steppenwolf

Marshall JTM45 Problem
« on: July 25, 2007, 11:04:46 AM »
Hi everybody!
I know this is not really a guitar amp forum, but I was still hoping that somebody has a hint for me!

I have a problem with my Marshall JTM45 amplifier, and I need help cause I simply don't find the solution.
Let me describe it first:

The JTM45 ist handwired, no PCB, new tubes, BIAS ok all voltages seem to be all right.

The Problem:
The Amp doesn't have headroom, even with lower power singlecoils, it distorts to early compared to other JTM45, moreover the distortion is somehow fuzzy, scratchy for lack of better words...
The components are all top notch, SOZO caps, Mercruy Magnetics Iron, swapped tubes, nothing changed...

First, I tried to put a 1kHz sine through it and watch it on the scope.
I noticed, that there is a tiny 'notch' on the edges of the sine that rises up or down depending on the setting of the volumeknob...
This phenomena is there already after the phase inverter, so it is no cross over distortion.
When I go up in frequency(I stopped @ around 4kHz), this notch is more and more apparent. When I turn the presence knob all the way up to ten, the notch nearly disappears...

has anybody an idea what it could be, I searched for days but I simply don't find the problem...

Thank you!
Have a nice day,
Stefan


pucho812

Marshall JTM45 Problem
« Reply #1 on: July 25, 2007, 11:11:12 AM »
New Tubes? Did you re-Bias? Whenever you change power tubes, You should re-bias the tubes. What tubes are in the power section....
You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is.

Kit

Marshall JTM45 Problem
« Reply #2 on: July 25, 2007, 12:30:15 PM »
Quote
it distorts to early compared to other JTM45


How early?

Most  JTM45´s I have played are pretty weak. To weak IMHO.

You said all voltages were fine......... bias?

If there is indeed something wrong I suggest that you monitor the signal from input all the way to the speaker with a scope. Watch you fingers!!

Has it always been weak or did this happen suddenly?

Oh, and check that the filter can caps are secured properly.
They can rattle loose from the holder and start to make noise/distortion that transfers into the circuit.

I know this from experience. :mad:
"Relaxing on the axis of the wheel of life."

VacuumVoodoo

Re: Marshall JTM45 Problem
« Reply #3 on: July 25, 2007, 01:34:40 PM »
Quote from: "steppenwolf"
Hi everybody!
When I turn the presence knob all the way up to ten, the notch nearly disappears...

has anybody an idea what it could be, I searched for days but I simply don't find the problem...

Thank you!
Have a nice day,
Stefan


This tells me that primary winding of the output transformer is probably wrongly connected. Swap the primary connections to output tubes plates.
It's a fairly frequent error when installing replacement transformers.
I've even seen some old Laney Klipp amps that had OT factory wired wrong way.
Alex Niemand
_____________________________________
Life's a party but you get invited only once...
Tubewonder amps
"L

Marshall JTM45 Problem
« Reply #4 on: July 25, 2007, 02:35:34 PM »
Just a quicky - is it a master volume mod'd JTM45?

steppenwolf

Marshall JTM45 Problem
« Reply #5 on: July 25, 2007, 05:12:06 PM »
Quote
New Tubes? Did you re-Bias? Whenever you change power tubes, You should re-bias the tubes. What tubes are in the power section....

I tried it with new KT66, BIASed them around 40mA both, should be OK...


Quote
How early?

Well, when both channels are patched, with both volumes at around 3, it begins to distort with a strat...


Quote
Has it always been weak or did this happen suddenly?


Well, I never really realized it, but I was never happy the way it sounded. But than I heard a JTM45 by a forum member, and thats really another world...We use basically the same layout and schematic, but his distorts around 4 on the volume and that with a les paul...and it has a beatiful dist tone...


Quote
Oh, and check that the filter can caps are secured properly.
They can rattle loose from the holder and start to make noise/distortion that transfers into the circuit.

