I am in the process of building a PP 6550 integrated amp - approx 40-50W pc CW.
Output stage is fixed bias, but (possible unique?) the grid voltage is controlled by a micro computer board (RPi) that allows the bias current to be varied by the control panel from 20 - 90 mA.
It works great, and the bias self adjusts for varying B+ voltages in "real time" and keeps the current to within 0.5mA.
Here is a pic a few months ago showing the control of one valve:
and here is a pic of the current status - one channel, the red box in the foreground is an ex-Woolies biscuit tin that houses the tone control (active 4 triode Braxandall circuit) and the middle board is the driver board (differential pair using 12GN7A's in pentode mode)
Output stage is fixed bias, but (possible unique?) the grid voltage is controlled by a micro computer board (RPi) that allows the bias current to be varied by the control panel from 20 - 90 mA.
It works great, and the bias self adjusts for varying B+ voltages in "real time" and keeps the current to within 0.5mA.
Here is a pic a few months ago showing the control of one valve:
and here is a pic of the current status - one channel, the red box in the foreground is an ex-Woolies biscuit tin that houses the tone control (active 4 triode Braxandall circuit) and the middle board is the driver board (differential pair using 12GN7A's in pentode mode)
