'77 JMP 100W MKII Red Plating

Spanngitter

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That plate voltage is pretty low for a 100w Marshall from the 70s. Is it the original PT
Indeed, I missed that...However, I had a few early 80s JCM800 which peaked around 400V but these had been JCM800/2204 Versions, so for them this is normal.
Beside a possible PT issue it could be also the Standby Switch, I had so far 2 cases where the Switch had some internal resistance and "stole" 30-40Vdc...
 

Matthews Guitars

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If you disconnect the bias supply tap off the transformer from the bias supply circuit, what is the open circuit AC voltage on the tap?
 

Matthews Guitars

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If the transformer is damaged, it will probably run hotter than it's supposed to. Have you checked that?

I can provide reference numbers if need be. I can turn on my amps, let them warm up for half an hour, and use my thermal camera to get
the operating temperatures of the transformers.
 

Steve Fundy

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I was experimenting with different resistor values and combinations from 72K to 110K ohms
 

Steve Fundy

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So, I have gotten this amplifier to work without red plating. My friend wants to know about replacing the PT as it seems to be a bit light on secondary voltage. What is the current replacement make/model, so I can price it out? Thanks everyone for all of your help!
 

Matthews Guitars

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You haven't even posted a photo of it yet. We don't even know if the PT is a laydown style or standup style. How about some pics?
 

playloud

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Original PT should be Dagnall T4145 I believe. Can you see any markings?
 

Steve Fundy

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Original PT should be Dagnall T4145 I believe. Can you see any markings?
It says T4074 I guess also Dagnall. I am trying to test the secondary & all of these schematics suck. What colors ate the secondary..green, black & yellow?
 

Steve Fundy

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I think I figured out which wires coming out of the PT are the secondary...whew! Yes it looks like he really should replace it.
 

Steve Fundy

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I haven't measured current again since getting things more or less calmed down. The plate voltage is pretty low at just under 400V. The filter caps were replaced a few years ago & the diodes are good. When I measured the secondary (if I have the correct points!) the voltage was just below 400. I had expected over 500V. It came to me running hot and red plating. I have now gotten that back to "normal". My friend is going to try it as is and see if it works ok for him, though he will probably want to replace the PT in the future. Thanks!
 

Dblgun

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400v or a bit less would be correct for a 50w in that time frame but not certain on the 100W. How do the pre-amp voltages look. I would think they would be low as well if the PT is weak. I would also think the filaments could be lower than expected but would be dependent on what is actually going on with the PT. 50w versions omitted one of the 10k droppers and replaced with buss wire to maintain the voltages to pre-amp and PI.
 
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Pete Farrington

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I don’t understand what failure mode people are thinking a ‘weak’ PT may have?

A PT works just the same from the day it was made till the day it dies.
There’s no mechanism by which it can put out a lower secondary voltage, on the HT winding at least. Shorted turns there will collapse the primary inductance and blow the fuse.

My hypothesis is that the HT voltage is low because the bias is still inadequate, so the valves are drawing very high idle current, and Marshall HT supplies are very saggy.
A 100W typically drops 100V at high demand.

I fail to comprehend the reluctance to check idle current. It’s a fundamental metric of the amp’s health.
 

Dblgun

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I don’t understand what failure mode people are thinking a ‘weak’ PT may have?

A PT works just the same from the day it was made till the day it dies.
There’s no mechanism by which it can put out a lower secondary voltage, on the HT winding at least. Shorted turns there will collapse the primary inductance and blow the fuse.

My hypothesis is that the HT voltage is low because the bias is still inadequate, so the valves are drawing very high idle current, and Marshall HT supplies are very saggy.
A 100W typically drops 100V at high demand.

I fail to comprehend the reluctance to check idle current. It’s a fundamental metric of the amp’s health.
I agree with you Pete and think voltages throughout the amp are likely lowered by the bias drawing excessive current. I was curious if this hypothesis is supported by these readings.
 

Matthews Guitars

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I agree, it's too soon to condemn the transformer. Disconnect the bias tap from the bias circuit, pull the power tubes, turn the amp on, and measure the plate voltage. If it STILL isn't getting close to 500 volts on the plate, THEN it's either the wrong PT or a bad one.

But still I'd double check to see that the correct line voltage tap is in use. There may be up to six primary taps.
 

william vogel

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When or if you replace the transformer, message me. I’ll send you the shipping information and a label for you to ship me that “bad” transformer. Worse case scenario, I’ll strip it down and rewind it.
 

playloud

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I don’t understand what failure mode people are thinking a ‘weak’ PT may have?

A PT works just the same from the day it was made till the day it dies.
There’s no mechanism by which it can put out a lower secondary voltage, on the HT winding at least. Shorted turns there will collapse the primary inductance and blow the fuse.

My hypothesis is that the HT voltage is low because the bias is still inadequate, so the valves are drawing very high idle current, and Marshall HT supplies are very saggy.
A 100W typically drops 100V at high demand.

I fail to comprehend the reluctance to check idle current. It’s a fundamental metric of the amp’s health.

What if you get the amp rocking so hard that some of the windings fall out of the PT? :p

@Steve Fundy, the bit about idle current is very important. You should be using that to bias your tubes, not negative voltage.
 

Matthews Guitars

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I don't wish to disagree with Pete but I HAVE seen an amplifier that was still (kind of) running and yet its power transformer was running too hot to touch, and that transformer was definitely damaged but yet not completely dead. In fact the transformer was running so hot that it had de-potted itself, leaving a puddle of molten and re-solidified varnish in the bottom of the cabinet. Yet it wasn't blowing fuses quickly. Its current draw was just high enough that fuses would last an hour or two and then blow. So a partial transformer failure is rare, but not impossible.
 

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