Bias Problems With Marshall DSL100

[wakeskater]

I purchased this amp off of Ebay.

When I received it one of the tubes was broken during shipping.

I ordered a new set of power tubes for it and installed all 4 of them today.

One of the first things I noticed is that when it powered up it had a hum.

I played on it a few minutes and it popped the HT fuse which made me start to think something was wrong.

I did a Google search and it brought me to this forum.

I searched this forum and found the procedure for adjusting the bias with a voltmeter, the 3 prongs, and two pots inside the amp.

As I was adjusting the bias I noticed the tubes on the left side of the amp were glowing pretty bright and one of them was really red completely up and down the tube.

One side of the bias will adjust out to .078V DC.

The other side was way too high at around .450V DC.

I thought maybe a tube was bad so I replaced the very bright glowing tube with one of the old tubes.

That allowed me to adjust the bias down to .078 for a little while but the longer the amp stayed on the higher the bias went it started to get close to .200V and I noticed the tube was also starting to glow again so I turned the amp off.

Does anyone have any idea where I can start to look for what might be causing this?

I would gladly take this amp to a tech, but the closest repair center is 2 hours away from me.

Any help would be appreciated!

Thanks
Jayson

[Amp Kat]

Try switching the good side tubes with the bad side and measure the bias again. If it starts changing again and red plating the tubes it's possible you have a problem. Could be a coupling cap failing or a resistor problem either screen or current sense resistors. Measure from pin 8 to the chassis(Power off) and holler back. Also try leaving the two outside tubes in on both sides and measure the bias voltage on pin 5 of the empty sockets on each side to see if it changes on one side or stays steady on both sides. Should be around -45 to -55 and steady.

[Twist]

Don't get confused on the bias adjustment trimmer pots. The center pin is common to ground but as you look at the back of the amp the right pin is reading the left set of tubes and the furthest left pin is reading the right set of tubes. If your comfortable working while the amp is on then power up the amp with the power tubes out and check for a negative voltage in pin 5 of each tube socket. This is what changes as you turn the trimmers. If you see no voltage on any pin 5 then that's where your runaway current is happening.
If you have a bad tube it can cause a loss of bias voltage in the adjacent tube on that side. If I'm weeding out a bad tube I will leave a speaker load on the amp and plug in one tube at a time and watch the current for about 15 minutes (this is of course after you have resolved that you have no other bias problems). This is also convenient for seeing how closely your tubes are matched. Also try gently pecking on the tube to see if it has an intermittent short.

[wakeskater]

Thanks for the replys.

I dont have a lot of electronic experience, but hopefully I took these measurements right.

Regarding measuring pin 8 to chassis with power off, I am guessing this is ohms? If so I got 0 on all pin 8s or the exact same reading I would get if I touched the leads together. I attached the neg (com) lead to a screw head that holds the tube plate down, hopefully that is considered the chassis.

Measuring the bias on pin 5 with outside tubes in. I left the black lead on the screw head, this time the meter is set on VDC.

On the open pin 5 I started with -46V on the left and -42V on the right. I let it warm up for about 10min and I got -42V on the left and -41V on the right.

With all the tubes out the two pin 5s on the left hand side measure -46 and the right two are -43.

Twist you mentioned watching the current with one tube in at a time for 15 min, by watching current do you mean the bias measurement from the 3 pins or something else? Sorry for asking so many questions...

Thanks again for your help!

[wakeskater]

Ok I think I had my meter on the wrong setting. I took the amp apart and checked the resistance on R6 and R9 I get about 1.3 ohms then after I realized I had the meter set wrong I went back and checked pin 8 and got similar 1.2 to 1.3 ohm readings.

[Twist]

Barring an issue with cold or cracked solder joints it appears you just have a bad tube. Sorry for the confusion when I said to watch your current. Since your actually reading millivolts across the 1 ohm resistors by Ohms law your current is the same. Voltage divided by resistance (1 ohm in this case) equals current. So if you read 70 millivolts (.07 volts) across the 1 ohm resistor then that means there is 70 milliamps flowing through that resistor. Go ahead and power up the amp with a speaker load and plug only one tube in and adjust it for around 25 millivolts. Watch it for a while and keep checking to see if the voltage drifts up. If the voltage at the test terminals starts rising then your current will rise too. Repeat this process with the other tubes. I bet you have a bad tube or two. Do not play through the amp while your checking the tubes. I hope this helps. There are still some other issues that could cause problems but they should be checked out by an experienced tech. Prodding around inside the amp is dangerous. Once you've established it's just bad tubes go buy you a new matched quartet and rebias.

[wakeskater]

Thanks again for your reply.

I resoldered all joints on the powerboard. The humming is not nearly as bad as it was, but it still has the bias upward drift problem.

The seller sent me a pair of new Mullard tubes so I replaced both tubes on the side that was giving me a problem. Same story the longer the amp is on the higher the voltage goes.

Wish I could figure it out, the amp sounds great when played. But obviously its going to eat tubes and give me problems as is.

Barring an issue with cold or cracked solder joints it appears you just have a bad tube. Sorry for the confusion when I said to watch your current. Since your actually reading millivolts across the 1 ohm resistors by Ohms law your current is the same. Voltage divided by resistance (1 ohm in this case) equals current. So if you read 70 millivolts (.07 volts) across the 1 ohm resistor then that means there is 70 milliamps flowing through that resistor. Go ahead and power up the amp with a speaker load and plug only one tube in and adjust it for around 25 millivolts. Watch it for a while and keep checking to see if the voltage drifts up. If the voltage at the test terminals starts rising then your current will rise too. Repeat this process with the other tubes. I bet you have a bad tube or two. Do not play through the amp while your checking the tubes. I hope this helps. There are still some other issues that could cause problems but they should be checked out by an experienced tech. Prodding around inside the amp is dangerous. Once you've established it's just bad tubes go buy you a new matched quartet and rebias.

[Amp Kat]

Hard to say without looking at it as everything you checked is panning out. I think you either have a bad connection around that one tube or a blown resistor or bad coupling cap. Look on the top of that board and see if any of the resistors look brown or burn in the middle. If the 1 ohm resistors are bad it will draw current so make sure they are good. If you continue to play it like that you are in serious danger of costing yourself some serious bucks to get it fixed so we need to go maybe deeper or you can take it to a tech.