marshall dsl 100w tubes red plate

[wizard60]

Hi there everyone....what a great place to visit for some info....i'm new to this....if any one can help me with a problem on one of those dsl 100w things.... you know... the early ones with the bias drift circuit board....i think i have fixed the bias drift..but the other problem i have is when i power up the amp in play mode .... the tubes go very blue and hum ..then red plate...in the early stages the amp would crackle a bit then quickly calm down and only one tube would be really blue for a short moment and then go normal......the amp would hold a steady bias and play very well....even if you turn it off then shortly after back on again.... but if you let the amp cool totaly... the crackle comes back and the tubes takes off. Now the problem has gone to all the tubes...(red plate)

With the power tubes out of the amp and not in play mode...i have -15v on (i think it's called pin 5) on v5 and v6... and -2v on pin 5 on v7 and v8..... bridge rectifier tested good...new coupling caps....the resisters were replaced with metal film before the crackling started....all solder joints have been done... if anyone would like to help ....that would be so good.... thanks

[bajaman]

what value is the bias set to?
you are sure it is not drifting upward?
what is the revision number of your circuit board?
what make of output tube are you using?
cheers
bajaman

ps: the blue is nothing to worry about - just a space charge phenomenon. The red plate indicates either bias set too low or bad tube.

[wizard60]

Hi ... thanks for the reply.... bias set at 85 mv when the amp was functioning ok....but now.. doesn't seem to matter where the bias pots are set to ....soon as you hit the standby for play mode... the tubes go blue like a gas cooker.. then red plate... you don't get a chance to bias it... i was told... as in my case..if you have no or very low voltage at pin 5 (i think it's called pin 5 ...the one that normaly has a voltage of around -42v with standby off as well as on.... i have -15v on v5 and v6.... -2v on v7 and v8...the heaters seem to work fine on all tubes. Some part in there has stressed over time and got to the stage where you can't even turn the amp on to play.

In regards to an upward bias drift....the answer is no... i played the amp for an hour or so with constant check of the bias.. all ok.... looke like the new metal film resistors did the trick.

Tubes are el34 sed c

Board revision nunber is jcm2-60-00 iss 345

ps ... the blue on the tubes is not normal... there is blue flames coming out of the slots of the metal housing that sits inside the tubes.... and a loud hum
thanks again

[bajaman]

change the tubes - one or more of them is bad
bajaman

[wizard60]

That was the first thing i did ....a good quad set of svetlana.... one thing i do know ...it's not the valves. it started with one tube ...then two tubes swap them to the other side ...no different...problem stayed in the same position... then it went to the four tubes.

I read somewhere that there needs to be a negative voltage in the range of -15v to -60v at pin 5... which in my case i remember very clear having -42 on all the output tubes with the standby on or off.... and as i mentioned ...at the moment i have -15v on v5 and v6 ...and -2v on v7 and v8 The grid and screen voltages have been there all time....

[Enzo]

Yes, there should be something like -45v on the grid, which is pin 5. 15v or 2v will most certainly result in melting tubes. Hopefully you have not melted the screens in your new tubes.

[wizard60]

low grid voltage

Thanks for the message.... when i get the grid voltage back on track ...it should turn out a good amp....those couple of hours of playing it after resoldering the board and replacing those 220k grid resistors... to 5k6 was bliss.... i'm hoping someone here may have some info on that low grid voltage.

[52 Bill]

I don't know if there is a language problem or an understanding problem here, but the low grid voltage is the bias problem.

The negative voltage bias supply is what sets the output tube current draw. With -2 and -15 volts on the grid pins (#5), your output tubes are drawing way too much current. That is why they are red plating and overheating.

Go back and check the bias supply circuit. This is not a different problem, you have not fixed the original bias problem.

[wizard60]

Thanks for the info.... not sure what to call that voltage... grid or bias????... but those low negative measurements were taken with the power tubes out of the amp... so to my understanding there is a problem before the tubes... if the reading was even on all #5 pins and up at -40v to -45v it should then be ok.... do you agree??

[52 Bill]

Yes both sets of output tubes should have somewhere around -45 on pin #5 before and after the tubes are installed.

As I said before, if there is only -2 or -15 volts there the tubes will overheat and die.

[wizard60]

yes..... i have mentioned the low and un even negative voltage on pin #5 in all of my replies including the start of this thread.... sorry guys but i'm not good at this electronic talk.... and i do appreciate everyone's help