Marshall JMP MKII non master volume

[pnut5150]

I am need of some help..Amp sounded horrible, I found a bad 2n2 cap and replaced it...all I had was 22n, will that make a huge difference in the tone? I found 2 more 22n that were cracked that read good but I changed them out anyhow. At volumes 2.5 and greater it sounds pretty good, anything under that it doesn't. really no clean at all, it starts breaking up pretty quick. I can also hear a oscillating sound at low volumes and the notes have no sustain and just kind of crack. All the tubes are new and I have changed out the preamp tubes to double check, the 1K resistors across the power tubes are good. one of the 12ax7's have a resistor across it and it reads 100k which is correct from what I can find. All other resistors on the board read good, the 10uf electrolytic caps on board read good as well. Any suggestions? Oh yea, I have jumped the two channels together as well...still doesn't sound right

[lowell]

Please find a schematic or link to one so we can help.

What part of the circuit was the 2nf cap you replaced in? That might clue us in on the problem. ALWAYS check the bias in a tube amp. It's the first thing I do. It can tell you a few things right off the bat. So is the bias good on both ends of the OT primaries? (Both power tubes...if it's50watts)

Is this a 2203 or 2204? Which JMP MKII?

[Leo_Gnardo]

IIRC 0.0022 were used at the plates of the preamps in "guitar" Marshalls and 0.022 in "bass" models . . . you'll get a noticeable bump in bass response, nothing to worry about, lots of guitarists like it and no harm to the amp.

As lowell says, insufficient bias current will leave things sounding lousy all the time. One bad output tube, or (horrors!) an open output transformer primary winding will leave you underpowered and sounding awful. Broken connection to ground or a loose screw causing same at output tube pins 1&8, same. On rare occasions I see a pre tube with only one filament glowing, there should be two, you'll get some signal thru but sounds distant & distorted. Couple quick things to check, if it isn't one of these stay tuned for more suggestions.

[pnut5150]

I haven't even checked the plate voltages..both sides of all the pre amp tubes are lit, figured the cap value wouldn't matter much. Amp sounds great when cranked, just absolutely no clean volume at all. There is a bad ground somewhere as I can hear the hum. I am not familiar with this vintage Marshall circuit at all, I know it is somewhere between a 73-74 can't remember what the original owner did say. I'll do some voltage tests and post back. Thanks for the info!

[Leo_Gnardo]

Amp sounds great when cranked, just absolutely no clean volume at all. There is a bad ground somewhere as I can hear the hum.

A push pull output section with one non functioning tube or open OT primary will hum too and never be able to deliver a clean signal.

Also measure the OT resistance from center tap to each plate, that would be a good start. I certainly hope the OT is not the problem but takes only a couple seconds to check this with amp out of the box and eliminate it as a suspect. Or not. Once that check is made, what's the bias current. If it's zero on one tube it may very well be kaput, with a fresh pair you're back in business.

Easy enough to punch up a schemo on Google, heck they might lead you back here. Model 1987 is what you have I suspect, or something very close to that.

[J M Fahey]

1) "amp sounded horrible" ... you replaced a cap but it didn't improve, so we don't know whether that was the actual problem or not.
How do you now the original 2n2 cap was bad?

2) to boot, you did not replace same value but 10X larger.
That said, sound was bad earlier, even without the 22n cap.

I suspect it's the coupling cap in the Guitar/High_Treble/High/whatever channel.

The exaggerated small value is not there by chance (nothing in a Classic amp is) but, probably by trial and error, it was found that cutting lows improved overdriven sound big time.
So for a proper test you need the proper one.

That said, you had a 2n2 cap before and it was already sounding "horrible" .

Can you please be more specific or perhaps provide an MP3 or short YT video showing it?
1 minute is more than enough.

Meanwhile do post voltage measurements, basically to detect some leaking cap which might bias next stage completely out of whack and also check power tubes biasing and screen voltages, the latter at the socket pins themselves.

[Chuck H]

DO- As Leo says and check the OT and power tubes.
DO- Check bias voltage and current.
DO- Check power supply, plate, cathode and grid voltages throughout.
DO- Stop cranking the amp until you figure out what's wrong with it.

Pretty much in line with Leo, the .002 (or .0022) cap is for the bright channel. It offers an alternative to the tone depending on which input is chosen and, as you mentioned, some guys like to jumper and blend them. I'd be inclined to put the proper value in there. If you don't want to order a capacitor you can just parallel two .001 caps and it'll be fine.

[pnut5150]

Yea at first I would not pass much sound at all..I testsd the caps and found the 2n2 reading 0.1 and it was cracked, I also found 3 22n's cracked and one of them with a low reading so I changed them as well. Also 1 330uf cap was leaking and I changed it as well. Major improvement after those parts were changed. Still not right though. Somebody has already changed the 10uf caps at the bias wheel, I testdd them and mounted them properly as they had just snipped the leads and soldered them on top. Now after reading your guys posts I looked at the power tubes upon turning the amp on and on from standby and discovered the power tubes are changing when flipped from standby..there is no nice blue inside the tube. However this is the first amp I have used that has 6550 tubes in it as well, but still assume they should get brighter. Amp does sound strong still at high volumes.
Here are the power tube voltages

First power tube:
Pin(1) -000.0mv
Pin(2) -000.0mv
Pin(3) 427v
Pin(4) 427v
Pin(5) -069.5
Pin(6) 427v
Pin(7) -000.0mv
Pin(8) -000.0mv

Tube 2
Pin(1) -000.2mv
Pin(2) -000.2mv
Pin(3) 429v
Pin(4) 429v
Pin(5) -069.5v
Pin(6) 429v
Pin(7) -000.0mv
Pin(8) -000.0mv

[pnut5150]

[lowell]

Those voltages look fine....normal. I'd like to add that hum doesn't necessarily mean bad grounding. As leo said it could be the power tubes...having only one working. But voltages look fine. We need to know the bias current though. That could cause distortion. A bad coupling cap can cause hum too if leaking DC to a following grid. That could also cause distortion. So we still need answers to initial questions. And it definitely wouldnt hurt to list all cathode, grid, and plate voltages of the preamp tubes.

[pnut5150]

Are these the bias resistors? The two 1 watters

[pnut5150]

On the bias filter caps I have 000.2mv and 000.1mv on the + side and -78.4v and -71.5v on the ground side of the cap

[pnut5150]

Found it, the 220ohm resistor in the bias circuit is junk
Thanks for the help guys! Now the tubes are back to pretty and blue.

[pnut5150]

I need some schooling now..although it is working, that 220 ohm resistor was reading correct..with it removed it works correct..I put a new one in and it's working fine. My question is how could the ohms be correct on that resistor but it still be bad? Is that possible?

[DrGonz78]

I count at least three 220k resistors that were tack soldered onto the legs of old components. I would bet that the solder joints were not the greatest and that it was a connection issue. If the resistor is measuring 220k or some where in that ballpark then it would not appear to be bad. I guess anything is possible when it heats up but seems rather unlikely to me.