Marshall Super Lead Bias Problems

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Marshall Super Lead Bias Problems

Postby chrischarge on Tue Aug 08, 2017 8:43 pm

Gentlemen, please be gentle, it's my first time....

So i'm having some bias issues with a 1976 100w Super Lead, it belongs to a friend of mine who got it in the 90's when he played in a popular British band. Since then it's been sitting cased in an attic...
Having a look inside it's clear there have been some modifications - caps/resistors removed, old caps replaced etc. this seems to have been done in '81 (i'm guessing but it seems like the tech signed the board) - before my friend purchased it, he loved the sound of it though and, now his old band are doing some reformation shows, would like to use it again.

It powers up ok (i checked all the tubes first - EL34's - and they're all ok) but quickly red plates on all the power tubes, bias currents range between 120-140mA and this is as low as they'll adjust to!! Plate voltage is 510V so I figure you're looking at 30-35mA ideally.

Can anyone suggest what may be going on here and what best to check first? also any input as to what the mods do would be greatly appreciated.

It's great at toasting bread from across the room but it'd be sweet to get her working properly...

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Re: Marshall Super Lead Bias Problems

Postby Baron Von Machinenmann on Mon Aug 14, 2017 5:56 pm

Sounds like you have no bias supply voltage. Given the age and condition as you described (mods etc) the best advice would be to get it to a competent tech before you fry something expensive.
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Re: Marshall Super Lead Bias Problems

Postby pdf64 on Mon Aug 14, 2017 7:38 pm

chrischarge wrote:...It powers up ok (i checked all the tubes first - EL34's - and they're all ok) but quickly red plates on all the power tubes, bias currents range between 120-140mA and this is as low as they'll adjust to!! Plate voltage is 510V so I figure you're looking at 30-35mA ideally.
Can anyone suggest what may be going on here..

How did you test these tubes?
EL34 that have redplated heavily are likely damaged and unable to operate stably in this amp again (I guess it may be possible that they may work ok in some lower voltage / cathode biased amps).
These amps have always been fussy about their power tubes; many current production tubes may just not work ok in there.
Aiken demonstrates that they push their EL34 way beyond their limit http://www.aikenamps.com/index.php/idle-current-biasing-why-70-percent
The best thing would be to take it to a tech with a specific competency in these SLs.
These are not good amps for the inexperienced to experiment with.

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Re: Marshall Super Lead Bias Problems

Postby Sloclone100 on Fri Sep 22, 2017 6:07 pm

The tubes are probably deader than fried chicken BUT you can still test the bias. First you must troubleshoot the bias circuit. I believe one leg of the bias tap goes to goes to a 27K resistor so check that part. Next is the diode to convert the bias voltage to DC - CHECK THAT PART. Then there is what some techs call a pi circuit made up of a resistor (15k) and two capacitors with their POSITIVE SIDE TO GROUND, check those parts as well. Finally they connect to a resistor and a variable resistor, in series. These resistors are fed from the voltage divider from your power tubes.
Here, I found it: Image
Check the first resistor (47k). Then check the pot (25k). You are bound to find a problem in one of those components. If not there are other things you can do. If the first resistor (15K) is decreased, the negative grid voltage will increase thus making the tube current decrease and if the second resistor is decreased, the negative grid voltage will go down, thus making the current go up - you don't want that. I am suspecting either a faulty component or the mods that were done have thrown the bias out of whack. I would substitute a large value potentiometer (250K or 500K) for the resistor right before the bias pot. Set it for max resistance and check your bias. Move the pot down to the correct setting and remove it. Measure he pot and that is the value resistor you should use in place of the 47k resistor that is right in front of the bias pot.

Then, of course, you'll have to buy a new set of tubes.
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Re: Marshall Super Lead Bias Problems

Postby cedarchoper58 on Fri Oct 06, 2017 12:52 am

interesting I run 460V aprox on my plates and bias my EL34's at 33ma
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Re: Marshall Super Lead Bias Problems

Postby pdf64 on Tue Oct 10, 2017 8:02 am

cedarchoper58 wrote:interesting I run 460V aprox on my plates and bias my EL34's at 33ma

Can you identify when it was made?
Was it an export model with 6550?
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Re: Marshall Super Lead Bias Problems

Postby LD50 on Tue Oct 10, 2017 7:50 pm

pdf64 wrote:
cedarchoper58 wrote:interesting I run 460V aprox on my plates and bias my EL34's at 33ma

Can you identify when it was made?
Was it an export model with 6550?


It is a '68 so would run EL34s so at idle that is around 60% of max, so plenty cool enough if it is sounding good.
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Re: Marshall Super Lead Bias Problems

Postby pdf64 on Wed Oct 11, 2017 5:28 pm

It would be really useful if someone could jot down a quick overview of how the 1959 SL HT varied over the years of production :cheers:
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