3 channel preamp hum issues

[shooter]

you can use it "backwards" also, stick a cd player's output (volume controlled handy) into the amps input jack, plug headphones or small amp into "test box jack" and probe the same points to evaluate input signal.

[Captain chunkulus]

Okay, made the probe tried it. Volumes from the test tone I used seem to be pretty consistent across all of the points that you showed me.

[Willabe]

Okay, just made an interesting discovery. Coming off the input stages, I decided to try an experiment. So coming off the coupling cap where it has a 2meg to ground, it connects to all three gain pot circuits. I lifted the coupling caps on all three, one at a time. Here's the weird. I'm still getting guitar signal on all three  channels. No distortion but clean on all three. Way less on the clean channel but pretty loud on both distortion channels. This should have killed all the signal entirely because nothing is connected to push signal down the line.

This has to fixed.

This could cause the buzz/hum. Even if it's not causing the buzz/hum, the 3 channels signal have to be isolated from each other.

I think it's probably cross wired in the relays.

[Willabe]

Have you tried putting the tube shields on the tubes?

If so any help at all?

[Captain chunkulus]

Have you tried putting the tube shields on the tubes?

If so any help at all?

Yeah, no difference.

[Captain chunkulus]

Okay, here's a discovery. So as previously stated the Transformer hums. With no signal at all, with the gain pot and volume control all the way down and just the output of the preamp to the front of this amp, there is hum Coming through the amp. I tried lifting the ground, the hum remains. I tried a completely new power supply , the hum still remains. If I unplug the guitar cable from the preamp to the front of the amp the hum goes away. Yet, when I lift the ground from the preamp to the amp the hum remains.

[shooter]

If I unplug the guitar cable from the preamp to the front of the amp the hum goes away. Yet, when I lift the ground from the preamp to the amp the hum remains.

I consider an amp "hum free" when it's all by itself, nothing jacked in and gain/vol/ts ~~ 80% of max and NO hum.
by "lifting" the ground with nothing plugged in your pre is floating, susceptible to all manner of room/power hums n buzzes

[Captain chunkulus]

I hear ya. I ordered a new transformer today. I figured I'll give it a try. I still can't understand how I have hum even when the preamp isn't even turned on. With just a cable plug from the preamp to the front of an amp that's turned on there's hum. Yet,  I can plug a guitar into the front of the amp direct and there's no harm whatsoever. I really feel like this preamp is cursed. LOL

[shooter]

I really feel like this preamp is cursed

when I was "figuring tubes out" I did a lot of quick n dirty builds, about 1 in 8 were evil, n since I trouble-shot for a career the LAST thing I wanted to do to relax was chase sneaky gremlins for FUN, so I'd just reduce to parts, come up with another wheel n move on 

[Captain chunkulus]

Thanks so much for this Shooter. I try to do my best for layouts but I'm obviously still learning. I've made a few movements twords killing the hum. I'm down to 1 high gain channel like one of you guys suggested. I have very little hum now. Almost enough that I could just move on but I think I want to keep plugging at it. A couple of questions I have though. I have merlins book and in his section on filaments  he says that in order to supply 1 12x7 with dc it doubles the current needed for the circuit. So in other words, instead of it taking 300 milliamps of power it takes 600 milliamps of power. Question I had is if you were going to run the entire 12 X 7 tube circuit on DC, what that requires 600ma of power times 7 tubes? So 4200ma? My Transformer is rated for 4000ma of power on the filament winding. So can I get away with running all 7 tubes on DC and it be okay? Or is it just too much to push it that far? Thanks.

I really feel like this preamp is cursed

when I was "figuring tubes out" I did a lot of quick n dirty builds, about 1 in 8 were evil, n since I trouble-shot for a career the LAST thing I wanted to do to relax was chase sneaky gremlins for FUN, so I'd just reduce to parts, come up with another wheel n move on 

[shooter]

running all 7 tubes on DC and it be okay?

if you do this math, I'd probably just go for it 
4000/4200*100 = answer in %

Glad you're getting hum free, leaves more time for strumming 

[sluckey]

Before committing to building this circuit you may want to temporarily connect a 12v battery to the filaments to see if it eliminates hum. Lawn mower, portable generator, kids toy car, even your automobile battery. You may even have a backup battery in your computer UPS or security system.

[Willabe]

I've made a few movements towards killing the hum. I'm down to 1 high gain channel like one of you guys suggested. I have very little hum now.

What exactly did you do to kill the hum?

[Captain chunkulus]

I've made a few movements towards killing the hum. I'm down to 1 high gain channel like one of you guys suggested. I have very little hum now.

What exactly did you do to kill the hum?

I moved the input jack ground from the grid ground of the first tube to the chassis ground near the input jack. Im allso running all the tube on filtered dc. I also moved the eq components for the high gain channels farther away from the gain pot inputs and outputs.

[Captain chunkulus]

Okay, still working on this preamp. I have lyme disease and had to drop it for awhile cause I've been sick. Anyway. I have very, very little hum. I'm running the heaters on filtered dc. Also elevated the heaters to 78v and am using a humdinger as well to balance the heaters. I'm using a none isolated input jack. The first tube is a parallel triode and that entire circuit node is grounded right at the input jack. The rest of the circuit is fully bussed and grounded from the very end of the buss nearest the right side of the chassis to the very first stage's filter cap which is tied to the input jack ground. Now here's what I can't find. There is a very low leve hum that isn't affected  by the volume pots. The minute  I plug the preamp output into the amp I'm using even if the preamp isn't powered on creates the hum. I've used an ebtech hum eleminator to kill the hum but it persists. It's just there no matter what I do. Any ideas? Thanks.