Marshall MA 100C 100 Watt All Valve Amplifier

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Re: Marshall MA 100C 100 Watt All Valve Amplifier

Postby DrNomis » 16 May 2014, 23:35

deltafred wrote:Simon. You could always build yourself a valve tester, something like this http://www.vintage-radio.net/forum/show ... hp?t=48853

It is something I think of every time I have a piece of valve gear on my bench. Once I have fixed it the desire to build one goes away.




Cheers for that mate, I'm going to download all the info to build one of those, might even start a thread in the workshop section...... :thumbsup
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Re: Marshall MA 100C 100 Watt All Valve Amplifier

Postby phatt » 17 May 2014, 12:55

DrNomis wrote:
phatt wrote:I have no idea as to which models this refers ,, But I did read way back on AGGH (I think)
that some of the later marshal's had inferior circuit board material that actually absorbed moisture. :shock:

And you do live in one of the most humid places in Aust. The symptoms you describe could well be this sorta issue.
Those boards slowly absorb moisture and after time you get a short between pins.
I do recall at least one Aussie Amp tech refusing to work on those amps as it's Marshalls problem but they refuse to recognize they screwed up or something along those lines. Apparently The big M won't cover the cost of repair. :evil:
Hope it's not that but thought I'd mention it.
Phil.



Cheers for that mate, my Marshall is an MA100C and the power valve sockets are actually mounted on the chassis rather than a PCB, the preamp valves are PCB mounted though...... :thumbsup


Cool mate, just thought I'd mention it. :slap:

Couple of points that have come to light over the years of struggling to get some idea of what is worth pursuing and what ends up cat chase tail.
Delta fred said re valve testers:
"It is something I think of every time I have a piece of valve gear on my bench. Once I have fixed it the desire to build one goes away."
I think there is much wisdom in that line somewhere. nudge wink and I tend to agree.
Unless you are about to embark on running a Valve store house and you need to know every last parameter it's hardly worth the effort.
I'd spend my money on a few extra valves and forget the duds or suspects. :popcorn:

I've noted quite a few comments on valve testers from reputable Valve savvy geeks and the best line was:
Best valve tester is a real working Amplifier as Vtesters only give you a hint of the health of the internals.

And this gem from Blackbunny;
"OTOH, nobody was making guitar amps with the very high gain structures and inbuilt distortion capabilities
that modern 'shredder' amps have, so maybe older valves had an easier job."

Bingo hole in one. :thumbsup

Interesting also to note that the old RCA books give working HT as say 300Volts for a said Valve yet you measure most modern gear
and it's closer to 400 Volts. And the Valve gets the blame when if runs out of magic puff 6 months later.
Just like motor cars yes you can get 2,000 horsepower out of an engine but it will have to be rebuilt every 440 yards. [smilie=lol_xtreme.gif]
An expensive exercise if you just need to go a mile to the shop for a bottle of milk. :block:
A well designed Valve Amp will likely last for years but if you push the limits then expect early failure rates.
My few cents worth,,Carry on.
Phil.
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Re: Marshall MA 100C 100 Watt All Valve Amplifier

Postby DrNomis » 19 May 2014, 05:17

phatt wrote:
DrNomis wrote:
phatt wrote:I have no idea as to which models this refers ,, But I did read way back on AGGH (I think)
that some of the later marshal's had inferior circuit board material that actually absorbed moisture. :shock:

And you do live in one of the most humid places in Aust. The symptoms you describe could well be this sorta issue.
Those boards slowly absorb moisture and after time you get a short between pins.
I do recall at least one Aussie Amp tech refusing to work on those amps as it's Marshalls problem but they refuse to recognize they screwed up or something along those lines. Apparently The big M won't cover the cost of repair. :evil:
Hope it's not that but thought I'd mention it.
Phil.



