Marshall Amp - Tube Overload Incident

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ido1957

ido1957

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I turned on my marshall last night and turned on the speakers using the rocker switch. I got a loud hum through the speakers (not right away but after a minute or so).

I turned the speaker rocker switch on and off and it kept humming when turned on.

I fiddled with the cables and power cord. No change.

I then noticed there were two power tubes glowing really bright orange. Typical smell of burning electronics too. I fiddled with the high / low switch output on the back - some popping/crackling and the tubes went back to "normal" (i.e. not glowing just a bit of orange and blue).

The problem seemed to go away although on "low" it seemed to have a slight hum. Played for a couple of hours with no problems. "I left it on high".

Anyone ever have this happen? Any idea why a power output switch might affect the tube to make it overheat?

On a side note I noticed my tuner had fallen off the amp and the sheet I had draped over it was slid partly off - I attribute that to the cat. :rolleyes: Not sure he can screw up an amp though...lol...
 
i'd need to see a schemo to speculate on what may have happened... but if i were you i would not drive it hard the next couple times you use it... may well be a fluke... or something is getting read to blow... ya may need to replace those tubes anyhow... after they've been cherry they dont last long anyhow...
 
Marshall Repair for Hum said:
Repair Details:
Replaced second preamp tube.
Replaced low voltage supply zener diodes and master volume switching opamp.
Replaced factory installed (in 1990) 56K bias resistor with 47K bias resistor as per 1994 schematic to lower increase the negative bias voltage from -50V to -38V to properly bias the EL34 tube configuration.
Resoldered misc components and organized wire layout to help with noise further.
Rtv’d misc components.
Test ok

Mesa Boogie 12AX7 preamp tube 13.95
YSL Marshal Part Valvestate IC M5201 5.95
YSL Part 1N5245B 15VO OW5 Z (2) .50
YSL Part 1/2W 47K5 T&R RES .25
Repair Labor (60) 65.40
Subtotal 86.05
GST 5.16
Total 91.21 (CDN)
Click to expand...
I'm wondering if the mods/repairs done above might have an impact on this. Or it might be a dirty or worn power/output switch. The thing is, I never flip that switch, it's been on high output almost exclusively since I bought the amp.
Guess I'll expect a repair or at least a retube coming up. This amp usually starts cutting out volume wise when it needs a retube so I'm anticipating that.
 
You should have a tech check it out. There are many things that could cause the redplating. Don't put new tubes in 'till you do.
 
I don't like the idea of a switch between your amp and the speakers. Not because of the extra components , but because of having to remember to flip the stupid thing. If you turn the amp on and then turn off standby before flipping the switch, you have no load. Then you flip the switch and you have just dumped a load on something running free. Not the same as if the load was already there.

You should see a generator with a diesel engine running when a transfer switch kicks over and the generator assumes the load. Its like the same diesel engine could be in a truck and go almost a million miles, but in a generator the average life might be a couple hundred hours.
 
Outlaws said:
I don't like the idea of a switch between your amp and the speakers. Not because of the extra components , but because of having to remember to flip the stupid thing. If you turn the amp on and then turn off standby before flipping the switch, you have no load. Then you flip the switch and you have just dumped a load on something running free. Not the same as if the load was already there.
Click to expand...

Are you talking about the standby switch (which is what I thought the OP was talking about)? A standby switch isn't between the amp and speakers; typically it interrupts the high voltage supply to the tubes. With it off the amp is doing nothing no matter what you do at the input; the only juice to the tubes is the heater supply. No tube amp should be designed with a way to generate power into no load; that would be very bad for the power tubes and output transformer.

I don't mean to be insulting; am I missing something?
 
ido1957 said:
I'm wondering if the mods/repairs done above might have an impact on this. Or it might be a dirty or worn power/output switch. The thing is, I never flip that switch, it's been on high output almost exclusively since I bought the amp.
Guess I'll expect a repair or at least a retube coming up. This amp usually starts cutting out volume wise when it needs a retube so I'm anticipating that.
Click to expand...

You don't by any chance have 6550's instead of EL34's in that amp? That's one reason a tech would change that bias resistor, and if it's set for EL34's and you are running 6550's, that could be some of your problem.
 
ggunn said:
Are you talking about the standby switch (which is what I thought the OP was talking about)? A standby switch isn't between the amp and speakers; typically it interrupts the high voltage supply to the tubes. With it off the amp is doing nothing no matter what you do at the input; the only juice to the tubes is the heater supply. No tube amp should be designed with a way to generate power into no load; that would be very bad for the power tubes and output transformer.

I don't mean to be insulting; am I missing something?
Click to expand...

No. I am referring to the switch he has installed between the amplifier and the speakers that he is flipping with the amp on, opening and closing the connection to the speakers. I seem to recall another thread where he explained why it was added.
 
