Measuring Bias Voltages

[drillher]

I have a Mc240 that I replaced the selenium rectifier with a silicone diode and a 470 ohm dropping resistor.I replaced the bias filter cap,and the two main power supply diodes.I also replaced the bumblebee cap on the bias board.I hope to borrow a variac.Can somebody please tell me how to use the variac?I see in the Mc240 schematic that I need -46 volts on pin 5 of a 6L6 (which one?) with 430 volts on pin 3.The above voltages come from the schematic.Thanks,
Dave

 

[skippy_ps]

With the amp turned on and plugged into the variac, adjust the variac voltage to 117. Use your vom.

Murray

 

[dshoaf]

On the right track, Murray. I'll add a bit more since Dave and I have been corresponding offline.

Dave, I'll assume you know that you are connecting the variac between the AC outlet and the amp. The amp is plugged into the variac, which varies the AC input voltage to the amp (Vari-AC, get it?;-).

Also, I'll clean the tube socket contacts _very_ well and make sure they are tightened to ensure all the tube pins are being contacted well. No need to try to take readings if there are mechanical contact problems to begin with.

The trick to confirming proper bias after rebuild:

- First, make sure that the power supply voltages are in spec with the variac set to about 117 or so. Generally, I'll monitor the high voltage power supply as I bring the amp up on the variac. I do not trust the little meters in most variacs for accuracy so I'll switch over and check the AC voltage once the amp has been up and running and the high DC voltage is at spec.

**Warning**: Be aware that if the old power supply caps are still in place, the amp may _not_ get to spec or you may have to crank the variac way up to get the high voltage rails in spec. That is, you are reading waaaaay higher than 125vAC when you check the AC voltage. Don't proceed with the following if this is the case. Shut it down and rebuild the power supply. ********

- Next, let the amp cook for a bit - 15 minutes. If the old coupling caps are still in the output stage, monitor the big tubes for red-plating. I keep my hand on the power switch - for good reason. Not a good idea to leave the room, then......

- If all is still ok, then I will set up the DC voltmeter to monitor the plate voltage of the output tubes - pin 3 @ 430V. Adjust the variac so that you get this reading on the meter.

- Then connect the positive meter probe to pin 5 and you should have something close to the -46vDC there if all is well.

As I am sure you're becoming aware, we're dealing with rather high voltage on pins that are close together. The typical meter probes have lots of ways for things to slip and cause all sorts of arcing/shorting problems. I hate it when I'm almost through and then slip the probe across 2 pins. Some of my probes still carry the scars of my inattention.

To help with this, I have taken to using a set of really cool Fluke probes with little wire pullers that you can slip around the tube lugs and they'll grab it. See them here: http://www.web-tronics.com/surpinclipac.html. They also have long, flexible shafts so that you don't have to stick your hands down into the chassis when moving the probes around. This helps with the number of times you have to shut the amp down to reach in and move an aligator clip-type probe. Note that these require the pretty standard probe wire system that Fluke markets.

Hope that helps.

Cheers,

David

 

[skippy_ps]

Lol, David. I find that short answers are better at keeping me out of trouble. Yours is a thorough reply.

Murray

 

[drillher]

David,

Many thanks again.It seems pretty straight forward.The power supply caps were replaced in '07,so I am OK there.The coupling caps are original and are next on the list.I will let you know when I get the variac and measure what I have.
Dave

 

[dshoaf]

Yeah, Dave. Things feel different once you're standing there with the amp heated up, the high voltage is running and you have the probes in your hand - as opposed to just planning and reading about the process.

Cheers,

David

 

[drillher]

I'm sure that is true,especially the part about high voltages.One last question,looking at the Mc240 schematic,I'm not completely clear which tubes in particular I need to take the measurements from.
Dave

 

[dshoaf]

For the small signal tubes and the larger output tubes, you want to take voltage measurements on all the active pins. No need to both with the 2 filament pins, though.

I'd suggest you make a copy of the schematic and write your voltage measurements beside the pins numbers. When you are done, then, you can easily see whether there are problems elsewhere in the amp. This might mean a resistor or 2 are out of spec or there's leakage in the one or more coupling caps. Anything beyond +/-10% of spec calls for attention. Remember, though, that the line voltage affects all this and is why the variac is important.

Lastly, if you care to, scan it and post here so those 'arm-chair' bench techs around here can comment.

Cheers,

David

 

[drillher]

I am only getting 25v,1/2 of what I should be getting on pin 5 of the last 2 6L6 output tubes.All other measurements are pretty close to specs.That calls for attention,I'm not sure what to look at first.
Dave

 

[drillher]

That is -25v,on pin 5 of last pair of 6L6's vs the -50v that the specs.say that I want.

 

[dshoaf]

Sent you an email.......are you reading that on both channels of output tubes or just one set?

If one set, then swap out the 2 sets of output tubes and test again. See if the problem follows the tubes first.

Cheers,

David

 

[drillher]

The low bias voltage is just on one set of 6L6's.I will swap them and re-measure this evening.Thanks,
Dave

 

[drillher]

I swapped the 6L6's from the good channel to the bad and the bias voltage
was about the same,way low.I worked fast on the tip that the -25v would fry
the pair.I switched them back and then found 147.8v at the R52/R53 junction
which was good (the spec.is 150v) and it was the same in the good channel.I
derive that this means that I need to change out the caps and then test
again.I may not be able to start that until this weekend.I also noticed that
the red print on the JJ 6L6's in the bad channel is much darker than the
bright red print on the good channel tubes.I'm thinking that they suffered
over heating and could be toast.This is still
fun for me and I look forward to making it right.
Dave

 

[drillher]

Caps And Diodes Replaced.

I replaced the caps other than the 160P's as Terry D. advised as well as the selenum rectifier and main power supply diodes.I bought all of the components as well as new tubes from Jim Mcshane.I have only a few hrs. on it since the re-build and it is starting to open up and the bass is becoming a bit more defined.Thanks for the help that fellow AK'er's provided to allow me to accomplish this restoration.
Dave