Marshall MA 100C 100 Watt All Valve Amplifier

[DrNomis]

Update:


I've just come back from a jam session where I had the overdrive channel volume on my Marshall amp turned up to just below half way, my amp didn't break down on me this time, it behaved itself really well so I'm happy with it, and you should have heard what my new PRS guitar sounded like through it....it sounded great!!, nice and fat, so I'm doubly happy......

So my amp and guitar was definitely worth the money I paid for them, I've estimated at least $2700.00 all up in total....

Looks like my search for a good guitar and amp has finally come to an end.....

[DrNomis]

Update:


I finally found out what was causing the reverb effect in my Marshall amp to malfunction, turns out the cause was pretty mundane....a broken wire in the reverb tank, before I fixed the problem I decided to take a pic of what I saw when I removed the reverb tank from the amplifier and looked inside...this is what I saw (you might need to magnify the pic):

Notice how the red wire is hanging in mid air?, this is one of the wires that goes to the transducer that's connected to the input socket, my guess is that it broke because of mechanical flexing-stress and the break was right inside the black heat-shrinking, the only reason why I found it is because the wire pulled-out easily from the connection when I gently tugged on it, replacing the red and black wires on both of the transducers fixed the problem completely....

[lead2203]

Update:


I finally found out what was causing the reverb effect in my Marshall amp to malfunction, turns out the cause was pretty mundane....a broken wire in the reverb tank, before I fixed the problem I decided to take a pic of what I saw when I removed the reverb tank from the amplifier and looked inside...this is what I saw (you might need to magnify the pic):

Notice how the red wire is hanging in mid air?, this is one of the wires that goes to the transducer that's connected to the input socket, my guess is that it broke because of mechanical flexing-stress and the break was right inside the black heat-shrinking, the only reason why I found it is because the wire pulled-out easily from the connection when I gently tugged on it, replacing the red and black wires on both of the transducers fixed the problem completely....

From the problems you've posted about, I would say that the amp has taken a good fall or was dropped at some point even when it was in the box. It seems OK, but I would check everything over at some point.

[DrNomis]

Update:


I finally found out what was causing the reverb effect in my Marshall amp to malfunction, turns out the cause was pretty mundane....a broken wire in the reverb tank, before I fixed the problem I decided to take a pic of what I saw when I removed the reverb tank from the amplifier and looked inside...this is what I saw (you might need to magnify the pic):

Notice how the red wire is hanging in mid air?, this is one of the wires that goes to the transducer that's connected to the input socket, my guess is that it broke because of mechanical flexing-stress and the break was right inside the black heat-shrinking, the only reason why I found it is because the wire pulled-out easily from the connection when I gently tugged on it, replacing the red and black wires on both of the transducers fixed the problem completely....

From the problems you've posted about, I would say that the amp has taken a good fall or was dropped at some point even when it was in the box. It seems OK, but I would check everything over at some point.

Cheers and that's a very good point, I had a good look at the amp but didn't see any obvious damage that could have been sustained from being dropped, all the valves were fine, no telltale whitening of the getter, I did a play test after I had fixed the reverb and everything on the amp worked normally, so I'm convinced that the amp is okay....touchwood, I'm really liking the amp everytime I use it, it just works for me and I think it improves my overall guitar tone, I know some people don't like the sound of the clean or overdrive channels, but I it works for me......

I did a small mod to the footswitch, I replaced the stock TRS plug with a gold TRS plug, the reason being is that the original TRS plug had a black plastic cover on it and I've seen these kinds of plugs fail and need replacing so the mod was a bit of preventative maintenance, the footswitch works perfectly......

[lead2203]

Update:


I finally found out what was causing the reverb effect in my Marshall amp to malfunction, turns out the cause was pretty mundane....a broken wire in the reverb tank, before I fixed the problem I decided to take a pic of what I saw when I removed the reverb tank from the amplifier and looked inside...this is what I saw (you might need to magnify the pic):

Notice how the red wire is hanging in mid air?, this is one of the wires that goes to the transducer that's connected to the input socket, my guess is that it broke because of mechanical flexing-stress and the break was right inside the black heat-shrinking, the only reason why I found it is because the wire pulled-out easily from the connection when I gently tugged on it, replacing the red and black wires on both of the transducers fixed the problem completely....

