| Author: | Dan McCormack (registered user: 106 posts ) |
| Date: | Wed, May 23rd, 2012 @ 11:26 ( . ) |
On 05/23/2012 @ 05:28, Steavis wrote :
: I don't think your problem is caused by the heater wiring but judging from the picture you should redo the heater wiring anyway. Perhaps use solid core 18g wire as you can bend that in to shape easier and it stays in to the same shape. Twist the wires tightly. Either make heater wires float and keep other wiring close to the chassis, or keep the heater wiring close to the chassis and 'float' the other wires. : : Does the PT have a CT for the heater wiring? If so you do not make a connection to ground. Huh, I accept the criticism, but I didn't think my heater wiring was very messy. I used green wire for both the heater wiring and signal wiring, so maybe it's hard to make out what's what? Here's an earlier picture, before I'd added most of the internal components, where the heater wiring is more visible: [link] (and, as always, a high res version: [link] ). I guess I see a few places where I could have kept the twisting a little closer to the tubes, but for the most part it's about as tight as it can get, I'd have thought. Does it still look like it needs improvement? If so, I'll certainly try again, I just want to make sure there wasn't any misunderstanding before I do all that work :) And the heaters are elevated to about 60 vdc using two 100 ohm resistors: [link] (full res: [link] ) On 05/23/2012 @ 05:28, Steavis wrote :
: > You guys seem to be agreeing that the most likely location of the problem is in the heater network for the long tailed pair. : : We were thinking you forgot a ground connection in the PI circuit. Or I think possibly the reverb section as you get the problem when dialing in the reverb as well. Oops, yes, by "heater network for the long tailed pair" I meant "resistor network for the long tailed pair." Now, I went over the wiring related to the PI, but the only connection to ground in the October Stage schematic ( [link] ) anywhere near the PI is at pin 3 of the presence knob (and that's connected securely). Is there supposed to be any other connection to ground in the circuit of an LTP PI? On 05/23/2012 @ 05:28, Steavis wrote :
: I notice from your picture there doesn't seem to be much solder in the turrets. I assume you soldered it from the bottom before installing the board. Are you sure the connections are all properly soldered? Maybe resolder them with a bit more solder. I soldered from the top and applied what I thought was a decent bit of solder to each turret, but I guess gravity drew it all towards the bottom. I'll add more to them all, to make sure it comes up to the top of each turret. On 05/23/2012 @ 05:28, Steavis wrote :
: Anyways, to sum it up, what I would do to try and tackle the problem: : : - redo the heater wiring (see above) : - add some more solder to all turrets : : : - double check *all* ground conncetions, checking from schematic. (this is where alligator leads come in handy) Can you expand on that? What exactly would I do with the alligator leads? Tie one end to ground and go around clipping it to points that should be grounded, with the amp on (with one hand behind the back and all that), to see what makes the hum go away? By the way, I noticed that when I use my multimeter to measure the voltage at either of the two grids on the PI, the hum goes almost entirely away. But I imagine that's just because the multimeter is applying a load that is effectively pulling the grids down to ground, right? Or does that reveal anything significant? Does that mean the hum is present at the grids of the PI, so it must be originating somewhere before that? On 05/23/2012 @ 05:28, Steavis wrote :
: - bypass reverb section completely, see if problem goes away. Actually, yeah, I did that the other day to see if that eliminated the problem. The hum is still there, but it did eliminate the weirdness with the reverb knob. So I assume there's another issue is how I integrated the reverb circuit, and I'll address that after I solve the hum issue. In the meantime, I'll just leave the reverb section out of the circuit to keep things simpler. Dan |
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