'77 JMP 100W MKII Red Plating

Steve Fundy

Member
Joined
Dec 20, 2016
Messages
32
Reaction score
5
Hello All. I am working on a Marshall 1977 JMP MK II 100 watt. It was brought to me by a friend who said it was running hot and melted the plastic grill above the tubes! It was definitely running hot. Red plating the power tubes (6550s). He told me he had it worked on by another tech years ago, and it was never quite the same. I pulled out the chassis and found that previous changed the bias resistor to 100K. I thought restoring it to the proper value of 47K would solve the problem. Now it sounds quite good but it's still red plating even with the bias set as cold as possible. I am still measuring bias current. I see 393V (V4-5) & 394V (V6-7) on plates. Thoughts? Thanks in advance!
 

Matthews Guitars

Well-Known Member
Joined
Aug 17, 2019
Messages
6,389
Reaction score
10,322
What's the bias VOLTAGE range? Pull the power tubes out, all four of them, and check the range of bias voltage and report it here.

A 6550 100 watt JMP (1959) or 2203 should have a 15K bias range resistor and 150K tube grid leak resistors.
An EL34 version should have 27K bias range resistor and 220K tube grid leak resistors.

Resistors can be bad, even wide open, while looking normal.

If the bias rectifier diode opens up, you'll have no bias voltage.
 

playloud

Well-Known Member
Joined
Dec 28, 2020
Messages
795
Reaction score
943
Did you replace the tubes? If they were being red plated for some time beforehand, they might be shot.

Ignore this. What @Pete Farrington said
 
Last edited:

Steve Fundy

Member
Joined
Dec 20, 2016
Messages
32
Reaction score
5
Hmmm, interesting. The bias resistor value of 47K is silkscreened onto the board in series with an adjustable 25K. The 2203 schematic I am working from (albeit blurry) shows that arrangement. I measured -20V at pin 5 and thought that would suffice. Apparently not. I cannot find any 150K resistors as tube grid resistors. I see a 5.6K on each pin 5? With power tubes removed Bias Voltage range is -18.6V to-25.5V
 

Seanxk

Well-Known Member
Joined
May 14, 2018
Messages
975
Reaction score
909
Once an output valve has been into a red plating state they will always show the symptom again, so you need to take this into account when you have corrected your bias.
 

Steve Fundy

Member
Joined
Dec 20, 2016
Messages
32
Reaction score
5
That probably barely half what’s needed.
Don’t refit the output valves until adequate bias is available.
It’s fortunate that a transformer hasn’t failed yet.
OK, so as I consider all of this, I began thinking the same thing. Apparently the bias voltage isn't sufficient. So, how should I get it to what's needed? One thought is to check the bias supply voltage at the transformer.
 

Pete Farrington

Well-Known Member
Joined
Jan 18, 2021
Messages
1,443
Reaction score
1,591
Location
Staffordshire UK
I suggest to check and report all PT primary and secondary winding voltages.

Have you verified that the amp has the the correct rated fuses in place?
 
Last edited:

Matthews Guitars

Well-Known Member
Joined
Aug 17, 2019
Messages
6,389
Reaction score
10,322
Is the amp wired correctly for your line voltage? If it's wired for 240 and you're plugged into 120, then all voltages in the amp will be half what they should be.

You should have 6.3 volts on the filaments and about 470-ish volts on the plates.

The bias tap right off the transformer should be about 90 volts AC before hitting that rectifier diode.
 

Spanngitter

Well-Known Member
Joined
Jan 22, 2019
Messages
268
Reaction score
275
Location
Upper Palatinate / Bavaria
I would also verify the Bias Voltages at Pin 5 of each tube as a leaking coupling cap can cause your Bias to drift away as well...and I have to second @Pete Farrington , with a Bias Voltage that low the amp pushed the sh*t out of those 6550 and I really hope the OT did not take it personal...to do a first check pls measure from either OT tap on Pin3 of the Power Tube Pair to the CR and between Pin 3 resistance (turn amp off for that and disconnec from wall!), IIRC you should measure something around 16 Ohm per side (it is normal that they are not 100% identical) resp 32 Ohm between Pin3s...
 

Steve Fundy

Member
Joined
Dec 20, 2016
Messages
32
Reaction score
5
Is the amp wired correctly for your line voltage? If it's wired for 240 and you're plugged into 120, then all voltages in the amp will be half what they should be.

You should have 6.3 volts on the filaments and about 470-ish volts on the plates.

The bias tap right off the transformer should be about 90 volts AC before hitting that rectifier diode.
Well it's been a US amp for its entire life I am to understand, so I doubt it's ever been set up for 240. The plate voltage is just a hair under 400V (394). I plan to measure the PT voltages today.
 