Cheers for that mate, my Marshall is an MA100C and the power valve sockets are actually mounted on the chassis rather than a PCB, the preamp valves are PCB mounted though...... :thumbsup


Cool mate, just thought I'd mention it. :slap:

Couple of points that have come to light over the years of struggling to get some idea of what is worth pursuing and what ends up cat chase tail.
Delta fred said re valve testers:
"It is something I think of every time I have a piece of valve gear on my bench. Once I have fixed it the desire to build one goes away."
I think there is much wisdom in that line somewhere. nudge wink and I tend to agree.
Unless you are about to embark on running a Valve store house and you need to know every last parameter it's hardly worth the effort.
I'd spend my money on a few extra valves and forget the duds or suspects. :popcorn:

I've noted quite a few comments on valve testers from reputable Valve savvy geeks and the best line was:
Best valve tester is a real working Amplifier as Vtesters only give you a hint of the health of the internals.

And this gem from Blackbunny;
"OTOH, nobody was making guitar amps with the very high gain structures and inbuilt distortion capabilities
that modern 'shredder' amps have, so maybe older valves had an easier job."

Bingo hole in one. :thumbsup

Interesting also to note that the old RCA books give working HT as say 300Volts for a said Valve yet you measure most modern gear
and it's closer to 400 Volts. And the Valve gets the blame when if runs out of magic puff 6 months later.
Just like motor cars yes you can get 2,000 horsepower out of an engine but it will have to be rebuilt every 440 yards. [smilie=lol_xtreme.gif]
An expensive exercise if you just need to go a mile to the shop for a bottle of milk. :block:
A well designed Valve Amp will likely last for years but if you push the limits then expect early failure rates.
My few cents worth,,Carry on.
Phil.




While I was doing some servicing work on my Marshall at home, I decided to measure the HT and it measured something like 600V DC.... :shock: ,I was surprised since the two main power supply filter caps were only rated to 550V DC, and I thought to myself, had the two caps gone faulty in some way?, because I read somewhere that the HT was supposed to be something like 450V DC...... :thumbsup
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Re: Marshall MA 100C 100 Watt All Valve Amplifier

Postby phatt » 19 May 2014, 15:38

Hi Drnomis,
I'm sure folks here will know more that myself but the few Marshalls I've delt with the working voltage on most of the later units is way over 500VDC. 550 on the last one I looked at. Yes if you refer to much older models I think you would be closer to 400VDC which will likely last longer.

Again better minds feel free to tune in but it is my understanding that a good rule of thumb for circuits like this;
The higher the Plate voltage the lower the Screen voltage other wise expect trouble.
If my memory serves me well with EL34 you basically halve the plate voltage for the screens and it will last a lifetime,,
but few amp builders read the data books.

I'd guess no amount of biasing will help once you go past a certain point. :shock:
I would assume the higher the working voltage the narrower the bias margin and at some point it will all fall over. :hmmm:
The screen is the weakest link in the Valve and maybe by the time your plates are gently glowing the screens are already white hot and close to death as they are only fine wires not plates of metal. I think it has become common practice now to increase the screen resistors on most marshal's as even the older units was pushing the screens to the limit at 400VDC working voltage with only a few volts drop across screen R's
That tone lizard guy has some interesting comments about Marshall's might be worth a look.
If you want i'll try to find the links tomorrow,, too late now.
Phil.

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Re: Marshall MA 100C 100 Watt All Valve Amplifier

Postby lead2203 » 19 May 2014, 19:52

DrNomis wrote:

While I was doing some servicing work on my Marshall at home, I decided to measure the HT and it measured something like 600V DC.... :shock: ,I was surprised since the two main power supply filter caps were only rated to 550V DC, and I thought to myself, had the two caps gone faulty in some way?, because I read somewhere that the HT was supposed to be something like 450V DC...... :thumbsup


Is that unloaded? I hope. With tubes in it.. it should run with the right voltages. What are filament voltages? The caps should be fine.. they "should" be able to take 100v over the rating.. unloaded. I run JJ EL34s at 550 volts with no problems. Again the main problem with this amp is ....its built like shit..and even more problematic is that its a combo...built like shit and combo = problems
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Re: Marshall MA 100C 100 Watt All Valve Amplifier

Postby bajaman » 19 May 2014, 23:08

totally agree with Phil on this one Simon ;-)
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Re: Marshall MA 100C 100 Watt All Valve Amplifier

Postby DrNomis » 20 May 2014, 09:56

lead2203 wrote:
DrNomis wrote:

