JCM 900 questions and issues

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iownrocknroll

New member
Within the past 8 months I had my JCM 900 heavely serviced. New Power Tubes JJ Tesla 6l6, New Pre amps, Groove Tube Gold 12ax7 in V1 and V2 and Groove Tube gold 12au7 in V3

Also had the filter caps replaced with JJ Tesla 50/50 500v

I also had all the pots cleaned, and a few items in the pre-amp replaced. a few resisters and caps

The amp is degrading in sound quality. It’s not warm or full sounding. It’s not very loud. It has no top or bottom.
I would say it is as loud on 10 when run through a 4x12 as my 15 watt Vox AC15 running through it’s 1x12 speaker.

Any thoughts?
 
well, off the top of my head I'd say that whoever replaced "a few items" in the preamp didn't do a good job or made poor choices but it's hard to say without knowing exactly what was done.

It's also possible that there is a bad tube or two in the ones you put in.

Even new good quality tubes can have a bad one.

I think probably a bit more info about what was done in the pre is in order.
 
If it worked fine after the service, and then went downhill over time, the first thing I would suspect is a tube. I have had a lot of tubes that were bad in one way or another out of the box, including JJ/Tesla brand, and some that worked fine for a short while and then went bad.... Of course tubes go bad in so many ways, more often than not, I'd say, when they go "bad", they are semi-functional.

I would put yer old tubes back in and try it - one at a time on the preamp tubes, and the power tubes as a set. If that does not fix it, or isolate the problem, the next step would be to take it to a tech, unless you are qualified.
 
MajorTom said:
If it worked fine after the service, and then went downhill over time, the first thing I would suspect is a tube. I have had a lot of tubes that were bad in one way or another out of the box, including JJ/Tesla brand, and some that worked fine for a short while and then went bad.... Of course tubes go bad in so many ways, more often than not, I'd say, when they go "bad", they are semi-functional.

I would put yer old tubes back in and try it - one at a time on the preamp tubes, and the power tubes as a set. If that does not fix it, or isolate the problem, the next step would be to take it to a tech, unless you are qualified.
Click to expand...
good advice!
 
You certainly don't "Own Rock & Roll" playing through that amp! :p

Seriously, that amp sounds like it needs an overhaul. I personally don't think it's worth it. Here's why.

An older amp gets worn out. Lots of parts go out of tolerance and things might need so much work that it's not worth the $$ you have to shell out.
Now, If we are talking about the OLD Marshalls (point-to-point/hand wired) it might be worth it.

I have owned a WORKING JCM 900 50w head many years ago. (1988)
It's not the most powerful 50 watts I ever heard and it got buried on stage so I sold it.
Had a great tone for recording though...sweet singing sustain, too.
But I've played through several amps that rival it's tone.
True, if you absolutely MUST HAVE that 80s Marshall sound...there is no substitute.
However, consider this:
I bought a Polytone Fusion 1-12" 100/50w combo in 1986. Great little tube amp. (paid $595 for it)
After 5 years, it broke down. I spent over $600 trying to return it to service.
It's still broke.
They stopped production after a few years and never reissued it. (kinda rare)
And I still hold on to it just in case (of what I don't know)
 
Depending on who your tech is, he should be able to run a battery of tests on the amplifier, and have the problem licked in no time. However, there are many techs who fly by the seat of their pants, and their test equipment consists solely of a Fluke DVM. Replacing the filter caps in an amplifier not all that old, or changing pots (unless the original pots were actually 'broken') are a first sign to me that a tech learned all his smarts from a Gerald Weber book. But, ask anyway. If he can;
  1. Hook up the amplifier to a suitable dummy load.
  2. Hook up a signal generator to the amplifier input.
  3. Hook up an oscilloscope across the dummy load.
  4. Set the controls for the biggest clean sine wave possible, and measure that signal across the dummy load.
  5. Calculate the output power. If you actually have 50-watts or 100-watts, all of this degrading sound quality is in your head, and not the Marshall head.
You might first test all the tubes, and compare the idle Plate current through all the 6L6's. Another angle; most tube manuals tell you that you need approximately 72VAC Grid-to-Grid to get full output. Your tech could also check the output of the phase inverter.
As you can see, this is a lot of test gear, and a lot of testing. But it separates the men from the boys, so to speak.
 
I am good with an iron and I know electronics but I am not too good with amps.

I have re-tubed and biased a few however I am don't feel confident trouble shooting this guy.

In the past I have used this amp live. Though lately I've been using a 4x10 deville hooked up to a 4x12 cab.

However I want to keep the marshall working in the studio. So this week it'll hit the shop.

