Marshall 1972 1987 noise reduction
- Thread starter wesgee
- Start date
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Hey, @wesgee, 
to the forum!
Are you experiencing hissing or humming? If it’s humming you’re worried about, you might be able to reduce it with a better grounding scheme and shielded conductors from input to V1.
Apparently there are artful ways to implement better grounding without drawing attention to the fact that the amp has been altered. I think @neikeel knows a bit about this.
I’ve been meaning to try shielded cable in my 1987. I don’t really find the hum in mine objectionable, except at very high volume. Even then, it’s totally obscured by playing. Anyway, shielded cable is probably effective, but the amp’s guts will look altered. It might slightly affect value, if you ever decide to sell it.
We would love to see pics or hear clips of your amp, if you get a chance! Cheers!
to the forum!Are you experiencing hissing or humming? If it’s humming you’re worried about, you might be able to reduce it with a better grounding scheme and shielded conductors from input to V1.
Apparently there are artful ways to implement better grounding without drawing attention to the fact that the amp has been altered. I think @neikeel knows a bit about this.
I’ve been meaning to try shielded cable in my 1987. I don’t really find the hum in mine objectionable, except at very high volume. Even then, it’s totally obscured by playing. Anyway, shielded cable is probably effective, but the amp’s guts will look altered. It might slightly affect value, if you ever decide to sell it.
We would love to see pics or hear clips of your amp, if you get a chance! Cheers!
wesgee
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Hi. thanks for the replies. Definitely not concerned about resale value. this piece of gear isn't going anywhere. I was considering the shielded cabling from the inputs to the 12AX7 pre-amp tubes to start.... good suggestion. I'm thinking of doing some re-routing of other wiring past that and just wondered if any specific experience/suggestion. It doesn't seem like the original is wired with a lot of attention to possible cross-coupling in the wiring? I've already dealt with the major 60Hz hum but knocked that down with new 50uF caps on the high voltage. Attaching a pic for ref. thx!
Looks like 73-74 to me.
Trying to work out what funny business is going on with those blue things (splices)
Sean points out where to start without replacing wires
There is absolutely no need to replace the grid wires with shielded wires if you have hum.
Lots of those wires are in the wrong dress. The input wires need to come out from the board down to the chassis and then up to the pots (plenty of pics of tidy ones around.
The heater wires need to be straightened and set to come to the sockets parallel.
Then the blue plate wires need to run in the same way down from the board along chassis to the socket. The grid wires parallel from the board and then down to the socket.
Make sure that the switching jacks on the inputs are clean and lubed (Deoxit gold).
Are your output tubes horribly mismatched?
If you have replaced your main filter caps then you should probably have replaced your bias caps (10uF with 150-250v rating) at the same time.
Most amps will be quieter if you do the above carefully.
After that I can suggest some subtle grounding mods.
Trying to work out what funny business is going on with those blue things (splices)
Sean points out where to start without replacing wires
There is absolutely no need to replace the grid wires with shielded wires if you have hum.
Lots of those wires are in the wrong dress. The input wires need to come out from the board down to the chassis and then up to the pots (plenty of pics of tidy ones around.
The heater wires need to be straightened and set to come to the sockets parallel.
Then the blue plate wires need to run in the same way down from the board along chassis to the socket. The grid wires parallel from the board and then down to the socket.
Make sure that the switching jacks on the inputs are clean and lubed (Deoxit gold).
Are your output tubes horribly mismatched?
If you have replaced your main filter caps then you should probably have replaced your bias caps (10uF with 150-250v rating) at the same time.
Most amps will be quieter if you do the above carefully.
After that I can suggest some subtle grounding mods.
wesgee
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- Feb 5, 2023
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thanks for the detailed feedback! i did replace the 10uF electrolytic bias caps along with the HV caps. I'm going to incorporate the wiring/routing advice provided and see if that cleans things up.... thanks again.
p.s. '72 is the date on the QA stamp on the amp chassis so it may very well be a '73.
p.s. '72 is the date on the QA stamp on the amp chassis so it may very well be a '73.
Trouble Free
Well-Known Member
Marshall didn't switch to PCB boards till mid 73' and they used mustards when they did switch, those caps didn't show up till around '79
maybe someone switched out the board? Or installed later caps on a earlier board ? What letter does the serial number end with?
maybe someone switched out the board? Or installed later caps on a earlier board ? What letter does the serial number end with?
Take a close look at the caps - they were around 73-74, mustards were more typical up to 76 when the green beans appeared, followed by the white beans, the red legos and WIMA varieties followed that.
In 73 you see all sorts of odd cap combinations (parallelled micas, all 0.1uF caps on bass versions etc) so I think the supply of caps lagged behind the orders for amps. Of course you could be correct that the caps only were replaced.
In 73 you see all sorts of odd cap combinations (parallelled micas, all 0.1uF caps on bass versions etc) so I think the supply of caps lagged behind the orders for amps. Of course you could be correct that the caps only were replaced.
Trouble Free
Well-Known Member
The PCB is green which suggests the amp was built sometime after Feb. 74'
before that it would have been yellow.
before that it would have been yellow.
