'77 JMP 100W MKII Red Plating

Steve Fundy

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Well, I don't understand why I don't see more than 400V? I returned the amp to it's owner and I'll see what happens later. Many thanks for all of your help!
 

Matthews Guitars

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Did you do the tests that were suggested? It's like you're not even reading our comments, because you certainly aren't responding directly to ANY of them.

If you disconnect the bias tap from the transformer to the bias supply, pull the tubes, turn the amp on, and measure plate voltage, you will then have a really solid indicator of the condition of the transformer. It should deliver more than 480 volts with no bias tap load. Probably over 500.

You returned the amp too soon. You made the problem go away for now but my suspicion is that you neither found the root cause problem nor did you fix it. And that will come back to bite you in the ass when the amp melts down another set of expensive 6550s.
 
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Pete Farrington

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I am still measuring bias current
So did the idle anode or cathode current ever get checked?
To return the amp without doing that is rather poor practice, especially so given that the fault and remedial action you took.
Having had plenty of opportunity to do so, it’s kinda on you if the PT fails.
 

Steve Fundy

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Did you do the tests that were suggested? It's like you're not even reading our comments, because you certainly aren't responding directly to ANY of them.

If you disconnect the bias tap from the transformer to the bias supply, pull the tubes, turn the amp on, and measure plate voltage, you will then have a really solid indicator of the condition of the transformer. It should deliver more than 480 volts with no bias tap load. Probably over 500.

You returned the amp too soon. You made the problem go away for now but my suspicion is that you neither found the root cause problem nor did you fix it. And that will come back to bite you in the ass when the amp melts down another set of expensive 6550s.
I did and got a bit under 400V
 

Steve Fundy

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I did and got a bit under 400V
According to the owner this problem began years ago and hasn't ever been "solved" by others before me as well. I told him to keep an eye on it and if the problem returns we will delve deeper. I suspect the previous techs also were baffled why the voltage is so off and bumped up the bias resistance to keep it working. Apparently the problem began in the 80s.
 

Matthews Guitars

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You've got a bad power transformer. Replace it. It wouldn't be the first time someone had a bad PT in a Marshall. Far from it.
 

Matthews Guitars

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Under 400 volts B+ with good supply capacitors, no tubes installed, and the bias circuit disconnected? Coupled with the other reported bias supply issues, it's likely to be a bad transformer, assuming that the technician has done his due diligence and ruled out leaky or shorted coupling capacitors.

So..yes that's something to final check before condeming the transformer. Check every coupling capacitor on the B+ circuits and be sure none are leaky or shorted.

However, a leaky or shorted coupling capacitor would not explain the bias supply issues. If the bias circuit is OK and the transformer is OK, leaky coupling capacitors on the B+ line would not have such a dramatic effect on the available voltage and current capacity of the bias circuit.
 

Pete Farrington

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With 4x6550 redplating -
I see 393V (V4-5) & 394V (V6-7) on plates.

But with no output valves fitted -
I did and got a bit under 400V

Under 400 volts B+ with good supply capacitors, no tubes installed, and the bias circuit disconnected?
I’m somewhat sceptical the info is good. My guess is that to redplate 4x6550 at nearly 400V will take well over 600mA continuous HT current, yet the HT voltage barely drops a volt compared to a only few mA loading from the preamp valves?
I don’t think there’s enough solid info to condemn the most expensive part in the amp.
 

Matthews Guitars

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If there are no power tubes in it, and no caps are leaking to ground, how else can you explain the low B+ voltage? With no load it should be sky high. I'd expect 500+.

It's very doubtful that the line voltage jumper got moved to the wrong tap.
 

playloud

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If there are no power tubes in it, and no caps are leaking to ground, how else can you explain the low B+ voltage? With no load it should be sky high. I'd expect 500+.

It's very doubtful that the line voltage jumper got moved to the wrong tap.

As Pete says, the voltages stated don't really make sense. I'd wager it's also at least worth checking the primary tap is correct before condemning the PT.

I'm a total amateur, but there are a few red flags here in what the OP is saying:
1. Used negative voltage to set bias (rather than idle current).
2. Asked for help identifying secondary wires on PT (while in circuit, which should be obvious)
3. The discrepancy in voltages reported with no/redplating load.

Now there's nothing wrong with helping a friend - in fact it's commendable! - but it would be a real shame if this seemingly original PT is written off on this basis.

@william vogel has made an offer to test the PT, and even rewind it if necessary. As @Steve Fundy is in the US, that sounds like a sensible option to consider.
 
