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MC30 Help

K

kerkalianer

New Member
I recaped a MC30 an measured all the resistors.
All caps now have correct values with very low ESR and Vloss readings.
Now, the amp is playing very nice, powerfull and undistorced.
But ... I measured those voltages...and found that the last 12AX7 must have -42V on the grids
(pins 2 and 7), actually both have -20V !
Both output 6L6 tubes have -40V on grids.
I tried everything to find why that voltage is so low, with no sucess!

Please, help!
 
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Is it possible that your voltmeter is loading down the voltage on the grids when the meter is connected to them? Do you notice excessive heating of the amplifier when it is in use? If the amplifier is working normally and not overheating, then I'd tend to blame the voltmeter for causing the low readings.
 
kerkalianer said:
I recaped a MC30 an measured all the resistors.
All caps now have correct values with very low ESR and Vloss readings.
Now, the amp is playing very nice, powerfull and undistorced.
But ... I measured those voltages...and found that the last 12AX7 must have -42V on the grids
(pins 2 and 7), actually both have -20V !
Both output 6L6 tubes have -40V on grids.
I tried everything to find why that voltage is so low, with no sucess!

Please, help!
Click to expand...
I will ask you to do what I ask everyone who requests help on this forum; please show us pictures "under the hood" (show us your work). Also, did you replace the bias rectifier (selenium) with a silicon diode? Did you change R30 (3.3K) to something higher? What is the serial number? What is your AC input voltage?
 
What's the voltage on the cathodes of the AX7s? If you have -20 on the grids and -40 on the cathodes something is seriously wrong, that means your 12AX7 grids are 20V positive with respect to the cathodes.

Agree with egellings - seems like a measurement error of some sort.
 
MC30bias.jpg
egellings said:
Is it possible that your voltmeter is loading down the voltage on the grids when the meter is connected to them? Do you notice excessive heating of the amplifier when it is in use? If the amplifier is working normally and not overheating, then I'd tend to blame the voltmeter for causing the low readings.
Click to expand...
w3jn said:
What's the voltage on the cathodes of the AX7s? If you have -20 on the grids and -40 on the cathodes something is seriously wrong, that means your 12AX7 grids are 20V positive with respect to the cathodes.

Agree with egellings - seems like a measurement error of some sort.
Click to expand...

Good idea!!!
May be my multimeter.
I will check it later whem at home
Thank you all, egellings, ramjet and w3jn, for the fast answer!
 
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