Marshall jmp-50 2204 issue Please Help :-(

[Tiliev]

Hi guys!
My name is Steve, and I have JMP-50 2204 amp with EL34’s!
Well.... it sounds horrible!
I have replaced everything I could think of and I could!
Tubes, filter caps, trim pot,bias caps, pots, jacks tube sockets, a lot of other caps!
.....still sound awful:-(
On low volumes it is not that bad, but the more I push the preamp when master is more then 50% (5-6) , the worse it sounds!
Well....with Marshall tube amps is supposed to be the other way around isn’t it !
And I have tried it with all different preamp and power tubes!
I was checking and rechecking the bias from time to time on a new purchased installed well matched pair of Svetlanas, and the last time I did it, I left the probe and meter on V5 , and found something that definitely didn’t look right to me!?
Before that :My amp plate voltage is 400V!
Most JMP’s are on 600V, but few run at lower : at 400V!
My JMP is one of them!
So I have set the bias around 37mA idle!
There was around 0.3~0.4mA difference between V4 and V5 at idle!
I have one of these socket probes that goes between tube and amp socket with probes to be plugged in meter! I am biasing the amp, on mV setting on the digital multimeter!
After about 15 min waiting for the bias settling in ,I left the probe and the meter on the way it was, and start playing!
The readings on the meter went up, which it should!
When I was swapping the probe back and forth between V4 and V5
, I found V5 voltage was jumping a lot higher then V4!
And as I said above,the more I was pushing the amp, the bigger that difference became!
For example:
The highest readings on louder lower notes on V4 was reaching 45mV, while V5 was going way higher to 130mV!
If I stop playing, both will go back to 37.0 37.4!
The lauder the amp is pushed, the bigger the voltage reading differences between V4 and V5 !
If I swap the tubes, the higher readings are still on V5!
I would guess, there is crossover distortion caused by the big difference in the current draw between V4 and V5!
What possibly can cause such a dissbalance Between the two tubes when amp is pushed!??
All transformers are room temperature!
No hum or any other signs of something being wrong?!

I have Marshall JMP, and now i’am recording with amp simulators .......yuck !
Please Help!
Any suggestions would be highly appreciated!
Thanks for Your Help, and Kind Regards!
Steve!

 

[Lake Placid Blues]

Re: Marshall jmp-50 2204 issue Please Help :-(

Some ideas.

The supplier of electrons to each power tube is the output transformer. One side of the primary windings goes to one tube plate and the other to the second tube's plate. The imbalance might be caused by a faulty OPT.

Another idea is that there's a negative voltage supplied from the bias circuit to the control grid of each power tube. This negative voltage regulates the current flowing through the tubes. As audio signal is received from the phase inverter the negative voltage becomes more positive allowing more flow through the tube. Is the imbalance caused by too much positive signal reaching the hotter tube's control grid? Perhaps positive DC is passing through a faulty coupling cap? Is + voltage coming from the negative feedback? Is there arcing from other pins to the control grid?

Anyway this will probably require a good tech with the proper tools.

 

[Tiliev]

Re: Marshall jmp-50 2204 issue Please Help :-(

Thank You Lake Placid blues!
I have asked for help in various net forums, and You Are the first with specific suggestions!
....And I appreciate it so much!

First!
We have to write off the possibility of faulty OT!
I have considered that possibility only by starring at the schematics and using my best available special tools!
Instinct and intuition:-(
I have replaced the original OT with brand new “Classic Tone” OT!
Nothing has changed!
But don’t regret it! I am looking at the bright side!
I will have spare OT )
I have two techs around me!
One is beyond my price range, the other was the guy who was retuning and biasing my amp in the last 10-15 years!
Since I was studying around in the last six months and learned this and that, I found the bias he was setting was way too cold!
Assuming he didn’t consider my amp plate voltage is not 600V, but 400V!
So now I have decided to learn more, and find my way out from that problem!
I don’t have special tools, but I can replace components!
:-(
That is definitely not the best scientific approach, but according to the circumstances, it could come practical!
Looking at the schematics there couldn’t be so much possible issues that would affect the load of each power tube!
I could eliminate them one by one, and with better luck and instinct, that could happen in the beginning of the row!
I could start with that sucker: The coupling cap on the phase investor circuit!
I’am not so shure which one is it , but I am sure I have not replaced caps on the phase inventor circuit!
What value should be that cap, and with what is directly connected?
Thanks for Your Help again Lake Placid Blues, and Kind Regards!
Steve!

