone.

Larry

Crunchboy wrote: I wasn't referring to you George. I think you're a good honest businessman. But your bright. Surely you can see how biased some of your supporters are. I could hear nice tonality in the stock clip. With some tweaking of the tone controls, I felt it could have been very nice indeed. What would have been fun is if you would have asked for opinions on the clip without divulging what it was.

In fairness, perhaps you could point out some positive aspects of the HW to offset the pointing out of the choke screw and solder joint.

Certainly, every little change affects tone. Best of luck with the tweaks.

I certainly don't want to turn this into an argument, but come on. Outside of how the amp sounds, based on the price tag, loose solder joints and unbolted components are flat out acceptable. With this line of amp, you are supposed to "get what you pay for". They're obviously NOT taking quality control seriously and to me, it would lend me to believe they've skimped on other design aspects.

Poor construction quality in this amp for this amount of money should not be heard of. Period.

Besides, why shouldn't we have some bias here? We've all had the real Marshalls, and yet here we are buying and building George's stuff....FOR A REASON.
My last real Marshalls got sold within the last few months. Why? Because they sat under my bench while I played amps that made me happier -- my METROS.

I got the Dagnall clone installed tonight. But it was too late to make a sound clip. I'll make one tomorrow.

I did like the amp better with the new OT. And it does sound pretty much exactly like my 12 series. At least it does attenuated to hell.
I'll check it out with fresh ears tomorrow.

It's 2:20 AM, I'm going to bed.


VG

wdelaney72 wrote:Poor construction quality in this amp for this amount of money should not be heard of. Period.

No doubt! In this case, however, I'll wager that The Artist received the amp through an endorsement deal. So hopefully the only cash involved is what he's investing with George.

This one belongs to a working musician like you or I. But he did say that he got a good deal on the amp.

I'll crack open the other HW that's here today. Hopefully those things I found were the exception and not the rule.

VG

Parasitic oscillation is that sound I was talking about months ago Mark that you described as "digital-distortion-like". That's what made the Unchained clip sound crap - I always play that riff to find PO as if you've got it, that riff will bring it out every time. The sound usually hangs around the G, B and e strings.

I'm getting it now with my PPIMV even though I've done all the usual methods to get rid of it (shielded cable for PPIMV, shielded output tube input wires, shielded preamp etc). I'm pretty sure the noise is coming from the PI or further on in the amp, NOT from the preamp. The preamp just amplifies it. When I turn down the tone control on my strat or the treble pot on the amp, it goes away (mostly). I believe the noise may be amplified extraneous noise from a leaking filter cap in the mains filtering, but that's just a theory - if it's not that, I haven't a clue what it is as I've tried practically everything else. There's a constant quiet but well audible buzz present when I'm not touching the guitar and grounding myself out.

Maybe the PO is why they biased the amps to 100% plate dissipation? I've noticed more bias voltage tends to lessen the effects.

Isn't there a way to mask PO by putting 100pf (or similar) caps over the 100k V1 filter resistors in the preamp? I seem to remember seeing that on the "chip/SIR" mod.

of course shielding all grids, yes even power tube grids will help PO.

placing caps across plate resistors - lower the better before it robs tone so start low and work your way up. 100p 120p ,220p , 250 ,500p 1000p.

try a 220k( really anywhere from 820 ohm to 10k) resistor on the grid of v2a - right on the socket ( pin2) with zero lead length is what u r going for as well.

also upping the power tube grid resistors ( thats why they are there) from the stock plexi 1.5k's shared by each pair to 5.6k on each socket. some go as hig as 10k -20k on each but start low again as that this will eventually effect the tone.

i have also read of instead of grounding the shielded cable to ground you would connect the shield to the plate( pin3)- something about freqs 180 degrees out of phase will cancel the oscilation happening.

food for thought

Mark

Well all the grids are shielded, and I've got (apparently) 5.6k grid resistors on each power tube.

How will adding an input grid resistor to V2a affect the sound?