Screen grid resistor and grid stoppers missing

[bahady30]

Hello there,

I'm a newbie at this forum and I hope I put this question in the right categorie.I have a Marshall combo JMP 2104 from 1980 . It always had a killer tone but recently a power tube (El34) has gone bad I think...I'm a technician so I opened the amp , drained the filter capacitors , searched for a decent schematic, cleaned the amp because it was a mess with a lot of dust. Cleaned the tube sockets and pins with contact cleaner. One thing I noticed that it was missing the screen grid resistors ( 1K, 5 watt) . I use the unicord incorporated schematic of a jmp 2204 50w master model. It should be the same but the 2104 is the combo version. The amplifier has yugaslavia Siemens El34 and ECC83 tubes in it. Is it nessecary to put in the screen grid resistors in conjunction with modern mass production tubes? What about the 5K6 grid stoppers which I see in a lot of schematics from the eighties? They are not in my amp.....Will this mods will increase stability if power tubes fail? Will it alter the tone? Hope I get some answers because everybody is saying different things and that confuses me. Thanks for the help.

[?Mark]

Strange, no 1k? you sure about that?
The schematic I looked at shows um.

http://www.drtube.com/schematics/marshall/2204u.gif

And yes 5.6k grid stoppers are ALWAYS
a good idea. link to explanation.

http://www.aikenamps.com/index.php/grid ... -they-used

[bahady30]

No the amp didn't come with screen grid resistors. I sent Marshall an email about that and they confirmed that they didn't include them on a short period of time in their 50W marshals . They were not very consistent in following the schematic. the grid stoppers were also not included. The tubes were better those days they told me. It was not necessary in 50W amps those days.I soldered the 1k resistors in, new tubes, cleaned the amp, installed new filter caps and the amp is more quiet now. I gigged with the amp last week. She's freakin loud but has the best overdrive I heard from a Marshall combo,and has some great clean tones too from a Marshall . She's a little to bright for my taste, I can't raise the treble above 1 but I read that's the nature of the JMP. I'm gonna experiment with another tube in V1. Now that's an Tong Sul ecc83 especially for V1. I received some old Philips Miniwatts but still in good shape.Maybe they are a little less brighter. I don't want to mod her because the amp has a sentimental value for me. I got this amp from a friend who sadly passed away...Tips are always welcome.

[bahady30]

The schematic you showed me is not the right schematic of my amp. My JMP is from Feb/1980. The schematic you showed me is an earlier JMP. The preamp section is a little different and the output section section also. Mine is more a 2204 schematic with a few different values of some components and mine has a cascaded pre amp section.

[danman]

There is a bright cap and a treble peaker that could be unsoldered (simply lift one leg to make for easy reinstallation later) to help tame some of the brightness. You may also want to get yourself a few RFT ECC83's to try out. They really help to tame a bit of the higher frequencies.