Buzzing on High Input of JCM 800 2204 Horizontal Input Version

Bardagh

Bardagh

Well-known member
So I resolved one issue with this amp with the help of the forum, now I'm onto the other, which is a very annoying buzz that appears when plugged into the high input. Plugging into the low input is almost dead silent. Obviously the high input is going through another gain stage with a bright cap and such, but I don't think the buzz is normal.

Poking around with a chopstick, the input wire from the jack to grid on v1b crackles and pops with any slight touch, and the parts of the socket for both the grid and cathode pins seem very loose and move around a lot when I poke the solder tabs. None of the other preamp tube sockets have that kind of looseness. I have tried cleaning the sockets with deoxit on the v1 preamp tube pins once already with no apparent improvement. I'm trying to find something small enough that it might allow me to put some more tension back in those pin sockets, but I kind of doubt that will be the source of the problem.

I have already ordered a couple new input jacks to replace the old ones since I have read these can be a point of failure and noise, but I may not have those in hand for several days. Anything else should I consider looking at or doing in the meantime? None of the solder connections appear bad in any obvious way.
 
The pins of the v1 socket are loose and need re-tensioned.

Drain the filter caps and unplug the amp before you go poking around in a tube socket.

You don’t need new input jacks IMO. The originals normally just need cleaned with deoxit which you’ve done so they should be fine. You can test them with a multimeter to confirm.

Your replacements are likely made in China, unless you found Cliff jacks made in UK, which during the pandemic, have been hard to come by. There’s many more knockoffs than originals around.
 
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glpg80 said:
The pins of the v1 socket are loose and need re-tensioned.

Drain the filter caps and unplug the amp before you go poking around in a tube socket.

You don’t need new input jacks - they are likely made in China anyway unless you found Cliff jacks made in UK, which during the pandemic, have been hard to come by.
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Fortunately, Tube Depot has Cliff jacks in stock. If I don't need to replace them it won't hurt to have the extras on hand I suppose.

Alright, I'll see what retensioning achieves - and of course drain the caps!
 
Bardagh said:
Fortunately, Tube Depot has Cliff jacks in stock. If I don't need to replace them it won't hurt to have the extras on hand I suppose.

Alright, I'll see what retensioning achieves - and of course drain the caps!
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Since the low input Jack works as intended the problem is isolated to the first gain stage and anything before it. You’ve cleaned the input Jack and verified the solder doesn’t look like cold solder joints. After retensioning the pins, if you still hear the problem, I assume you’ve changed the preamp tube to verify it doesn’t follow the tube?

I’d probably also touch up the solder joints of V1 with a small dab of solder just to make sure it is good, since obviously someone has been in the amp.

If after all of that and you still hear it, I’d start looking at the plate resistor for V1 and see how far it’s drifted, as they are known causes of shot noise when they begin to drift due to age and overheating.
 
glpg80 said:
Since the low input Jack works as intended the problem is isolated to the first gain stage and anything before it. You’ve cleaned the input Jack and verified the solder doesn’t look like cold solder joints. After retensioning the pins, if you still hear the problem, I assume you’ve changed the preamp tube to verify it doesn’t follow the tube?

I’d probably also touch up the solder joints of V1 with a small dab of solder just to make sure it is good, since obviously someone has been in the amp.

If after all of that and you still hear it, I’d start looking at the plate resistor for V1 and see how far it’s drifted, as they are known causes of shot noise when they begin to drift due to age and overheating.
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Yeah, I've changed to verifiably good, low noise tubes.

So I gave it another dose of deoxit and tried to get those little pins tighter and it does appear to done something as I now don't get pops and crackles upon prodding the input wire, just the faint glassy impact sounds that are normal. HOWEVER that buzz is still there. It is at least partially due to the filament wires. If I separate the grid wire from those as much as they allow with what slack they have, it reduces a little bit. But it is pretty loud, and gets louder along with the preamp and master volume. I would say it's as bad or maybe worse than what I've heard from those Jet City JCA50H amps that have notoriously bad heater hum.

Touching the solder joints up will be no problem. There are not too many components between the high and low inputs either, so maybe I should just start pulling and testing.
 
Sounds like a wiring issue under the board where you’re getting EMI coupling from the heater wires.

What does the heater wiring look like? Make sure your grids cross orthogonally to the heater wires so that coupling is minimized
 
Could be a grounding issue on the high gain input jacks possibly as well. Wiggle the grounding wire for that jack either at the jack or at the grounding lug or a the PC board. This is addition to the other suggestions which are all plausible as well.

