Super Lead 1959SLP

[tonefr3ak]

Hey, new on the forum after lurking a long while.
Acquired a PCB '90s Marshall Super Lead recently with a bias drift/runaway issue, +0.1mv every two seconds, checked with both probe and 1 ohm to ground method to rule out faulty probe.
The bias rises continually as long as the amp stays on, but with a fan pointed directly at the power tubes, the drift is slowed substantially to the point it's not a problem.
Also, with no V2 installed the drift stops. Ideas?
Understandably, I'd like to use the amp without having to use a fan, even though it's very much capable of drowning out the noise.

The filter caps have been replaced, including bias supply caps, change of bias pot, and all resistors measured and checked by me.
Tried with several sets of output tubes, same issue.

 

[Matthews Guitars]

How long are you waiting for the amp to stabilize before making the bias measurement?

Does it actually stabilize or does it truly runaway and start to red plate?

In a healthy amp with no bias issues, it stabilizes with no signal applied (quiet amp) in about 10 minutes, at which point the tubes are at operating temperature and bias remains steady. If you play it and turn it up, you'll see the bias increase and the tubes get hotter. And then, stop playing and turn the volume back down, and the tubes will cool down and bias will come down.

Reading the bias in the first ten minutes from cold start, it WILL be seen drifting until it stabilizes.

BIAS CHANGES WITH OUTPUT. This is important to understand.

The bias supply is a simple circuit, there's not much to go wrong with it. It's a diode, two caps, a few resistors, and of course the feed from the transformer. If it's not stable, replacing every part in the whole bias circuit costs pocket change. The diode may be compromised, the resistors may be damaged, of course you check the solder connections. The caps don't last forever, be sure to use high quality ones.

Carefully clean the PC board to eliminate contaminants that MAY be causing high resistance shorts. Use a small medium bristle brush like
a toothbrush, to efficiently scrub off any dirt. Isopropyl alcohol is good for a cleaning agent.

 

[rtcook]

I have a complete board with pots and jacks still attached for that amp if you find you are in need of one. It is a pull from my amp when I converted it to a turret board bass amp.

 

[Purgasound]

Those boards do not have known issues of that nature. At least not that I'm aware of... Not to say it couldn't happen.
However, a component can be subject to thermal drift. These are hard to identify but a heat gun and multimeter will reveal a bad one. Sockets can also become conductive if a previous arc has left a carbon trace. They may not be conductive at lower voltages but as voltage and/or current and heat increase then they can start to conduct.
V2, the second preamp tube should not be affecting bias whatsoever. If you have a rising bias with V2 installed it sounds like there's an oscillation happening in the amp. Its possible that the oscillating frequency is above human hearing so it may not be audible. You would need to probe the circuit with an oscilloscope to find it. Can be caused by poor lead dress.

 

[Matthews Guitars]

A can of freeze spray is cheaper than a heat gun, if you don't have a heat gun. A can of compressed air is the same basic thing, just turn it upside down when spraying and you get very cold refrigerated liquid that will cool down a part and cause it to change value.

Heat or cold will cause any component to change value, but will cause a GREATER value change in a defective part.

 

[tonefr3ak]

How long are you waiting for the amp to stabilize before making the bias measurement?

Does it actually stabilize or does it truly runaway and start to red plate?

In a healthy amp with no bias issues, it stabilizes with no signal applied (quiet amp) in about 10 minutes, at which point the tubes are at operating temperature and bias remains steady. If you play it and turn it up, you'll see the bias increase and the tubes get hotter. And then, stop playing and turn the volume back down, and the tubes will cool down and bias will come down.

Reading the bias in the first ten minutes from cold start, it WILL be seen drifting until it stabilizes.

BIAS CHANGES WITH OUTPUT. This is important to understand.

The bias supply is a simple circuit, there's not much to go wrong with it. It's a diode, two caps, a few resistors, and of course the feed from the transformer. If it's not stable, replacing every part in the whole bias circuit costs pocket change. The diode may be compromised, the resistors may be damaged, of course you check the solder connections. The caps don't last forever, be sure to use high quality ones.

Carefully clean the PC board to eliminate contaminants that MAY be causing high resistance shorts. Use a small medium bristle brush like
a toothbrush, to efficiently scrub off any dirt. Isopropyl alcohol is good for a cleaning agent.

Thank you for your thorough reply.
There is no stabilisation, it rises continually as long as the amp is on, and it does redplate eventually when there’s out of adjustment on the bias pot to try to stabilise.
Will try to clean the board.

 

[tonefr3ak]

Those boards do not have known issues of that nature. At least not that I'm aware of... Not to say it couldn't happen.
However, a component can be subject to thermal drift. These are hard to identify but a heat gun and multimeter will reveal a bad one. Sockets can also become conductive if a previous arc has left a carbon trace. They may not be conductive at lower voltages but as voltage and/or current and heat increase then they can start to conduct.
V2, the second preamp tube should not be affecting bias whatsoever. If you have a rising bias with V2 installed it sounds like there's an oscillation happening in the amp. Its possible that the oscillating frequency is above human hearing so it may not be audible. You would need to probe the circuit with an oscilloscope to find it. Can be caused by poor lead dress.

Oscillation from the preamp is what I’m thinking so I’ve trying to lift and recoordinate wires around V2, but to no avail.
Power tubes have arced when it red plated before I owned it, those sockets have been replaced.
Gonna try the temperature methods and see if it leads anywhere. Thank you for your reply!

 

[tonefr3ak]

I have a complete board with pots and jacks still attached for that amp if you find you are in need of one. It is a pull from my amp when I converted it to a turret board bass amp.

I will keep this in mind, thank you very much

 

[tonefr3ak]

12AT7 in V3 worked great, no drift. And no, the previous 12AX7 was not a bad tube as a different 12AX7 showed the same problem.

Unless I’m just really unlucky today

 

[79JMP100]

Check the cap between the two plates of the phase inverter.

 

[tonefr3ak]

Those have been replaced earlier. Will try to raise the 47pf shunt to 150pf and see what happens

 

[Mastershon]

Any luck resolving this issue? It’s possible you have a leaky bias cap, or even a leaky filter cap. A leaky filter cap can cause node voltage drift and B+ drift (hence the possibility of ‘stable when you pulled V2’ comment).

Try pulling all preamp tubes see if you drift or not. If not it points to a filter cap. If it still drifts try replacing the bias caps. Also, the bias pot can cause drift if dirty and/or oxidized and/or has a defective wiper/track. I suppose a leaky bias diode is possible also.

 

[tonefr3ak]

Any luck resolving this issue? It’s possible you have a leaky bias cap, or even a leaky filter cap. A leaky filter cap can cause node voltage drift and B+ drift (hence the possibility of ‘stable when you pulled V2’ comment).

Try pulling all preamp tubes see if you drift or not. If not it points to a filter cap. If it still drifts try replacing the bias caps. Also, the bias pot can cause drift if dirty and/or oxidized and/or has a defective wiper/track. I suppose a leaky bias diode is possible also.

Working on it.. I’ll make sure to post the fix for my amp when I find it

 

[tonefr3ak]

Bias drift seems to have stopped after retensioning the pins on all the preamp sockets.
The issue now seems to be it's redplating when playing fast stuff. Rhythm stuff is no problem. And it always goes back to the set bias voltage after I stop playing. Any ideas?
All the voltages at the power tubes are fine and seems in order.

 

[tonefr3ak]

Stable as can be now. Tracked it down to the right side PI coupling cap. Weird since it was already a new cap before

 

[Mastershon]

Glad you sorted it out!