Marshall MA 100C 100 Watt All Valve Amplifier

blackbunny · Post by **blackbunny** » 06 Mar 2014, 12:50

+1 ...what bajaman said, your problem is highly likely to be a bad EL34. JJ EL34s seem to have consistently low failure rates in Marshalls and they sound good, Svetlana WInged C (SED) are expensive but have been the best of the EL34's until recently. There are some reports of bad sounding and badly built new Winged C EL34s since New Sensor (Sovtek / EH / Mullard reissues) bought the Svetlana brand and factory to add to their empire.

Some fussy amp owners have been buying up all the NOS SED Winged C's that were made pre 2010. But I digress. Anyhow, JJs are easy to find and not too expensive. I use them all the time now.

DrNomis · Post by **DrNomis** » 06 Mar 2014, 13:38

blackbunny wrote:+1 ...what bajaman said, your problem is highly likely to be a bad EL34. JJ EL34s seem to have consistently low failure rates in Marshalls and they sound good, Svetlana WInged C (SED) are expensive but have been the best of the EL34's until recently. There are some reports of bad sounding and badly built new Winged C EL34s since New Sensor (Sovtek / EH / Mullard reissues) bought the Svetlana brand and factory to add to their empire.

Some fussy amp owners have been buying up all the NOS SED Winged C's that were made pre 2010. But I digress. Anyhow, JJs are easy to find and not too expensive. I use them all the time now.

I wonder if one or maybe all the EL34s in my Marshall went faulty in some way after the amp being transported in a Taxi to room 5 at Charles Darwin Uni?, I don't recall it being a rough ride in the Taxi......

I'm probably going to have to organize a Taxi to take the amp back to my home so I can work on it.....

DrNomis · Post by **DrNomis** » 06 Mar 2014, 13:46

bajaman wrote:NO - try the Chinese 12AX7B for the preamp tubes and either the JJ EL34 or the Winged C type for output tubes

Just wondering, how much would you charge me for a set of 4 matched JJ EL34 power valves and three Chinese 12AX7B valves?......

blackbunny · Post by **blackbunny** » 06 Mar 2014, 14:23

DrNomis wrote:Just wondering, how much would you charge me for a set of 4 matched JJ EL34 power valves and three Chinese 12AX7B valves?......

Not sure what valves cost over in NZ, or if bajaman sells them, but this guy is the best I've found in Oz for low prices and great service:
https://www.ebay.com.au/itm/JJ-Tesla-E3 ... 540493955d
He doesn't seem to have any Chinese 12AX7s at the moment, but I've had good results with these Mullards:
https://www.ebay.com.au/itm/Mullard-Rei ... 54055a7e5b

If you buy a set of valves he combines the postage costs and it ends up being very reasonable.

My valve cupboard is pretty bare at the moment, but I will have a few sets arriving at the end of the month - PM me if you still need some in 3 weeks or so.

DrNomis · Post by **DrNomis** » 06 Mar 2014, 14:33

blackbunny wrote:
DrNomis wrote:Just wondering, how much would you charge me for a set of 4 matched JJ EL34 power valves and three Chinese 12AX7B valves?......
Not sure what valves cost over in NZ, or if bajaman sells them, but this guy is the best I've found in Oz for low prices and great service:
https://www.ebay.com.au/itm/JJ-Tesla-E3 ... 540493955d
He doesn't seem to have any Chinese 12AX7s at the moment, but I've had good results with these Mullards:
https://www.ebay.com.au/itm/Mullard-Rei ... 54055a7e5b

If you buy a set of valves he combines the postage costs and it ends up being very reasonable.

My valve cupboard is pretty bare at the moment, but I will have a few sets arriving at the end of the month - PM me if you still need some in 3 weeks or so.

Cheers for that and will do....

deltafred · Post by **deltafred** » 06 Mar 2014, 19:19

DrNomis wrote:I wonder if one or maybe all the EL34s in my Marshall went faulty in some way after the amp being transported in a Taxi to room 5 at Charles Darwin Uni?, I don't recall it being a rough ride in the Taxi......

It should stand a Taxi ride, I know Taxi drivers are specially selected and trained but they aren't that bad.

My 100w Carlsbro (poor man's Marshall) spent 4 years bouncing around in the back of a hard sprung van (as did 2 more guitar amps and a PA amp - all valve) and fell off the top of a 412 and the only damage was bent transformer mounting lugs, but that was 60's valves I suppose.

bajaman · Post by **bajaman** » 06 Mar 2014, 20:34

No - the taxi ride didn't do it

Unfortunately the Sovtek /EHX/ russian tubes are not very well made - I avoid using ANY of these in my amp servicing - the failure rate is just TOO damn high

The JJ tubes are far better made, however i have struck the occasional premature failure with some of their EL34 tubes in recent times - probably because they are rushing these out due to the increased demand for them from OEM manufacturers who all have mostly given up on the EHX tubes

Apparently Peavey used to buy truckloads of these Russian tubes \until they saw the light and switched to JJ

My tube tester does not tell me how a tube is going to "sound" , but it sure shows up the tubes with extremely low emission qualities such as the EHX 12AX7WA,WB,XT,XT+,LP,LPS, etc. Don't start me on those "Mullards" made in the same factory

bajaman · Post by **bajaman** » 06 Mar 2014, 20:37

If you can find the Chinese 12AX7B preamp tubes - get them - they are currently the best and most consistent emission and quality wise on the market at present.
Stay away from the Chinese 12AX7 however - these are the ones that didn't make the cut

DrNomis · Post by **DrNomis** » 06 Mar 2014, 23:14

I sooooo need to get myself some kind of valve tester.......

