Marshall Lead 12 // Faulty Overdrive

[Chris]

Hello!

I've got a Marshall Lead 12 (Stack Version) which has quite a strange "overdrive behavior" - I recorded it via Mic and Line out (mp3 attached - Mic+Line-out/ Mic only/ Line out only) - the "brzzzzzzz" is what i mean....

A shematic and rough information about the amp can be found here:
http://www.ssguitar.com/index.php?topic=2826.msg20791
http://www.drtube.com/schematics/marshall/3005.gif

Giving a singal to the power-stage only gives a clear and *loud* sound - This lead me to look a the preamp section:
Measured to Chassis/0V

C18: 17.8V
C17: -17.8V
IC1/Pin8: 13.4V
IC1/Pin4: -13.4V

From Tranny: AC 13.7V (x2)

Done so far (Problem & Voltages were monitored and stayed the same): New Bridge Rectifier, ElCaps, InputJacks, Gain Pot.

Do you have any advise for me?

Thanks in advance
Chris

[Roly]

Hi @Chris - Welcome!

Giving a singal to the power-stage only gives a clear and *loud* sound - This lead me to look a the preamp section:

That's good, but after listening to your clip, just to be sure about the output stage; unplug the speaker and measure and post the voltage across the speaker socket.  Start on your 20V range, but it should be very low, a fraction of a volt.


Assuming that you have cleaned and checked all your leads and sockets...

If that's okay, then, as the preamp supply voltage seem reasonable, there isn't much in the preamp to do really.  If the preamp dual op-amp IC is in a socket I'd first try levering it up a mm or two and reseating it a few times to scrub the contacts, but if it's soldered in I'd be taking it out, putting in a socket, and plugging something like a TL072 or LM833 in there and see what happens.

Keep us informed of your progress.   :tu:

HTH

[J M Fahey]

What I hear is normal SS clipping on signal peaks, dissappearing as soon as signal calms down a little.

Also: miked sound is smoother, raw line out (which is just an unfiltered  speaker out tap) is raspy and thin.
Again normal.

I wonder why you replaced

Bridge Rectifier, ElCaps, InputJacks, Gain Pot.

[Chris]

Morning!

First I want to thank you for the replys;

@Roly:
Voltage across speaker socket(s) is (-)0.06v - ok to my eyes.

I have to admit here that this is my first SS-Amp I really "use" - So I was also wondering by myself, if the behaviour is normal ... (Roly seems to hear something "unusual" too... ??)

BUT to my subjective feeling the "SS-Clipping" should be MUCH smoother - If this amp sounds like it should sound, its the wrong amp for me 

(Parts have been changed because of silly and desperate feelings, s.a.)

---> I'll get myself a TL072 / LM833 and give it a last chance ... (and solder in a socket)

Chris

[J M Fahey]

Tube amps are "easy", even a simple crude Champ into a cheap crude 8" speaker sounds good and clips relatively smooth; an SS amp can sound very good, but you have to work more ... and you need a good speaker.

To begin with, *forget* that "line out" , unless you pass it through a speaker simulator or cab sim (there's a thread on that)

Your miked speaker demo is much smoother, but you need a better speaker.

FWIW I'll post a video showing one of my amps (what I made about 25 years ago), which is *very* similar to a 3005, has a similar single TL072 or RC4558 preamp, BUT:
a) drives a 100W amp (SS of course)
b) which drives 1 or 2 12" speakers in a proper cabinet (not a "shoebox" .
Might also use 4 x 10" or at least a couple *good*  10".

Regular guitar, no pedals, straight into a Fahey 3600 head (which is what I described above) into, in this case, a cheap Fender single 12" speaker, into a 20" x 20" x 12" cabinet.
If you listen carefully, even on "clean" there is some of that "SS distortion" present ... but the speaker smooths it ***A LOT***

If it's only for practice or recording, you won't need to drive an external power amp, the 12W one will be fine, but hook a good speaker there.
Say, some mid priced Eminence Legend, a Jensen MOD1250 or 1270, or at least some 12" "pull" from , say, a Peavey Bandit or something.
Many replace such speakers with an "improved" expensive Celestion and sell the old one for $20/30 .

Enjoy:

http://youtu.be/FNehpDVExU8

If you need more, THESE guys tore Marshall Lead 12 guts from their cabinets,rack mounted them (driving beefy SS poweramps and lots of 4x12"cabinets) and used them both live and to record:

[Roly]

And they give pretty damn good video as well.  Do you remember that story OneBaldBob told about playing in front of a Marshall stack ... but was actually plugged into a small amp behind that was miked to the PA?   :lmao:

One of the reasons that valve amps sound the way they do is because the output transformer has "soft" magnetic saturation and compresses the signal somewhat before it goes into clipping.  It is possible to simulate this in a s.s. amp by having two or three diode clipping networks somewhere along the signal chain, but each with different load resistors arranged to progressively flatten signal peaks before the output stage itself overloads.