While I was doing some servicing work on my Marshall at home, I decided to measure the HT and it measured something like 600V DC.... :shock: ,I was surprised since the two main power supply filter caps were only rated to 550V DC, and I thought to myself, had the two caps gone faulty in some way?, because I read somewhere that the HT was supposed to be something like 450V DC...... :thumbsup


Is that unloaded? I hope. With tubes in it.. it should run with the right voltages. What are filament voltages? The caps should be fine.. they "should" be able to take 100v over the rating.. unloaded. I run JJ EL34s at 550 volts with no problems. Again the main problem with this amp is ....its built like shit..and even more problematic is that its a combo...built like shit and combo = problems




Actually, the amp was connected up to a 50 Watt / 8 Ohm Dummy-Load when I measured the HT, all the valves were installed in their respective sockets at the time, no signal was being fed through the amp, so basically it was idling...... :thumbsup


Oh, the filament supply voltage is 6.3V AC..... :thumbsup


Blackbunny is going to do a deal with me and get me some replacement power valves for my Marshall so we'll see how things go once the new valves are installed, hopefully they'll sort my amp out, I haven't really had the chance to fire-up my Marshall much since I replaced the suspect valve so far, so I don't know if I've fixed the fuse-blowing or not, I'll see if I can give it a workout this week, will keep you posted....... :thumbsup


Just want to say thank you guys for all the help you've given me so far....much appreciated, and I'm also learning a lot from all this too..... :thumbsup


Just had a thought....could my Marshall be biased a bit too hot, and furthermore, could it be that the biasing is shifting over time (ie, unstable) where the power valves start conducting more current than they're supposed to?, there's a paper sticker on each power valve which reads 42mA 11/45, I'm assuming that each power valve is biased so that at idle each one is conducting 42mA, that would mean a total current-draw of 168mA from the power supply while the amp is idling, the original HT fuse was rated to 630mA, or thereabouts, so that means that the fuse could take another 462mA before it would blow (630mA-168mA=462mA)....... :hmmm:
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Re: Marshall MA 100C 100 Watt All Valve Amplifier

Postby lead2203 » 20 May 2014, 19:29

DrNomis wrote:
lead2203 wrote:
DrNomis wrote:

While I was doing some servicing work on my Marshall at home, I decided to measure the HT and it measured something like 600V DC.... :shock: ,I was surprised since the two main power supply filter caps were only rated to 550V DC, and I thought to myself, had the two caps gone faulty in some way?, because I read somewhere that the HT was supposed to be something like 450V DC...... :thumbsup


Is that unloaded? I hope. With tubes in it.. it should run with the right voltages. What are filament voltages? The caps should be fine.. they "should" be able to take 100v over the rating.. unloaded. I run JJ EL34s at 550 volts with no problems. Again the main problem with this amp is ....its built like shit..and even more problematic is that its a combo...built like shit and combo = problems




Actually, the amp was connected up to a 50 Watt / 8 Ohm Dummy-Load when I measured the HT, all the valves were installed in their respective sockets at the time, no signal was being fed through the amp, so basically it was idling...... :thumbsup


Oh, the filament supply voltage is 6.3V AC..... :thumbsup


Blackbunny is going to do a deal with me and get me some replacement power valves for my Marshall so we'll see how things go once the new valves are installed, hopefully they'll sort my amp out, I haven't really had the chance to fire-up my Marshall much since I replaced the suspect valve so far, so I don't know if I've fixed the fuse-blowing or not, I'll see if I can give it a workout this week, will keep you posted....... :thumbsup


Just want to say thank you guys for all the help you've given me so far....much appreciated, and I'm also learning a lot from all this too..... :thumbsup


Just had a thought....could my Marshall be biased a bit too hot, and furthermore, could it be that the biasing is shifting over time (ie, unstable) where the power valves start conducting more current than they're supposed to?, there's a paper sticker on each power valve which reads 42mA 11/45, I'm assuming that each power valve is biased so that at idle each one is conducting 42mA, that would mean a total current-draw of 168mA from the power supply while the amp is idling, the original HT fuse was rated to 630mA, or thereabouts, so that means that the fuse could take another 462mA before it would blow (630mA-168mA=462mA)....... :hmmm:


So it has 600v with the tubes in it ..off standby?? Whats the AC reading for the b+ windings? If it is that high.. you may have a bad power transformer...OR you have a bad multimeter ..check it with a different DMM
The number on the tube is for matching ..not what its biased at..you need to check the bias.
Go read Merlin's page to get a better idea of how amps work ...theres allot of good info there. http://www.valvewizard.co.uk/index.html
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Re: Marshall MA 100C 100 Watt All Valve Amplifier

Postby deltafred » 20 May 2014, 20:06

Could be an over reading meter but Simon's heaters voltage reads 6.3v, unless it over reads on DC and not AC.

There was a guy on FSB a while back who's meter was over reading as the battery died, I have one that does the same.
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Re: Marshall MA 100C 100 Watt All Valve Amplifier

Postby DrNomis » 21 May 2014, 07:20

The battery in my multimeter was good so I don't think my multimeter was over-reading, although you never know...... :thumbsup
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Re: Marshall MA 100C 100 Watt All Valve Amplifier

Postby DrNomis » 21 May 2014, 07:28

lead2203 wrote:
DrNomis wrote:
lead2203 wrote:
DrNomis wrote:

While I was doing some servicing work on my Marshall at home, I decided to measure the HT and it measured something like 600V DC.... :shock: ,I was surprised since the two main power supply filter caps were only rated to 550V DC, and I thought to myself, had the two caps gone faulty in some way?, because I read somewhere that the HT was supposed to be something like 450V DC...... :thumbsup


Is that unloaded? I hope. With tubes in it.. it should run with the right voltages. What are filament voltages? The caps should be fine.. they "should" be able to take 100v over the rating.. unloaded. I run JJ EL34s at 550 volts with no problems. Again the main problem with this amp is ....its built like shit..and even more problematic is that its a combo...built like shit and combo = problems




Actually, the amp was connected up to a 50 Watt / 8 Ohm Dummy-Load when I measured the HT, all the valves were installed in their respective sockets at the time, no signal was being fed through the amp, so basically it was idling...... :thumbsup


Oh, the filament supply voltage is 6.3V AC..... :thumbsup


Blackbunny is going to do a deal with me and get me some replacement power valves for my Marshall so we'll see how things go once the new valves are installed, hopefully they'll sort my amp out, I haven't really had the chance to fire-up my Marshall much since I replaced the suspect valve so far, so I don't know if I've fixed the fuse-blowing or not, I'll see if I can give it a workout this week, will keep you posted....... :thumbsup


Just want to say thank you guys for all the help you've given me so far....much appreciated, and I'm also learning a lot from all this too..... :thumbsup


Just had a thought....could my Marshall be biased a bit too hot, and furthermore, could it be that the biasing is shifting over time (ie, unstable) where the power valves start conducting more current than they're supposed to?, there's a paper sticker on each power valve which reads 42mA 11/45, I'm assuming that each power valve is biased so that at idle each one is conducting 42mA, that would mean a total current-draw of 168mA from the power supply while the amp is idling, the original HT fuse was rated to 630mA, or thereabouts, so that means that the fuse could take another 462mA before it would blow (630mA-168mA=462mA)....... :hmmm:


So it has 600v with the tubes in it ..off standby?? Whats the AC reading for the b+ windings? If it is that high.. you may have a bad power transformer...OR you have a bad multimeter ..check it with a different DMM
The number on the tube is for matching ..not what its biased at..you need to check the bias.
Go read Merlin's page to get a better idea of how amps work ...theres allot of good info there. http://www.valvewizard.co.uk/index.html




Not sure what the AC reading for the B+ winding was as I never checked, but, I'm assuming that it wouldn't be too far off the DC voltage, I don't think the power transformer is bad since the amp wouldn't power up at all if it was bad..... :thumbsup
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Re: Marshall MA 100C 100 Watt All Valve Amplifier

Postby lead2203 » 21 May 2014, 17:04

It WOULD still power up and run ..but..Whatever is going on ...600v is not right.... You should check this out......but ok...good luck with it....
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Re: Marshall MA 100C 100 Watt All Valve Amplifier