Outlaws said:
No. I am referring to the switch he has installed between the amplifier and the speakers that he is flipping with the amp on, opening and closing the connection to the speakers. I seem to recall another thread where he explained why it was added.
Click to expand...

Oh. Bad idea. IMHO, of course.

His problems could be coming from damage to the amp brought about by using that switch. Which is what you were saying, I believe.
 
This is a Marshal 4100. The rocker switch I was talking about was the standby switch. When I turned off standby (and engaged the speakers) I got the hum. When I flicked the High/Low Output (50/100 W) switch in the back, it fixed the problem.

BTW - I played this afternoon for a couple hours. I turned off the standby and got no hum - everything worked normal. I only engaged the speakers for a few minutes, as I normally don't use the speakers, I use the back recording jack to the board through the PA.
 
ido1957 said:
This is a Marshal 4100. The rocker switch I was talking about was the standby switch. When I turned off standby (and engaged the speakers) I got the hum. When I flicked the High/Low Output (50/100 W) switch in the back, it fixed the problem.

BTW - I played this afternoon for a couple hours. I turned off the standby and got no hum - everything worked normal. I only engaged the speakers for a few minutes, as I normally don't use the speakers, I use the back recording jack to the board through the PA.
Click to expand...

Does that jack disable the power section of the amp? If you are running the amp with the power section enabled but no speaker load, it's no wonder you are having problems with the amp.
 
To clarify: You can not run a tube amp unloaded! If the standby switch is in the ON position (plate voltage to the tubes), you must have either speaker(s) or a dummy load plugged in.
 
Outlaws said:
No. I am referring to the switch he has installed between the amplifier and the speakers that he is flipping with the amp on, opening and closing the connection to the speakers. I seem to recall another thread where he explained why it was added.
Click to expand...

thats the stupidest thing i've ever come across as a mod... incredibly dumb boy designs inc...
 
it sounds as though he also has a misunderstanding about the standby... hope that's all it is... the stand by simply removes the high voltage line from the tubes.... it has nothing to do with "engageing" the speakers...
 
dementedchord said:
it sounds as though he also has a misunderstanding about the standby... hope that's all it is... the stand by simply removes the high voltage line from the tubes.... it has nothing to do with "engageing" the speakers...
Click to expand...

Yes; he says that it is a rocker switch, and on Marshalls with rocker switches, one is the power and the other is standby. But he also says that he uses the amp without speakers, using the line out to go to the PA. In standby, the preamp output wouldn't work, either, would it? On Fender amps, anyway, the standby switch shuts of the plate voltage to the preamp tubes as well as the power tubes.
 
There seems to be a lot of miscontrued information here so I will try to clear it up.
1) The standby switch is the on located on the front of the amp, next to the power switch. With standby engaged there is no sound coming out of the speakers. With standby turned off, the sound comes through the speakers.
2) I did not mod this amp by putting a switch between the speakers and the head. Other than the soldering done by the tech (shown above) this amp is prety much stock.
3) The "Recording Compensated" output jack sends the preamp + effects loop signal out of the amp head. I run this to the board.
4) With the standby engaged I still get signal out of the Recording Compensated output jack on the back of the amp.
5) I don't need a dummy load with standby engaged. The speakers are connected to the amp they just don't put out any sound when it's in standby mode.
6) The switch I was toggling refers to the High/Low output switch on the back. Toggling that fixed the hum problem and the tubes stopped glowing bright orange.

Hope this will clarify....
 

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Thanks for clarifying. As long as you stay in standby mode, you should be able to run the amp with no load.
 
Last edited:
ok if that/s true then the mod must have been to not lift the B+ from the preamp tubes when the standby is on...as normally happens with a standby switch...
 
dementedchord said:
ok if that/s true then the mod must have been to not lift the B+ from the preamp tubes when the standby is on...as normally happens with a standby switch...
Click to expand...

Nope - these are the mods done to the amp (below - taken from earlier post in this thread) and that wasn't one of them. The ability to get signal out of the Recording Compensated jack when the amp is in standby has always been there.

Marshall Repair for Hum said:
Repair Details:
Replaced second preamp tube.
Replaced low voltage supply zener diodes and master volume switching opamp.
Replaced factory installed (in 1990) 56K bias resistor with 47K bias resistor as per 1994 schematic to lower increase the negative bias voltage from -50V to -38V to properly bias the EL34 tube configuration.
Resoldered misc components and organized wire layout to help with noise further.
Rtv’d misc components.
Test ok

Mesa Boogie 12AX7 preamp tube 13.95
YSL Marshal Part Valvestate IC M5201 5.95
YSL Part 1N5245B 15VO OW5 Z (2) .50
YSL Part 1/2W 47K5 T&R RES .25
Repair Labor (60) 65.40
Subtotal 86.05
GST 5.16
Total 91.21 (CDN)
Click to expand...
 
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