Marshall MA 100C 100 Watt All Valve Amplifier

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Re: Marshall MA 100C 100 Watt All Valve Amplifier

Postby deltafred » 18 Oct 2014, 10:04

Have you checked you meter battery lately Simon, some meters read high when the battery voltage drops.

I fell foul of this on my car, I was having alternator problems so hooked up my digital multimeter to the cig lighter socket and it was reading almost 20V yet the battery was not charging. Turned out the alternator was knackered as was the meter battery but the car battery was fine.
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Re: Marshall MA 100C 100 Watt All Valve Amplifier

Postby phatt » 23 Oct 2014, 08:39

Others may know more but I recall reading that some British makers used transformers that were wound for 220VAC and if used on 240~250VAC power would over voltage the circuit. :hmmm:
(may have been Laney , but I can't recall the detail)
Just thinking it maybe a similar problem here as to the best of my knowledge most later Marshall's run around 550VDC for HT, so IF? the pwr Tr is as above then it would indeed lift the HT voltage up around 600VDC which would cause problems.
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Re: Marshall MA 100C 100 Watt All Valve Amplifier

Postby DrNomis » 13 May 2015, 01:45

deltafred wrote:Have you checked you meter battery lately Simon, some meters read high when the battery voltage drops.

I fell foul of this on my car, I was having alternator problems so hooked up my digital multimeter to the cig lighter socket and it was reading almost 20V yet the battery was not charging. Turned out the alternator was knackered as was the meter battery but the car battery was fine.




Good point Deltafred, next time I get my Marshall home from Uni I'll do another check of the HT voltage, but, I'll make sure the multimeter has a fresh 9V battery in it beforehand, will post my findings here.... :thumbsup


What I can't work out is that when the amp blows the HT fuse, then blows the replacement fuse after being powered up, I take the chassis out of the cabinet, disconnect the centre-tap of the output transformer from the HT supply and measure the DC resistance of the output-transformer's centre-tap to circuit-ground and I measure just over 200k Ohms, what's causing that?, I have been able to narrow it down to being caused by either one of the output valves, or one of the output-valve sockets, the funny thing is, when I do a visual inspection of the insides of the chassis, I can't for the life of me see anything abnormal like carbonizing, or evidence of severe arcing, but when I remove one of the output valves the 200k Ohms reading rapidly increases to infinity (open circuit) and stays that way when I put the valve back in it's socket.... :scratch:


The best theory I can come up with now is that something inside the socket, or something related to it is causing the 200k Ohms resistance reading, and whatever it is must be getting disturbed, or moved, when I remove the output valve from it's socket..... :hmmm:


This has to be the weirdest, and most frustrating fault I've ever had to fix in a piece of electronic equipment, that I've ever come across, but I'm not beaten yet...... :hmmm:
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Re: Marshall MA 100C 100 Watt All Valve Amplifier

Postby bajaman » 13 May 2015, 07:13

try some new EL34 tubes Simon - NOT Electro Harmonix ;-)
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Re: Marshall MA 100C 100 Watt All Valve Amplifier

Postby DrNomis » 13 May 2015, 09:03

bajaman wrote:try some new EL34 tubes Simon - NOT Electro Harmonix ;-)




What's your recommendation?, JJs?, what about Groove Tubes?,Ruby Tubes? , I'm pretty new to selecting good tubes for amps, yeah I know...I seriously need to get myself a decent tube tester of some sort..... :thumbsup


If installing a set of new power tubes in my amp ends up fixing it for good, I'll have to do something really nice for you as a way of thanking you, Bajaman (I meant that in the nicest possible way by the way)....... :thumbsup
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Re: Marshall MA 100C 100 Watt All Valve Amplifier

Postby bajaman » 13 May 2015, 11:00

I have had a good run with the JJ tubes in the past and yes you do need to get atube tester to take the guesswork out of the fault finding ;-)
Good luck :D
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Re: Marshall MA 100C 100 Watt All Valve Amplifier

