Marshall plexi copy question

[cberry]

Hi All,
I own a London City 50W Marshall copy (4 inputs) and can't get the thing to sing properly.
It's got me really baffled but not having that much experience with amps, I guess that's pretty normal.
The problem is that when cranked even into relatively mild distortion, I'm getting some wierd phasing effects from the amp. It sounds a little brittle at first very unlike what I expect a mild overdrive to sound like, cleaning up rapidly thereafter. I'm not sure if f it's a ghost note that's there, not being too familiar with what that sounds like or how to test for it...
Anyway , maybe how the circuit differs from the plexi helps...
First of all, It's running on 373V HT - according to the voltage charts, the Marshall runs at 400V. Under serious overdrive this sags to 320V - a likely candidate I assume is the PT but the ratings on this are unclear at best. I'm assuming there's 200mA available.
Overall, there's 50uF of extra stabilisation in the marshall before the choke and it's aranged slightly differently thereafter.
Next, my copy has 1K screen grid resistors where these are conspicuously absent in the plexi.
The control grids also have 1k resistors on them and the bias seems more stabilised on my copy.
Further down towards the input side of things, the tone stack is slightly different and a few components have been saved (slight capacitor changes and common cathode capacitors are used).
There's a schematic here:
http://www.drtube.com/schematics/londoncity/dea100.gif
This one's got me baffled like I said. anyone have any ideas as to where to start? Or do you think I should just go ahead and change everything that's not quite plexi about it?
Thanks.
cb

[Plexi]

Yes it follows a marshall /bassman type circut,but,I think the drawing may be incorrect..Compared to a marshall.
So you might check your amp,look at V2.
Is the cathodes wired like is shown in the schematic? Your amp may be wired right,and they just numbered the cathode pins wrong in the drawing.
Pin 3 should have the 820 ohm and pin 8 should have the 100k and tone slope tone stack caps.

Richie

[cberry]

Whoops wrong schematic... It's nearly the same...
http://www.drtube.com/schematics/londoncity/dea70.gif

Pin 3 has the 100k + tone stack, pin 8 the 820k and the link's between 2 and 6.
So it's just a mirror right?
Thanks
cb

[Plexi]

Yes..thats is correct.

Richie

[cberry]

OK, I did some experimentation to see "what if"...
First thing I did after measuring voltages last time was to put a variable
resistance from the tone network output to earth. (Yes a pot wired from
the treble slider to earth (about 10k worked well)
a) this renders the tone control "useless" in conventional terms but allows
some wierd and wonderful effects - just rename the knobs whatever you feel
like sort of thing...
b) It can greatly attenuate the volume of the preamp section.

(aside) - Having fiddled with it for a bit, I wrote down the settings and
ran them through Duncanamps tone stack calculator and found that it's a
typical Marshall EQ curve - albeit at -20dB

OK so I basically measured the same stuff as I did before - droop and
whatnot, messed with the controls until I found what I thought was a good
level and wrote it down - then repeated the process with an overdrive pedal in the input...
So, far from a conclusion but an observation: When the HT drops to 340V from 373V,
that phase sound comes into play again - that's 10% droop - not a lot.
Is it possible to draw any conclusions from this? well, I don't think so but
what I can do is test a few more things tomorrow.

1) Test hypothesis that if HT drops below 340V - bas sound occurs:
- Use the 315V winding of the mains transformer - same current.
- Use a different transformer (I think I've got a Partridge 400VA mains
transformer lying around somewhere I can try. That will probably decide the
current V Voltage story.

2) It's possiuble that I just don't like the sound of overdriven pentodes -
although all my "guitar heroes" use them aparently.

3) Check mains filter caps - although they seem to be holding up
fine. I could put a little more filtration in there.

4) Attempt the laborious task (well it doesn't appeal to me right now) of
converting everything to Plexi standards.

Ideas? Comments?

Thanks.
cb

[cberry]

Last update for the weekend...
Tried the 325V winding to see what's happening...
Frustration had me playing chromatic scales with micro-bends to see if I can isolate whatever or wherever there's an annoying frequency causing all of this...
200Hz!!!! Bingo. It seems that playing the octave of the mains ripple helps me identify the cause of my troubles...
Again a 10% drop in HT brings the "ghost note" or "phasing" sound to annoying levels. so It's not the voltage but the stabilisation.
Like I said before, the mains transformer was replaced by a previous owner (and in these amps the trannies were a bit on the weak side) so it doesn't surprise me that the current delivered is a little weak...
I'll have to check all the capacitors as well but I think I'm well on my way to getting this beast to sound good after all.
Another thing I'm going to do to at least bring it under a bit more control is to hard-wire the presence control (same as Plexi) and put a master volume in the hole.
cb