Are my Grid Resistors Causing Hum?

[The New Steve H]

I'm uploading a shot of one of the tube sockets on my new amp. This is a 5F6A preamp coupled to a Little Wing output section, with a PPIMV included. And it has four output tubes and two OTs.

It has a little hum in it. Not as much as a store-bought amp, but enough to be irritating. I didn't notice any hum until I cleaned up the wiring. Most of that centered around the grid and plate wires for the output tubes.

I have read that it's a good idea to keep heater wires far from grid wires. But the tubes I used--6BM8--are arranged so the grid pins are right next to the heaters (3 & 4).

Originally, I had the grid resistors attached to the pins, with the other ends attached to wires, with no support. In other words, it went pin-resistor-wire-PPIMV. Then I noticed that I could get more support if I put the grid resistors through unused pins. Now it goes pin-resistor-unused pin-wire-PPIMV.

I'm wondering: did I increase the hum when I did this? There is no way to get the tube-side end of the resistor away from the heater wire, but running the resistor across the socket keeps the resistor closer to the wire.

[Jana]

There are no unused pins on that tube--pin 8 is the cathode of the triode section.

http://tdsl.duncanamps.com/show.php?des=6BM8

[Super_Reverb]

So you are not using the triode in that tube and Jana's note indicates you are using the cathode pin of the triode as a wire solder point. Is it possible that you are getting induced 60 Hz noise due to this connection? In the physical tube construction, the cathode is the closest element to the heater, right? Maybe try using the triode unused plate.

Another idea for grid stoppers and other smaller series elements is: after you solder the resistor to end of wire, use several layers of heat shrink tubing to stiffen up the wire/resistor joint. I overlap the shrink tubing about half way over the resistor and have never had any joint failures.


cheers,


rob

[Gaz]

If I were you I'd isolate the hum before desoldering stuff.

[The New Steve H]

The funny thing about this is that I thought I was being professional by eliminating the wacky resistor-hanging-off-the-tube setup. Originally, the resistors were soldered to the tubes, and I ran them through shrink wrap for support, as someone just suggested.

I guess I could run them through pin 1 or 9, or just go back to the original arrangement.

Maybe I should work on two of the tubes and turn the others down while I'm locating the hum. That would beat unsoldering four resistors and rewiring.

[The New Steve H]

If I unplug the guitar and the hum goes away, can I assume the problem is in the input/preamp area? I would think hum from the output heaters would be impossible to get rid of by killing the input.

[Gaz]

Ok, that means it's either the guitar/cable, or input wiring. To determine if it's the input wiring you can build a "dummy plug" by taking a 1/4" jack (like from a busted cable), and short the tip to sleeve. When you plug it in, it will un-short the first grid (usually shorted by the input jack when nothing is plugged in), but won't pick up any additional noise.

In general, noise rarely comes from the output stage because the signal to noise ratio high at that point in the amp. I always suspect the preamp if I have ever have spurious noise issues.

[The New Steve H]

It was just a general question. I haven't worked on the amp today, so I haven't been checking.

[briane]

If I unplug the guitar and the hum goes away

to me that says it all. Problems not in amp - its in your cable or guitar.

In my experience, given a well built amp - typically 99% of the hum comes from the guitar. Ussually bad pots or caps. I dont know what axe your using...But does it have WIMA or PIO caps and CTS pots? Thats required for a hum free guitar. of course singles will always have some hum no matter how well the guitar is built electronically.

cheap pickups - same problem. Have you verified that your guitar build is hum free?

[The New Steve H]

Okay, whoa, let's hang on. It was a general question. I wasn't saying it came from the guitar.

I've been fiddling with the amp, and it looks like I get a small amount of 120 Hz noise when I plug the guitar in. I'm not worried about that. But I have a little 60 Hz noise and 120 Hz noise all the time.

I have a new question. Merlin Blencowe says to run heater wiring across tube sockets to avoid hum. I did that. I looked at my tubes just now, and I realized that this means resting other connections ON the heater wires. I am posting two photos. In one, there is a jumper across pins 3 and 8 (12AX7 cathodes), and in another, there is a resistor across pins 1 and 6 (12AX7 outputs). This doesn't look smart to me. It sounded good in theory.

What's the answer? Is it okay the way it is? Should I move the heater wires around the tubes? Should I leave the heater wires in place and raise the jumper and resistor off the heater wires?

[briane]

ok - didnt mean to jump the gun - just going on experience. I've had to rebuild a lot of guitars electronics over the years to get an amp to be dead quite.

connections ON the heater wires

anytime components or wires physically touch, you will have signal injection - or more appropriately signal flux.

Wires and components should never touch.

[The New Steve H]

I decided not to wait for a reply.

I made loops around V1 and V3, replacing the jumpers that went across the tube bases. I put a new 100K resistor on V2, jacked up off the heater wires about half an inch. The hum seems greatly reduced, but here is the weird part: the amp sounds incredible now. Much clearer, without the synthesizer-y sound it had before. I don't know how moving these wires could do that, but it sings like a bird now, and the PT doesn't seem to get as hot.

I must have corrected something else accidentally while I was tidying up in there. Anyway, I love the amp now. And the PPIMV is a great addition.

Just saw Briane's last post. I am wondering if moving that 100K off the heaters did all this.

[Structo]

You were probably getting some oscillation by having the wires too close.

A quiet amp is very dependent on design, lead dress and technique.

[briane]

yep, structo's got it there. I consider lead layout to be another component like a resistor or capacitor...

wait wire length influences inductance - so yes in an odd way wires are capacitors (though not very good in their dielectric properties).

little tweaks can change everything, glad its working out.

[The New Steve H]

This amp is addictive now. I have to give Da Geezer props for coming up with the output section.

I still have to mess with it. The way it's set up now, you can use the PPIMV to turn down two tubes at a time, but unfortunately, it doesn't turn off one OT at a time.