I know this from experience. Mad


I'll check that, thanks!!

Quote
This tells me that primary winding of the output transformer is probably wrongly connected. Swap the primary connections to output tubes plates.
It's a fairly frequent error when installing replacement transformers.
I've even seen some old Laney Klipp amps that had OT factory wired wrong way.


Strange, if I change the wires coming from the KT66 anodes to the OT, it squeals...


Quote
Just a quicky - is it a master volume mod'd JTM45?


No, it is the original 1960 schematic...

Thank you guys for your support!!!
Have a nice evening!
Stefan

12afael

Marshall JTM45 Problem
« Reply #6 on: July 25, 2007, 08:14:27 PM »
check all cathode R and caps.
heavy metal is the law!!!

pucho812

Marshall JTM45 Problem
« Reply #7 on: July 25, 2007, 09:45:34 PM »
How many KT's are in there? Can you post a link to the schemo

Check voltages going to the pre amp tubes. I know simple but a few weeks ago I had a Marshall on my bench. The problem stated was the amp starts to play then sound goes away. On the scope I saw it and on the scope with 1K I had that similar peek you describe The problem ended up being a bad bridging rectifier which was wrecking havoc to the voltages on the 12AX7 pre amp tubes....
 I just figured the amp was trying to tell the guitar player something :green:

Correct me if I is wrong but don't the channels piggy back one another meaning if I have it in high gain mode it goes from pre amp channel 1 into channel 2 then to the rest of the amp :?  If that is the case then your problem sounds not to far from normal but I could be wrong.

were the sozo's worth it :?
You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is.

steppenwolf

Marshall JTM45 Problem
« Reply #8 on: July 26, 2007, 05:50:44 AM »
Here a link to the schematic I use:
http://marstran.com/JTM45.gif

Quote
check all cathode R and caps.

Well, the only cathode caps used is the 250uF on V1, I also checked the Rs and they seem to be ok.

I traced the signal path with a scope and I noticed, that the sine was pretty OK before the phase inverter. Even with the volume on full, it didn't distort that much and there were no strange notches...
But after the PI, measured after the 0,1uF Caps I noticed, that the signal is distorted much more than in the preamp...Even with volume on half, it begins to cut off sharply. Moreover this notch appears and it is more noticible on higher frequencies. 1k you can see it, 2k even more and at 4k it is obvious....but it is only there, when the distortion begins and grows, you can't see it with a clean sine...
I really don't get it :sad:

Quote
How many KT's are in there? Can you post a link to the schemo

2 KT66. Link on top...

Quote
Correct me if I is wrong but don't the channels piggy back one another meaning if I have it in high gain mode it goes from pre amp channel 1 into channel 2 then to the rest of the amp Confused If that is the case then your problem sounds not to far from normal but I could be wrong.


Well, the channels are bridged, because channel 1 is bright and 2 is dull...
So, you can combine those sounds and get the deep bass and a bright top end...I mean, nearly all (old)Marshall users I ever saw live bridged their channels...And even if I use only the bright channel, it distorts the same...

Quote
were the sozo's worth it Confused


I'm not sure of anything right now :cry: , but I felt that those caps are more transparent than the Mallorys and have a woddier low end, but really not night and day...

Thank you guys!

Johan

Marshall JTM45 Problem
« Reply #9 on: July 26, 2007, 06:21:46 AM »
if you pull the powertubes and send a signal in through the amp, do you get a strong clean signal on the P.I. anodes?  ( or on the gridpin of the powertube sockets)...if so, you might be looking at a faulty OP-TX.
to backcheck.power off and send a sinewave IN through the output and look at the primary with your scope...you probably know, or can look up the turn ratio of the TX. if there are shorts in the windings it should show.
on the marshalls I've replaced OP-TX on, they would only deliver about 3-4 Watts and be very fuzzy
j
EDIT: by the way "45" is just a modelnumber..they usualy output around 28-30watts..


steppenwolf

Marshall JTM45 Problem
« Reply #10 on: July 26, 2007, 06:42:06 AM »
Hi!
What do you mean with OP-TX?
Outputtransformer, what means TX, sorry :wink:

Thanks!

