Marshall Super Lead Reissue Master Volume Pop

[GoldtoneWorkshop]

Hey Everyone,

I have a Marshall Super Lead 100 reissue on the bench that has a weird popping noise. On the normal channel, when you turn the volume knob all the way up you get an audible pop at around 9.5 on the dial. After the pop there is considerable hum/noise in the circuit. Turning the knob back and forth around that spot, you can make it pop consistently. It kind of sounds like a cable is being disconnected in the speaker or something. I replaced the pot, but the new pot is doing the same thing.

This amp was originally in for me to remove a master volume mod. Originally the mod jumped the two channels (like you would with a patch cable on the front, but internally) and replaced one of the inputs with a potentiometer for MV. I replaced the pot with a jack and got everything back to stock, but now there is this popping sound in the speaker.

The customer says that he doesn't ever run the amp this way, but wants me to get rid of the popping and noise, if possible.

Model is 1959SLP
Board is labeled ST1 PCBP-00118-03

Let me know if there is anything else y'all need to help narrow it down. Thanks in advance!

[Jon Snell]

I would expect it is going unstable due to wiring position etc.
Where the X is, insert a 22k resistor to help with stability and parasitics.

[GoldtoneWorkshop]

Awesome! I'll try it out and report back!

[Helmholtz]

Also thinking instability.

Does the popping stop if you turn up the presence?

[GoldtoneWorkshop]

Alrighty. Got back to the amp this evening after some other projects and installed the 22k resistor, but the amp is still popping when you get to the end of the volume pot's range.

I also tried turning down the presence, and the popping is still present.

[Helmholtz]

I also tried turning down the presence, and the popping is still present.

Question was about turning presence fully up (to 10).

[GoldtoneWorkshop]

Sorry, I misread your original post. Same issue with the presence maxed out as well.

[Helmholtz]

Did you replace the normal channel vol pot?

Not clear if the popping occurs with or without anything connected to the input and if it only pops when you turn the pot.

You might have some bad (ground?) connection somewhere.

Try if pulling the input tube makes a difference.

[GoldtoneWorkshop]

Did you replace the normal channel vol pot?

Not clear if the popping occurs with or without anything connected to the input and if it only pops when you turn the pot.

You might have some bad (ground?) connection somewhere.

Try if pulling the input tube makes a difference.

Normal channel pot has been replaced, and still makes the same popping sound. This is with nothing plugged into any of the inputs.

Pulling the input tube eliminates the popping sound. Replacing V1 with a new tube brings the popping back.

I'll check through the amp to see if I have a bad ground somewhere.

Thanks for following up!

[Greg_L]

Could be an oscillation. Check the wire routing. You might need to shield the run to the grids.

[Chuck H]

I've seen a few 1959's that were mildly unstable. That is, the amps were basically functional unless the treble and volume were full up, or some such. Lead dress can be important in these amps (as already noted). I couldn't find any info on this that I would consider comprehensive enough to link. Basically all leads should be as short as is practical (so any that have been replaced should be examined for excess length). Purple "presence" lead should be routed out and away from the preamp. In the preamp, cathode and plate leads should be on the chassis while grid leads are raised up. The one exception would be the input grid lead on the bright channel. Some later Marshall models had the input grid and plate leads parallel against the chassis and tacked in place with a glob of ??? (hot glue? silicone?) This probably creates about one or two pF of extra capacitance between the grid and plate. It might be worth a try and it's a stock Marshall implementation. Check ground scheme against a known stock amp just to be sure it's correct.

[GoldtoneWorkshop]

Could be an oscillation. Check the wire routing. You might need to shield the run to the grids.

Great idea! I swapped out the shielded cable that was going from the normal input jacks to the tube grid and replaced it with a different shielded cable. The popping noise is gone now. Maybe there was a break in the old wire somewhere?

[GoldtoneWorkshop]

I've seen a few 1959's that were mildly unstable. That is, the amps were basically functional unless the treble and volume were full up, or some such. Lead dress can be important in these amps (as already noted). I couldn't find any info on this that I would consider comprehensive enough to link. Basically all leads should be as short as is practical (so any that have been replaced should be examined for excess length). Purple "presence" lead should be routed out and away from the preamp. In the preamp, cathode and plate leads should be on the chassis while grid leads are raised up. The one exception would be the input grid lead on the bright channel. Some later Marshall models had the input grid and plate leads parallel against the chassis and tacked in place with a glob of ??? (hot glue? silicone?) This probably creates about one or two pF of extra capacitance between the grid and plate. It might be worth a try and it's a stock Marshall implementation. Check ground scheme against a known stock amp just to be sure it's correct.

The grounding scheme definitely doesn't appear stock, as there are a few ground lugs that look previously used that aren't in circuit any more. The bright channel definitely has a lot of hiss.

Purple wire is bound with a bunch of cables routed under the rectifier section, so many that wasn't touched.

I'm trying to unravel what's been done to this thing in the past, because the caps in the tone stack were replaced with Mojotone vitamin T caps at some point too.

[Greg_L]

Great idea! I swapped out the shielded cable that was going from the normal input jacks to the tube grid and replaced it with a different shielded cable. The popping noise is gone now. Maybe there was a break in the old wire somewhere?

Awesome. Glad that worked. Yeah who knows, maybe that old shielded wire was faulty and picking up some crap from the cosmos or something. In my experience shielded wire is great when it's good and implemented properly. It's really bad though when something goes wrong with it.

[Chuck H]

Or it could be that the new shielded cable just has a closer spacing between the shield and conductor. Making it more capacitive. Small shakes now as the problem is solved. Sort of...

I never saw a stock 1959 with a shielded lead for the inputs. But this IS a reissue with a different board and I can't say if Marshall did this or it was added. Regardless, like I mentioned before, the bright channel on these amps can be a marginally stable circuit in my experience. Maybe there's some mojo there? As in when it's only just stable it's good and when it's more stable it's just ok.?. I don't know, food for thought though.

At any rate I'm glad the problem is solved. As for hiss...

Not much to be done for hiss with lead dress, grounding or lead length. Hiss is generally a byproduct of gain and "Johnson noise". About the best you can do is for this is to reduce series resistance wherever possible. In this case that would mean altering the amp from stock. But since the stock 1959 circuit doesn't really have any gross series resistance I probably wouldn't chase that method. Tubes can be noisy sometimes. You could try different tubes in the V1 position and choose the best one that balances gain and hiss most acceptably (or least offensively as it were).

EDIT: Carbon composition plate load resistors are notorious for hiss. They have a different construction that is a prone to very tiny arcs in higher voltage circuits that can sound like crackling, but sometimes hiss. If there are carbon comp resistors in the plate load circuits you could change them to metal film and it MAY make an improvement. Maybe not.