Help diagnosing hiss/noise on '77 JMP 2100 combo?

[drasp]

I've had the good luck of finding a very solid/original '77 JMP 2100 Lead & Bass combo. Absolutely lovely amp. I've already replaced the original electrolytics on the PCB & all 3x of the Daly 50/50s hoping that would drop some pretty obnoxious noise/hiss on the normal/I channel at low volumes. On the bass/II channel, I can open things up quite a ways & the amp idles nice & quiet. Bring in the normal channel & its instantly quite noisy. I've tried swapping in known good preamp tubes (currently has what I assume are the original RFTs + GE 6550s). Plates are ~395V, biased at ~50% (40mA) as suggested here & other places. The amp sounds just fantastic at low & high volumes - if it weren't for the high noise floor, I wouldn't touch a thing!

SO, can anyone suggest where I might start? Any parts that would cause noise only to show up on the lead channel, or it is more likely an EQ issue where its simply more noticeable there? Also, is V1 the only really critical tube here w/ V2 being the cathode follower, or is there a gain stage in V2 as well? Noise seems to change a bit as I saw through other tubes, but not much - I really don't think its simply preamp tube noise. Also, I've been through the amp pretty obsessively w/ DeOxit red & pots/jacks/etc. see clean. Lead dress seems pretty good already, but I tried to tidy up anything that looked messy. OH! And though the whole amp is full of mustard caps, the 680n/160v cap on the presence pot had been changed to a red chicklet cap - only part I could see that was obviously changed. . . here are a few circuit pix for anyone who is curious!!! (before I recapped, used some NOS LCRs from '99 for the 50/50s & Vishay BC 118s for the caps on the PCB)

[img:800:437]http://delaraspelle.com/misc_pix/77_mar ... 2100-c.JPG[/img]

[img:800:600]http://delaraspelle.com/misc_pix/77_mar ... 2100-d.JPG[/img]

[img:800:600]http://delaraspelle.com/misc_pix/77_mar ... 2100-e.JPG[/img]

[mhartman]

Try subbing some different 12ax7 tubes in V2. That CF is very sensitive to tube types and will be noisy if it doesn't like your tube.

[Jana]

Nice score!

These amps do have some hiss but it shouldn't be overwhelming.

These amps are very sensitive with respect to the grid wire for V2. The only "mods" I would do to this amp is to shorten the green grid wire a little bit, making sure it is up off the chassis. The yellow cathode wire(s) should be shortened and rerouted a tiny bit so they are not laying across the green grid wire.

Do the same with V1. Marshall always had more wire than needed for the grids on these two tubes.

Short and direct with grids off of the chassis is what you want.

[drasp]

Awesome, THANKS!!!

Will play with the lead dress a bit more & see if that helps. Short leads there do make it a PITA to get to the solder side of the PCB. . .

Any recommendations for an AX7 that would do better in V2? RFT is a pretty tough tube, right? Also tried a Mullard and a JAN Philips, could try a few other things.

[Stevem]

replace the resistors off of the input jacks (likely 68K) and all the preamp and plate load resistors with metal film types, the two plate load resistors really need to be bumped up to 1 waters.
It also will help to replace all the power supply node dropping resistors with metal film types and upping them by 1 watt from what they are now.

Lead dress wise on all the preamp tube sockets you want to pull up all the wires to the socket and get them to sit up above the heater filament wires.
Do this with neddle nose plyers and the amp on thru a speaker and you will hear the 60 hz hum level drop.
Note that with the amp out of the cabinet and off of the metal plate the chassis sits on the 60 hz hum level will be higher by a tad until you bolt it all back down.
while you are in there make sure that impedance slelector goes into the reciver nice and tight as that connection point is the souse of a ton of blown OTs in Marshall amps!

[drasp]

Thanks again, all! Dug in a bit this afternoon. Got all the non-heater leads up & away from the heaters, no massive improvement, but perhaps a tiny bit. The noise is definitely hiss, not 60hz hum - like a gentle "swisssssssssh" - only audible when the volume on the normal/lead channel is up, much louder when the treble & presence knobs are up. Easy to play over, but when the amp is just idling its pretty obnoxious - my other amps (all Fenders) idle dead quiet in the same room/same outlet/etc. Perhaps just a Marshall thing? This is my first Marshall, only UK-ish amp I've owned is a HIWATT. . .it also idled dead quiet. Hmm. . .really nessary to replace a bunch of resistors? I've certaily replaced resistors in my Fenders (is it the 1k/1W parts that are arranged in a triangle & prone to getting noisey?), maybe that's the next step?

[Jana]

The next step is to play it. It sounds normal for those amps. A handpicked tube for V1 will help a little bit but other than that, it is the nature of the beast.

[drasp]

The next step is to play it. It sounds normal for those amps. A handpicked tube for V1 will help a little bit but other than that, it is the nature of the beast.

Beautiful!!! I can totally live with that, LOL.

[danman]

My 50watter has quite a bit of hiss to it also when the volume is up. One thing I did notice is that the hiss is much louder when using the stock value 4700pf bright cap on the volume pot. I lowered mine down to 500pf and it helped to quiet the noise considerably and gave a bit more range to the volume pot before breakup starts.