Usually you need to use something like a steel dental pick or something similar to tighten the 12ax7 pin halves together.

As glpg80 said drain your filter caps before mucking around in there. You could also poke around with a wooden chopstick to try to isolate the exact location that is giving this issue.
 
All the above advice is solid. I’d recommend using a chopstick to move around the wires in the first gainstsge to see if you can identify the problem along with tapping on the resistors and taps. Might be interference between the 68k and the 0.022. Does it squeal at high volume? Or have more of a constant hum? If it’s a consistent hum I’d check your grounds. Pics may help as well
 
Sounds like it's more a mechanical noise issue since you seem to have isolated it to the input grid wire which is a signal carrying wire. There could be corrosion at either soldered ends of the wire due to age. As glpg80 said you could reflow the joints as they could be cold solder joints sometimes even solder joints that look good are cold joints.

If none of these suggestions remedy the problem, then replace the input jack and or replace the input grid wire with a shielded wire or a new wire of the same gauge.

Buzz is usually associated with 60 cycle hum induced from the heaters or RF interference like a radio frequencies. Sounds like you are homing in on the problem.
 
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Just my 2¢ here...the best thing I've found for cleaning jacks is a .22 caliber gun barrel wire brush. .22 caliber is 3/100's smaller than a 1/4" jack, which is perfect. Spray contact cleaner on the brush, go in and out about a dozen times, done. The contact surfaces will be like new.
 
Well, the new jacks should get delivered today and that will be a simple thing to pop those in. We’ll see if that helps at all, although some of the noise is definitely from the heaters on V1. If I push those wires down with a chopstick until they touch the chassis the buzz is significantly reduced.

A guy was selling 10 feet lengths of RG188 on eBay for $12 so I went ahead and got some in case I need to try replacing that wire to grid with a shielded one.

Oh yeah and I picked up a set of various electronics tools. All kinds of tiny pokers and things with tines, sharp super tiny tweezers that actually can pick up those snipped component leads and some tiny wire brush things I assume are for applying flux? I usually use a toothpick… :LOL:
 
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Seems like it's really just that black heater wire coming off pin 9 of V1 as it goes clockwise under the plate, grid, and cathode wires of the first gain stage before it joins and is twisted with the red heater wire from 4 & 5. Maybe I just need that shielded wire on the input to grid.
 
Sounds like you are on the verge of solving your issue. Let us know how it all goes...:2thumbsup:

Are the heater wires twisted in this amp? The twist in meant to cancel AC hum of the heaters. In some Marshall's the techs got lazy and ran straight lengths of or heater wire to the tube sockets. It may or may not be contributing to your issue but I thought I would mention it.
 
harddriver said:
Sounds like you are on the verge of solving your issue. Let us know how it all goes...:2thumbsup:

Are the heater wires twisted in this amp? The twist in meant to cancel AC hum of the heaters. In some Marshall's the techs got lazy and ran straight lengths of or heater wire to the tube sockets. It may or may not be contributing to your issue but I thought I would mention it.
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They are semi-twisted. :LOL: Not the nicest job, although given the layout of the amp I don't know how to achieve much better results (this is a horizontal input with the pot mounted board).
 
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Well, new jacks didn’t really change anything but I figured as much. Oh well. I reflowed some joints here and there and tried to reposition the wires on V1 a bit and it definitely has helped bring the buzz down a notch. Hopefully going to a shielded wire on the input will make a further improvement.

I also went looking at voltages and b+ is almost 491.8, which seems like a lot given the numbers you usually see people give for these, more around 465, 475. Plate voltage of V4 was 491.1 and V5 was 490.5. I went to test a new pair of EL34's I got from Nesstone and I couldn't set the bias cold enough for them. With the bias pot set at its coldest setting, they were still in the mid 40's.
 
Those voltages are correct for JCM series amp starting in 1982 through the late 80's, I would imagine a horizontal input was 1984 and later. Modern EL34's may have short lives at 491 DCV so I wouldn't bias them over 55-60%, these days I bias at 50% and I notice no loss in tone.

Also the input grid signal carrying wires should cross the cathode and or plate voltage wires at 90 degree right angles to each other to minimize noise induction it's called lead dress.

