70's Marshall JMP 2199 Master Lead 2x12 combo

[Wicked_Tone]

I've had this amp for several months and am finding that the overdrive is on the fuzzy, buzzy side. I've ran some pedals through all 3 inputs and the bright channel is extremely trebly to the point of no bottom end what so ever, which sounded horrible. I've opened the amp recently and found that the original Daly brand filter caps are still there and appear to not be leaking or bulging, but I am wondering about just replacing them anyway. I placed an order for Mallory 150 caps through Tubedepot.com to see if replacing the original cap values in the amp will help to smooth out the tone a bit. I tried different diode combinations which did boost the gain, but it still had that terrible quality to it. For example, when bending a string it just did not have a pleasant dynamic even for a solid state amp. The character was like a combination of ping and buzz ringing, if that makes any sense. There is potential in this amp, as turning down the gain on the amp or guitar volume yields a usable clean sound. I want to mod this amp with a bluesbreaker or old JMP type crunch, if possible. Any suggestions or thoughts on eliminating that wierd buzz quality would be immensely appreciated.

[phatt]

I'd back off on replacing Caps that are obviously not broken and has little to do with the problem you describe as You will likely complicate the problem further.

On the other hand if you want to resolve the issue A schematic would be a big big help.

The problem is likely channel/system equalization but just where and how would require a schematic or bare minimum a photo of both sides of the pcb board/s.

Cheers, Phil.

[Enzo]

Oh, please don't just throw parts at it.   If it is humming really bad, you might need new filter caps, but really, solve the problem first, THEN once the amp is working think about wholesale cap swaps and other upgrades.   Otherwise, making a lot of extra uninvolved parts changes is just that many more opportunities to confuse the troubleshooting.

Schematic:  http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/marshallamps/jmp_masterlead__30w_2199.pdf

Before any mods, have you disconnected the amp's own speaker and played the amp chassis through some other speaker you know the sound of?  Do that, it might be the speaker you hate.

You tried different diodes and that improved gain?   What does that mean??  I see only two diodes in the power amp plus a zener there.    Those plus the main bridge is all I see, none of which would affect gain. 

And has the bias been adjusted?  Too cold bias results in crossover distortion.

[Wicked_Tone]

Oh, please don't just throw parts at it.   If it is humming really bad, you might need new filter caps, but really, solve the problem first, THEN once the amp is working think about wholesale cap swaps and other upgrades.   Otherwise, making a lot of extra uninvolved parts changes is just that many more opportunities to confuse the troubleshooting.

Schematic:  http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/marshallamps/jmp_masterlead__30w_2199.pdf

Before any mods, have you disconnected the amp's own speaker and played the amp chassis through some other speaker you know the sound of?  Do that, it might be the speaker you hate.

You tried different diodes and that improved gain?   What does that mean??  I see only two diodes in the power amp plus a zener there.    Those plus the main bridge is all I see, none of which would affect gain. 

And has the bias been adjusted?  Too cold bias results in crossover distortion.

Yes, I tried the amp through a set of Celestion G12-80s with the vented magnets and the amp still had that same character I described.

I changed the two diodes in the power amp with leds. This gave the amp more perceived gain to my ears. I changed the diodes back though. I tried adjusting the bias pot with gradual adjustments and played the amp with each small adjustment, but it still didn't smooth out the sound. The amp doesn't hum at all, so I would imagine the filter caps are fine then.

I'll hold off on changing out the caps. Anything else that could be a culprit?

[Roly]

I changed the two diodes in the power amp with leds.

This is not at all a good idea.  The diodes D1 and D2 set the current down through the output stage bias network and Voltage Amplification Stage (VAS).  Changing 1N914's for LED's will result in a much increased current down this path and serious danger of letting the smoke out of sundry parts.  It is also unlikely to have any significant effect on the amp sonics.

In fact what you describe sounds to me much more like it is due to the preamp, not the main amp.

