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Spoiler alert. It isn't the first stage.
This is frustrating because feel like I'm sooo close. After we solved the case of the wandering center tap voltage being the cause of some real nasty transient distortion, the amp opened up and really started demonstrating the predictable behavior I've been looking for. Leaving the noise issues aside for the moment, the amp is transitioning into and out of Overdrive/clipping beautify, with sensitive response to pick attack. I'm getting a wide range of control using the guitar volume in the transition from clean to overdrive. But the back to the source of the noise...
It's really strange. I have 3 balanced stages in the output which include a long tail phase inverter, cathode followers, which directly drive EL84 output tubes. On the front panel, there's a toggle to make Global Feedback selectable or not. Yet the common mode noise rejection is dialed in and if I put my ear to the speaker I can hear the faintest 60Hz but it's inaudible a few feet away. Microphonics are null and all these stages seem impervious to acoustic vibration.
Yet when I install the V2 tube (which supplies two stages), this seems to be the source of most if not all of the noise problems (V1, which is a parallel input stage, does not seem to suffer any sensitivity to microphonics and removing/installing that tube seems to have no effect on the noise).
This leaves the second or third stage. Lifting the 100k plate resistor in the second stage eliminates the noise almost entirely. This leaves the tone stack and second gain stage as the noise source.
Here are the symptoms: Highly sensitive to microphonics. The noise could be described mainly as a hiss kind of noise, but there is a little bit of hum as well. The amplifier is LOUD anyway, but noise I'm talking about is significant when the volume is down completely.
I'm feeding both the first and second stages from the same filter node, and I'm going to make some changes to how I've physically grounded them. I'm suspicious that I'm getting some coupling and this may be contributing to the issue. If I had to do it again, I would probably decouple these stages and will if I build more.
Here is a schematic of the power supply, and first two stages along with the wiring layout:
Here is the layout as it currently stands. I've tried to make the illustration as clear as possible to show how it is wired. I omitted the shielded wire from the input jack to the grid stopper on V1 and the shielded wire from the volume pot to the grid of V2. (translucent lines indicate under-board wiring, which seemed like a good idea at the time).
Here I've made changes which I feel are a better ground scheme, bringing all stage grounds to a common point at the base of the cathode resistor. Then running a connection to the grounded filter node. Plus I've changed the physical layout of the tone stack.
I've tried different tubes in V2 as well as replaced the grid resistor, plate resistor, and cathode resistor.
I'm open to and appreciate all insight/suggestions. Thanks
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