The only other thing I can think of would be the pins of the socket being loose and rocking on the pins of the tube causing an audible mechanical noise because of the amplification factor or the audible signal when you are moving the identified wires with the chopstick. I have seen some older Marshall where the pins of the tube socket are very loose and do not hold the tube well. You may have to re-look at this if installing the shielded wire does not clear up your noise issue. I guess you could twist the heater wires some more or redo them if the noise is still bad.

The other thing you could try is STAR grounding methods of the different sections of the circuit sometimes Marshalls original grounding scheme could produce noise. It would be an undertaking but might be worth your time. There are pics of layouts of the STAR grounding on Metro and on the internet to download. I have used the standard Marshall grounding in some high gain builds but with good lead dress and shielded wire with DC elevated heaters and the amp was as quiet as high gainers can get. Even with DC heaters the Friedman BE100 has some pretty good hiss when the gain is cranked.

Does your amp have chassis mounted tube sockets or PCB mounted?
 
Oh... I thought I would mention that really old Ceramic caps can start to leak DC causing noise issues. I have a 1972 Marshall superlead that had been really noisey/hissy for years since it was all original I didn't want to mess with it much. It was all original with the original bias supply caps that weren't leaking or bulging. The mains screend/PI filter caps were updated and replaced with NOS LCR'sa few years ago so I though it was all good and it was just a noisy amp. But I finally forced myself to change the 10uf bias caps out. When I did this I also changed out the ceramic 500pf mixer bright cap on the 470K reisistor with a NOS CDE 560pf andthen installed a new 5000pf Mallory ceramic bright cap in lieu of the 40+ year old Lemco ceramic cap on the volume pot. I also changed out the 500pf ceramic cap on the tonestack with a NOS CDE 560pf mica cap.

All the noise and hiss of that amp disappeared and it sounds so much better and less shrill in highs more in line with my 68 plexi. I didn't thinkit was that big of deal but it does appears that the 500pf and 5000pf aged ceramic caps were all contributing to noise and hiss.

Anyway I figured this might be some food for thought for you.
 
All the tube sockets are chassis mounted, just the pots and inputs are pcb mounted on these. The wiring looks pretty much like any other stock horizontal input 2204/2203 I’ve seen pictures of. It would certainly have taken more time and a good bit more wire per amp to have done it in a way to minimize noise more.

As it is, the grid, cathode and plate wires for the first gain stage of the high input channel don’t cross, they just run adjacent to one another since the pcb layout accounted for that. The heater wire from pin 9 however travels by its lonesome clockwise all the way around and underneath those wires.

I reduced some of the buzz just by making sure the heater wires were pushed down as far as they would stay, lifting up the input wire and changing it’s relative position. If the shielded wire reduces the buzz further it might be ok. When it gets to be a problem to me is if you’re running loud and gainy so the buzz is drowned out, but then you roll down the guitar volume to get clean/semi-clean tones and the buzz is now easily heard with the guitar.
 
harddriver said:
Oh... I thought I would mention that really old Ceramic caps can start to leak DC causing noise issues. I have a 1972 Marshall superlead that had been really noisey/hissy for years since it was all original I didn't want to mess with it much. It was all original with the original bias supply caps that weren't leaking or bulging. The mains screend/PI filter caps were updated and replaced with NOS LCR'sa few years ago so I though it was all good and it was just a noisy amp. But I finally forced myself to change the 10uf bias caps out. When I did this I also changed out the ceramic 500pf mixer bright cap on the 470K reisistor with a NOS CDE 560pf andthen installed a new 5000pf Mallory ceramic bright cap in lieu of the 40+ year old Lemco ceramic cap on the volume pot. I also changed out the 500pf ceramic cap on the tonestack with a NOS CDE 560pf mica cap.

All the noise and hiss of that amp disappeared and it sounds so much better and less shrill in highs more in line with my 68 plexi. I didn't thinkit was that big of deal but it does appears that the 500pf and 5000pf aged ceramic caps were all contributing to noise and hiss.

Anyway I figured this might be some food for thought for you.
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Yeah I may end up looking at some of those old ceramic caps, but right now I’m definitely eyeballing those bias caps. The bias range doesn’t seem right to me. I put the Svetlanas it already had in it back in and returned them to a healthy bias, but noted that the bias pot was still almost completely to the cold side to achieve that. I’m going to go ahead and order the set of replacement electrolytics since those big 35 year old LCR’s aren’t going to last forever either.
 
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