JMP: popping, crackling noise and volume drop

[Chris-in-LA]

It's a very interesting issue because after we replaced the two faulty V1 100K resistors, the problem still keeps coming back. Playing for half an hour with my usual settings (very high master) and than the V1 starts acting up. Volume and gain drops, crackling noise, I tap on the tube and it's good for another 5 min then starts again. Does it with every preamp tube and I've tried about a dozen one.

So I opened up the amp, turned it on, cranked the master and started tapping on the all of the caps, resistors and solder joints in the amp with a plastic pen. One of the old 22n Mustards (V1 cathode cap?) has an amplified thud when I tap on it (I marked that cap on the pic below) while all of the other resistors and caps are noiseless. Can this Mustard cap cause all of my problem or is this normal?

That cap might be leaking and is letting some DCV through. Put a meter on it.

 

[LPman]

I am confused on the wiring of the 'HI input' cathode (typically called v1b); I don't see a partial bypassed cathode (typically 2k7R//680nF) I see a fully bypassed cathode. Maybe the 2k7R runs under the board?

v1a (cold clipper) is not bypassed, which is correct but, I can't make out the value, should be 10kR

I see many areas that look like clipped in replacements could have cold solder joints e.g. in the circled mustard coupling cap and the 680nF v1b cathode cap.

What is the value of the baby blue coupling cap?

I admit I don't look at many jmp 2204s so I could be wrong on these observations.

just lookin' for clues

The partially bypassed cathode is there, it's just hard to see on the photo because the 2.7k resistor is under the big Mustard cap.

The baby blue cap is .0022U and the Mustard that I marked is also .0022U. Shouldn't these be .022U by the way? What difference does this make in tone?

So is that normal that the Mustard I marked gives a thud sound? I've just bought a cheap .022U Wima cap to try it but if that sound is normal I won't bother to change them.

 

[XTRXTR]

.022uF is the same as 22nF where u=micro and n=nano. Its a convention to express Farads in micro, nano, and pico as the capacitance in Farads is smaller in component value in descending order. Thus 22nF=22000pF (pico Farads)

You stated that baby blue cap is .0022uF=2.2nF. If your stated value is correct those caps should be replaced with a 22nF=.022uF per 2204 spec.

The others say 22n which is 22nF. The schematic calls for 22nF in both locations. Actually all coupling caps on a 2204 are 22nF by spec.

I would say it is not impossible for a mustard cap to make noise like you are describing like a thud. Usually the disc caps make those noises. I would replace both of those first two coupling caps to the proper values.

I would also state that the fundamental issue of the amp losing volume and having to wait for a cool down before it returns to normal needs to be diagnosed. You could replace those caps to 22nF and still find you have that issue. Perhaps show the tech in person so he can get a better sense of what is happening.

Intermittent issues are the hardest to track down and may require a well experienced tech to repair properly. As @neikeel pointed out it could be an arc situation between the pin 3 and 2 on the output tubes. pin 3 about 400 VDC to pin 2 at +3.15 to -3.15 VAC is near enough to ground, for an arc if other variables are present that diminish the natural insulation qualities of air. e.g. carbon buildup on the socket, pins bent too close together...etc.

 

[Chris-in-LA]

The partially bypassed cathode is there, it's just hard to see on the photo because the 2.7k resistor is under the big Mustard cap.

The baby blue cap is .0022U and the Mustard that I marked is also .0022U. Shouldn't these be .022U by the way? What difference does this make in tone?

So is that normal that the Mustard I marked gives a thud sound? I've just bought a cheap .022U Wima cap to try it but if that sound is normal I won't bother to change them.

Again, put a meter on the side going towards the pots and check for DC voltage.

 

[mAx___]

Not an amp circuit I'm familiar with, are V1a and b cascaded at Volume 2? I can't figure some connections well enough from the picture.

 

[XTRXTR]

2204 in general is cascaded v1b into v1a at the point where the preamp volume 1 is placed. Volume 2 is usually called the master volume and controls the AC preamp'ed signal going into v3 the ltp type PI.

There were a few early 2204 JMP Marshalls that were not cascaded from the factory. Similar to a 1989 except that only two input holes and both ran into Volume 1, volume 2 was still a Master Volume. 1989 was a four hole input two each were run into its own volume 1 and 2 without a Master Volume.
Unicord 2204:
Unicord 1989

Marshall 1987 and 1989 were similar without the cascading inputs like the JCM 800 2204:

FYI V1B per ecc8xx datasheet is the more quiet of the dual triode, which is why you typically see it on the high sensitivity input. Less susceptible to hum.

