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Yea exactly, the noise is still at PREOUT when you pull V4.
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Good, that means the noise is coming from IN the V3 stage.
Find C11. On either side of it are R26 and R85. I would short across each one - one at a time. We are thus rounding the signal path. Does the sound stay or leave at each? Then again those are before V3, and pulling V3 killed the noise. SO I start to suspect a noisy R5 or R14.Education is what you're left with after you have forgotten what you have learned.Comment
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Also, how about a leaking C49?Comment
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I've already replaced R5, R14, C11, R26, R85, C158 coming off V3A awhile back (this was before I even made the original post, I got a bit ahead of myself)Originally posted by Enzo View Post
However, I will still short across to see if anything changes.
C49 is a 400v mylar capacitor, there are a handful of those on the preamp board.Originally posted by doombass View Post
I will test them tonight on my heathkit and see what I find.Comment
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It may not appear leaky on a static leakage test, as the noise would just be tiny little dips.
I hope you realize when I suggest doing something to a particular resistor, while it is possible the resistor itself is bad, my purpose is to isolate the source of the noise. ie the noise is coming in before or after this point. SO by shorting grid resistors, I am also shorting the signal path at that point. C49 for example, we cannot directly ground it, but what we can do is shunt it to ground with a cap. SO instead of a clip wire to ground, I add a cap in series with my wire. I use 0.047/630v caps for this because I have a bunch. But the value isn't critical. With a cap I can "ground" pin 1 of V3 - the rightmost end of C49 - and see if that kills or diminishes the noise.Education is what you're left with after you have forgotten what you have learned.Comment
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Yes I get where you are coming from which is why i'm going to do what you suggested. Could a damaged trace be the cause of this issue? I ask because that preamp board is quite flimsy and doesn't have a lot of support in the middle of it. I also have a.047/630v cap that I can use to test C49Originally posted by Enzo View Post
thanks,Comment
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Possibly. Or poor contact on a ribbon cable connector.Education is what you're left with after you have forgotten what you have learned.Comment
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I attached a .047/630v cap to pin 1 of v3. This significantly diminished the noise, still apparent but I have to get close to the speaker to listen for it. I set it up so I could do a quick A/B comparison and when I remove the cap the noise comes back full force.
I still need to short those resistors though.Comment
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Maybe, maybe not. SO shunting pin 1 kills the noise. I don't care it is not 100% dead. Clearly the noise is already on the signal path there. Does grounding pin 2 - shorting whichever resistor that was - R26 - kill the noise?
Sometimes grounding a grid causes hum. We ignore the hum listening only for the old noise and does it go away.Education is what you're left with after you have forgotten what you have learned.Comment
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Grounding pin 2 connected to R26 of V3 introduces hum but the noise remains the same. No changeComment
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I'm just going to throw my very limited knowledge into the mix. I presume you've cleaned the jacks (including ground tabs), pots, and sockets with contact cleaner, and did a visual check for all the socket receptacles ( female ) themselves closely ?
I've seen new ones that needed to be re-tensioned, after having very little real world use (from a cheap design). Some sockets you really have to press the tube in to get it to seat, and I would use a contact cleaner with a lube for that tight a socket. If the tubes are too lose it causes issues, but uneven grab is also a culprit.
Chopstick every joint in the offending section, to make sure nothing is loose or not soldered properly.
Just very easy stuff to check and do, and my apologies if you've done all that already. I've fixed a few amps with those easy "tricks", but sometimes it's something else..." Things change, not always for the better. " - Leo_GnardoComment
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SO the noise is coming in after R26/pin2.Education is what you're left with after you have forgotten what you have learned.Comment
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Yea, the amp came to me in rough shape. I cleaned all of those components awhile back so the easy stuff has been done. I actually replaced all the preamp tube sockets because they were so abused that re-tensioning the contacts wasn't an option. The problem area is narrowed down, Enzo has been extremely patient with me in this process. I'm going to do a few more checks over the weekend so I can finally put this thing to rest.Originally posted by HaroldBrooks View PostComment
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Excellent. Yes, Enzo and several others here are quite proficient and intellectual in their diagnostics, to say the least.Originally posted by aetherresonance View Post
Glad it was narrowed down through a bisection of symptoms. Best way to start so you are not chasing things on the wrong end of the chassis.
I still have a lot to learn, but it's a certainty I will hanging out around here." Things change, not always for the better. " - Leo_GnardoComment
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I discovered something over the weekend which I believe is worth noting.
I shorted R56 to ground at node 2 (where c60 connects to r56) and the noise significantly diminished. After this I started messing with the resonance and presence knob. These knobs BOTH effect the sound of the noise but it's quite subtle. I missed this previously because it of how slight the change was but the resonance knob introduces more low end into the noise when turned up.
This amp has independent reso and presence for each channel. Each knob behaves the same on both lead and clean channel.Comment
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