Postby DrNomis » 13 May 2015, 11:18

bajaman wrote:I have had a good run with the JJ tubes in the past and yes you do need to get atube tester to take the guesswork out of the fault finding ;-)
Good luck :D



I'll also need to teach myself how to correctly set the biasing in tube amps too, I owe it to myself as a tube amp owner...... :thumbsup


I'm going to see if I can buy a set of new output tubes from blackbunny once he's been able to source some, will let you know how it all goes....... :thumbsup
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Re: Marshall MA 100C 100 Watt All Valve Amplifier

Postby DrNomis » 29 May 2015, 09:07

Okay....time for a new update:


I've finally "Bit the bullet" and have gotten blackbunny to do some servicing work on my Marshall amp, he's going to try replacing all the power valves and the output transformer and give the amp a really good soak test, so it'll be interesting to see how things go as I'm curious to find out exactly what was causing the main HT fuse to blow, was it a fault in one of the original power valves, the output transformer, or both?, we shall see..... :hmmm:
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Re: Marshall MA 100C 100 Watt All Valve Amplifier

Postby danbrown182 » 21 Aug 2015, 03:58

Hey there,
I stumbled across this thread after searching Google for answers about problems with my Marshall MA100c. Finally someone that has had exactly the same grief with theirs as I have! I know nothing about amps so I can't repair it myself and I'm not joking when I say this amp has been back for repairs more then once every year and I have had it since 2011. I bought it for £598 and I'm finally trying to sell it for 150 as the amp is terrible. Notice you can't even buy it from many shops now. It weighs a ton and blows fuses like nobodies business. It absolutely hates being played loud and distorted and is even more likely to blow when you put it through a distortion pedal which is a huge problem considering I play in a punk band. I'm now on a huge budget so am buying a laney 2x12 cab with a orange micro terror head. The 2 for under 300 pound! Much lighter to carry around. I actually feel sorry for the unlucky buggar that is gonna buy it off me because altho working fine now..i know just around the corner is going to be the next time... But I need the cash. By any chance when you have been playing yours a long time and it gets warm if you turn the power and stand by buttons off together do you ever get a horrendous sounding squeeling noise? That happens to mine everytime now unless you turn the stand by button off first. It's just a really badly made amp and a huge name like Marshall should be ashamed! No wonder it never stayed in circulation long. Hope you manage to get to the bottom of it. I gig every week now so cannot afford to have such a liability of an amp.
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Re: Marshall MA 100C 100 Watt All Valve Amplifier

Postby DrNomis » 21 Aug 2015, 10:27

danbrown182 wrote:Hey there,
I stumbled across this thread after searching Google for answers about problems with my Marshall MA100c. Finally someone that has had exactly the same grief with theirs as I have! I know nothing about amps so I can't repair it myself and I'm not joking when I say this amp has been back for repairs more then once every year and I have had it since 2011. I bought it for £598 and I'm finally trying to sell it for 150 as the amp is terrible. Notice you can't even buy it from many shops now. It weighs a ton and blows fuses like nobodies business. It absolutely hates being played loud and distorted and is even more likely to blow when you put it through a distortion pedal which is a huge problem considering I play in a punk band. I'm now on a huge budget so am buying a laney 2x12 cab with a orange micro terror head. The 2 for under 300 pound! Much lighter to carry around. I actually feel sorry for the unlucky buggar that is gonna buy it off me because altho working fine now..i know just around the corner is going to be the next time... But I need the cash. By any chance when you have been playing yours a long time and it gets warm if you turn the power and stand by buttons off together do you ever get a horrendous sounding squeeling noise? That happens to mine everytime now unless you turn the stand by button off first. It's just a really badly made amp and a huge name like Marshall should be ashamed! No wonder it never stayed in circulation long. Hope you manage to get to the bottom of it. I gig every week now so cannot afford to have such a liability of an amp.
DAN


Was your MA100C a Vietnamese manufactured one?


To be honest, I never noticed any squealing when powering-down my MA100C since I always switched it off using just the power switch, I came to three possible conclusions as to why my amp was blowing fuses:

1, Possible lose connection in one of the power valve sockets.