Johan

Marshall JTM45 Problem
« Reply #11 on: July 26, 2007, 07:09:19 AM »
Quote from: "steppenwolf"
Hi!
What do you mean with OP-TX?
Outputtransformer, what means TX, sorry :wink:

Thanks!

..sorry..OP:output  TX: transformer..so yes..op-tx output transformer..
j

Junction

Marshall JTM45 Problem
« Reply #12 on: July 26, 2007, 09:47:06 AM »
Just picked up on this thread and would have to agree that this does sound like a faulty output transformer.

The other observations you have provided sound pretty normal, the 45's do start to break up early on the gain control, normally around 4 to 5 but depends on the output levels of the specific guitar. The audio thru the input stage generally is nearly always clean evan cranked to max volume and on a scope you will clearly see distortion around the phase inverter. However there is feedback from the output back to the phase inverter, so problems at the output will then be reflected back.

It was not clear if this amp is a new build or an older amp that used to work well before. If it's a new kit, then you really need to check all wiring, components and voltages very carefully, most problems in kits tend to be one simple mistake.

See if your friend will allow you to rip his OT from his amp to try in yours.

Cheers
Michael (happy JTM45 owner)

steppenwolf

Marshall JTM45 Problem
« Reply #13 on: July 26, 2007, 10:17:16 AM »
Quote
The other observations you have provided sound pretty normal, the 45's do start to break up early on the gain control, normally around 4 to 5 but depends on the output levels of the specific guitar.


Yeah, right, but at 3 with a vintage Strat...that seem to be really soon...

Quote
It was not clear if this amp is a new build or an older amp that used to work well before. If it's a new kit, then you really need to check all wiring, components and voltages very carefully, most problems in kits tend to be one simple mistake.


Sorry, didn't mention that...

It is a newer build by Ceriatone but without OT PT and choke, I installed those on my own. All Iron is Mercury magentics JTM45 replacement...

The wiring is neat, all components seem to be fine, I checked and double checked them...
Thanks again!
Stefan

pucho812

Marshall JTM45 Problem
« Reply #14 on: July 26, 2007, 10:25:45 AM »
It's a mercury output transformer? Shoot they are down the road from me. I seriously doubt it's the output transformer.

As for my question about the sozo caps, I maninly ask because I know some guitar guys who say they have major Mojo.

If everything is good up to the PI, then check the PI bad tube?
You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is.

steppenwolf

Marshall JTM45 Problem
« Reply #15 on: July 26, 2007, 10:37:00 AM »
Quote
   
PostPosted: Thu Jul 26, 2007 2:25 pm    Post subject:
It's a mercury output transformer? Shoot they are down the road from me. I seriously doubt it's the output transformer.

Really, do you know them? I was told those Trannies are really good...

Quote
As for my question about the sozo caps, I maninly ask because I know some guitar guys who say they have major Mojo.

They defenitly have the "Mojo". I'm not that vintage voodoo kinda guy, but a friend from the forum recorded his superlead one time with malloys, one time  
with the sozo caps, as he wanted to have a reference after recapping. Same tubes, same setting, same speakers and I listened to the recordings without knowing and I picked the Sozos :grin: They are tighter in the bass and have that shimmering top end...that sounds like voodoo :wink:
BTW, I have them in my Les Paul, too. Bumble bees are just too expensive!!

Quote
If everything is good up to the PI, then check the PI bad tube?

Tried allready, even with a special balanced selected tube, still the same...

