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MB DC5 repair

7/30/2003 11:08 AM
Chris ( CMW amps )	MB DC5 repair Hi all Got a DC5 on the bench with hum/volume-loss ..... Don't have a layout-drawing so this takes some more time . The plate-current at idle does vary so .... - oscillations ? - bad bias-supply ? Before I did get the amp the reverb-OT was replaced and some solder-joints were solderd again . Anyone ? Chris

8/14/2003 10:59 AM
Chris ( CMW amps )	bttt Chris

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8/14/2003 1:23 PM
Wild Bill	Chris, what do you mean by "vary" with the idle current? Changing slow or fast by itself or after turning a control? An oscillation is usually like a constant signal. The idle current would simply be higher because the tube is working harder just like having an input signal. This is tricky because the only indication would be reading a higher bias voltage than you'd expect to bring the idle down to the desired point. Does the bias voltage vary? Maybe a bad bias filter but this usually means a constant voltage with bad ripple causing hum. A leaky coupling cap would be my first suspect but only after I had checked for poor/intermittent solder joints. ---Wild Bill

8/14/2003 1:23 PM

Wild Bill

Chris, what do you mean by "vary" with the idle current? Changing slow or fast by itself or after turning a control?

An oscillation is usually like a constant signal. The idle current would simply be higher because the tube is working harder just like having an input signal. This is tricky because the only indication would be reading a higher bias voltage than you'd expect to bring the idle down to the desired point.

Does the bias voltage vary? Maybe a bad bias filter but this usually means a constant voltage with bad ripple causing hum.

A leaky coupling cap would be my first suspect but only after I had checked for poor/intermittent solder joints.

---Wild Bill

8/14/2003 6:02 PM
Chris ( CMW amps )	Hi Wild Bill The plate-current ( using the shunt-method ) did change between two values . I'll check all the things you did mention ( thanks ! ) , did put this amp "away" 'cause of all other amp-jobs over here ..... Chris

8/15/2003 1:53 PM
Wild Bill	Good luck with it, Chris! Another trick that may be helpful in tracking down which stage is generating the oscillation is to measure the voltage drop across the plate resistor. With a 12AX7 usually running 1.5 ma or so you can easily calculate how much voltage will be dropped. If the tube is oscillating you get more drop than you'd expect. I've had problems in an early stage drive the later ones exactly as if it was a real signal. You could watch the voltage drop increase as you cranked up the volume, even though you heard nothing because the signal was really an ultrasonic oscillation. If there was no signal applied to the stage you'd expect to see no change in the plate resistor voltage drop at all. HTH ---Wild Bill

8/15/2003 1:53 PM

Wild Bill

Good luck with it, Chris!

Another trick that may be helpful in tracking down which stage is generating the oscillation is to measure the voltage drop across the plate resistor. With a 12AX7 usually running 1.5 ma or so you can easily calculate how much voltage will be dropped. If the tube is oscillating you get more drop than you'd expect.

I've had problems in an early stage drive the later ones exactly as if it was a real signal. You could watch the voltage drop increase as you cranked up the volume, even though you heard nothing because the signal was really an ultrasonic oscillation. If there was no signal applied to the stage you'd expect to see no change in the plate resistor voltage drop at all.

HTH

---Wild Bill

8/16/2003 9:17 AM
Chris ( CMW amps )	Thanks Wild Bill ! Most of the amps I do work ( repairing + modding ) on are ptp Fenders and 2 or 4 inputs Marshalls ..... Have a nice weekend , I'll post the results asap Chris

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