JCM800s Questions

TheAmpNerd

Member
Messages
1,056
I'm curious y'all...

As to why the checkered history of these amps?

Ones that I've encountered typically have three
types of caps...

Some with the following types of signal caps:
  • Vaunted yellow mustard caps
  • Medium yellow box caps
  • Crappy green thin caps

These amps are consided one of the "icons" of tone.

Most all I've encountered have:
  • Hum when cranked.
  • The master volume tone problem (changing EQ when quiet to loud)
  • Changed PTs, OTs, Sockets and other stuff changed replaced.

Wondering what is best for reliablilty in these amps. I know it is
good to hard wire the output selector.

In Retrobob's case he's pulled the faulted power tranny, so what else is
good while he's in there?

Does replacing the diodes with better and with suppressor caps help?
Along with the bias stuff?

I'm sure new filters all around are in order...

is it worth while to replace
the signal caps while he's got the board up?

As Mike suggested replacing the stuff around the power tubes....but
there isn't much there...the screen resistors almost never go out and
the grid stoppers are mounted differently, plus when the amp is in
service they are below the heat of the tubes...I'm not saying they
don't get hot in there.

So as you can tell retrobob's thread got me thinking.
 

John Phillips

Member
Messages
13,040
Are you sure you mean JCM800s and not the earlier JMPs?

The 800s are some of the most reliable amps I know of, possibly the absolute most reliable considering the numbers made.

I'm not saying it can't happen, but I've never seen one with a blown transformer. Apart from tube failures and the odd bit of outright physical damage (eg smashed pots) - which is actually quite hard to do, they're pretty tough - I don't think I've ever had to fix more than half a dozen or so 800s for actual 'failure' in the last twenty years, out of two or three hundred older Marshalls. I have come across a few that have been hacked by other 'techs', so I have to assume they failed in the first place, but it's hard to tell why stuff was done sometimes...

The impedance selectors on the 800s are the modern rotary switch types, and although I've come across one that failed and several others that needed cleaning to be sure they wouldn't cause trouble, the real problems are with the pull-out type used until nearly the end of the JMPs (the even earlier 'bean' type used on the Plexis aren't quite so bad, it's the ones with the flat disk and a little square window that are the worst ones - these are total crap, and should never have been used in the first place IMO). I honestly think they're responsible for a lot of the reputation older Marshalls have for 'weak' transformers. I always hardwire or replace these.

They certainly do change tone when the MV is turned up or down, I agree - it's just part of what they are. Some people find it a problem to get a good tone with it turned down, but I've never found them all that bad. I've also never noticed particularly excessive hum when they're turned right up though - they're not 'quiet' amps, but not what I would call noisy for such a powerful amp with a fair amount of gain and a very simple and 'raw' circuit.

I'm expecting to see more with failures as time goes by though... they're all getting to be 20 years old or more - I've started seeing quite a lot with leaking bias caps, more than main filters actually. There's also a problem with 'user bias adjustment' as is becoming more common - if you bias them up to 70% of the dissipation for an EL34 as has become the 'received wisdom', you'll blow tubes when you crank it. They shouldn't be set anywhere near that hot, they're really operating the EL34s way outside what they will take anyway (the 50s typically put out 70W and the 100s 130W - clean). You need to keep them down around 50%, and watch what happens to the screens when you crank it.

The most common actual failure I see on them is the power switch light not working :). (Fixable without replacing it usually too, it's normally just a wire come off inside.)
 

Swarty

Member
Messages
1,130
When I open a JCM800 I expect the vast majority of signal caps to be red box types with a few white ones.
 

tmac

Goldmember
Gold Supporting Member
Messages
2,235
It seams to me that the JCM800 master volume amps from the late 70's through the 80's were very well made like JP said. I just "modded" a special run "small box" JCM800 for a guy (an '86 right Kelly?) and it has the red box caps.

The only thing I did to it was remove the bight caps C4 and C5 and it now sounds fabulous and a lot quieter too. The amp becomes usable at sane volumes and the harsh gritchy tone (IMHO) is gone.
 

TheAmpNerd

Member
Messages
1,056
John,

Yes, you are correct I am thinking of the JMP 2203. Now aren't these
virtually identical to the JCM800 with the two verticle inputs?

Are the Vert JCM800s considered the most desirable?

Ah yes, the cheesey window plug-in connection. Perhaps
you are right this being the cause of most of the
older amp's and their tranny problems.
 

John Phillips

Member
Messages
13,040
Yes, you are correct I am thinking of the JMP 2203. Now aren't these virtually identical to the JCM800 with the two verticle inputs?
Circuit-wise, yes. I'm not sure the transformers are identical in internal spec though - I could be wrong, but even given the difference in impedance selectors, the 800s seem more reliable. I wonder if they changed the insulation type or something... it might also explain why I think these amps sound slightly better than the 800s.

