My Marshall's standby switch went bad...any ideas why?

EADGBE

Member
Messages
12,337
It was making a popping sound when I turned it on to play. I played this amp the least. But yet it failed. Any ideas why? My amp repairer replaced it and now the popping sound upon turning it on to play is gone. It's a Marshall 100 watt master volume from 2002.
 

Mighty Melvin

Member
Messages
2,711
Well, it's a long time since I was fixing amps for a living but the standby switches then were turning on and off a zillion volts. It's a miracle that they all didn't blow up.

If a switch doesn't open and close very quickly and sharply it will burn out. Maybe yours was like that. Or wasn't, to be more accurate.
.
 

Jeff Gehring

Gold Supporting Member
Messages
7,667
Every switch exhibits contact bounce to some degree. That means when you close the switch, the contacts slam together, then bounce a few times before settling into the fully closed position. When you first close the standby switch, the caps on the load side of the switch are discharged. Voltage cannot change instantaneously in a capacitor, just as current cannot instantaneously change in an inductor. So when the contacts first close to connect the rectifier HV output to the load filter caps, those caps appear initially as a near short. There is an initial high current surge as those caps charge. This is taking place during the period of contact bounce, so there is some arcing going on inside the switch as the load is connected/disconnected/connected etc, until the switch has settled. If the switch is a cheap one, or poorly made, the contacts may begin to pit from the arcing over time, and there may be an accumulation of carbon deposited from the arcs. All of which can lead to a switch acting up, as you experienced.
 

MKB

Silver Supporting Member
Messages
9,458
That standby switch is likely rated for voltage well below the operating voltage it's seeing. Surprised they don't fail more often.
This. Just another of the questionable design details of some Marshall amps:
  • inadequate cooling design on some amps, especially vintage NMV's
  • High power dissipation on power tubes
  • They get awfully close to the heater/cathode voltage limit on cathode followers
 
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Mighty Melvin

Member
Messages
2,711
Every switch exhibits contact bounce to some degree. That means when you close the switch, the contacts slam together, then bounce a few times before settling into the fully closed position. ..... All of which can lead to a switch acting up, as you experienced.

This is why I don't like rocker switches, especially in difficult applications. By and large, their snap is less snappy than that of a toggle switch, and as the rocker switch's contacts mush together, there's a lot of burn time.
 

Mighty Melvin

Member
Messages
2,711
This. Just another of the questionable design details of some Marshall amps:
  • inadequate cooling design on some amps, especially vintage NMV's
  • High power dissipation on power tubes
  • They get awfully close to the heater/cathode voltage limit on cathode followers

I remember telling a customer "You know why Marshalls sound like they're going to blow up any minute? Because they are"
 

Badside

Member
Messages
1,635
This. Just another of the questionable design details of some Marshall amps:
  • inadequate cooling design on some amps, especially vintage NMV's
  • High power dissipation on power tubes
  • They get awfully close to the heater/cathode voltage limit on cathode followers

The Standby Switch rating issue is common to all amps. See, you can't find a switch rated for more than 250V that I know of. We all make do with those and pray for the best.

As for the high power dissipation on power tubes, they are typically biased at 70%, pretty average. A Vox AC30 is typically over 100%...

The heater/cathode on the cathode follower tube IS an issue though. Some tubes just don't last in that position (usually V2). On my home builds I elevate the heaters to make up for it.
 

J M Fahey

Member
Messages
2,926
I played this amp the least. But yet it failed.

It's a Marshall 100 watt master volume from 2002.
Call your Lawyer NOW!!! and Sue for Breach of Contract!!!!
They refuse to honor the Warranty !!!!!
Oh!!!! , he reminded you it prescribed 14 YEARS ago?
Lousy Lawyer !!!!! :eek: they are all the same !!!!! :mad:

Just another of the questionable design details of some Marshall amps:
  • inadequate cooling design on some amps, especially vintage NMV's
Armchair designer detected :rolleyes:
Or is it a case of sour grapes? :D

besides, those poorly designed Classic amps seem to have stood the test of Time, for some 60 years so far.
How long do you say those Kemper you are endorsing will last?
Just curious about what you claim. :rolleyes:
 

MKB

Silver Supporting Member
Messages
9,458
Call your Lawyer NOW!!! and Sue for Breach of Contract!!!!
They refuse to honor the Warranty !!!!!
Oh!!!! , he reminded you it prescribed 14 YEARS ago?
Lousy Lawyer !!!!! :eek: they are all the same !!!!! :mad:


Armchair designer detected :rolleyes:
Or is it a case of sour grapes? :D

besides, those poorly designed Classic amps seem to have stood the test of Time, for some 60 years so far.
How long do you say those Kemper you are endorsing will last?
Just curious about what you claim. :rolleyes:
In my day job, I have been designing circuitry for decades, and routinely have to perform thermal analysis and testing of new equipment designs. I can tell when something is likely getting too hot. And I suppose somewhere Dudley Craven or Ken Bran performed the math to tell if the cooling openings they put in the amps were sufficient. Just because an amp can be maintained to operate for decades does not mean the design has the components in SOA.

I have a Marshall Vintage Modern, which is a good example of reasonable thermal design in that it has a sizable grill right above the power tubes. The heat can rise out of the cabinet and not get trapped or concentrated, and you can get airflow through the back panel and through the top. But some Marshall NMV's, especially some of the 50W amps, have no significant air flow into the cab or across the tubes. There are many examples of techs for touring bands that cut holes in the sides of Marshall heads to mount fans, or have the fan sitting behind the amp to blow into the back, or remove the back panel entirely to improve cooling.