 

[Tiliev]

Re: Marshall jmp-50 2204 issue Please Help :-(

Is the 0.022uf that goes in to the grid on each power tube!?
I think I have a spare one!
I will pray that to be the problem!
;-)

 

[FuseG4]

Re: Marshall jmp-50 2204 issue Please Help :-(

yes I believe there is supposed to be a .022 cap on each power tube!

and if you were asking about which is the PI coupling cap it looks like its the .022 cap from the master volume to the PI v3

 

[FuseG4]

Re: Marshall jmp-50 2204 issue Please Help :-(

also um, if the screen resistor on v5 going bad could cause this? The screen could be intermittently shorting to the plate if the resistor is worse at high temps or if a solder joint is bad here, I think?

Or the cathode resistor on v5, same thing, could be bad resistor or joint that gets worse as the amp heats up or vibrates from playing. I think.

other things, loose or dirty tube socket for v5?

 

[Tiliev]

Re: Marshall jmp-50 2204 issue Please Help :-(

Thank You FuseG4!
I have replaced the tube sockets with Benton’s!
I have installed new ceramic 1K/5W resistors over pin 4 and 6, also aded 5.6K Grid resistor to grid 1!
Originally wasn’t like that, but I’ve was advised to do it!
Today I have replaced the coupling cap to V5, and even the 47pf cap between the two circuits, but didn’t change much!
I did not have 47pf, so I put 75pf as some people were recommending even 100pf for the so called “brown sound”! I didn’t see it brown , but I heard it more like murky or washed out and didn’t like it much!
Don’t get me wrong! I like dark sound, so when I push the strings to brighten it through dynamic envelope!
It just lacked that Marshall punch! It was like ....Viagra hangover :-(
It was kind of compressed,but did not like the character,and will put the 47pf back!
In my amp cathode (pin 8) and pin 1 are bridged to ground!
I had bunch of other issues, and solved them, but this .....oh boy ...real nightmare:-(
I think V4 doesn’t work right either! In fact barely moves few mV, at the same time, V5 takes the brunt!
Thanks again folks!
Will keep in touch!

 

[LLL]

Re: Marshall jmp-50 2204 issue Please Help :-(

I did not have 47pf, so I put 75pf as some people were recommending even 100pf for the so called “brown sound”!

Yeah, that's definitely a myth/internet rumor part change there.

Here's Ed's actual amp specs:

Ed's 12301 Specs

 

[Lake Placid Blues]

Re: Marshall jmp-50 2204 issue Please Help :-(

In my amp cathode (pin 8) and pin 1 are bridged to ground!

That is correct and required for EL34s. It doesn't matter if your running 6L6s..and most beam tetrodes because that connection on those tubes is internal.

 

[Tiliev]

Re: Marshall jmp-50 2204 issue Please Help :-(

Hi Guys !
I think I got it!
The 100K piher connected to coupling .022 inverter cap on V5 circuit was way off!
It couldn’t show measurement within 200K range on the meter! I pulled it out ,and same thing!
I didn’t have 100K and I put two 220K in parallel instead!
Well now it sounds like Marshall
It measured ~110K!
Should I buy and replace it with exact value, or I can leave it like that?
What I see now when I live the bias probe in while playing is V4 works much harder then before, and crunch is much fuller and smooth!!
Before V4 was barely moving!
10-15 mV Evan the amp was pushed, while V5 was going over the roof!
OK!
What mV/mA -age would be considered acceptable as a highest pick the meter would reach when amp is pushed really hard?
I know the way I measure it with the digital multimeter is far from scientific but still, i was still able to notice the difference in the load between the two tubes!
I also got in the mail the two 47pf NOS caps, and put back the right value!
With this cap is better no matter what the people say ;-)

I would like to thank You All for the help, and express my deapest appreciation

It was a shame!
To have JMP, and to record guitars with amp modeling devices:-(
Kind Regards to all of You!
Happy Steve!