From the problems you've posted about, I would say that the amp has taken a good fall or was dropped at some point even when it was in the box. It seems OK, but I would check everything over at some point.

Cheers and that's a very good point, I had a good look at the amp but didn't see any obvious damage that could have been sustained from being dropped, all the valves were fine, no telltale whitening of the getter, I did a play test after I had fixed the reverb and everything on the amp worked normally, so I'm convinced that the amp is okay....touchwood, I'm really liking the amp everytime I use it, it just works for me and I think it improves my overall guitar tone, I know some people don't like the sound of the clean or overdrive channels, but I it works for me......

I did a small mod to the footswitch, I replaced the stock TRS plug with a gold TRS plug, the reason being is that the original TRS plug had a black plastic cover on it and I've seen these kinds of plugs fail and need replacing so the mod was a bit of preventative maintenance, the footswitch works perfectly......

Since it a combo I would check and tighten the screw's nuts and bolts ...even go over the cab too..but it sounds like its good.
I agree that the clean and crunch sound best. The gain channel sounds small compared to the the clean or crunch channel.
Foot switches ....mostly...go bad from the cord being wrapped around the box. If you have space ...put a stereo jack on the box too....then you can use any TRS/stereo cable.

[DrNomis]

Update:


I finally found out what was causing the reverb effect in my Marshall amp to malfunction, turns out the cause was pretty mundane....a broken wire in the reverb tank, before I fixed the problem I decided to take a pic of what I saw when I removed the reverb tank from the amplifier and looked inside...this is what I saw (you might need to magnify the pic):

Notice how the red wire is hanging in mid air?, this is one of the wires that goes to the transducer that's connected to the input socket, my guess is that it broke because of mechanical flexing-stress and the break was right inside the black heat-shrinking, the only reason why I found it is because the wire pulled-out easily from the connection when I gently tugged on it, replacing the red and black wires on both of the transducers fixed the problem completely....

From the problems you've posted about, I would say that the amp has taken a good fall or was dropped at some point even when it was in the box. It seems OK, but I would check everything over at some point.

Cheers and that's a very good point, I had a good look at the amp but didn't see any obvious damage that could have been sustained from being dropped, all the valves were fine, no telltale whitening of the getter, I did a play test after I had fixed the reverb and everything on the amp worked normally, so I'm convinced that the amp is okay....touchwood, I'm really liking the amp everytime I use it, it just works for me and I think it improves my overall guitar tone, I know some people don't like the sound of the clean or overdrive channels, but I it works for me......

I did a small mod to the footswitch, I replaced the stock TRS plug with a gold TRS plug, the reason being is that the original TRS plug had a black plastic cover on it and I've seen these kinds of plugs fail and need replacing so the mod was a bit of preventative maintenance, the footswitch works perfectly......

Since it a combo I would check and tighten the screw's nuts and bolts ...even go over the cab too..but it sounds like its good.
I agree that the clean and crunch sound best. The gain channel sounds small compared to the the clean or crunch channel.
Foot switches ....mostly...go bad from the cord being wrapped around the box. If you have space ...put a stereo jack on the box too....then you can use any TRS/stereo cable.

I found that to my ears when I engaged the boost function of the overdrive channel, the crunch tone got fatter, maybe that's a quirk of my amp but it does seem to get fatter in boosted crunch mode, all the tubes are stock but I'm thinking of trying some better quality preamp and power valves, like some 12AX7LPS, and some Groove Tube EL34s when the stock valves wear out and need replacement, what would you suggest as good replacements?.....

[senkrongazi]

Do you have the wiring diagram? Marshall MA 100C 100 Watt All Valve Amplifier and PCB ve Circıit diagram?

[DrNomis]

Do you have the wiring diagram? Marshall MA 100C 100 Watt All Valve Amplifier and PCB ve Circıit diagram?

