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Author Topic: Marshall 6100 LM Lead channel Not Working please help  (Read 5732 times)

evil bunny

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Marshall 6100 LM Lead channel Not Working please help
« on: September 30, 2012, 04:58:30 AM »
 
I have a Marshall 6100 LM 30th anniversary head where just the lead channel has no output. 
Clean and crunch channels work fine.
All the pre-amp tubes glow/get hot.  ( i dont have a tube tester some cheap good recommendations would be welcomed).  Also for future reference which tubes are for which channel?

the tone controls for the channel are working, When I put the master and channel volume up and twiddle bass, mid and treb the tone changes.  But the gain knob for the channel doesnt have any effect, and feels a little scratchy to turn. 
Is this simply the gain pot needs changing?  Or is this a dead tube?
Will tubes still get hot and glow if they are not functioning correctly? 
Also if anyone has a schematic for this amp please link or send I have had no luck finding one online. 
« Last Edit: September 30, 2012, 08:42:41 AM by evil bunny »

polo16mi

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Re: Marshall 6011 LM Lead channel Not Working
« Reply #1 on: September 30, 2012, 06:29:24 AM »
Hi Evil:

Take a look here http://www.drtube.com/marshall.htm

I see several 6100´s, but not 6011´s.

Maybe you can find it.

« Last Edit: September 30, 2012, 06:30:41 AM by polo16mi »

evil bunny

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Re: Marshall 6011 LM Lead channel Not Working
« Reply #2 on: September 30, 2012, 08:29:45 AM »
sorry typo, 6100.

Roly

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Re: Marshall 6100 LM Lead channel Not Working please help
« Reply #3 on: September 30, 2012, 09:42:03 AM »
Apart from heater failure and going microphonic, preamp valves tend to have a long life and are generally not given to just stop working.  In fact socket connections are far more troublesome, so I don't think that testing them would do anything much except confirm that they are still working (you should be able to see two heaters in each valve).

I've attached a Zip which contains four circuits, the preamp being on sheet 6200A.jpg, but it isn't clear which channel is which.  It appears to use both physical switching and FET switching.

Does the faulty Lead channel have two two-position switches associated with it?

In the meantime you can try extracting each preamp valve in turn with a gentle rocking motion, and re-inserting it the same way, being careful to note the "missing pin" alignment.  This may clear a dirty contact on the base.
If you say theory and practice don't agree you haven't applied enough theory.

evil bunny

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Re: Marshall 6100 LM Lead channel Not Working please help
« Reply #4 on: October 04, 2012, 03:02:44 PM »
hey Roly,
thanks for the recommendations.
Have had a go at the amp, and the pre-amp valve that is seperate from the others (which i believe is the LM part) was incredibly loose in the socket.  I gave it a wiggle and now there is sound on the lead channel.  Not perfect mind, there is some of the guitar sound in there but it sounds like some kind of weird synthy spaceship.  Is this what a tube that has gone microphonic sounds like?
I will replace the socket as it seems like the pin receptacles are incredibly worn,  and put a new tube in. 
Thanks again.

=:B

Roly

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Re: Marshall 6100 LM Lead channel Not Working please help
« Reply #5 on: October 05, 2012, 01:27:24 AM »
Well, microphonic sound like ... a microphone.  You tap it and it goes "clunk" or "jangle" or "boing", or can even sound a bit like wind chimes.  Loose connections sound very crackly.

"incredibly loose" and "the pin receptacles are incredibly worn" suggests that something has been going on here.  Assuming you bought it second hand I'd suspect that the previous owner was into swapping preamp bottles for some reason (generally tone).  You can re-tension the contacts, but if you're up for it then replacing the socket with a new one is certainly a better way to go.

Anyway it's quite clear now what is wrong with the lead channel.  :dbtu:
If you say theory and practice don't agree you haven't applied enough theory.

Jack1962

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Re: Marshall 6100 LM Lead channel Not Working please help
« Reply #6 on: August 03, 2013, 05:24:52 PM »
retension that loose socket that's your problem

Littlewyan

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Re: Marshall 6100 LM Lead channel Not Working please help
« Reply #7 on: August 15, 2013, 05:25:19 PM »
The pre amp valve seperate from the others? This is the phase splitter valve so this should affect all channels really. The lead channel uses pre amp valves 2 to 5, whereas the crunch channel uses valves 2 and 6. The valve you say you touched is number 7 so thats very odd.

I've got one of these amps myself and I've never had an issue with the Lead channel like that, HOWEVER I do have it every now and then where the crunch channel doesn't make any sound. I usually fix it by going back and forth between channels or if I leave it a second sound will eventually come through. Probably same issue as yours, valve contacts are loose.