Thanks for all your help with this guys, hopefully I'll be able to get my Marshall up and running again, will keep you posted......

blackbunny · Post by **blackbunny** » 06 Mar 2014, 23:41

bajaman wrote:If you can find the Chinese 12AX7B preamp tubes - get them - they are currently the best and most consistent emission and quality wise on the market at present

I've found them to be consistently good, but they're a bit hard to find in Oz ATM for some reason. Only found one Oz seller so far: EVATCO online store

http://evatco.com.au/product-list/audio ... 12ax7b-sh/

Definitely a fine choice for any amp with a high gain front end, like your Marshall Simon (and that Soldano you KNOW you want).

DrNomis · Post by **DrNomis** » 07 Mar 2014, 09:50

blackbunny wrote:
bajaman wrote:If you can find the Chinese 12AX7B preamp tubes - get them - they are currently the best and most consistent emission and quality wise on the market at present
I've found them to be consistently good, but they're a bit hard to find in Oz ATM for some reason. Only found one Oz seller so far: EVATCO online store

http://evatco.com.au/product-list/audio ... 12ax7b-sh/

Definitely a fine choice for any amp with a high gain front end, like your Marshall Simon (and that Soldano you KNOW you want).

I've got that website bookmarked now, what I'll do is take things one step at a time with servicing my Marshall, have to get it home so I can service it first, but once again thanks for all your help guys.....

DrNomis · Post by **DrNomis** » 29 Mar 2014, 05:35

Blackbunny,

I've been thinking it over and I'd like to take you up on that offer of some new JJ EL34 Power Valves, I checked all the rectifier diodes in the power supply and they all look good, I can't find/see anything else that could be causing the main HT fuse to blow so it looks like the stock Power Valves may indeed be bad even though the amp appears to run fine when it is working, I just got it up and running again on Thursday 27/03/2014, tested it on Friday and it powered up normally again....

Could it be possible that the stock EL34 Valves could be failing intermittently due to the high HT?, I measured the HT when the amp was working and it was about 550V or so.....

bajaman · Post by **bajaman** » 29 Mar 2014, 06:04

Simon - normally when the HT fuse blows after switching the standby on (off ?

) it is a sure sign of a bad power tube - ALWAYS check the screen resistors before replacing bad power tubes

A tube tester takes the guess work away too

The HT sounds about right - did you measure it with the tubes connected or out of the amp ?

DrNomis · Post by **DrNomis** » 29 Mar 2014, 07:09

bajaman wrote:Simon - normally when the HT fuse blows after switching the standby on (off ? ) it is a sure sign of a bad power tube - ALWAYS check the screen resistors before replacing bad power tubes A tube tester takes the guess work away too
The HT sounds about right - did you measure it with the tubes connected or out of the amp ?

When I measured the HT, all the tubes were in their sockets and the amp was off standby(ie HT was getting to the anodes of all the valves).......

I also checked the screen grid resistors, none looked like they were burnt-out.....

If one of the screen-grid resistors had it's HT end touching one of the heater-connections, would that cause the HT fuse to blow?, because when I had a look at each of the screen-grid resistors, one of them looked like it's HT end was making contact with one of the heater connections on the valve sockets, after checking with a multimeter for any dangerous voltages, I carefully re-positioned the resistor so it wasn't making contact with the heater connection....

bajaman · Post by **bajaman** » 29 Mar 2014, 07:14

measure them with the amp turned off and an ohmmeter !

DrNomis · Post by **DrNomis** » 29 Mar 2014, 07:19

bajaman wrote:measure them with the amp turned off and an ohmmeter !

Will do when I get a chance cause the amp is at uni.....

Will let you know how it goes....

DrNomis · Post by **DrNomis** » 30 Mar 2014, 01:44

One thing I did do was to measure all the 5k6 control grid resistors on the EL34 power valves, they all measured correctly and none were open-circuit, so, the plot thickens.....

blackbunny · Post by **blackbunny** » 30 Mar 2014, 04:05

bajaman wrote:Simon - normally when the HT fuse blows after switching the standby on (off ? ) it is a sure sign of a bad power tube - ALWAYS check the screen resistors before replacing bad power tubes A tube tester takes the guess work away too The HT sounds about right - did you measure it with the tubes connected or out of the amp ?

