(read Part 1 if you haven't already)
So, I managed to have a look at the circuit board yesterday. Only one of the screws holding the main PCB required pliers to extract it-as a result of some rust at the end of it.
The other side:
Pretty much as I expected from the other side. BAC1 and BAC2 are 47uF 350V types, textbook plate voltage filter cap values. Interesting to note that the circuit ground is isolated from the chassis by a 10 ohm resistor for noise isolation plus a diode pair as a safety switch in case signal ground goes "live" to a voltage over 0.5 volts. The huge red capacitor is a 220nF 400V DC blocking capacitor to couple the passive EQ board to the high-voltage plate of one of the valve amplifiers sections. Lastly, the "unbypassable" output DC coupling resistor is 1Mohm, so it should not affect "muh toan" too badly.
Looking under the can, we find this:
A Groove Tubes 12AX7, so a decent brand for the time, and from an era before Metasonix introduced adventurous valve choice to guitar pedals and amps. At this point people were overwhelmingly still just using the same four or five valve types that Leo Fender or Vox had used in 1960 to design guitar amps.
As mentioned before - safety...one can say this is the product of a more innocent time, but even then people would put fibre insulator cards between a circuit board with exposed 200 volt solder points and a metal case that is less than a centimetre away. This guy didn't. While I'm sure it is safe, I would not use this at a gig where there might be drink spills...
Part 3 coming soon!
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