18 watt trainwreck

[ampbldr2]

I looked at all my grounds and all look good and all star to one lug on the PT. I took scope measurements and found B+1 has a 12Vac P-P @ 10ms wide sawtooth signal. B+2 has a 400mv P-P @ 8ms wide sine wave signal and do not see noise on B+3 or B+4.

When I probe the trex channel at the volume control pin2 I am getting some loud pops and further into the output it pops when I probe the signal pins. If I turn up the trex volume it induces the noise but when i turn up the stock volume it does not. I am not sure what is going on here. The amp plays fine through both channels just at idle there is a high noise coming out the speaker even with no inputs connected.

Is the noise 60 cycle hum or a high voltage line? How should I go about properly diagnosing the problem here?

[ampbldr2]

This is a good read. http://www.geofex.com/ampdbug/hum.htm

My quest to attack this hum problem may still be at the preamps because when I turn up the volume on the trex channel the hum increases and that is with no input applied. V1 trex input resistors cross over the tube instead of entering v1 pin7 from the side. I have a bunch of other things to try as well. The leads from v4/v5 for pin2 are in parallel with the output leads. Probably a bad idea and I will change them tonight and see if that helps.

[surfsup]

I see there are two RC combos at the end of your board, these must be the V1 cathodes? On the shot of the bottom fo board, there are no leads under the baord connected to them, and its hard to tell from your full top photo where the leads are going. I see the two black leads off the tube at 3 and 8.

EDIT: To minimize soldering, and separate V1 grounds, if you have identical Cs and Rs to make two new RC combos, maybe you can desolder the pin 3/8 leads, tape em off, and temporarily solder two new RC combos at the pin, then run a lead off each to the input jack ground? I'm thinking the other two RCs on the board would not come into play even though they are still hooked up on the backside. oops wait I don't see an input jack ground but you have a hole there already maybe you can separate the input jack and V1 grounds and put em on a new star?

Also your heater wires make some close loops around the preamp tubes?

Your PI cathode looks like its sitting on top of a the heater wires. Also nearer the socket, the cathode lead runs parallel to the grid lead and they seem to sit on top of each other? Not sure how that would affect hum.

[ampbldr2]

I did alot of rewiring and elevated the heaters. At idle I am still getting a good amount of hum and can see it is clearly 120Hz at the output which is the 60Hz after recitified turns into 120Hz. The layout on the trex is tight. I found alot of electro magnetic interference where v2pin2 150ks connect to each other so i moved the resistors closer to each other and that helped when I turn up trex volume it does not squeal anymore. I did not separate v1 grounds from the power sections ground. Instead I tried separating the power section grounds but that did nothing to reduce the hum.

I was wondering if I could elevate the heaters from ground reference with 100 ohm 1w resistors right at the 6.3 taps and connect to the cathode of the output tubes would help?

I am also wondering if I can change the amount of signal into the PI by changing the value of 820 ohm to something greater? The volume on the trex is hair trigger.

[ampbldr2]

I found a good post on 60 or 120 Hz hum and seems the 100 Potentiometer did the trick. I may have to go this route.

https://tubeamparchive.com/viewtopic.ph ... sc&start=0

This was another description "Balancing the Filament Circuit w.r.t Chassis"
http://www.lynx.bc.ca/~jc/hum.html