2, Possible bad power valve (likely, since there's been reports of Vietnamese manufactured MA100C amps coming from the factory brand-new with bad EL34 power valves).

3, Possible partial-short in the output transformer.


Blackbunny has had my MA100C on his workbench, and so far he reckons it has been working fine.
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Re: Marshall MA 100C 100 Watt All Valve Amplifier

Postby danbrown182 » 21 Aug 2015, 10:45

I have no idea if mine is a Vietnamese manufactured one or not but its pretty embaressing when you are playing a gig to a packed audience and then boom! Nothing.
I think head and cab is the way forward now anyways. Especially if you are in original bands. That way you can share cabs.
I've had the valves replaced several times and still the same problem. It just doesn't seem to be able to handle the power very well. You can always tell when it is nearly time for it to blow again because after about 20 mins of playing the amp will get quieter and seem to lack power (sometimes have to have the volume on almost 3 quarters and my mates blackstar 30 watt tiny amp is louder) I just hate the amp and it has put me off Marshall for life. Think we should actually copy and paste this thread and send it to Marshall because it seems obvious to Me that they have manufactured a crap product.
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Re: Marshall MA 100C 100 Watt All Valve Amplifier

Postby DrNomis » 21 Aug 2015, 11:08

danbrown182 wrote:I have no idea if mine is a Vietnamese manufactured one or not but its pretty embaressing when you are playing a gig to a packed audience and then boom! Nothing.
I think head and cab is the way forward now anyways. Especially if you are in original bands. That way you can share cabs.
I've had the valves replaced several times and still the same problem. It just doesn't seem to be able to handle the power very well. You can always tell when it is nearly time for it to blow again because after about 20 mins of playing the amp will get quieter and seem to lack power (sometimes have to have the volume on almost 3 quarters and my mates blackstar 30 watt tiny amp is louder) I just hate the amp and it has put me off Marshall for life. Think we should actually copy and paste this thread and send it to Marshall because it seems obvious to Me that they have manufactured a crap product.



The first time my MA100C blew a fuse when I took it to a jam night at the Happy Yess club at Brownsmart in Darwin, I had transported the amp in the boot of a taxi, I only noticed that the fuse had blown when the amp stopped producing sound right in the middle of a jam with a drummer, so I can understand how embarrassed you must have felt, at the time I was pretty annoyed with it considering I had paid about AU$1300.00 for my MA100C, to be honest I think I paid way too much for it given the trouble I'd had with it, when I bought it I was unaware that the MA100C suffered from that issue, hope Marshall are aware of the issues with the Vietnamese MA series of amps and take steps to sort them out.


Here's something to try next time your MA100C blows a fuse, replace the blown fuse then, before putting the chassis back in the cabinet, remove the EL34 power valves one at a time from their sockets and then re-install them, then put the chassis back in the cabinet, does that get the amp going again till the fuse blows?, if it does then it looks like our MA100c amps definitely have the same issue..... :thumbsup
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Re: Marshall MA 100C 100 Watt All Valve Amplifier

Postby danbrown182 » 21 Aug 2015, 11:36

I'm selling it mate so I won't be messing with it anymore. You can seriously pick up better amps for half the price. Far too heavy to lug about. Better off getting a 50 watt or even less because nowadays most venues mic up the amps anyways. I thought I was getting an amazing product with a tube amp. I was silly really because I only really play distorted power chords. Certainly didn't need this kind of amp.
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Re: Marshall MA 100C 100 Watt All Valve Amplifier

Postby lead2203 » 22 Aug 2015, 13:50

DrNomis wrote:
danbrown182 wrote:I have no idea if mine is a Vietnamese manufactured one or not but its pretty embaressing when you are playing a gig to a packed audience and then boom! Nothing.
I think head and cab is the way forward now anyways. Especially if you are in original bands. That way you can share cabs.
I've had the valves replaced several times and still the same problem. It just doesn't seem to be able to handle the power very well. You can always tell when it is nearly time for it to blow again because after about 20 mins of playing the amp will get quieter and seem to lack power (sometimes have to have the volume on almost 3 quarters and my mates blackstar 30 watt tiny amp is louder) I just hate the amp and it has put me off Marshall for life. Think we should actually copy and paste this thread and send it to Marshall because it seems obvious to Me that they have manufactured a crap product.



