Unicord 50 watter 1970 schematic?

[6adam7]

Hey guys,
Can anyone tell me, is this schematic correct?

This guy:
http://drtube.com/schematics/marshall/1987u.gif

Anything in your experience, that should be changed?

Cheers,

Steve

[Tazin]

What year is the amp that your trying to repair?

[neikeel]

Bias feed wire should be off the HT supply direct (i.e. before, not after the HT switch)

Other than that it is correct for a mid 69on amp until the mains became neutral and live switching arrangement (around 75/76 I think) to accommodate modern safety standards.

[6adam7]

Thank Neil, good/interesting to know...
Tazin, it's a kit set up according to that schem, has all good stuff in it, ( xf2's, Marstran trannies, sozo's, Tungsram in v1, a quad of Weber Stan spec 1225's) but I just seem to be fighting it a bit....I use it as a pedal platform, so run it clean ( maybe 2 to 3 volume high treble input and Alex attenuator on -6db) but it just seems extremely muddy/ bassy, and lacking mids ( body?)
I kind of know this is characteristic of this circuit maybe but heck, sometimes I think it sounds more JTM45 than plexi - Thus me asking if anything should be changed and if schem is correct....

[6adam7]

Oh, btw, Neil, this amp has no bright cap...
I remember reading some posts/ replies of yours about different values and effect on high mids etc - Could I trouble you for suggestions? ( silver mica?)

Cheers,

Steve

[bill bokey]

For a clean sound I find small values work best. I use 100-120pF and that adds a touch of sparkle as well as tightening the bass end.

[Tazin]

I'm kinda surprised to hear that this circuit sounds muddy specially at low volumes. I have an amp setup very similar to this circuit and it's kinda bright and crisp sounding. Anyhow, I agree with Bill's recommondation regarding the 100pF - 120pF volume pot bypass cap.

[neikeel]

If it is muddy something is wrong.
I agree 100 to 150pF helps give a bit of sparkle.
A leads spec JMP50 should be capable of taking your head off

[6adam7]

Awesome, thanks for the replies guys,
Yeah, I'm suprised too, It's something I've been fighting for a long time...Its been getting better as I find stuff out....
One thing I've just now realised (??!$&&?) after going through notes is, both channels have .022uf coupling caps...
I use the bright channel, which should be .0022uf yes?
Could that be it??
It's not that there is too much bass per say, ( or is it low mids?) it's more its width, if that makes sense, so if I was to turn up the volume, as she starts to clip, the bottom end/ low mids lose definition-turn to mud....It's like, I want/need to turn bass knob up to give it some body, but can't because it turns to mud...
So when I'm playing it clean and use a gain pedal I get this subtle ' awww' thing happening, which I fricken hate - it's better with single coils but humbuckers, I lose a ton of definition, and that's what Im chasing....
Lately, I've been wanting to get rid of this amp because of this, but at the same time, man!, it's sooo close!!...I can hear a really nice sounding amp there, just need to take the blanket off the speakers!!!
Hope that makes sense???

Cheers,

Steve

[bill bokey]

A 2n2 coupling cap would help, also what's the B+ ?
Can you post a few pics ?

[neikeel]

A 2n2 coupling cap would help, also what's the B+ ?
Can you post a few pics ?

+1

[6adam7]

Unfortunately no, no gut shots and no idea of B+.
I'm a player not a tech, sadly for my wallet...
It's a small box Weber head, which started out as a bastard 6m45 that I took a punt on and bought off the bay for peanuts... I converted it to Jtm45 specs, tried kt66's, 6l6wgc's, 6l6's,.. And finally years later worked out that I'm chasing a later, tighter sound...(70 spec)
My techs alway complain about the lead dress and what a crap job the tech before him did...

So, it's a mongrel but I'm pretty happy with it, besides this annoying thing I've described...
So!... If I change out the .022 for a .0022 on the bright channel, is it going to reshape it tone wise drastically?.... Or just shelve bottom end and make it sound thin? ( don't think I'd want the mids/trebs to sound any thinner than they already are...)
What do you guys think?

[neikeel]

Yes it will shelve the bottom end but will make it tighter and more focussed with more perceived gain too, I would always use at least a 1500pF and preferably a 5000pF cap on volume 1 pot. They always sound best with the volume around 6 or 7 and tamed with a bit of PPIMV and/or a good attenuator.

Have you tried playing with bass control off and mides on 10 and treble between 6 and 8 (same with presence)?

[6adam7]

Thanks again Neil,..
Yes, that does sound more in the ball park of what Im chasing - gonna get it done I think..
Do you have a cap suggestion?.. Sozo?, or?
And, yes I have mucked around with eq..
It seems like bass affects body so if I turn it down it's too thin, so maybe the .0022 will do the trick....
So you suggest those bright caps values even though I'm running it so clean?...

[6adam7]

Have you tried playing with bass control off and mides on 10 and treble between 6 and 8 (same with presence)?
[/quote]

Best setting Ive tried is bass - 2 ( for body) mids dimed and about 4 treble, presence about 5/6 any more trebs and it's a bit peaky..
I've found my gain pedal doesn't like too much presence either...