Need some help in Dallas TX ...scoping and amp (new pics)

[rdjones]

Where would you ground the filament CT...Currently is shares a ground with the preamp cap can. Also, the B+ CT is grounded on a 50v zener bolted to the chassis.

The filament CT can probably be left for now.
Heater hum will be 60 Hz and you have to sort out your hash first.

Since you're using both a can and zener this precludes some of the more effective grounding schemes, as suggetsted by tubelectron in point #2.
The output section's cathodes, filter cap (and the bias system) should be tied as close as possible to the negative of the zener so as little (or no) current flows through the chassis.
If you can't connect a lug under the diode's chassis mounting get it as near as possible.

Merlin has as good as any reference to grounding:

http://valvewizard2.webs.com/Grounding.pdf

http://www.freewebs.com/valvewizard2/Grounding.html

reddog

[MGW]

Rewired the amp using Bruce Egnater's ground schem with two exceptions..I have nothing sharing the mains ground, and the CT on the filaments is not used. Used a pair of 100 Ohm resistors on V1 for artificial CT. B+ CT, both cap cans are grounded to one location. Used a ground bus on the pots. Here is a pic. The wire on the bias supply grounds between the bass and presence pot. Definitely 120HZ noise. 60hz goes away when I put the lid on the bottom. ALso removed the zener on the B+ CT...getting 505vdc at the revervoir cap. Is that going to be an issue or should I put the zener back in?

[Phil_S]

What is that yellow wire from the tip of the #2 input jacks to the outer lug of the 1st pot?

Where is the ground buss actually grounded? I don't see it in the picture.

You can probably drop 5-10V on the power rail by using a wirewound resistor of proper ohm/watt rating. I don't recall seeing what power tubes are in use for this amp.

[LOUDthud]

If there is an isolated ground terminal for the preamp filter cap(s), tie that to ground somewhere in the preamp, not near the main filter cap.

[MGW]

What is that yellow wire from the tip of the #2 input jacks to the outer lug of the 1st pot?

Where is the ground buss actually grounded? I don't see it in the picture.

You can probably drop 5-10V on the power rail by using a wirewound resistor of proper ohm/watt rating. I don't recall seeing what power tubes are in use for this amp.

The low input jack is in between the first two gain stages. The yellow wire is feeding the gain pot. When you plug into the low jack it bypasses the first gain stage.

As to the ground, I assume it is just grounded to the chassis through the pots. There is no other ground connection on it.

The tubes are JJ EL34.

If there is an isolated ground terminal for the preamp filter cap(s), tie that to ground somewhere in the preamp, not near the main filter cap.

I can ground it to the ground bus attached to the pots if that is appropriate. That is the only ground location on the preamp.

I also noticed that when I take the amp of I hear the transformer itself hum.

[Phil_S]

We're blind here. Picture please that shows all the grounds. Otherwise, it is a WAG.

[MGW]

Here's the pics. I've added comments in them.

[img768]http://millerguitar.com/forumpics/photo2.JPG[/img]
[img768]http://millerguitar.com/forumpics/photo3.JPG[/img]
[img768]http://millerguitar.com/forumpics/photo4.JPG[/img]
[img768]http://millerguitar.com/forumpics/photo5.JPG[/img]
[img768]http://millerguitar.com/forumpics/photo6.JPG[/img]
[img768]http://millerguitar.com/forumpics/photo7.JPG[/img]
[img768]http://millerguitar.com/forumpics/photo8.JPG[/img]
[img768]http://millerguitar.com/forumpics/photo9.JPG[/img]

[Phil_S]

To my amateur eyes, you've got a ground loop. The B+ filter cap & CT ground is separate from the cathode & speaker ground. As I understand it, these are at the same ground potential, yet make contact with the chassis at two different points. I'd pick one spot and that's the ground for all of those. You've got plenty of room between the PT and the board. There is no need to tuck in the ground point in such cramped quarters.

I don't see, maybe I missed it, where the preamp ground buss is grounded to the chassis. If there is none, I'd run a wire from the end closer to the PT to the ground point established for all the stuff in the previous paragraph. Some people like to ground this one near the input jack, which I've never understood, but they swear by it. In any case, I think you need to bring the buss to the chassis somewhere.

I have one safety comment on the mains ground. That wire should have enough slack so that it will be the last to get yanked should there be a mishap and the IEC inlet breaks free from it's mount. It looks taught to me.

[LOUDthud]

If there is an isolated ground terminal for the preamp filter cap(s), tie that to ground somewhere in the preamp, not near the main filter cap.

I can ground it to the ground bus attached to the pots if that is appropriate. That is the only ground location on the preamp.

That should work at least better than what you have now. Do it at the point where the V1 and V2 cathodes connect.

[TUBEDUDE]

I see you have the cathodes and speaker grounded on the chassis. i don't like to ground to aluminum chassis except at one point where my bus bar originates, near the chassis ground. All other grounds go to the bus bar, the cathodes of the power tubes i ground at the filtercap ground location on the bar. Aluminum has good magnetic qualities for a chassis, but is not a great conductor. Some small resistance can exist between two chassis grounds. Even small resistances in a ground circuit can generate noise, and the more current, the greater the developed voltage.

[MGW]

I moved the cathode and speaker ground to the lug with the filter cap on it. Grounded the ground bus from the front pots to the same lug as the first cap. The second cap is grounded on the bus bar at the same location as the first two gain stages. No change.

[Phil_S]

Are you doing all this testing with the open side of the amp exposed, that is, not in the cab and not shielded? I know we've been looking at 120Hz ground problems. Maybe you can humor me yet one more time and do something really simple for 60Hz hum. If the amp is open, make a shield for the open side. You can do this by gluing a sheet of regular kitchen aluminum foil to a piece of cardboard. Cover the open side of the amp and see if the hum goes away. It will take you 10 minutes and cost you about 10¢. The effort:potential ratio on this is quite good and that makes it worth trying.

[ToneMerc]

Aluminum has good magnetic qualities for a chassis, but is not a great conductor.

A common misconception that is continuously passed around.


http://www.wisetool.com/designation/cond.htm


http://www.kp44.org/ftp/ElectricalCondu ... erials.php

TM

[TUBEDUDE]

Thanks ToneMerc. Thinking about it, I don't know why. I accepted that as fact. Must have been from a "trusted" source. Mea Culpa. I like to ground everything off the bus bar however.

[MGW]

Phil...thanks for the tip. However, that was one of the first things I did. Only not aluminum foil. I used and actual sheet of aluminum that will go in the head cab. In short it is shielded. That makes a big difference in 60hz...I think it pretty much eliminates it. However, there is still a fairly significant hum at 120hz. Is it possible that the transformer could be causing it? Would putting two pairs of 1n4007 diodes instead of just the pair make a difference?