Yep, I would replace a screen resistor that is more than 5% higher than rated value (which is usually 1k / 5 watt in a Marshall), and it's a very good indicator of an EL34 that's been drawing too much screen current.
Open circuit screen resistors usually happen when an EL34 has been red-plating for a while or sometimes when there is an internal short or near-short inside the EL34 - hopefully the HT and mains fuses will blow before there is any serious damage.

For some time I've been wondering about the factors that make HT fuses blow, so here's one discussion I've been following on Aussie Guitar Gearheads, about the pro's and cons of Standby switches.
http://www.guitargear.net.au/discussion ... ic=40125.0

The discussion starts off with analysis of a voltage doubling power supply, but develops into discussion of various Standby methods toward the end of page 1 and through page 2 & 3. Food for thought...

I can tell you that my Marshall TSL100 (and other TSLs & DSLs I have I have worked on) blows more mains fuses & HT fuses than any other guitar amps I've ever come across, even when they have new / good EL34's fitted and seem to be running well.

I'm guessing that the unusually large power supply filters combined with the highly filtered DC filament supply might lead to huge peak currents when the amp's turned on, and from Standby to operate.

Your Marshall MA100C might have inherited some of the less desirable characteristics of the notorious older TSL & DSL models, such as the high peak currents.....

DrNomis · Post by **DrNomis** » 30 Mar 2014, 04:25

blackbunny wrote:
bajaman wrote:Simon - normally when the HT fuse blows after switching the standby on (off ? ) it is a sure sign of a bad power tube - ALWAYS check the screen resistors before replacing bad power tubes A tube tester takes the guess work away too The HT sounds about right - did you measure it with the tubes connected or out of the amp ?
Yep, I would replace a screen resistor that is more than 5% higher than rated value (which is usually 1k / 5 watt in a Marshall), and it's a very good indicator of an EL34 that's been drawing too much screen current.
Open circuit screen resistors usually happen when an EL34 has been red-plating for a while or sometimes when there is an internal short or near-short inside the EL34 - hopefully the HT and mains fuses will blow before there is any serious damage.

For some time I've been wondering about the factors that make HT fuses blow, so here's one discussion I've been following on Aussie Guitar Gearheads, about the pro's and cons of Standby switches.
http://www.guitargear.net.au/discussion ... ic=40125.0

The discussion starts off with analysis of a voltage doubling power supply, but develops into discussion of various Standby methods toward the end of page 1 and through page 2 & 3. Food for thought...

I can tell you that my Marshall TSL100 (and other TSLs & DSLs I have I have worked on) blows more mains fuses & HT fuses than any other guitar amps I've ever come across, even when they have new / good EL34's fitted and seem to be running well.

I'm guessing that the unusually large power supply filters combined with the highly filtered DC filament supply might lead to huge peak currents when the amp's turned on, and from Standby to operate.

Your Marshall MA100C might have inherited some of the less desirable characteristics of the notorious older TSL & DSL models, such as the high peak currents.....

I'm thinking you could be right, a transformer is an inductive device, that is, it has two inductors magnetically coupled by an iron core, what could be happening is that at the very moment when you switch the standby switch on to allow HT to the valve anodes it generates a huge Back-EMF which can be some order of magnitude higher than the DC level of the HT, I'm guessing that it could easily exceed something like 1000V or so, maybe putting some kind of a Snubber-Circuit around the Standby switch may help things?......

Or maybe modding the amp so that it utilizes the slow turn-on characteristics of a Valve Rectifier?......

Just had a thought, it looks like C116 (100nF/630V) is in parallel with the Standby switch, and is there to filter out transients generated when the Standby Switch is switched on or off, could that have gone faulty and would replacing it with say a 100nF/3KV help things?.......

Post by **Ice-9** » 30 Mar 2014, 12:06

I usually replace cheap output valves with decent ones as soon as I get a new amp and bias it correctly. The left over cheap valves can be handy in your gig bag in case of a valve failure at a gig.

With all the trouble you have had DrNomis I would have been taking the amp back to the shop and asking for a new replacement amp, not a repair. We spend a lot of money on our gear and they should work out of the box without problems. Cheap valves I can accept but broken wires and non working parts is a sign the amp is used, either on display in the shop or another customers return. I have had a similar problem with an Fender EVH 5150 combo, It is now 7 months since I bought it and finally am getting a replacement, the thing is i'm still waiting for the replacement coming into stock and the shop asked me to return the original weeks ago as they have to be able to re sell it. (re sell it as new ! Remind me not to buy anything from this shop again).
Sorry to droll on a bit off topic, but my point is some music shops have bad business practices and you should of had a brand new replacement, I always check when unpacking things now weather they look like they have been re packed or looks used in any way.

EDIT- WOW, ive just found the Marshall MA100C amp is now available in the UK at £399. That is a good price for a 100w valve amp. Send yours back for a refund and buy a new one for half price (nearly).