 

[Darg1911]

Re: Marshall jmp-50 2204 issue Please Help :-(

Haven't been following this thread but ... Are you talking about the 100K plate resistor for the pi? Or, the bias splitter resistors for the power tubes? The splitters are usually 220K for EL34's in the Marshall. The phase inverter has 100K on one plate and 82K on the other. If you are talking about the 100K for the pi plate, personally, I would replace the paralleled 220K resistors with a single, in spec, 100K resistor. If you are talking about the bias splitters, I'm not sure why they would be 100K's in the first place.

So which resistor exactly was it that was out of spec?

 

[Tiliev]

Re: Marshall jmp-50 2204 issue Please Help :-(

Thanks for Your Help darg1911!
Here it is on the schematics!


I just did that because I didn’t have 100K at the moment!
I will sure take Your advice!
What type of 100K resistor You would advise me to buy?
Thanks again for Your Help, and Kind Regards!
Steve

 

[FuseG4]

Re: Marshall jmp-50 2204 issue Please Help :-(

Carbon composition for plate resistors on phase inverter, this is a stage with large enough voltage swings that carbon composition resistors will add pleasing distortion.
I would look for at least 1/2 watt. If one or two watt will fit without the leads being too long, go for it. Higher wattage resistors have less inherent noise

And if you want, get a 110k resistor instead of 100. As resistors drift in vintage amps, 100k resistors often end up around 110k so going for the value can a little warmth

If you go carbon composition on the one, you may as well do the 82k resistor on the other plate to match if it doesn't look like carbon composition. Carbon comp is dark brown, carbon film is beige, and metal oxide is light blue.

 

[Darg1911]

Re: Marshall jmp-50 2204 issue Please Help :-(

I'd match the resistor as close as I could. May be tough finding a Piher but plenty of AB's still around, although 5% is getting harder to find. I'd buy several so you can get one close to spec. I'd stick with 100K, it's gonna drift upward over time as it is. If you want to go with current production, there are new carbon comps being made. They'll look the part, but forget about using them if you believe in the old carbon comp "mojo".

 

[Tiliev]

Re: Marshall jmp-50 2204 issue Please Help :-(

Thanks a lot Guys!
I bought some NOS carbon comp to replace all plate resistors!
They look like AB, but they are probably not!
I don’t believe in anything
I trust;-)
Now i trust You
As far as the “mojo” goes, the most I trust is the fingers
I will experiment though!
I will live it open for some time as experiment laboratory!
Actually it was opened for quite a while, but it was more like experiment laboratory for providing Murphy’s law’s )))
It was long struggle until I found the issues I was dealing with, but hey....it feels now like orgasm after long and tiresome sex

I will stay in touch and share experience the way You did with me!
Thanks again, and Kind Regards!
Steve

 

[Darg1911]

Re: Marshall jmp-50 2204 issue Please Help :-(

Glad you found it and it seems to be working well now.

Just my opinion but, really, I wouldn't replace all the plate resistors just for the hell of it. If it's pretty close to spec value and it's not noisy (ticks, pops, crackling ...), I'd be inclined to leave it alone. But that's just me. And for the record, I'm not a tech.

 

[Tiliev]

Re: Marshall jmp-50 2204 issue Please Help :-(

I know what You mean!
If it works, don’t mess with it
Actually I have some quiet but noticeable cracking noise showing up from time to time!
But when it doesn’t, the amp is amazingly quiet!
I will replace only the ones i put in during the search of the problem, and keep spares around!
I don’t have fancy knowledge and equipment for that purpose!
I actually do that for first time!
And that was my only option: to replace components with highest chance to be the cause!
I search a lot on forums ,and most of the times I find things that happened to other people along with solutions given to them!
But I was never able to find anything on dissballanced load on power tubes!
And here We Are!
We got it ;-)
Thanks to all of You Guys!
And i’am learning!
And I LOVE TO LEARN THINGS!

 

[muttznmongrelz]

Re: Marshall jmp-50 2204 issue Please Help :-(

Great amps! Had one for about 15 years. Let’s see a pic or two of it if you have any.