Postby phatt » 22 May 2014, 15:07

[quote="DrNomis"
Not sure what the AC reading for the B+ winding was as I never checked, but, I'm assuming that it wouldn't be too far off the DC voltage, I don't think the power transformer is bad since the amp wouldn't power up at all if it was bad..... :thumbsup[/quote]

Err??? I doubt that statement would be correct. :hmmm:

If your AC secondary reads 500 VAC then after rectification the HT would be around 700VDC.
This is assuming full wave bridge Which means ACV secondary x 1.4 = DCv

How one arrives at DC voltage is very much dependent on the type of rectification used.
different types of recto circuits give quite different VDC results.
Current capacity is also quite different.
Phil.
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Re: Marshall MA 100C 100 Watt All Valve Amplifier

Postby dai h. » 23 May 2014, 04:09

B+ sounds awfully high (I'm assuming screens are lower(?)). My general sense is that anything that pushes specs (esp. in new production tubes) is asking for trouble. EL34 are rated for 800V on plates (less for the screens) but with the state of new production seems being conservative would make more sense. Also, FWIW my understanding was that with some of the vintage Marshalls, the PT would sag a lot under heavy load, helping to keep the output tubes alive (i.e. not overdissipating).
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Re: Marshall MA 100C 100 Watt All Valve Amplifier

Postby DrNomis » 23 May 2014, 08:29

In any event, the main conclusion I've been able to come to so far is that one, or all of the stock power valves had some issue which was causing the HT fuse to blow, my guess is that the issue only manifested itself when I was using the overdrive channel and when I turned the channel volume up past half-way, so, I'm going to replace all the power valves with a new set as soon as I get a new set of replacement power valves and see if the amp still blows it's HT fuse, will let you guys know how it all goes...... :thumbsup


It definitely looks to me like a problem with one or all of the stock power valves, I've had a good look inside the chassis and can't find anything wrong, no shorts, no burnt or carbonized parts...... :thumbsup
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Re: Marshall MA 100C 100 Watt All Valve Amplifier

Postby DrNomis » 17 Oct 2014, 21:22

Time for an update:


I just got my Marshall home from uni earlier this week and it has been powering up, and working properly so far.....touchwood... :thumbsup

Note that I have re-installed the original power output valve that I thought could be faulty.... :thumbsup


Been studying the schematics and noticed that all the stage coupling-caps are all rated for 400V, I had measured about 600V or so previously when the amp was on my workbench, I'm wondering if replacing them all with new caps rated for 630V might be worthwhile, particularly the two coupling caps for the phase-splitter...... :hmmm:
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Re: Marshall MA 100C 100 Watt All Valve Amplifier

Postby sinner » 18 Oct 2014, 00:08

600v??? Where?
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Re: Marshall MA 100C 100 Watt All Valve Amplifier

Postby DrNomis » 18 Oct 2014, 04:39

sinner wrote:600v??? Where?



The main HT, it measures about 600V all the way up to the + terminal of C 11, I find that a bit odd since C 11 is only rated to 350V..... :thumbsup
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Re: Marshall MA 100C 100 Watt All Valve Amplifier

Postby electrip » 18 Oct 2014, 09:01

With or without the preamp tubes in place? :shock:
As I remember old marshall schematics had voltages at some nodes writen down.
What are the voltages at the marked 'H40T, H50T etc.' testpoints in the schematic?
Could be helpful just now.

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Re: Marshall MA 100C 100 Watt All Valve Amplifier

Postby DrNomis » 18 Oct 2014, 09:45

electrip wrote:With or without the preamp tubes in place? :shock:
As I remember old marshall schematics had voltages at some nodes writen down.
What are the voltages at the marked 'H40T, H50T etc.' testpoints in the schematic?
Could be helpful just now.

electrip



I measured the voltages with all the valves in place and with the amp taken off standby so that it was getting HT to all the valves, I've got a sore back at the moment otherwise I'd remove the chassis from the amp so I can measure the HT at those node points, I'll have to wait till my back comes good again cause it's painful when I try to lift heavy things..... :thumbsup
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