Really strange!

pucho812

Marshall JTM45 Problem
« Reply #16 on: July 26, 2007, 10:55:11 AM »
I put mercury transformers in my supro amp that I built.  The amp forums say they got Mojo. I dunno about that but I do know they sound good, really good and worth the money. WHen I did my buid I had a choice between the mallory's and the sozo, I had not used the sozo's before so I went with the mallory's. A week after our air condition guy(also a guit-player) got some sozo's and couldn't stop raving about them. I just assumed because of the high dollar price tag but when I heard them I was impressed.

Looking over the schematic I would venture to say you have 3X12AX7(V1 A+B,V2 A+B,V3 A+B) tubes the 2 KT 66 power tubes and a 5y3 rectifier(v6)?

O.k. so are any tubes microphonic? again simple and I am sure you check for it.
Check your reference to ground and make sure the amp has proper ground.
Also check that 47pF cap by the PI mainly out of the fact I dunno if you checked that one.

In my time of doing gear repair I find that caps are usually the first to cause problems and on the bottom of the list would be the transformers. Not to rule them out as a possible colepret but usually not a problem.
You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is.

steppenwolf

Marshall JTM45 Problem
« Reply #17 on: July 26, 2007, 11:10:49 AM »
Quote
Looking over the schematic I would venture to say you have 3X12AX7(V1 A+B,V2 A+B,V3 A+B) tubes the 2 KT 66 power tubes and a 5y3 rectifier(v6)?


Yes, right, the rectifier is a GZ34, but thats the same I think!

Quote
O.k. so are any tubes microphonic? again simple and I am sure you check for it.
Check your reference to ground and make sure the amp has proper ground.
Also check that 47pF cap by the PI mainly out of the fact I dunno if you checked that one.


Microphonic. I swapped the (preamp)tubes, but what I noticed, that when I power on the amp, I can hear some sort of rattling I think from the Power Tubes and when I tip them with my fingers I can hear this rallting again, I think out of the tube...But the tubes do that even without power...Could that be a problem?

Just a question. I was talking about testing caps. My testing consits in measuring if there is any DC passing through the cap...How could I ckeck the 47pf?

pucho812

Marshall JTM45 Problem
« Reply #18 on: July 26, 2007, 11:28:36 AM »
GZ34 is a full wave rectifier and is closer to a 5AR4 tube then a 5y3. They way they are close is they both do full wave rectification.

Power tubes sounding like there is something loose inside is bad... Have you ever known a rattling to be good.   :green: think about it rattle in your car, rattles on a rattle snake,etc,etc.
   Does the tube have a blueish glow to it? If so then it has a leak and needs to be replaced. I would replace it for the rattle alone as it means something is broken or there is a crack somewhere probably on/in the ceramic base and not visable.

Testing a cap... Do you have a multi-meter with capacitance testing on it? If you do test the cap and see if it comes with in the tolorace of what is labeled. I only mentioned that one as well thats the only one that wasn't mentioned. Besides good to test all the caps in the above fasion.
You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is.

steppenwolf

Marshall JTM45 Problem
« Reply #19 on: July 26, 2007, 11:35:32 AM »
Quote
Power tubes sounding like there is something loose inside is bad... Have you ever known a rattling to be good. Mr. Green think about it rattle in your car, rattles on a rattle snake,etc,etc.
Does the tube have a blueish glow to it? If so then it has a leak and needs to be replaced. I would replace it for the rattle alone as it means something is broken or there is a crack somewhere probably on/in the ceramic base and not visable.


Yes, it has a blueish glow, when I turn on the HT and it reacts on my playing, especially on high volume. It dimes and gets brighter when I play bass notes...I was told that blueish glow is pretty normal in power tubes, is that right?

Quote

Testing a cap... Do you have a multi-meter with capacitance testing on it? If you do test the cap and see if it comes with in the tolorace of what is labeled. I only mentioned that one as well thats the only one that wasn't mentioned. Besides good to test all the caps in the above fasion.


Yes I have, but I can't measure capitance in circuit, right, so I should desolder it...


 

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