JCM 900 slx bias trim pot

[Leevi]

I should bias the following amp but met an issue there


https://el34world.com/charts/Schematics/files/Marshall/Marshall_jcm900_slx_50w_2500.pdf


The trim pot does not support the setting and the maximum value I can get is only 11mA (5881).
The corresponding maximum voltage  is -46V. So something should be done.


It is a PCB model and thought to ask which resistor you recommend to change around the trim pot?
Decreasing the value of the R28 (22K) does certainly have some effect?


/Leevi

[Leevi]

10K parallel with 22K (R28) solved the problem. Control area is now from -41V to -35V
/Leevi

[sluckey]

R28 should be 47K for 5881s. 22K is used for EL34s.

[Leevi]

47K in this case moves the voltage to wrong direction  I.e. to more negative voltage. Probably there is something wrong elsewhere?
/Leevi

[sluckey]

Maybe weak tubes?

[shooter]

just typing along those lines, did you try the range without tubes?

[Leevi]

Maybe weak tubes?

I'll verify that with new tubes. But I don't believe that is the reason. The negative voltage was adjustable to -46V (in standby) so the reason must be in the bias circuit.
/Leevi

[2deaf]

Ya know, the reason to draw a schematic is so that other folks can know how the circuit is.  The guy that drew that SLX schematic should have been taken out and horse-whipped.  Then fired.  With no pension nor benefits.  Maybe he was and he went to work for Fender.  I'm starting to think Giogio is right and we were genetically engineered by ancient aliens.  Our intelligence has been declining ever since so that the younger people can't figure out why the older people drew schematics the way that they did.  The same thing applies to state and federal forms.

My best interpretation of the schematic has the positive end of the bias bridge elevated +6.3V by the DC heaters.  That would reduce the negative bias voltage by 6.3V.  I am leaving the negative sign off of the bias voltages so that we are only dealing with a magnitude.  The LT is 40V_rms and in an ideal world that would make 56.6V_p less a couple of volts (from the diodes) for the bias voltage at the bias reservoir capacitor.  Well now, if you took 6.3V off of that, you ain't going to be very likely to get 50V after a voltage divider.  Having to interpret a schematic is a pretty good indication that you are dealing with a p!$$-poor drawing.

If the 6.3V_dc was actually the opposite polarity, it would add to the bias voltage.  That would make a lot more sense.  The hottest bias voltage in an ideal world with an R28 22K resistor would then be 44.4V.  That's pretty close to the observed 46V.

A 10K resistor in parallel with the R28 22K resistor is 6.9K.  The extreme voltage dividers are 8.2K/53.9K and 8.2K/6.9K.  The observed 41V with the first divider would make 47.2V at the bias reservoir capacitor.  Running with that 47.2V, the largest voltage possible would be 21.6V with the second voltage divider.  The observed 35V is not consistent with this.

I know you ain't owing me but if at all possible, I would love to see some hi-def pictures of the guts.

Oh yeah, did I mention that I was not amused by the schematic? 

[Leevi]

A 10K resistor in parallel with the R28 22K resistor is 6.9K.  The extreme voltage dividers are 8.2K/53.9K and 8.2K/6.9K.  The observed 41V with the first divider would make 47.2V at the bias reservoir capacitor.  Running with that 47.2V, the largest voltage possible would be 21.6V with the second voltage divider.  The observed 35V is not consistent with this.

You are absolutely right, BINGO. The R28 is not as stated in the schematic. The resistor is 56K instead of 22K. When connecting the 10K resistor parallel with that gives ~8.5K. What comes to the control range i gave you wrong info, it is from -25.9V to -41.8V.

See the pictures


/Leevi