 

[Pete Farrington]

This link is nice as it has the Nov 76 non cascaded and Feb 78 cascaded JMP 2204 schematics https://el34world.com/charts/Schematics/files/Marshall/Marshall_50w_master_vol_2204.pdf

 

[XTRXTR]

@Pete Farrington
Interesting how the Rev '78 version puts (2) 10k 1W between the screens and PI. Also its between the 50uF/50uF cap terminals there. Did they actually have four 10k dropping resistors? Or is that referring to some unseen note. I only just noticed that.

Many different wirings, V1a is the cold clipper on the JCM but its the Hi sens input on the JMP. Not much different in regard to a high gain circuit perhaps. I've always followed the JCM schem and Mark Huss nicely drawn schem.

https://mhuss.com/MyJCM/JCM800_2204.pdf

 

[Pete Farrington]

Interesting how the Rev '78 version puts (2) 10k 1W between the screens and PI

My guess (based on the rationale https://www.marshallforum.com/threa...xx-series-weird-voltages.128889/#post-2308409 ) is that the 6550 version uses 10k HT dropper to V3 LTP, whereas the EL34 version uses 20k.

 

[LPman]

Thanks guys for all the great input, very helpful community!

I resoldered some funky solder joints at around V1 and I also put some cloth material under and around the V1 tube into the socket so that it sits much more stable now. Since then, the issues have not reappeared, fingers crossed.

Wouldn't replacing those two .0022 coupling caps to .022 cause the amp to become more muffled and bassy? This amp has a plate voltage of 400VDC so everything that makes it stay tight and bold sounding (or even increase these features) is a good thing for me.

I'm currently in the process of ordering some NOS mustard caps and resistors to restore the amp a little. I don't know if I should order .022 or .0022 Mustards. Am I correct in thinking that changing to .022 would muffle my tone?

 

[Pete Farrington]

Am I correct in thinking that changing to .022 would muffle my tone?

I don’t think so.
Coupling caps are part of high pass filters, they roll off low end.
To make tone muffled usually requires low pass filters, to roll off high end.
However, everybody perceived things differently.
If the lows are excessively extended, the tendency to bias shift when overdriven may cause flub etc.

 

[LPman]

Update: I took the amp back to my tech (he's a really great guy by the way, 40 years in the field, still working at big festivals). We measured all the existing components of the amp, the values of all caps and resistors, all of the solder joints. We gave a good tugging at every wire and tube and socket while the amp was on - not a single popping sound came out of the amp. Every value is on point and all wires and joints are rock steady. All throughout this examination I played the amp cranked (attenuated) like I usually do. We made the usual measurements again: plate voltage (400 VDC), bias (a little below 30 ma for my taste), heater voltage on the power tubes (6,5 V), OT resistance, etc. etc. He tightened the preamp and power tube socket-pin connections as well.

Then I went back to my studio and what happens is the thing crapped out on me like it's been always doing in this past month. Totally flabbergasted. We just can't ever reproduce the issue at my tech's workbench. I also noticed that while the amp sounded tight as a jackhammer at my tech's like a JMP should, now it sounded noticeably saggy. I tried the amp with 3 different cabs.

What are the remaining rational explanations?
- At non-attenuated high volumes, the components might vibrate at my place - but we tugged and gave a good chopstick kick at every wire and connection... so out of question.
- Bad speaker cable: while the current one is fairly new, I had problems before with speaker cables.

So this morning I bought a new speaker cable at an audio store. Just by looking at it, it's much better quality than the usual overpriced guitarstore ones. So far it seems that the issue is solved with the new speaker cable. FINGERS CROSSED.

 

[XTRXTR]

This doesn't make sense to do if you used the same speaker cab, guitar, speaker, and power cables, at your techs place.

Maybe you will get lucky.

If the same cab and cables, etc. was used I would look at wall voltage dips, spikes and, maybe a bad breaker or fuse in the electric box, outlets. Otherwise something in your location maybe causing it.

Bad speaker cable, or guitar cable, bad grounded power outlet.

 

[LPman]

This doesn't make sense to do if you used the same speaker cab, guitar, speaker, and power cables, at your techs place.

Maybe you will get lucky.

If the same cab and cables, etc. was used I would look at wall voltage dips, spikes and, maybe a bad breaker or fuse in the electric box, outlets. Otherwise something in your location maybe causing it.

Bad speaker cable, or guitar cable, bad grounded power outlet.

No, my own speaker cable was never used at my tech's place. That's why it came to me that it could be the culprit. We'll see in the following days.

If it's not then its twilight territory because the problem has been happening not only in my studio, but at home and at my office too. But could never reproduce it at my tech's. Really frustrating.

Another thing that I don't know if has any relevance: yesterday another band played at the neighboring studio and my amp picked up the guitarist's signal very loudly. I couldn't even record a track, it's like he was playing through my own cab from the other insulated room. I tried different guitar cables with no success. Could the reason for this also be my faulty speaker cable?

