Adding VU-METER to guitar amp

[carrejans]

After installing this VU-meter, I notice an increase in hum.
Then I measured the plate voltage of the 6L6GC's: 970V!!

I must have done something really wrong. Any ideas?

Thanks...


Heres some pictures:

[guitician]

970v DC?

Were did you have the ground probe connected when taking this measurement?
Did you have this circuit working before you installed it in the chassis? The only problems I could see is your lead dress my be causing a positive feedback from the output to the input of the power section. That's why I asked if you had it working outside the chassis. Maybe add some caps to shunt high frequency oscillations to ground.

I noticed that on one of your tubes the location pin is broken. Are the pins in the right place?

[carrejans]

OMG, I had my multimeter at the wrong setting. My appologies.

But still, before installing the VU-meter and LED, the amp was almost dead-quiet.

Chopstick method doesn't reduce hum.

Funny thing is, that when I cut all the wires from the vu and led; there is still this "new" hum. I think I didn't remove the tubes during soldering. Could this be the cause? (I don't solder long time; no overheat)

[guitician]

Heat damage!... to a vacuum tube!.....That's a good one....lol

You may want to ground some of the preamp grids to see where the hum is starting. Does it hum with the volume controls set to 0?

[carrejans]

Heat damage!... to a vacuum tube!.....That's a good one....lol

You may want to ground some of the preamp grids to see where the hum is starting. Does it hum with the volume controls set to 0?

I just don't understand why it hums even when I remove the new connections. So there must happened something during soldering, was what I was thinking.

I checked with grounding the preamp grids. I first grounded them all, then releasing them one at a time.
The hum situates itself on pin2 of the last ECC83.

The hum increases when increasing the volume, yes. When set to zero, there is still a small hum. (but not irritating)

So, what can be the cause of this?

Thanks...

[guitician]

Did you replace/swap the last tube to see if maybe the tube is causing the hum?

[carrejans]

Ok, so I did a second check.

- no grids grounded => annoying HUM

- pin2 of tube3 grounded + low volume => no HUM at all
- pin2 of tube3 grounded + high volume => still no HUM

- pin7 of tube3 grounded + low volume => no HUM
- pin7 of tube3 grounded + high volume => very little HUM

- interchanging tube2 and tube3 => no difference

[guitician]

If you ground the phase inverter you are muting the power section and there is no way hum can reach the speaker. Are the input jacks a shorting type? By that I mean do they have more than two connections? I the grids of tube #1 are grounded and the volume is up is there still an annoying hum?

[carrejans]

If you ground the phase inverter you are muting the power section and there is no way hum can reach the speaker. Are the input jacks a shorting type? By that I mean do they have more than two connections? I the grids of tube #1 are grounded and the volume is up is there still an annoying hum?

The input jacks, I used are like these:


I connected them in exactly the same manner as here: http://www.ceriatone.com/images/layo...one_100508.jpg


If I ground the grids (pin2+7) of tube1 (12AY7), there is still an annoying hum, even at lower volumes. (but increasing, with increasing the volume)


Thank you guitician for all your kind help so far!!

[guitician]

Those are stereo jacks and you should have left the center pair of contacts alone, or tied them to ground. How are your P-P output tubes biased? If they are running too hot(not enough negative voltage) it could load down the power supply and cause hum. Could your 10uf 450v capacitor be bad?

[carrejans]

Those are stereo jacks and you should have left the center pair of contacts alone, or tied them to ground. How are your P-P output tubes biased? If they are running too hot(not enough negative voltage) it could load down the power supply and cause hum. Could your 10uf 450v capacitor be bad?

-Yes, I used stereo jacks, because I had them laying around. But I didn't use the middle connection.

- The voltage over the 1Ohm resistor is set to 40mV, so I have a dissipation power of 17.2W (plate voltage is 430V). The 6L6GC (JJ) can handle up to 30W, if I remember correctly.
The negative bias voltage (pin5) is -34V.
If I turn my bias to the most negative (-43V), there is a slight decrease in hum, but not enough. But them I'm playing too cold, I think. (only 8W static dissipation power)

- How can I test the 10uF capacitor, to check if it's broke?

- I compared with a tone generator, and it looks like a 100Hz hum. Maybe this can help finding the problem?

[Steve Conner]

I don't really know what could have happened here.

All of the possibilities suggested so far seem wrong, since the amp was working just fine before the meter was installed, and you presumably didn't change anything else while you were installing it.

I think it's most likely that you moved some other wires around while installing the meter, and the wires are now picking up hum. So try poking the wires around with a stick while the amp is turned on, and see if you can find which one is picking up the hum.

Or maybe some wire or component wasn't soldered properly, and you knocked it loose while you were working.

Also, some amps will hum on the bench, but quieten down when you put them in the cabinet, if the cabinet has tin foil on it.

[guitician]

OK, 100hz hum would be on the DC line, from the full-wave rectifier. It can induce itself into your signal path from ground connections and from lead dress. Like Steve said check all your connections. Substituting parts is the easiest way of testing a bad cap. Your bias is fine.