JCM 800 Bias? Tubes? & wattage question

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mr. torture

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I have an 1983 Marshall JCM800 50 watt head. I do not know how old the tubes are or if they need to be biased? Not sure what that does.
Should I take this in to a tech every once and a while to have it checked out?
How much maint. Is required on these heads?

Also on a side note, I have a 4x12 Marshall cab with the 75 watt celestions.
The amp cant really push the cab to get it thumping like other 100 watt heads I have owned. I am suspecting this is the cause of the weak sounding guitar tracks I have experienced. Will a 100 watt head make a difference in the overall power of the recorded material?

Sorry for all the questions, Thank you for your time.
 
Your existing tubes don't need to be biased. You would only need to bias if you replaced the output tubes with set that had different specs.

Is this a US head with 6550s, or European model with EL34s?

I have a JCM800 50 watt head as well which still has the original 6550s. They are still kicking. They are a very hardy tube. I have replaced the preamp tubes though.

How does the amp sound? Does it feel "flat" or "dead"? Does it hum or make crackling noises? If so it could be time to re-tube. Replacing preamp tubes is easy: just change 'em out. No biasing required. Changing the power tubes will likely require a bias adjustment if the amp still has the OEM power tubes. If it's got 6550s you should seriously consider having a tech switch it over to EL34s if you want a "classic" Marshall sound. The 6550s are better suited to extreme metal tones at high volumes, and clean stuff.

I run mine into a 1960 Vintage cab (30 watt speakers) and like the tone. I actually traded the original cab that came with this amp (75 watters) to a friend with a 100 watt JMP head since we preferred the sound of each others cabs.
 
I am assuming it has the original tubes, I will open it up today. It does seem to sound a bit flat. I will get some new pre-amp tubes for it today.
Do you get a bigger push through the lower wattage speakers?
Maybe that is what I am looking for. I was thinking of a 2x12 with the vintage 30's.
Have you ever tried a plexi reissue head? I am going to play through one today and I would like to get your thoughts on it.

Thanks for the info.
MR. T
 
The sound of your amp/speaker combination should sound like an old Scorpions album (think Blackout) The 75 watt speakers are grainier sounding and have a tighter low end than the vintage 30s. the vint 30s break up quicker and smooth out easier. Back in the day, we used to either mod the amp for more gain (there wasn't quite enough) or use a Tube Sceamer or Overdrive pedal in front to push the amp right.
What kind of sound are you going for?
 
If I could acheive a scorpions sound I would be happy. The problem i have is the power of the track I record with the 800. I double track and get 2 hollow not very dynamic guitar tracks. They sound dull and lifeless.
The room is well treated with rigid fiberglass according to Ethan's designs on John Sayer's sight. I use a 57 and 421 mics, Great river pre's.
I thought I would get a bigger sound considering the equipment being used.
That is why I started suspecting the amp as not pushing enough air around to capture a powerful sounding track. I tried a 100 watt JCM 800 today and it is friggin loud. It seems to push alot harder than my 50 watter. Also seemed to sound brighter, Although it could have been the room I was in at the music store.
I tried a boogie rectifier which was in my opinion quite bad, Too much low end and muddy.
I guess I should take a look at the tubes in my amp, maybe part of the dullness comes from worn out tubes as suggested by
M.Brane
Thanks for the help so far guy's
 
If they sound hollow together are you perhaps getting some comb filtering due to your mic placement?
 
boingoman said:
If they sound hollow together are you perhaps getting some comb filtering due to your mic placement?
Click to expand...

Very possible.

I've never played through a Plexi re-issue. I have played through some early 100 watters. They sound good, but are friggin' loud!

The Vintage 30s have more "thump", and a "rounder" sound than the 75s to my ears. I like 'em. My friend prefers the the tighter, brighter sound of the 75s.

The treble response on a JCM should have no problem killing small animals with the presence control up. If it's sounding dull it could very likely be a tube issue.
 
I pulled the tubes and looked them over. I am no electrical expert, but these tubes look cheap. The 12ax7's are PRC (peoples republic of china?)
and one of them look to be from a different era than the other 2.
I have one GE6650 which looks really old and a china made 6550 which has no brand name.
Could you suggest some good replacement tubes and where to purchase them?
I pulled (3) 12ax7's and (2) 6550's is this all of them?
Thanks from an obvious tube amp repair rookie.
 
mr. torture said:
I pulled the tubes and looked them over. I am no electrical expert, but these tubes look cheap. The 12ax7's are PRC (peoples republic of china?)
and one of them look to be from a different era than the other 2.
I have one GE6650 which looks really old and a china made 6550 which has no brand name.
Could you suggest some good replacement tubes and where to purchase them?
I pulled (3) 12ax7's and (2) 6550's is this all of them?
Thanks from an obvious tube amp repair rookie.
Click to expand...


That is your problem. The output tubes should be matched (within reason) That GE tube has to be 20+ years old and the chineese tube is most likly from the early '90s. Get a set of JJ 6550s These seem to be the closest to what the US tubes were. Try www.eurotubes.com They have good prices and a knowledgeable guy to answer any questions. Replace the 12ax7s as well, they are probably old as well.
PS you can't tell if a tube is good by looking at it (unless it's obviously broken) Some of the best NOS tubes look old and cheap.
 
I ordered all new tubes from eurotubes.com. Thanks for the link!
And all your help.
 
I bet those mongrel tubes are a big part of your tone issues.

Couple things to be aware of when you install your new tubes:


If the amp is dusty inside blow it out with some compressed air. The cans like you can get at electronics/computer shops will work fine. If you use an air compressor be gentle. Stubborn dirt can be loosened up with a small, soft paint brush. Of course make sure the amp is unplugged when you do this, don't touch any wires or any of the contacts on the sockets. The large caps in a tube amp can store enough electrical charge to knock you on your ass even when unplugged.

Keep a close watch on the power tubes for a while after powering up. If the amp hums a lot and/or the large grey/black metal plates inside the tubes start to glow red shut the amp down immediately, and take it to a tech. It needs a bias adjustment. If you run it that way it'll cook the tubes. If the amp sounds flat, and the distortion seems "out of tune" it also needs to be biased, but you won't hurt it by running it that way.
 
M.Brane said:
The large caps in a tube amp can store enough electrical charge to knock you on your ass even when unplugged.
Click to expand...

...or they could kill you. Read up on safety before going under the chassis, but changing the tubes alone w/o a rebias can be done without going inside the chassis. If it needs a rebias, read up on safety and DIY or take it to a tech.
 
Thanks for the tips, Since I have no desire of reconstructing Frankensteins
Laboratory I have decided, once upon receiving the tubes I will be taking the head down to my local tube amp tech. And have him do the whole job.
Thanks again for your help!
-Mr. Torture
 
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