1973 Marshall Super Lead 100 1 power tube won't bias

[Mkjames]

I cannot seem to bias the amp as one socket (the outermost socket closest to the fuses) wont bias. I've got a new matched set of JJ's and new screen grid resistors, 5watt 1Kohm. I've checked the grid resistors and they all check out around right around 1K. My plate voltage is a very steady 524 dc. Doing the equation based on the plate voltage I should be biased right at 33mA which I can get dialed in on three sockets but the fourth is running at 57mA. To check, I swapped the tubes around with the same results.

[charmon]

I'm at work so I dont have a schematic or pin-out handy....but what voltage are you getting for the bias feed at that socket? I think its pin 5. Should be around -43 volts or so. Compare this one to the other 3 sockets and I think you will find your culprit.

[mickeydg5]

The negative grid voltage at pin #5 should be practically the same on all four power tubes.

Something else to check is cathode circuit. How are you measuring bias at the power tubes?
Anyway the ohm measurement from chassis ground to pin #8 on the socket should be zero at all four sockets. If you have 1 ohm bias resistors in place then that should be taken into account. Those resistors if in place should be 1% tolerance or less so again all four sockets should indicate about the same ohm measurement.

Any extra resistance in the cathode circuit could throw the idle current off if measuring directly to chassis ground.

Ohm measuerments should be done with power OFF.

[charmon]

What if one of the 5K6 resistors is going bad?

[mickeydg5]

Chuck, exactly as you said, if something in the grid circuit is going bad, like the 5.6k resistor, it will affect the negative voltage at pin #5 resulting in a different voltage and causing different operation.

That needs to be checked too. I agree. Also to check are any connections, traces and wires in the grid circuit.

[Mkjames]

I'm at work so I dont have a schematic or pin-out handy....but what voltage are you getting for the bias feed at that socket? I think its pin 5. Should be around -43 volts or so. Compare this one to the other 3 sockets and I think you will find your culprit.

Thanks for getting back. I went in to test and found that pin 4 was loose from the inside and had the potential to pull right out. I only have a phenolic from a fender rebuild that I did so I have to adapt it to the opening. Once I get that done, hopefully tonight, I'll be able to read the volts. If I didn't mention it before, I measured originally using a bias probe that measures milliamps. Prior to yesterday, I had the amp set up at 50 watts with the center two tubes driving and biased at 33mA while I was waiting on a new set of tubes to arrive. I received them yesterday and I'm getting a feel for what problems exist now that I can see the meters. I also recently recapped the whole amp and changed out the old grid resistors.

[Mkjames]

The negative grid voltage at pin #5 should be practically the same on all four power tubes.

Something else to check is cathode circuit. How are you measuring bias at the power tubes?
Anyway the ohm measurement from chassis ground to pin #8 on the socket should be zero at all four sockets. If you have 1 ohm bias resistors in place then that should be taken into account. Those resistors if in place should be 1% tolerance or less so again all four sockets should indicate about the same ohm measurement.

Any extra resistance in the cathode circuit could throw the idle current off if measuring directly to chassis ground.

Ohm measuerments should be done with power OFF.

OK. I changed out one socket and got the amp back online. The issue still exists but here's how it tested.

1. Pin 5 tested -47.7 volts across all 4 tubes
2. All four 5.6k ohm resistors tested good with power off
3. Pin 8/1 has no 1ohm bias resistor so it tested at 0 ohms at all 4 tubes
4. I'm using a Bias Pro MQ10 which shows me Cathode Bias current (mA) BTW: My apologies, I described the wrong tube initially. The tube in question was the tube closest to the preamp tubes. I'll call that tube number 1 for our reference. The tubes are also a new match set that I verified on my own tester.
- Tube 1 39.7 mA (Closest to preamp tubes)
- Tube 2 16.4 mA
- Tube 3 17.0 mA
- Tube 4 15.1 mA
5. All four 5w 1k screen resistors tested good and are new
6. I swapped valve 1 and valve 2 and tested voltage and mA with the same results on socket 1

[mickeydg5]

The Bias Pro is by A-sharp Fretworks I presume. If so it uses the 1 ohm cathode method for measuring.

So you changed out the power tube socket that was acting up.

Do you have four 8C type bias probes? Maybe one of the probes is out (of wack). That is if you were placing the same probe at the same tube location each time.

[Marshall Mann]

Good to see you here Mickey!

[Mkjames]

The Bias Pro is by A-sharp Fretworks I presume. If so it uses the 1 ohm cathode method for measuring.

So you changed out the power tube socket that was acting up.

Do you have four 8C type bias probes? Maybe one of the probes is out (of wack). That is if you were placing the same probe at the same tube location each time.

Unfortunately, I don't have any other probes. I did however move the probes (and tubes) around to different sockets and the results were consistent with with previous results.

[Anitoli]

Just a suggestion but im curious if youre having some type of voltage/thermal runaway on the screen resistor or control grid resistor. You could hit the resistor with some freeze spray and see iff the bias come down to where it should be.

Also could try with heat as well. heat the suspect resistors lead a little bit with a soldering iron and observe the bias draw.

[Herbvis]

Did you swapp the tubes around and see if the problem follows the tube?

Nevermind, just saww that you did that. I just woke up.

[Mkjames]

Just a suggestion but im curious if youre having some type of voltage/thermal runaway on the screen resistor or control grid resistor. You could hit the resistor with some freeze spray and see iff the bias come down to where it should be.

Also could try with heat as well. heat the suspect resistors lead a little bit with a soldering iron and observe the bias draw.

I don't have freeze spray, I've never heard of it, however I do have a soldering iron. Am I to assume that when I add heat to the resistor I should see the mA rise?

[Anitoli]

Just a suggestion but im curious if youre having some type of voltage/thermal runaway on the screen resistor or control grid resistor. You could hit the resistor with some freeze spray and see iff the bias come down to where it should be.

Also could try with heat as well. heat the suspect resistors lead a little bit with a soldering iron and observe the bias draw.

I don't have freeze spray, I've never heard of it, however I do have a soldering iron. Am I to assume that when I add heat to the resistor I should see the mA rise?

Yes. The resistance usually falls as the component heats up. It would be the opposite with freeze spray. Make sure you do this with the power off if you heat on the lead. If its one of those square sand resistors you can heat the body of it.
Any of these would work:

http://www.mcmelectronics.com/browse/Fr ... 0000000474

[mickeydg5]

Good to see you here Mickey!

Hi Marshall Mann
I visit this site from time to time and comment on few things here and there.
It is a very nice site.