|Author:||Steavis (registered user: 39 posts )|
|Date:||Wed, May 23rd, 2012 @ 12:49 ( . )|
: 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 :)
On the previous picture it looked like it was messy but now we see it looks good. ;-) No need to redo that I'd say..
: 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?
: 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.
: 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?
that's exactly what I meant. be carefull...! ;-) make sure you dont create shorts. perhaps put the amp in standby or off altogether to clip the alligator clip on the board then switching it on again...
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?
I once had the same in an amp with oscillation problems (SF bandmaster if I recall right). It threw me off completely.. with that particular amp I was expecting stray voltages over the fiber board in the PI section because of what you just described, turned out the problem was oscillation in an added presence contol (instead of MV) which caused motorboating. I had to remove the presence control completely..nothing else I tried in the wiring of the presence cured the problem. I dunno why it happens when measuring the grid of a PI.. I would think the same as you. Maybe someone more knowledgable can shed a light...I'm also curious.
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
okay so bypassing the reverb cured the oscillation problem, but there is still a hum? That's already one problem less. ;-) But it also means the oscillation and the hum are not caused by the same thing.
At this point I wouldn't know what to try next after you've tried all this without having the amp in front of me. you don't have a oscilloscope, do you?
--* Loud hum in October Stage w/reverb
5/23/2012 @ 15:16--Dan McCormack
5/23/2012 @ 15:33----Steavis
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