 

[LPman]

Believe it or not, I played for 3 hours yesterday night, recording and jamming both on my 412 (16 ohm) and 212 cab (8 ohm) and no issues! The amp sounded stellar. It was cranked like a mother all night. Would have started acting up before that only after half an hour.

I'm now 99% certain it was my speaker cable all along. How it could cause V1 to go haywire? I have no idea. Anyway I'm glad that crappy cable didn't blow my OT. The cable was less than a year old and I also took it to my tech on one of my visits to measure on it and the values were correct on both jack ends. I suspect that something must have started to melt inside or a solder joint go loose anytime I began to play the amp loudly. With the new audio store cable it's all fun again!

Does this make sense to anyone?

 

[PelliX]

I'm now 99% certain it was my speaker cable all along. How it could cause V1 to go haywire? I have no idea. Anyway I'm glad that crappy cable didn't blow my OT. The cable was less than a year old and I also took it to my tech on one of my visits to measure on it and the values were correct on both jack ends. I suspect that something must have started to melt inside or a solder joint go loose anytime I began to play the amp loudly. With the new audio store cable it's all fun again!

Does this make sense to anyone?

Any cable can start to break/fray inside thus reducing conductivity when bent or twisted a certain way - so, yes, it's plausible. That said, generally one can identify the fault by simply using the speaker cable as a signal cable and wiggling or doing a continous measurement while moving it around. Not all cables are equal, you can also get a dud or dodgy one from the factory (depending on their QC). Long story short; don't blindly trust cables, especially speaker cables on a valve amp. A spare is a good thing to have. If you're so inclined, disassemble or chop up the iffy one and attach new plugs. Test it before using it, of course.

Another thing that I don't know if has any relevance: yesterday another band played at the neighboring studio and my amp picked up the guitarist's signal very loudly. I couldn't even record a track, it's like he was playing through my own cab from the other insulated room. I tried different guitar cables with no success.

That's bizarre. Making some assumptions here:

- You plug straight into your amp (maybe with some pedals or whatnot between) all wired, no wireless.
- You have no 'connection' between your amp and any 'studio equipment' (like some rack FX processor in your FX loop or a DI out).
- None of your bandmembers is having a laugh at your expense and grinning the entire time.

The best I can think of is that you have a VERY microphonic preamp valve, probably V1. Then there's a really, really long shot and that's that the guitarist nextdoor is on a wireless set which by freak occurrence is being picked up by your amp. I've had a neighbor talk through my amp from his CB radio. Hearing voices is one thing, a seemingly possessed amp is another...

Could the reason for this also be my faulty speaker cable?

No, unrelated. That's either bad luck or a cable that got mistreated a bit.

 

[LPman]

That's bizarre. Making some assumptions here:

- You plug straight into your amp (maybe with some pedals or whatnot between) all wired, no wireless.
- You have no 'connection' between your amp and any 'studio equipment' (like some rack FX processor in your FX loop or a DI out).
- None of your bandmembers is having a laugh at your expense and grinning the entire time.

The best I can think of is that you have a VERY microphonic preamp valve, probably V1. Then there's a really, really long shot and that's that the guitarist nextdoor is on a wireless set which by freak occurrence is being picked up by your amp. I've had a neighbor talk through my amp from his CB radio. Hearing voices is one thing, a seemingly possessed amp is another...



No, unrelated. That's either bad luck or a cable that got mistreated a bit.

- Yes, I play straight through the amp when I practice alone, no pedals involved.
- No equipment. There's no fx loop or any bells and whistles on this old '69 amp.
- Haha, no I went to their room and told about the issue. Two guys were practicing, a drummer and a guitarist straight into a Roland amp.

My V1 is not microphonic, none of my preamp tubes are. I tested all of my stash throughout this journey of troubleshooting and only put the best ones in the amp. Next time these folks will be around, I'll see if this happens again with my new speaker cable.

Sometimes I've been hearing FM radio stations through this amp very faintly, but this occurrence 3 days ago was really surprising. My guitar is equipped with P90s, I don't know if that has any relevance. When I changed guitar cables from my long one to a really short one, the noise lessened somewhat.

 

[Pete Farrington]

my amp picked up the guitarist's signal very loudly

My guitar is equipped with P90s, I don't know if that has any relevance. When I changed guitar cables from my long one to a really short one, the noise lessened somewhat.

It seems to me it was more likely to have been your guitar, rather than your amp, that was the source of the issue.
Such that if the instrument cable was pulled out of the amp, the issue would have stopped.
Single coil pickups are completely non discriminatory, just as eager to sense whatever electromagnetic interference is in the local environment as the guitar strings.

A replacement / upgraded instrument cable may be beneficial.