Quick Hum question

[yalesmith]

I get hum when i turn on the amp, with the standby switch off. I can hear the hum coming through even though on standby. When the amp is turned on, it gets louder and louder as the volume increases. Any ideas? Also, i have an artificial center tap of my filaments (2 100 ohm caps to ground). I am thinking the fact i can hear it with the standby switch might be a tell tell sign.

Thanks

[xtian]

"On standby" is functionally the same as with no speaker attached, right? In this case, the hum is mechanical vibration from a magnetically unbalanced PT. There was a thread here about when to consider returning a noisy PT because of mechanical vibration.

But you've got more trouble if it gets louder with the volume control. Now were talking about the physical relationship of the PT and OT, and/or lead dress issues and filtering.

[Phil_S]

Pull the preamp tubes one at a time (only one out of circuit). See if that kills the hum. Report back. The fact that hum increases with the volume control suggests it is being amplified in the preamp.

What amp? 60Hz or 120Hz? 120Hz is close to the A string. 60Hz is below the low E.

[yalesmith]

when i pull the ef86 tube i still hear the hum, when i pull the 12ax7 the amp goes dead....interestingly enough there is no hum when played through the effects loop. I had gone through this problem before and board members help me out, and now it has come back but different. The amp btw is a dr z stangray clone

thanks

[Phil_S]

The suggests to me the problem is between V1 and the PI. Look at your lead dress and grounds.

[yalesmith]

Stupid question but I have one of those pt that gives you the option of 120/240 volts. You have to combine the wires etc to operate 120 volts which I did correctly ( I think) but my stupid question is does it matter what pt primaries goes to load and what glues to neutral....apologies in advance if this a dummy question

Dave

[xtian]

No, it does not matter which of your PT primary wires goes to hot and which to neutral.

[boots]

Would it matter if the primaries were connected in or out of phase?

I know that when you connect two secondary windings to get the full voltage of both windings, it does make a difference which direction youconnect the windings together. If you connect them out of phase, it cancels part of the voltage.

I'm not sure, but I would think the polarity of the windings matters - i.e., they might "humbuck" one way, and not the other.

[Phil_S]

You get 0 volts on the secondary if the two primary windings are out of phase. I think you get a lot of heat, too, and maybe some buzzing. I've never made the error in real life.

His amp is working, so it means he connected it correctly.

[yalesmith]

ok thanks, I decided to rewire the preamp, ground bus, etc .... Will let you know how that works out.

Thanks Again

[Phil_S]

You don't learn much with the shotgun approach. Change one thing at a time and test. That tells you whether the thing you change does anything. With the shotgun approach, you might fix one thing while introducing a different error.

[yalesmith]

Yes that occured to me, the things I did change was lead dress. Ok i have 3 questions:

1.) I have read that isolating the speaker jack might be useful, but I get confused when it says to ground the jack to negative feedback. Where is the negative feedback found?

2.) I have read I should try grounding the ground bus to the input jack rather than the chassis.

3.) Install a hum pot, I currently have an artificial CT ( 2 100 ohm to ground) The problem is I cant find a schematic on how to to wire the pot...any ideas?

Lastly, when I turn down the volume of the amp I still can hear the guitar, so i am pretty sure it is a ground issue.

Thanks in advance for all the help!

[Phil_S]

Yes that occured to me, the things I did change was lead dress. Ok i have 3 questions:

1.) I have read that isolating the speaker jack might be useful, but I get confused when it says to ground the jack to negative feedback. Where is the negative feedback found?

I am not sure if I found a reliable schematic for this amp. It looks like there is no NFB, so forget about this.

2.) I have read I should try grounding the ground bus to the input jack rather than the chassis.

You are getting into a compound question here. What type of jack did you use? How is it grounded? Maybe it is time for you to post a photo of your build? If you are using a Cliffs type jack, it is isolated. If Switchcraft, then it is not isolated unless you employed a nylon bushing. Some people ground the jack (sleeve) to the buss and then ground the buss to the chassis very near the jack.

3.) Install a hum pot, I currently have an artificial CT ( 2 100 ohm to ground) The problem is I cant find a schematic on how to to wire the pot...any ideas?

On the basis of what you say, I do not think you have filament hum. There is no need to fool with this unless that's the problem. The artificial CT almost always works fine. If you need to do it, it is simple. Just sub the pot for the two 100r. Ground the wiper. Each outer lug goes to one filament supply line.

Lastly, when I turn down the volume of the amp I still can hear the guitar, so i am pretty sure it is a ground issue.

<shrug> Could be a variety of things. Turning the volume to 0 sets it to maximum Ohms. Not necessarily a ground issue, but maybe just a wrong wiring issue. It is easy to overlook the obvious.

[Structo]

90% of hum problems is related to lead dress.

Keep heater wires away from grids.

Keep power supply wires away from sensitive areas in the preamp.

Make sure you have all the heaters in phase on the tubes.
(that is, best to use two colors for the heater wires and solder the
same color wire to the same pin on each tube.
So say you have blue and yellow, make sure the blue
goes to pin 9 on each preamp tube, and yellow to pins 4&5 as an example)

Same with power tubes, same color to same pin on each.