The only thing I have on my computer is a set of schematics which I'll post here:

[DrNomis]

Update:

I went to the Happy Yess club to play an open-mic gig last night, was jamming with some other musicians onstage when.....my Marshall amp died on me again, I decided to wait till this morning and have a look at it, turns out the T630mA HT fuse blew again, so rather than wait another month to get one measly 25c fuse replaced, I'm going to do it myself, I've already checked for dangerous voltages with a multimeter, as far as I can tell there isn't any so the amp appears to be safe to work on.......


Okay, since I have the chassis on my workbench I thought I would do something nice for anyone interested and take some pics of the circuit board, so here you go:

[bato001]

That fuse has blown twice at the same club. I'm wondering if they don't have a problem with their electrical system. It may be worth it to look into a surge protector or some other means of assuring your amp is getting a steady, consistent supply of current when you play that particular club.

[DrNomis]

That fuse has blown twice at the same club. I'm wondering if they don't have a problem with their electrical system. It may be worth it to look into a surge protector or some other means of assuring your amp is getting a steady, consistent supply of current when you play that particular club.

Very good point you make there bato001, and I might look into getting a surge protector next fortnight, I'm going to go to my local Jaycar Electronics store in the morning to see if I can buy a few small boxes of fuses of the correct ratings to cover all the fuses in my Marshall, should only cost me a few dollars, one thing though, while looking at the PCB in the chassis, to the best of my knowledge I couldn't see any burnt/cooked components that could have caused the fuse to blow.....

[deltafred]

Are the output valves (tubes) biased too hot with the bias getting hotter as you are using it in anger and so drawing more HT current.

It would be worth checking the biasing and the HT current.

[DrNomis]

Are the output valves (tubes) biased too hot with the bias getting hotter as you are using it in anger and so drawing more HT current.

It would be worth checking the biasing and the HT current.

It's possible, I seem to remember reading somewhere that the bias trimpots are a bit finicky to adjust in these MA series amps, one thing I did notice was that the amp stopped working after I turned up the overdrive channel just past half-way, I have a feeling that the T650mA fuse is a bit under-rated for the job it's doing.....


Here's why I'm feeling that the fuse is a bit under-rated, an EL34 Power Valve is rated to draw a maximum of 150mA when fully conducting, there are 4 EL34 Valves in the power amp section of my amp capable of producing 100Watts, each pair of EL34 Valves will therefore draw about 300mA when they are fully conducting (assuming they are well matched), so when the amp is producing full power the total current draw from the power supply is 600mA, I'm not sure if my maths is correct or not, that means the fuse is operating just below the point where it will blow, I seem to remember that fuses are designed to blow when the current exceeds the rating by 1.5 (the T650mA fuse is a Slow Blow type), so going by that the fuse should be one rated at 600mA X 1.5 = 900mA, that's probably assuming that the power amp is driven into clipping, I don't know if my maths is correct and,or valid, can someone check that?......


I do realize that the T650mA fuse is there for a good reason so I'm very wary of replacing the original one with one that's rated higher, I could risk expensive damage to the amp so I won't do it, but all the same, I feel that the fuse is a bit under-rated for the job.....


By the way, the fuse is the one marked FS1 on the PCB.....

[bajaman]

try a 1A slow blow fuse
good to see that Marshall have finally gone back to hand wiring and chassis mounting the output tubes again - if the new fuse blows you most probably have a bad output tube.
do you have a tube tester to measure for grid shorts or gas leakage ? Very handy piece of equipment

[DrNomis]

try a 1A slow blow fuse
good to see that Marshall have finally gone back to hand wiring and chassis mounting the output tubes again - if the new fuse blows you most probably have a bad output tube.
do you have a tube tester to measure for grid shorts or gas leakage ? Very handy piece of equipment

Thanks for your input bajaman, I may try that.....


I actually don't happen to have any kind of tube testing equipment at all so I seriously need to invest in one, by the way, how does my maths look to you?.....

[deltafred]

First thing to do is pull the HT fuse and connect a meter across the contacts and measure the current under no load conditions.

Your figure of 300mA per valve is way too high, somewhere between 30 - 45mA is more realistic for an EL34 IIRC (I don't carry bias current values in my head, I Google them when required it is much safer).