Maybe the worst amp of all is the vintage AC30. If your amp routinely melts the plastic in the cooling ports, it's probably getting too hot.

How long will the Kemper last? I don't know, haven't been inside of it or seen a schematic. I guess we'll see. I hope it won't need maintenance every few months like some Marshalls do. And I don't have any sour grapes towards Marshalls, I love them. But they do have a few issues from a design perspective.
 

MKB

Silver Supporting Member
Messages
9,458
The Standby Switch rating issue is common to all amps. See, you can't find a switch rated for more than 250V that I know of. We all make do with those and pray for the best.

As for the high power dissipation on power tubes, they are typically biased at 70%, pretty average. A Vox AC30 is typically over 100%...

The heater/cathode on the cathode follower tube IS an issue though. Some tubes just don't last in that position (usually V2). On my home builds I elevate the heaters to make up for it.
I've seen a few Marshalls biased mighty hot, mainly due to high B+. That and poor cooling in some of the designs probably make them eat output tubes.

I forgot to mention the bias voltage issue in some amps (putting the tap on the wrong side of the standby switch so the bias isn't present during standby).
 

gldtp99

Member
Messages
4,529
Yes, my 1976 NMV, 50 watt, Marshall 1987 has been running on the same Power/Standby switches, Transformers/Choke, and Mustard Caps for 41 years now.

What an unreliable mess !!!!!! (not really)

I did have a used old production preamp tube die in the amp while a friend was playing it at a jam once------ I switched the amp out with my '74 Orange OR80 to keep him playing----- It took all of @10 minutes the next day to find/replace the bad tube with another used old production 12ax7/ecc83. I think the old USA/Euro preamp tubes sound better in this amp, anyway that's what I run in it.

Here it is sitting under a 10 watt, Single Ended 6L6GC head I built a few years ago------ a beat-up old Marshall 1987...................that is very satisfying to play:

 

gldtp99

Member
Messages
4,529
Was/is the bias supply wired to the cold side of the standby switch, as per the 78331-2 schematic? http://bmamps.com/Schematics/marshall/Marshall_jmp_lead_50w_100w.pdf

No, the bias supply was wired to the Hot side (before switch) on this one----- this one came to me modded (oscillating/unusable)--- I fixed the problem and did further mods (One Wire Mod and others)--- then I did a Fenderish set up on V1 for a lower gain Blues/Rock sound and kept it that way for a few years.
Then I re-did it stock (with a Lar/Mar PPIMV)----- then later it was one of three amps (along with a 1987 clone I built and a JTM45RI) that I did Bright Cap and V2a cathode resistor "boost" bypass cap experiments with.
It now has a 5000pF Bright Cap and a "boost" cap on V2a---- and it still has the PPIMV.
This was always the best sounding, out of the three amps (no matter which set of mods), and most people love playing it.
A customer came over a couple of months ago looking for a "Party Amp" (his words)---- apparently this means an amp that one can play AC/DC songs on without a pedal and be loud enough to get over a loud drummer---- I let him try out this '76 1987, and many others, and he ended up buying an old PA head (Challenger 30) that I gutted/rebuilt into a Fender BF/Brownface hybrid.
He liked this Marshall but I wanted much more money for it vs the rebuilt PA head------ he made the best "Tone for Money" choice but I would have spent the extra cash on this Marshall------ It's a sweet Classic Rock amp................... gldtp99
 
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67super

Member
Messages
1,504
In my day job, I have been designing circuitry for decades, and routinely have to perform thermal analysis and testing of new equipment designs. I can tell when something is likely getting too hot. And I suppose somewhere Dudley Craven or Ken Bran performed the math to tell if the cooling openings they put in the amps were sufficient. Just because an amp can be maintained to operate for decades does not mean the design has the components in SOA.

I have a Marshall Vintage Modern, which is a good example of reasonable thermal design in that it has a sizable grill right above the power tubes. The heat can rise out of the cabinet and not get trapped or concentrated, and you can get airflow through the back panel and through the top. But some Marshall NMV's, especially some of the 50W amps, have no significant air flow into the cab or across the tubes. There are many examples of techs for touring bands that cut holes in the sides of Marshall heads to mount fans, or have the fan sitting behind the amp to blow into the back, or remove the back panel entirely to improve cooling.

Maybe the worst amp of all is the vintage AC30. If your amp routinely melts the plastic in the cooling ports, it's probably getting too hot.

How long will the Kemper last? I don't know, haven't been inside of it or seen a schematic. I guess we'll see. I hope it won't need maintenance every few months like some Marshalls do. And I don't have any sour grapes towards Marshalls, I love them. But they do have a few issues from a design perspective.
I'm a designer in my day job also and heat is the enemy of longevity in all things electronics. This may be to simple (and maybe some heat OCD) but when I gig I aim a small fan into the back of whatever amp I'm using, right across the power tubes and transformer. It makes a huge difference, to a great extent the entire chassis is a heat sink.

As far standby switches go, guitar amps are one of the few things that have them. Old TV's full of tubes didn't, radios full of tubes didn't. I've go a radio from 1941 that works great with original tubes and has no standby switch.
 
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gldtp99

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4,529
Two things:
  1. I LOVE that 1987! And...
  2. I would so have to add a little white dot under the remains of the final "L" in the logo.



Yes, then it would be a Marsha!

A previous owner put the black duct tape on the top right side of the head cab---- I lifted a corner once, saw bare wood underneath, and just stuck the tape back down.
I think it adds character to this one----- I have a brand new, still in the box, Marshall 1987xl type head cab that this chassis would bolt right in to but I'll save that for something else.......... gldtp99
 




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