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Thread: Marshall JTM60 with very low output

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    Marshall JTM60 with very low output

    Greetings from chilly Sydney.

    I’m hoping you can help me out with a Marshall JTM60. It came in as “not working”. I got that part going OK. It was a crack between pad and track on V1. I must say that this is probably the weakest PCB I have ever seen. Another tech before me had obviously struggled with the same issues. So many lifted and lost solder pads. I wouldn’t be surprised if this remaining fault lied in that cause.

    The output is exceeding low and I am going nuts trying to find the reason, so I turn to my mentors once more. As a picture tells a thousand words, I have entered voltages on the 2 parts of the schematic (PI and power tubes). The scope shots are: Blue trace monitors the speaker out. (The speaker is a Weber Tru-Load set to, and measures, 16ohm). The Yellow trace moves to different test locations. The numbers in the brackets are the values at clipping.

    The signal starts clipping at around 5VAC RMS at the speaker, and as you can see from the scope, its an already ugly signal at an output of 1.5w. The volume knob is at around 2.

    Increasing the volume knob, makes the signal very ugly, but not getting past the clipping voltage levels. I need to see around 25-28VAC RMS for the rated output.

    The bias is set to -39v at pin 5, which gives a bias current of 35 & 36mA at a plate Vp=462VDC. I checked the voltage at the pins on the 2 EL34s and all looks good. As you can see on the marked up schematic Vp=462 and Vs=454V and Vg=-39v.

    The plate has a lot of ripple even after I replaced the filter caps. 7.8VAC p-p with no signal increasing to 21.2VAC p-p at clipping (I noticed my typo of 2.12VAC on the marked up schematic). Oddly, at clipping the signal at the EL34 plates is 53VAC on V105 and 25VAC on V106.

    In addition to changing all the 28 year old filter caps, I have tried new tubes in the PI and power tube positions.

    And so after 7 hours on this piece of s**t, I’m hoping you can give me some advice.

    Chris
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    Senior Member Pedro Vecino's Avatar
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    Vp = 462, Vs = 454V, Vg = -39v and 35 & 36mA looks good.
    I d´ont know how to interpret the measures in AC that you put but associating it with what happens I have remembered that the output transformer in that amplifier is somehow weak. I found it broken in two units.

    Although it is not decisive you could measure the resistance between the primary to at least check for symmetry. With dynamic signals at the input a series ammeter on the plates can also show an elevation of intensity that is not matched with the developed power.
    If you have an accessible output transformer, you could connect it eventually to see what happens.

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    If the signal on power tube grids is ok,i'd say the OT is toasted.

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    I would
    - make sure that the speaker out switching jacks work properly
    - disconnect all OT leads and do a neon bulb test.

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    Quote Originally Posted by Pedro Vecino View Post
    V
    Although it is not decisive you could measure the resistance between the primary to at least check for symmetry.

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    Thanks Pedro
    I think you may be right. Between Red centre tap and the other ends of the Primary I have 100ohm and 153ohm

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    Thanks Alexradium. It's starting to look that way.

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    Quote Originally Posted by Helmholtz View Post
    make sure that the speaker out switching jacks work properly
    - disconnect all OT leads and do a neon bulb test.
    I gave the output jacks a blow with compressed air and a good spray flush of contact cleaner. The output transformer was so easily accessible if went straight on to testing the OT, after seeing the DC resistances were quite a bit out. I don't have a Neon in stock, so I put 10VAC across the secondary windings and measured 81- 0- 120 VAC on the primary.

    That would explain the differences in my Plate signal between the 2 tubes. Thank you Gents. For some reason I avoid testing OT's. I guess I always like to find a good news outcome for the owner. This one is now tasting expensive compared to the value of the amp.

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    Supporting Member Randall's Avatar
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    I had a JTM30 in last year with the same complaint, low output. It was the OT.

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    It's weird, because it WAS working fine.....

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    I gave the output jacks a blow with compressed air and a good spray flush of contact cleaner. The output transformer was so easily accessible if went straight on to testing the OT, after seeing the DC resistances were quite a bit out. I don't have a Neon in stock, so I put 10VAC across the secondary windings and measured 81- 0- 120 VAC on the primary.
    A DCR difference between primary halves of up to 20% is not unusual (depending on winding/interleaving scheme). But the AC voltages on both halves must be identical.

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    Quote Originally Posted by Randall View Post
    I had a JTM30 in last year with the same complaint, low output. It was the OT.
    I've just ordered a new Hammond OT. Not bad price at AUD70 (about USD45). My lesson: Don't be so resistant in suspecting an OT.

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    I'd like to complete the notes on this problem of low output on the JTM60. The OT from Hammond arrived during the week and I installed it this morning. The amp powered up beautifully and I saw a nice sign wave on the scope putting out 46w just before clipping. Thank you to those generous people who offered their expertise.

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    Bent Member Chuck H's Avatar
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    FWIW the Hammond replacement 50W Marshall OT (*sort of) took second place in a four OT shootout here some years ago. Mercury did (*sort of) take the top slot, but it should for the price. And the Hammond was preferred by many for it's clarity and definition. I've been a Hammond advocate since I started doing this stuff.

    * "Sort of" meaning that no census was actually taken, but the order of preferences was clear enough.

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    Well thanks for completing the mystery as that is what helps us all in doing the work we do.

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    Quote Originally Posted by christarak View Post
    I've just ordered a new Hammond OT. Not bad price at AUD70 (about USD45). My lesson: Don't be so resistant in suspecting an OT.
    well,that happens the most on inexpensive amps,where you know the company had to cut corners somewhere.

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    Quote Originally Posted by Chuck H View Post
    I've been a Hammond advocate since I started doing this stuff.

    * "Sort of" meaning that no census was actually taken, but the order of preferences was clear enough.
    I agree Chuck. We have 2 Hanmmond distributor in Australia, and I really like both the products and the service. The Mecury's need to come from the USA and that takes time and large shipping costs, and lets not forget the government putting out their hand for another share.

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    Quote Originally Posted by alexradium View Post
    well,that happens the most on inexpensive amps,where you know the company had to cut corners somewhere.
    Very true, mate. The JTM60 sounds pretty bloody good now.

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    Quote Originally Posted by drewl View Post
    Well thanks for completing the mystery as that is what helps us all in doing the work we do.
    Thank you for your thanks, nd the great support we receive on this Forum.

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    Supporting Member The Dude's Avatar
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    Quote Originally Posted by christarak View Post
    ...My lesson: Don't be so resistant in suspecting an OT.
    I don't think that's necessarily a bad thing. It's an expensive part. There are too many folks who go straight to ordering the OT without checking other things. Glad you got the amp up and running!

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    ^^^^^ THIS ^^^^^
    It might be LAST on the Order of Suspicion list, but it's still on the list.

    What The Dude refers to happens enough times around here that R.G. himself has to make a (semi-)joking post to remind all the newbies that come in ready to hang their OT as guilty before it got a trial.

    Justin

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