01-16-2018, 07:40 AM
I spent some more time with the amp yesterday. I've also spent some time thinking about it. I really should do both of those more before deciding to fix something. 
Listening to the clip that I posted, I'd have to say that the noise sounds like a resistor (or capacitor) sounds when it passes current unexpectedly. Think failing plate resistor. Except that the noise only happens when playing. The amp is really quire quiet when there is no input signal.
From my session yesterday, I couldn't get the noise to happen with channel 2. But I'm pretty sure that I'm not crazy, so I guess channel 2 is just more resistant.
But my time with channel 1 provided some interesting data. First, there is no question that the noise originates prior to the volume control. The noise can be turned down when it happens, and the signal to noise ratio remains the same. However, the noise is very much dependent on the volume pot on the guitar. Turn up that pot and at some point the noise happens. This explains why I hadn't noticed it earlier, as I tend to just set the volume on the amp around 5 and set the actual room volume using the guitar. In those sessions where the noise did not happen, I just never rolled the guitar up high enough. Working the two volume knobs (guitar and amp) has shown that it is the input signal, rather than the volume, that causes the noise. So, whereas I orginally thought of this as a "rattle", it's really a distortion.
Right now I have two theories:
1. The load resistor is failing. My understanding of load resistors is that they manage the current in the input circuit. Rolling up the volume on the guitar is going to increase the input current, so a failing resistor could cause current leakage wight at the grid, which would certainly be noisy. Replacing the load resistor at the input jack would resolve this issue.
2. I still haven't discounted the possibility of oscillation at the first amplification stage. I know that there is radio signal entering the amp from the pickups/lead (I can hear it). The HF signal could be driving the first tube into oscillation, which would significantly increase it's current draw and would cause it to distort. A grid stopper would resolve this issue.
But in recognition of the fact that there are people that are much smarter than I am, I also am reminded that slider asked about retensioning the tube sockets. A loose socket would cause arcing at the tube pin as current increases, which retensioning would resolve.
So, those three things give me something to work with. I'll report back.
Listening to the clip that I posted, I'd have to say that the noise sounds like a resistor (or capacitor) sounds when it passes current unexpectedly. Think failing plate resistor. Except that the noise only happens when playing. The amp is really quire quiet when there is no input signal.
From my session yesterday, I couldn't get the noise to happen with channel 2. But I'm pretty sure that I'm not crazy, so I guess channel 2 is just more resistant.
But my time with channel 1 provided some interesting data. First, there is no question that the noise originates prior to the volume control. The noise can be turned down when it happens, and the signal to noise ratio remains the same. However, the noise is very much dependent on the volume pot on the guitar. Turn up that pot and at some point the noise happens. This explains why I hadn't noticed it earlier, as I tend to just set the volume on the amp around 5 and set the actual room volume using the guitar. In those sessions where the noise did not happen, I just never rolled the guitar up high enough. Working the two volume knobs (guitar and amp) has shown that it is the input signal, rather than the volume, that causes the noise. So, whereas I orginally thought of this as a "rattle", it's really a distortion.
Right now I have two theories:
1. The load resistor is failing. My understanding of load resistors is that they manage the current in the input circuit. Rolling up the volume on the guitar is going to increase the input current, so a failing resistor could cause current leakage wight at the grid, which would certainly be noisy. Replacing the load resistor at the input jack would resolve this issue.
2. I still haven't discounted the possibility of oscillation at the first amplification stage. I know that there is radio signal entering the amp from the pickups/lead (I can hear it). The HF signal could be driving the first tube into oscillation, which would significantly increase it's current draw and would cause it to distort. A grid stopper would resolve this issue.
But in recognition of the fact that there are people that are much smarter than I am, I also am reminded that slider asked about retensioning the tube sockets. A loose socket would cause arcing at the tube pin as current increases, which retensioning would resolve.
So, those three things give me something to work with. I'll report back.