I've never had a decent set of tubes act strange. I had electro harmonix and tungsol do weird stuff. But I've never had issues with JJ Tesla, Groove Tube, or RCA. Perhaps I'm overdo.

The amp is around 20 years old. Perhaps an input/output transformer or a bad diode that is starting to fail?
 
i question the use of the au7 in V3... most marshalls just use 12 ax7 for all positions... even the driver/phase splitter... where some use at7's there (also called a function tube)... dont recall ever seeing a au7 there in any amp...
 
I tried all three ax7 at7 au7 same results

I've always used au7 in my phase invertors in all my amps
I've generally noticed cleaner clean less break up at high volumes
shinnier highs
 
which version is this??? mod number?? i'll check to see specificly what it's supposed to be... the only thoing i use au's for is mic pre's and hi-fi... they tend to be clean but lacking in gain...most amps other than marshalls use at's for phase/drive... their use of ax's helps with their overall high gain/distortion characteristics....IMO... ymmv..
 
4100 uses 3 12ax7's

ax's have a amplification factor of 100
at's 60... au's 20

so it's easy to see why it lost alotta it's power...
 
as I stated earlier it does it with all 3 models of AX tubes. actually because I tried and ay too.


also this should only be for input gain. my 15 watt vox that runs a 12ax7 and a 12ay7 should not be louder regardless of the coifencient of 3db+ in output volume per watts doubled
 
Just for giggles, take an instrument cable and hook the effects loop output to the effects loop return. Sometimes the switching jack in the loop gets corroded and all sorts of funny problems happen.
 
Farview said:
Just for giggles, take an instrument cable and hook the effects loop output to the effects loop return. Sometimes the switching jack in the loop gets corroded and all sorts of funny problems happen.
Click to expand...

True - I have a JCM900 4100 Dual Reverb and I have had it for 20 years. I've had a few things done to it but most of the time it starts losing volume and getting static-y it needs a tube or two or three.... Last month it was giving off static - I had a power tube go bad. Same a couple years ago. New Years 1999/2000 I had it go down - needed tubes....I think I just replaced them all to be sure (like headlights).

It seems it burns through them every couple years....

This is from a thread I posted a couple years ago when the tech did a bunch of stuff in addition to a tube (pretty cheap too)
Repair Details:
Replaced second preamp tube.
Replaced low voltage supply zener diodes and master volume switching opamp.
Replaced factory installed (in 1990) 56K bias resistor with 47K bias resistor as per 1994 schematic to lower increase the negative bias voltage from -50V to -38V to properly bias the EL34 tube configuration.
Resoldered misc components and organized wire layout to help with noise further.
Rtv’d misc components.
Test ok

Mesa Boogie 12AX7 preamp tube 13.95
YSL Marshal Part Valvestate IC M5201 5.95
YSL Part 1N5245B 15VO OW5 Z (2) .50
YSL Part 1/2W 47K5 T&R RES .25
Repair Labor (60) 65.40
Subtotal 86.05
GST 5.16
Total 91.21 (CDN)

I've also had the effects loop jack repaired as mentioned by Farview.
 
If you can't measure anything, you're just chasing your tail. I have seen more than one amplifier seem to not have any guts, and it turned out to be the bias set way too high. But if you can't even measure that, it's hit-and-miss. You have to measure everything that can't outrun you. You'll get it in a minute. The only thing I wouldn't realistic expect you to be able to do is test the low-voltage power supply. You'd need a schematic, and have to know were you were poking around.
It looks like the zener was replaced once; maybe it's down again? Nah, that would affect the IC's and relays. You'd have the default channel working, and sounding good. It looks like a tube problem, but which tube? Likely an output tube. If you can test the output tubes, and check the idle current, you'd know. Compare Plate and Screen voltages. Maybe soldering around the tube socket? Measuring everything would eliminate a lot of guess work.
 
100 watt amp? 4 6L6 power tubes! pull two. Either the outside two or the inside two. DON"T PULL ONE OUSIDE AND ONE INSIDE! If the amp sounds good with either set you now have narrowed the power tube problem to one of two tubes.
 
iownrocknroll said:
The amp is degrading in sound quality. It’s not warm or full sounding. It’s not very loud. It has no top or bottom.
I would say it is as loud on 10 when run through a 4x12 as my 15 watt Vox AC15 running through it’s 1x12 speaker.
Click to expand...

That REALLY sounds like worn power tubes to me. I know you last replaced them 8 months ago, but how hot is the amp biased and how many hours a week do you on average play it?
 
Wait! Shouldn't a Marshall be running EL34's? I know some of them had 6550's, but most ran EL34 power tubes.
 
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