The first time my MA100C blew a fuse when I took it to a jam night at the Happy Yess club at Brownsmart in Darwin, I had transported the amp in the boot of a taxi, I only noticed that the fuse had blown when the amp stopped producing sound right in the middle of a jam with a drummer, so I can understand how embarrassed you must have felt, at the time I was pretty annoyed with it considering I had paid about AU$1300.00 for my MA100C, to be honest I think I paid way too much for it given the trouble I'd had with it, when I bought it I was unaware that the MA100C suffered from that issue, hope Marshall are aware of the issues with the Vietnamese MA series of amps and take steps to sort them out.


Here's something to try next time your MA100C blows a fuse, replace the blown fuse then, before putting the chassis back in the cabinet, remove the EL34 power valves one at a time from their sockets and then re-install them, then put the chassis back in the cabinet, does that get the amp going again till the fuse blows?, if it does then it looks like our MA100c amps definitely have the same issue..... :thumbsup

Have you put a new set of tubes in this yet??
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Re: Marshall MA 100C 100 Watt All Valve Amplifier

Postby DrNomis » 22 Aug 2015, 14:46

lead2203 wrote:
DrNomis wrote:
danbrown182 wrote:I have no idea if mine is a Vietnamese manufactured one or not but its pretty embaressing when you are playing a gig to a packed audience and then boom! Nothing.
I think head and cab is the way forward now anyways. Especially if you are in original bands. That way you can share cabs.
I've had the valves replaced several times and still the same problem. It just doesn't seem to be able to handle the power very well. You can always tell when it is nearly time for it to blow again because after about 20 mins of playing the amp will get quieter and seem to lack power (sometimes have to have the volume on almost 3 quarters and my mates blackstar 30 watt tiny amp is louder) I just hate the amp and it has put me off Marshall for life. Think we should actually copy and paste this thread and send it to Marshall because it seems obvious to Me that they have manufactured a crap product.



The first time my MA100C blew a fuse when I took it to a jam night at the Happy Yess club at Brownsmart in Darwin, I had transported the amp in the boot of a taxi, I only noticed that the fuse had blown when the amp stopped producing sound right in the middle of a jam with a drummer, so I can understand how embarrassed you must have felt, at the time I was pretty annoyed with it considering I had paid about AU$1300.00 for my MA100C, to be honest I think I paid way too much for it given the trouble I'd had with it, when I bought it I was unaware that the MA100C suffered from that issue, hope Marshall are aware of the issues with the Vietnamese MA series of amps and take steps to sort them out.


Here's something to try next time your MA100C blows a fuse, replace the blown fuse then, before putting the chassis back in the cabinet, remove the EL34 power valves one at a time from their sockets and then re-install them, then put the chassis back in the cabinet, does that get the amp going again till the fuse blows?, if it does then it looks like our MA100c amps definitely have the same issue..... :thumbsup

Have you put a new set of tubes in this yet??




Not yet, Blackbunny still has the amp at his place and has had the amp on soak test... :thumbsup
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Re: Marshall MA 100C 100 Watt All Valve Amplifier

Postby lead2203 » 23 Aug 2015, 03:45

DrNomis wrote:
lead2203 wrote:
DrNomis wrote:
danbrown182 wrote:I have no idea if mine is a Vietnamese manufactured one or not but its pretty embaressing when you are playing a gig to a packed audience and then boom! Nothing.
I think head and cab is the way forward now anyways. Especially if you are in original bands. That way you can share cabs.
I've had the valves replaced several times and still the same problem. It just doesn't seem to be able to handle the power very well. You can always tell when it is nearly time for it to blow again because after about 20 mins of playing the amp will get quieter and seem to lack power (sometimes have to have the volume on almost 3 quarters and my mates blackstar 30 watt tiny amp is louder) I just hate the amp and it has put me off Marshall for life. Think we should actually copy and paste this thread and send it to Marshall because it seems obvious to Me that they have manufactured a crap product.