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william vogel

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As Pete says, the voltages stated don't really make sense. I'd wager it's also at least worth checking the primary tap is correct before condemning the PT.

I'm a total amateur, but there are a few red flags here in what the OP is saying:
1. Used negative voltage to set bias (rather than idle current).
2. Asked for help identifying secondary wires on PT (while in circuit, which should be obvious)
3. The discrepancy in voltages reported with no/redplating load.

Now there's nothing wrong with helping a friend - in fact it's commendable! - but it would be a real shame if this seemingly original PT is written off on this basis.

@william vogel has made an offer to test the PT, and even rewind it if necessary. As @Steve Fundy is in the US, that sounds like a sensible option to consider.
I didn’t mean I’d rewind it for them. They can buy a replacement for half what I charge for winding it. I want the lamination steel and I also want to properly condemn it as shorted windings before any dissection would occur. He appears to lack the interest testing it properly and maybe doesn’t have the equipment but I do and would love the opportunity for the transformer or the parts to make it good again.
 

playloud

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I didn’t mean I’d rewind it for them. They can buy a replacement for half what I charge for winding it. I want the lamination steel and I also want to properly condemn it as shorted windings before any dissection would occur. He appears to lack the interest testing it properly and maybe doesn’t have the equipment but I do and would love the opportunity for the transformer or the parts to make it good again.

Sorry, my mistake. Perhaps if we knew where the OP was, someone might be able to suggest a local tech who could offer a second opinion?
 

william vogel

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Sorry, my mistake. Perhaps if we knew where the OP was, someone might be able to suggest a local tech who could offer a second opinion?
It sounds like he’s through with it and returned it. I also think that was a mistake without properly biasing the amp with a current measurement but it was apparent that wasn’t happening.
 

Steve Fundy

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OK all. I did the best I could with my limited knowledge of Marshalls. I am a Fender amp guy and know my way around them pretty well. My friend wanted to get the red plating to cease. I managed to do that. I gave it back to him on the premise that he would test it and watch it to see if the condition would return. Since I really couldn't go much further with it it was the thing to do in my opinion. As for certain comments:

I am certainly not experienced with Marshalls, but quite experienced with Fenders. I have used the both negative voltage and idle current measurement method to bias many Fenders. If this was a Super or Twin, I wouldn't have needed to ask for help!

When I asked for assistance with finding the proper place to measure the AC secondary it was because someone before me had made a change to the wires coming from the PT. Different colored wires were butt spliced onto the leads! I had to trace back from the rectifier diodes etc to discover where those leads connected to. Perhaps it wasn't the original PT and when another was installed, the leads weren't long enough? I do not know. Again, in a Twin, Super, Deluxe et al, I would have found that in a second.

I did much of what was suggested here but also had the main goal of getting it back to useable condition without spending countless hours to try and completely understand why other techs as well as I encountered this issue. If it comes back to me I will continue to drill down into it and hopefully be able to do it on my own, so as not to have to bother any of you folks. I do appreciate the help. So, case closed. Have a great day or night wherever you all are and again thanks!
 

Matthews Guitars

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Just be aware, the basic Marshall schematic (JTM45 to Superlead) and the basic Fender schematic (Bassman) are very nearly identical. The first Marshalls were the Bassman circuit, with minor tweaks only. So if you can work on Fenders, you know how Marshalls work.

Of course there are differences in construction and layout, but basically they're the same amplifier concept.
 

Steve Fundy

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Just be aware, the basic Marshall schematic (JTM45 to Superlead) and the basic Fender schematic (Bassman) are very nearly identical. The first Marshalls were the Bassman circuit, with minor tweaks only. So if you can work on Fenders, you know how Marshalls work.

Of course there are differences in construction and layout, but basically they're the same amplifier concept.
Yes of course. I know that, except Bassmans had a 5U4 instead of solid state rectification. It's exactly what you said: Layout. Unfamiliar territory. The amps work the essentially same way.
 

Pete Farrington

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I have used the both negative voltage and idle current measurement method to bias many Fenders.
Due to valve and mains supply variance, it’s insufficient to just set the negative bias voltage to some nominal level. Rather its effect on anode or cathode current should be checked, and further adjustment made as required.

Through the tweed era, the Bassman iterations used 5U4, 83 and GZ34 rectifiers. The 5F6A that served as the basis for the JTM45 used a GZ34 (=5AR4).
 
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