JCM 800 6550's really "stiff"

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Kapo_Polenton

Kapo_Polenton

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Comparing this to my Randall Lynchbox that runs EL34's, I don't know what it is about this 800 but it really fights me. Even with a pedal I find it not very yielding. Is this a 6550 vs. EL34 feel thing? I tried a couple non evasive mods tonight:

2. Locate the 10K cathode resistor (R6 found on V1B) and replace it with a 4.7K resistor. This will beef up the gain. This seems to be the "sweet-spot" on these amps. If you want insane gain use a 1.5K.

and

3. Locate the 820ohm cathode resistor (R9 found on V2A) and place a .68uf cap across it. This will increase the gain a little. But mostly it gives you the nice "Plexi" midrange boost.

Not too convinced with #2 and #3 I like the voicing of but am thinking the last thing I need is any more mid range cut when the amp is not very yielding. I'll prob just stick with stock so as not to butcher this thing but as is, I am not really in love with it. So here are a few questions, 6550's cost way more than EL34's. Is it worth getting a new pair of these or should I just make the required changes to the amp so I can run KT77's and EL34's? Or do I just have one of the amps that has iron which lends itself to this particular sound?
 
Good point.. I suppose this might have been biased cold to preserve the life of the output tubes.
 
Well 6550s have much more clean headroom than EL34s. Sounds exactly what you would expect at low volume levels, stiff. Crank it and its another matter entirely. 6550s sound sweet breaking up so do your ear drums :lol: :LOL: .
 
Keep in mind that EL34's are a much softer sounding tube compared to a 6550. Also a EL34 is a pentode which is very sensitive to input voltage which means you will have more touch sensitivity within the amp as a whole, and that is what you are missing with 6550's. In fact when the US distributor switched to 6550s in the late 70's early 80's, guys hated that change, and of course they were biased pretty cold to compound things.
Click to expand...

I can def see that.. it really comes down to a feel thing more than just listening to clips on the net as well.. I 've listened to a few comparison clips and the difference is not as great as one might imagine but it is totally in the feel and how it reacts under the fingers that makes the diff. It is going to be a pain in the ass because I have to disconnect a lot of the leads to get at the underside of the circuit board but I think I am def. going to make the switch and have this bad boy run EL34's. Just too brittle and spikey for my tastes.
 
Try 6550's in your Randall....see how much difference you notice. Also you should try KT88's instead of 6550's....
 
Just a thought.. any of you guys wire up a PPIMV using the low input of a 2204? I'm trying to avoid drilling holes anywhere and now I'm flirting with incorporating some of the mod5 changes I was reading about and opening up the power section more to get the jam or sag I'm looking for.
 
Recently I hear a lot of guys running 6550 in their 800's and jmp's again(maybe due to slash?) I never liked 6550 in those amps and almost everyone back in the day switched them to el34. after all, only USA market Marshall's had 6550 due to shipping durability and availability, uk still had el34. 6550 are too clean and stiff in those amps which many are already short on gain so why sacrifice even more with 6550. I did the mod with the 10k to a 2.7k for more gain and changed the 6550 to 6ca7 and the amp totally transformed into a much more playable amp.
 
I also added a .47 uf bypass cap over the 4.7 k resistor after looking back on my notes and checking out the Caswell #34. He used a .1 uf but i am going for a little less high end. Hopefully I can get home later on today and really give it a good spin... Did you make the required resistor changes to run 6ca7 tubes or piggy back resistors to bring those 220k down in value?

I know Zakk was huge on the 6550's... Slash too you are correct. not sure if this contributes to the craze but I def find for gobs of gain, these tubes are probably great, I just don't find them very easy to play. I guess what i notice is no sag or compression.
 
Not sure if it just the 800. I have a 2004 SLP RI running GE 6550. I can run any tubes in it and these are the best for me. It is modded allot though by one of the greats. Killer amp.
 
stiff? YES! :yes: that's why i NEVER liked any Marshall 100w or 50w JMP's that i've owned over the years,which came with 6550's.....ditch 'em & BE HAPPY!!!



...EL34=CLASSIC MARSHALL TONE :rock:
 
Kapo_Polenton":3crkhxhb said:
Comparing this to my Randall Lynchbox that runs EL34's, I don't know what it is about this 800 but it really fights me. Even with a pedal I find it not very yielding. Is this a 6550 vs. EL34 feel thing? I tried a couple non evasive mods tonight:
Click to expand...
I can only guess, but I don't think changing from 6550's to EL34's is going to completely change the feel of the amp.

How loud are you running the amp? I know the 800 is a master volume amp, but the difference between MV 4 and MV 8 is VERY significant in the feel of the amp.

I don't know anything about the Lynchbox, but I know the 800, although being a master volume amp, is still not a unit designed for high preamp gain.
 
I don't run it that high up.. last time i used an attenuator the tone flubbed out which leads me to beleive I've also got tube isues here. Damn 6550's cost quite a bit though and that is why i was flirting with going to EL34's.. I have fresh spares on hand and some KT77's i am intersted in running again.
 
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