The first time my MA100C blew a fuse when I took it to a jam night at the Happy Yess club at Brownsmart in Darwin, I had transported the amp in the boot of a taxi, I only noticed that the fuse had blown when the amp stopped producing sound right in the middle of a jam with a drummer, so I can understand how embarrassed you must have felt, at the time I was pretty annoyed with it considering I had paid about AU$1300.00 for my MA100C, to be honest I think I paid way too much for it given the trouble I'd had with it, when I bought it I was unaware that the MA100C suffered from that issue, hope Marshall are aware of the issues with the Vietnamese MA series of amps and take steps to sort them out.


Here's something to try next time your MA100C blows a fuse, replace the blown fuse then, before putting the chassis back in the cabinet, remove the EL34 power valves one at a time from their sockets and then re-install them, then put the chassis back in the cabinet, does that get the amp going again till the fuse blows?, if it does then it looks like our MA100c amps definitely have the same issue..... :thumbsup

Have you put a new set of tubes in this yet??




Not yet, Blackbunny still has the amp at his place and has had the amp on soak test... :thumbsup

Just change the tubes ....soak test??? Stop over thinking this ..... You are makeing something out of nothing ....unless there is something I missed in this thread .....just put in a good set of tubes.

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Re: Marshall MA 100C 100 Watt All Valve Amplifier

Postby DrNomis » 11 Aug 2016, 00:51

Hey Everyone, thought I would post an update on this thread.


Just recently got my Marshall back from being fixed, as it turns out, apparently all the trouble was caused by the power transformer putting out too high +HT for the power tubes to handle, at least one or two of the stock power tubes suffered an internal flash-over which also caused some carbonizing inside the Octal tube-sockets (hence the resistance reading I was getting from the Output Transformer's Centre-Tap to chassis-ground), this in turn damaged a couple of Mosfets in the FX Loop circuitry.

I have been using my Marshall amp ever since I got it back home and so far it has been working fine, no more randomly blowing HT fuses now, been really happy with the amp's performance, it even seems to be sounding a lot better than it did before it was fixed, from what I can remember..... :D


The amp has a complete set of new tubes installed in it, the power tubes are a set of four Groove Tubes EL34s, not sure what the preamp tubes are, I'm happy with it anyway.
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Re: Marshall MA 100C 100 Watt All Valve Amplifier

Postby phatt » 12 Aug 2016, 11:51

Glad to hear it worked out for you,, :thumbsup
Yes those Marshall amp guys obviously don't read the data sheets. :roll:

I recently had a TSL100 here with the dreaded leaky PCB board with bias creep. With help from the net I fixed the PCB and some other issues but that turned out to be the easy bit. The Amp still sounded like crap and after a lot of double checking and head scratching,,,
A simple data sheet told the awful sad truth.

This Amp was one owner and never touched.
Voltages read;
Plate = 495VDC
Screen = 487VDC

The power valves were Svetlana and the Data sheet for those reads
MAX plate voltage = 800VDC
MAX Screen Voltage = 425VDC
Can you see a clue in there? :whappen:

Also recently fixed a dying Marshall Haze 40, power valves were Grove tube.
Plate = 447VDC
Screen= 435VDC
Data sheet for those likely the same as Svet's, I'd say that's livin' on the bleeding edge.