The max power of an EL34 is 25W so you need to know the anode to cathode voltage and current flow, multiply these together and that is the power it is dissipating. IIRC you should set the quiescent current to give about 70% dissipation (but don't take my word for it, do as I would and check first). 70% dissipation with 300mA anode current would require an anode voltage of no more than about 75 volts.

(As you are well aware there are potentially lethal voltages involved so take all necessary care and precautions).

[DrNomis]

First thing to do is pull the HT fuse and connect a meter across the contacts and measure the current under no load conditions.

Your figure of 300mA per valve is way too high, somewhere between 30 - 45mA is more realistic for an EL34 IIRC (I don't carry bias current values in my head, I Google them when required it is much safer).

The max power of an EL34 is 25W so you need to know the anode to cathode voltage and current flow, multiply these together and that is the power it is dissipating. IIRC you should set the quiescent current to give about 70% dissipation (but don't take my word for it, do as I would and check first). 70% dissipation with 300mA anode current would require an anode voltage of no more than about 75 volts.

(As you are well aware there are potentially lethal voltages involved so take all necessary care and precautions).

You're quite correct deltafred,the 30-45mA current you're quoting is the current flow from the cathode to the anode when each EL34 Valve is properly biased and when there's no audio signal being applied (Quiescent-state), the 300mA I was quoting is the current flow when each EL 34 is fully-conducting, or just before the onset of clipping in the power amp, I was basically trying to work out the maximum current the power amp will draw from the power supply, for me to do the test you're suggesting, I'm going to have to make up a dummy load, not really a big deal as I should be able to design something suitable, I do have the other bits of test equipment I'll need such as a Signal Generator, Oscilloscope, multimeter, so I will try your suggestion and see what results I get, might even post some screenshots, thank you for your suggestion by the way, much appreciated.....

So far what I've done is substituted a 1A/240V fuse for the T650mA fuse (FS1) as per bajaman's suggestion, I've just done an operational test and the amp appears to be working fine again, although I have only tested it at low volume, I'll have to try the amp out at a typical gigging volume (being careful, of course), and see if it blows the fuse again.....


From what I understand about the Push-Pull Power Amp stage typically found in Marshall amps, while the power amp is idling, that is, not receiving a signal from the Pre Amp, the current draw from the power supply is basically the sum of all the bias currents flowing through each power valve, when you feed a signal from the Pre Amp to the Power Amp, the current draw goes up as the Pre Amp signal goes up and therefore the power dissipation of the Power Valves goes up too, the biasing current is basically a balance between Power dissipation and crossover distortion.....

[DrNomis]

Just doing some googling for Marshall MA100C Issues and thought I would post some links to some relevant webpages:

This sounds awfully like what I've been experiencing (I didn't notice any red-plating of any of the power tubes though).

http://www.marshallforum.com/marshall-a ... tubes.html

Here's another interesting relevant webpage (I know it concerns the MA100H but as the MA100C is virtually the same the info is relevant).

Note: Read what it says about the 630mA fuse rating and also about the Filter Choke, or lack of.....

http://www.electronicspoint.com/vietnam ... 34293.html


And my MA100C happens to be one manufactured in Vietnam.....


Will post more links as I find them......

[bajaman]

Hey Dr
It is WELL WORTH fitting a filter choke in place of that stupid 7w resistor - will tighten the amp up in the bass end and improve the top end response when driven hard

[DrNomis]

Hey Dr
It is WELL WORTH fitting a filter choke in place of that stupid 7w resistor - will tighten the amp up in the bass end and improve the top end response when driven hard

My thoughts exactly bajaman, so, what I need to do is work out how much inductance the choke needs to have, and how much current it needs to be rated for, once I know that, then I can order one online, how much inductance do you think it needs to be, and the current rating?.....


It seems to me that the manufacturer of these Vietnamese Marshall MA100C amps decided to sacrifice the amp's reliability in order to cut costs, surely one filter choke and one measly 1A fuse wouldn't have significantly added to the cost of the amp, would it?, I bought the amp from Sounds Of Music in Casuarina for $1300.00.....