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.

[danman]

With the inconsistent quality of power tubes these days, and the fact that EL34's will draw considerable screen current when the amp is pushed hard, it is wise to install the screen resistors. The EL34's that you are currently running (Seimens EL34's were usually manufactured by RFT) are pretty stout tubes compared to the offerings these days. That is most likely why they have never given you any trouble. If you plan to use new tubes in the amp, and you like to play loud, it would be good insurance to install the 1k/5 watt screen resistors.

The 5.6k swamp resistors are used to squelch oscillation and make the amp more stable. If the amp has good lead dress and the circuit is relatively quiet, you may not need to install them. It is possible though, that you may run across certain EL34's that will give you problems and others that will work just fine without the swamp resistors. Somewhere in the late 70's to early 80's the quality of power tubes started to decline and that is most likely the reason why Marshall started using them in all of the circuits. Nowadays it is just good practice to install them both and any affect on tone should be small.

[bahady30]

Thank you for clearing this up for me. It''s a little bit strange to me that I'll have to modified the amplifier because the modern tubes are not that good anymore. What are the best brands you can buy nowadays without spending a fortune? I think I'll start off with some JJ El34 and see what happens , install the screen grid resistors and see what happens. When I need the swamp resistors I'll put them in too. Can somebody provide me a decent 2104 schematic to work with? I think that I have to solder the 5k6 resistors to pin 5 of the El34 and the 1K 5watt resistors between 4 and 6 . Correct me if I'm wrong.

[neikeel]

Thank you for clearing this up for me. It''s a little bit strange to me that I'll have to modified the amplifier because the modern tubes are not that good anymore. What are the best brands you can buy nowadays without spending a fortune? I think I'll start off with some JJ El34 and see what happens , install the screen grid resistors and see what happens. When I need the swamp resistors I'll put them in too. Can somebody provide me a decent 2104 schematic to work with? I think that I have to solder the 5k6 resistors to pin 5 of the El34 and the 1K 5watt resistors between 4 and 6 . Correct me if I'm wrong.

I'd try JJ EL34s too (the new ones might be worth a look too).

I would just put the screen resistors on first and try without grid stoppers if you can (they always attenuate a little signal)

[danman]

When adding the screen resistors, simply move the wire from pin 4 to pin 6 and then solder your 1k resistors across pins 4 and 6. Pin 6 is a free pin (no internal connection in the tube) so it makes for a handy tie in point for the resistor. Be sure to elevate the resistor above each socket a bit to allow for better cooling. They can get pretty warm when the amp is played hard.

FWIW, you can still find NOS power tubes from reputable dealers and the RFT EL34's are slightly lower in price than some of the other brands. Tesla also made a great EL34 back in the 60's and 70's that can be had for a little extra.

[bahady30]

Thanks for helping me out on this! Today I changed the position of the exsisting output tubes because there is a some popping going on when I put it on . One output tube is bad . When I change their places, the bad one is lightning up sometimes. so I ordered two JJ El34 . I also ordered a Tung Sol ECC83 specialy made for V1 in the pre amp section because the old Siemens don't sound right anymore to my ears. When I get the tubes I solder the screen grid resistors in and bias the amp properly. The amp is humming when I play on the high input so I've to look at the inputs to and clean them properly. Hopefully this and the new ECC83 solve the problem. I ordered a DIY bias probe because I like safety. This is very educational forum so I read a lot here and wanna learn from you. I let you know how it worked out. Sorry for my bad English.

[danman]

A bad power tube can also cause the amp to hum so hopefully with the new tubes that problem will disappear. Filter caps can also cause this but I wouldn't necessarily replace them unless you are sure they are bad. With the 2203 and 2204 circuits, the hi input passes it's signal through the low input because the preamp is cascaded. The switching contacts on the input jacks can occasionally go bad also and require cleaning or replacing to fix the problem. Let us know how it turns out.

