What purpose does the 100k/22uf circuit from heater CT to ground do?
This was original to the Cordovox circuit.
I believe the 100k is to elevate the heaters to reduce hum.
The 22uf is to filter any remaining ripple.
Maybe a dated approach...
Does anyone think this could be an issue? Rarely see it in guitar amps right?
below is a more detail explanation from Gemini
"In a guitar amp, a 100kΩ resistor and a 22μF bypass capacitor on the 6.3V heater center tap serve to "elevate" the heater voltage, which is a key technique for reducing noise and hum. This is most beneficial for sensitive high-gain preamp tubes, as it helps create a cleaner, less noisy signal.
The purpose of heater elevation
While a simple grounded center tap helps cancel some AC hum, it does not prevent capacitive coupling between the tube's heater and cathode, which can introduce a 60Hz hum into the audio signal. By elevating the heater to a positive DC potential (usually sourced from the high-voltage B+ supply), this coupling is significantly reduced.
In a guitar amp, this technique is particularly important in the following ways:
Silence high-gain stages: The first gain stages of a guitar amp are the most sensitive to noise, and any heater hum introduced here is amplified by every subsequent stage. Elevating the heater voltage is one of the most effective ways to eliminate this type of hum.
Manage cathode-biased tubes: In cathode-biased stages, the cathode is already at a positive DC voltage. Elevating the heater voltage ensures that the heater-to-cathode voltage stays within the tube's maximum rated limits, preventing damage and further reducing noise.
Reduce parasitic noise: The high resistance of the 100kΩ resistor, combined with the bypass capacitor, helps isolate the heater circuit from the potentially noisy main signal ground. This prevents ground loops and ensures a quieter operation.
The role of the 22μF bypass capacitor...
The capacitor is placed in parallel with the 100kΩ resistor to form a low-pass filter, which is critical for making the elevation effective.
Filters out residual noise: The 22μF capacitor filters any remaining AC ripple from the high-voltage B+ power supply. This ensures that the elevated potential applied to the heaters is a pure DC voltage.
Creates a stable reference: By providing a low-impedance path to ground for AC frequencies, the capacitor ensures that the DC elevation voltage remains stable, even when there are small fluctuations on the B+ rail. This prevents noise from the power supply from modulating the heaters and causing additional noise in the preamp."
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