Would you buy a Marshall? I certainly would NOT!
From what I've read the early units ran lower voltages and even back then they had issues. :hmmm:

These circuits were designed around a ~ 400Volt supply (+/- 10~20%) and once you go past that you have to rethink the whole design of the power supply or screen grids will suffer. :evil:

These amp makers must be the dumbest basturds to ever build valve gear. :slap:

Rant over,,back to normal transmission. Phil :popcorn:
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Re: Marshall MA 100C 100 Watt All Valve Amplifier

Postby blackbunny » 16 Aug 2016, 06:23

Dr Nomis asked me to try and fix his trouble prone MA100C amp after he had taken it back to the shop for "warranty repairs" ( that didn't fix the problems ) and then tried to fix it himself.
I took it on as a "rainy day" repair project and after I had a few attempts at it, the amp now seems to be running reliably for the first time since it was new. Phew.

As phatt said in previous posts, most current production EL34s are unreliable when hit with high screen voltages and the first thing I found when testing Dr Nomis' amp was 510-515 VDC B+ and 500 vDC screen voltages, resulting in one EL34 red plating and another EL34 drawing way too much screen current.
Playing with bias currents and fitting fresh EL34 valves didn't solve anything, so I knew there were some problems inside the amp. Here's what I found and what I did...

A] The Mains transformer only has 115 vAC and 230 vAC primary taps. Connected to our local mains supply which averages around 245vAC, the HT secondary measured around 365vAC at idle and heater voltages were 6.7 vAC.
I mounted a 7 way tag strip on the chassis near the rectifier circuitry and inserted 2 x 22 ohm 10 watt resistors (in parallel for 11 ohm total resistance / 20 watt ) between the mains input IEC socket Active and the mains power switch to drop the primary voltage under load.
This dropped the secondary voltages to around 345vAC HT at idle, "sagging" to 325 vAC at full power, or around 445-450vDC. Heater voltages dropped to 6.3 vAC for the EL34s and 6 vDC for the preamp valves.

B] I cleaned carbon off the inner pair of output valve sockets using acetone and non-residual contact cleaner and re-tensioned the pins on all the sockets. One EL34 (V7) had "flashed over" at some stage, which apparently hadn't been spotted by whoever did the previous repairs.

C] I replaced the heater ground reference 100 ohm 1 watt resistors with 2 watt metal oxide resistors. The factory 1 watt heater resistors looked like they had been running very hot and were reading over 120 ohms.

D] I changed the 1k / 5 watt screen resistors to 1.5k / 5 watt . This reduced the screen currents and gave the current production EL34 valves a better chance of surviving,

At this point, the amp (with newly fitted EL34's) was working OK up until around 70% of full power, then the output waveform would go badly asymmetrical with one half of the waveform going into hard clipping with some nasty spikes.

I had a couple of known good Marshall OT's at hand, so I de-soldered the leads from the MA100C OT and "temp-clipped" a SLP 100 reissue OT in. Same asymmetrical clipping, so I changed the driver / PI 12AX7 valve, checked the voltages on the pins and tried again. Still nasty asymmetrical clipping, so I unhooked the SLP OT and "temped" in a JCM 2000 OT, with the same results.

I now knew that the problem was somewhere on the PCB, and since the output at the FX Send was clean and symmetrical at very high levels, I started testing the FX Return circuit and viola! There was the asymmetrical signal between the output of the FX Return gain stage and the PI valve input.

E] Testing voltages and components in the FX loop indicated that several parts were faulty and needed replacing. TR91 (TN2404KL MOSFET) and T92 (J202 FET), D90 (24v zener), D93 & D94 (3v3 zeners) and R101 (2k2 0.6 watt resistor) were all suss or measured out of spec., as was R102 (220k / 0.6 watt resistor), which I replaced with a 220k / 2 watt metal oxide resistor.
I refitted the PCB, connected everything up and was rewarded with a nice symmetrical waveform through the FX loop and into the test load at all levels up to clip point, and textbook symmetrical clipping from about 90 watts RMS output.

After checking power supply voltages and setting output bias currents I ran the amp at full power for 10 hours with no problems. Yay!

Hopefully this will help other bunnies when they're trying to diagnose and fix these MA series amps.

Cheers, BB :blackeye
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Re: Marshall MA 100C 100 Watt All Valve Amplifier

Postby bajaman » 16 Aug 2016, 11:01

well done that man ;-)
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