[bahady30]

So I'm back with a update. I soldered the 1K 5watt grid resistors in place between pin 4 & 6 . The yellow wire that was originally on pin 4 is moved to pin 6. I've placed the new JJ El34 en a new tung stol ecc83 in v1. Bias the amp at 38mA . Plate voltage is 380V . The amp is brighter with the JJ El34, almost too bright for my taste. The popping is gone but there is a constant hum in the amp. With the pre amp pot to zero and the master volume at 10 with the guitar plugged I hear the basic hum coming through the speaker. When I crank the pre amp the hum pattern becomes more pronounced en eventually it goes squealing... First I,ve ordered new filter caps and I'm gonna replace the old ones which are 35 years old. I think there is something wrong in the first pre amp section....what do you think? Maybe a bad wiring ? I want to mention that this is more pronounced on the high input.

[neikeel]

Yes if the hum is there all the time then the caps maybe due for replacement. A poor preamp ground can also cause the symptoms you describe, does the hum go when you touch the input jack with your guitar plugged in? I presume you have LNE wiring on the amp and in your house (sorry don't know where you live)?
Oh and don't forget the 10uF 150v (or 200v) bias caps whilst you are at it!

[bahady30]

I rent an apartment and the living room is LN and has no earth. In the kitchen there is LNE so I tried the amp in the kitchen last night and the hum was gone.The amp is LNE wired. I also cleaned the low input contacts because it's indeed a cascaded pre amp . Oke there is some white noise when the pre amp is cranked but I think this is normal on high gain settings. I ordered some big 50uF+50uF 500V F&T caps because the old ones are 36 years old Daly capacitors. The high input is a very sensitive one. I saw that there is not much shielding going on in the pre amp stage and that surprises me... Did the sound improve after the pre and power tube changes? I have been replacing my old output tubes with JJ EL34 and I don't like them. To bright for my taste but i'll give them a chance....The vintage vibe is gone...It's a good all round sound but nothing special. I set my bias at 60% dissipation as a starting point. I have to tweak it a little and see what suits me the best. What modern output tubes are you guys/girls using in your Marshall?

[neikeel]

I rent an apartment and the living room is LN and has no earth. In the kitchen there is LNE so I tried the amp in the kitchen last night and the hum was gone.The amp is LNE wired. I also cleaned the low input contacts because it's indeed a cascaded pre amp . Oke there is some white noise when the pre amp is cranked but I think this is normal on high gain settings. I ordered some big 50uF+50uF 500V F&T caps because the old ones are 36 years old Daly capacitors. The high input is a very sensitive one. I saw that there is not much shielding going on in the pre amp stage and that surprises me... Did the sound improve after the pre and power tube changes? I have been replacing my old output tubes with JJ EL34 and I don't like them. To bright for my taste but i'll give them a chance....The vintage vibe is gone...It's a good all round sound but nothing special. I set my bias at 60% dissipation as a starting point. I have to tweak it a little and see what suits me the best. What modern output tubes are you guys/girls using in your Marshall?

If the JJs made the amp bright you may hate putting in new filter caps! They do change the tone, making the amp a bit crisper and a little brighter. You may need to calm the preamp with an old but new testing Mullard ECC83 in V1.

[bahady30]

Thanks for the answer. I've to admit that I don't have much experience in different brands of tubes and their characteristics. I do know that the first pre amp tube ,V1, is the most important one tone wise. Maybe it's a good thing to search for a NOS pre amp tube but over here in Holland I don't know a supplier who sells Mullard NOS tubes....

[neikeel]

Thanks for the answer. I've to admit that I don't have much experience in different brands of tubes and their characteristics. I do know that the first pre amp tube ,V1, is the most important one tone wise. Maybe it's a good thing to search for a NOS pre amp tube but over here in Holland I don't know a supplier who sells Mullard NOS tubes....

Holland is geat for Phillips Miniwatt or Mullards, you are very lucky - get on e-bay, find some with test results and a seller that will offer a return if no good and get x2 or x3 for the preamp. Same goes for EL34s, get good testing pulls or Siemens (RFT ) with dimples on top - best NOS EL34 for value at the moment.