Ah yes, the cheesey window plug-in connection. Perhaps
you are right this being the cause of most of the
older amp's and their tranny problems.
These things are a curse and should be shot on sight :). They were never good in the first place, and they stick out very slightly beyond the back edge of the amp, meaning they're almost certain to get bumped. Even if they don't fall out entirely, the contacts get stretched, and either get loose enough to not connect properly, or let corrosion get in. I never leave one operational in any amp intended as a 'working' piece - anything other than a strictly 100%-original collector's item, basically (and only then if I'm quite sure it's working properly). The easiest thing - since most users only run them with one cab - is to simply move the center wire to the correct tag for whatever impedance you want - the only loss of originality is the solder joints.
 

JamesHealey

Member
Messages
746
John I know running tubes at 70% dissaption isn't exactly great for Marshalls, I've got Splawn which is pretty much a modded 800 circuit is ridiculously close.. Is there any adverse effect to the amp other than blowing tubes left right and centre? coz I really like the tone of 70% diss. Im not too fused about blowing tubes aslong as the amps going to survive.
 

John Phillips

Member
Messages
13,040
Make sure you have wirewound screen resistors of at least 5W rating - the hard-glazed ceramic type, usually a dark green/grey cylinder in appearance are the most robust, and an HT fuse that you have checked the value of (T500mA in a 50W, T1A in a 100), and you'll probably be OK. If a tube blows it should take out the HT fuse without damaging anything else.

Be aware that even screen resistors that tough will probably start to cook if you thrash the amp continuously with that sort of bias setting though. Space them at least half an inch above the tube socket and wrap/crimp the resistor leads (and the supply wires) tightly through the socket eyelets to prevent any chance of them coming loose if they get hot enough to melt the solder.
 

TheAmpNerd

Member
Messages
1,056
Amazing.

Here we are discussing this and guess what walks in? You gessed it,
a JMP MKII, I kid you not. (This is almost like the time when I ended
with to a SuperTwin one week and then a SuperTwin the next week.

I put in a quad of Kt77s. Biased it up right about 14.5 watts per tube.
Let her settle in for a while then gave her a run though.

After cranking her for a good run this is what I measured
for each tube:

Tubes 4,5 Tubes 6,7
.035/15w .045/20w

B+ about 447, bais at -38v

I'm really surprised by tube 6,7 rising so much...I'm almost thinking
if you kept playing for a full set...you might run into thermal runaway.
 

Hacksaw

Time Warped
Platinum Supporting Member
Messages
10,217
Make sure you have wirewound screen resistors of at least 5W rating - the hard-glazed ceramic type, usually a dark green/grey cylinder in appearance are the most robust, and an HT fuse that you have checked the value of (T500mA in a 50W, T1A in a 100), and you'll probably be OK. If a tube blows it should take out the HT fuse without damaging anything else.

Be aware that even screen resistors that tough will probably start to cook if you thrash the amp continuously with that sort of bias setting though. Space them at least half an inch above the tube socket and wrap/crimp the resistor leads (and the supply wires) tightly through the socket eyelets to prevent any chance of them coming loose if they get hot enough to melt the solder.
John, The splawn has the 5 watt hard ceramics. they are spaced, not quite half inch but they do breath and they are wrapped. ;-) everything you mentioned, the Splawn does have. I've melted a couple sets of the reissue mullard in a Splawn. a visual inspection showed good health.

Thought you would like to know. + Sorry for the little thread derail..
Rik
 

Hacksaw

Time Warped
Platinum Supporting Member
Messages
10,217
Amazing.

Here we are discussing this and guess what walks in? You gessed it,
a JMP MKII, I kid you not. (This is almost like the time when I ended
with to a SuperTwin one week and then a SuperTwin the next week.

I put in a quad of Kt77s. Biased it up right about 14.5 watts per tube.
Let her settle in for a while then gave her a run though.

After cranking her for a good run this is what I measured
for each tube:

Tubes 4,5 Tubes 6,7
.035/15w .045/20w

B+ about 447, bais at -38v

I'm really surprised by tube 6,7 rising so much...I'm almost thinking
if you kept playing for a full set...you might run into thermal runaway.
karma?? lol... good to see ya back here ampnerd.
 

TheAmpNerd

Member
Messages
1,056
Interestingly the last two output tubes should not rise 5 watts
while playing...what gives?

That is what I was wondering when I noticed this...

Pulled for closer inspection:




I was thinking that is a whimpy splice if you ask me. Yes it was pulled from
tube six, pin three. So to keep investigating I pulled off the shink wrap...



Yep that's it. The "tech" that installed the new output tranny (2005) cut the leads to fit
and had them reversed. When ever he went to test the amp it squeeled.
So he put the leads proper then realized the brown lead was too short...
hacking on a part from the trash can using an inferior lap splice.

While everything tested out fine the splice must have generated a lot of heat...
causing the output tube to fry and take the tube socket out with it, tube 6/socket 6.

While I have the amp apart, I also replaced socket 4...as arching was
shown to be eating away at this tube socket.

I happen to have an exact match of Alpha wire and prceeded to do
a proper splice job...shown...



Finally here it is taped and ready for heat shrink before reinstalling it.

 

Hacksaw

Time Warped
Platinum Supporting Member
Messages
10,217
visuals inspection.. first thing to do.

nice and easy. :D
 




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