Steve Fundy

Member
Joined
Dec 20, 2016
Messages
32
Reaction score
5
I would also verify the Bias Voltages at Pin 5 of each tube as a leaking coupling cap can cause your Bias to drift away as well...and I have to second @Pete Farrington , with a Bias Voltage that low the amp pushed the sh*t out of those 6550 and I really hope the OT did not take it personal...to do a first check pls measure from either OT tap on Pin3 of the Power Tube Pair to the CR and between Pin 3 resistance (turn amp off for that and disconnec from wall!), IIRC you should measure something around 16 Ohm per side (it is normal that they are not 100% identical) resp 32 Ohm between Pin3s...
Thus far I have measured the same range of -18V to -25V on all four pin 5s. What I am curious about is the resistor on each pin 5. They are 5.6K. I cannot find them on the schematic. I thought they were supposed to be 1.5K. I will check OT today as well. Jeez, if this was a Fender I would have fixed it already LOL
 

Chappy

New Member
Joined
Sep 7, 2013
Messages
18
Reaction score
13
Location
Canada
Check out the following schematic:


hard to read but the resistors are there labelled 5K6 on the output tube grids. They are between the two screen resistors labelled 1K 5W.

As Mathews Guitars pointed out earlier you can also see the comments for USA 6550 models. In the bias supply note that the first resistor is 27K, or 15K for USA models with 6550. Reducing this resistor from 27K to 15K will increase your bias supply voltage (make it more negative). On the far left side of the schematic you can see where the 56K is replaced by a 47K, and the 220K grid leaks are replaced by 150K for USA amps. If all of these resistors check out then next I would be looking at the 10uF bias capacitors. If these go leaky or have dried up then your bias voltage will also be reduced.

Here is a link to the 2203 schematic which also shows the 5.6K grid stopper resistors, again labelled as 5K6.

 
Last edited:

Matthews Guitars

Well-Known Member
Joined
Aug 17, 2019
Messages
6,389
Reaction score
10,322
I was going to say, 400 plate volts is way low. 460 to 480 would be the broad range that I'd expect out of a 70s JMP.

This plus the low bias voltage suggests there may be a problem with the power transformer....not something you want to hear but the evidence does suggest such a possibilty.

Other possibilities include one or more bad rectifier diodes in the power supply or some really worn out filter capacitors.

You'd rather have those as problems than need a new power transformer.
 

Matthews Guitars

Well-Known Member
Joined
Aug 17, 2019
Messages
6,389
Reaction score
10,322
Please post some good clear photos of the chassis, both top and bottom.
 

Steve Fundy

Member
Joined
Dec 20, 2016
Messages
32
Reaction score
5
I was going to say, 400 plate volts is way low. 460 to 480 would be the broad range that I'd expect out of a 70s JMP.

This plus the low bias voltage suggests there may be a problem with the power transformer....not something you want to hear but the evidence does suggest such a possibility.

Other possibilities include one or more bad rectifier diodes in the power supply or some really worn out filter capacitors.

You'd rather have those as problems than need a new power transformer.
Yeah I agree. I expected to see a higher plate voltage. I think the caps were changed not too long ago, but I don't know when. I am much more knowledgeable with fenders but I would have expected 450 or more. I am looking at the schematic suggested and it is definitely different than the one I had that was supposed to be correct. I see the 5.6K's now. Also the 15K on the bias circuit. I will check out the remaining PS components and see what I can discover. I am going to attempt to get the bias voltage up to stop the red plate and advise my friend that the PT be might need to be replaced. If I can get it to that point and it's still sounding good, it will be up to him to decide what to do next. I don't have a clue why the person who worked on it years ago put a 100K in there? Thanks!
 

Matthews Guitars

Well-Known Member
Joined
Aug 17, 2019
Messages
6,389
Reaction score
10,322
Remember, the bias supply has its own filter capacitors and they are often left in place and not replaced when the main reservoir capacitors are replaced. But if they have reached the end of their lifespan, they're going to result in lowered bias supply voltage.
 

Steve Fundy

Member
Joined
Dec 20, 2016
Messages
32
Reaction score
5
Remember, the bias supply has its own filter capacitors and they are often left in place and not replaced when the main reservoir capacitors are replaced. But if they have reached the end of their lifespan, they're going to result in lowered bias supply voltage.
The caps on the bias circuit seem to be ok. I experimented a bit with different resistive values and was able to get the bias range from -25V to -50V and the red plating is gone, running much cooler & actually still sounds pretty darn good. I am not finished yet though.
 

Dblgun

Well-Known Member
Joined
Jan 6, 2019
Messages
491
Reaction score
897
Location
AZ
What changes did you make to get the additional bias voltage?
 

Latest posts



Top