Since this amp doesn't have Fx Send and Return I would be inclined to lift C16 off the top of the main volume control, patch in your CD or MP3 player, and listen to some known music just through the main amp and loudspeakers.  It you still have the objectionable sound when bypassing the preamp, I would also try connecting some other speaker cab to see if the unwanted effect is associated with the power amp or the speakers (which may be damaged, have a loose voice coil &c).

If these tests are clean however then you need to concentrate on the preamp (which is where I suspect your problem lies).  Is the objectionable sound common to both channels, or only in one?

This problem is quite unlikely to be a faulty or failing component, so much as a component of a value that gives you an unpleasant result, so simply replacing it won't solve anything.

[phatt]

 If the Amp still works after leds trick,  :loco  You are one VERY lucky person.
I'd go buy a lottery ticket.  :tu:

Maybe Stay well away from that part of the Amp until you understand what you are doing.

The Amp is likely not broken it probably works fine. The issue is not new and you will find similar issues all the way back to Valve Era.
In those days you used whatever input worked with your brand of Axe and if not happy ,,, find another amp. 8|
Phil.

[J M Fahey]

You were very lucky that the diodes in the power amp were (just by sheer chance) NOT the bias ones.

To have an idea of what you did and to understand the reactions around here, imagine you told us you poured a bucket of gasoline over yourself , just to clean some paint stains in your clothes, and then , while you were waiting, lighted a cigarette to pass the time. :loco

You were lucky that you misread the label and it was actually water .

OK, back to the amp.

No "magic" parts will help you here, what matters is parts *value*, not brand, colour, who-used-it, etc.

That amp has a seriously compromised "bright" channel, which should be called "Zero bass -  zero mids" instead.

The culprit?: what on the schematic is called C5 (0.022uF) , which must be replaced by a 2.2uF x 25V electrolytic, with the "+" side towards the Tr2 transistor collector and the "-" terminal (usually indicated by a stripe) towards the R9 resistor.

This will make that channel "flat - slightly bright" , quite "normal", but most important, will make distortion pedals usable.

As of

I want to mod this amp with a bluesbreaker or old JMP type crunch, if possible.

...     ... if it were easy somebody would already have 

Get a good distortion pedal, now it will be usable, that's more than you bargained for.

The amp by itself is not bad at all (after modding), but Marshall in the 70's was just starting the SS path .... an quite half heartedly.

Please post results.

[Wicked_Tone]

You were very lucky that the diodes in the power amp were (just by sheer chance) NOT the bias ones.

To have an idea of what you did and to understand the reactions around here, imagine you told us you poured a bucket of gasoline over yourself , just to clean some paint stains in your clothes, and then , while you were waiting, lighted a cigarette to pass the time. :loco

You were lucky that you misread the label and it was actually water .

OK, back to the amp.

No "magic" parts will help you here, what matters is parts *value*, not brand, colour, who-used-it, etc.

That amp has a seriously compromised "bright" channel, which should be called "Zero bass -  zero mids" instead.

The culprit?: what on the schematic is called C5 (0.022uF) , which must be replaced by a 2.2uF x 25V electrolytic, with the "+" side towards the Tr2 transistor collector and the "-" terminal (usually indicated by a stripe) towards the R9 resistor.

This will make that channel "flat - slightly bright" , quite "normal", but most important, will make distortion pedals usable.

As of

I want to mod this amp with a bluesbreaker or old JMP type crunch, if possible.

...     ... if it were easy somebody would already have 

Get a good distortion pedal, now it will be usable, that's more than you bargained for.

The amp by itself is not bad at all (after modding), but Marshall in the 70's was just starting the SS path .... an quite half heartedly.

Please post results.

First of all, I would like to thank everyone who replied to my post! I can't express enough how grateful I am with sharing your knowledge and expertise on this forum. I will be looking to take a serious study into electronics, as I have realized the potential for great error.

Thanks for the capacitor info. I will follow your suggestion.

[Wicked_Tone]

Update. I changed C5 for the 2.2 electrolytic and now the amp sounds so much better and takes all my pedals really well! Thanks for the suggestion J M Fahey!

[J M Fahey]

 :dbtu: