Try and check the voltages when in the fault condition.
The grid bias for the triode section is by/due to grid current, the high value grid return resistor being the giveaway for that type of bias arrangement, the bias for the pentode section is provided by the current flowing through all the valves, that current has to flow through the 70 Ohm resistor (R16) The voltage developed across that resistor being the pentode's grid bias voltage, supplied via the 1 meg and 1k resistors, R15 & R14)
https://nvhrbiblio.nl/schema/Grundig_80U.pdf
Lawrence.
The grid bias for the triode section is by/due to grid current, the high value grid return resistor being the giveaway for that type of bias arrangement, the bias for the pentode section is provided by the current flowing through all the valves, that current has to flow through the 70 Ohm resistor (R16) The voltage developed across that resistor being the pentode's grid bias voltage, supplied via the 1 meg and 1k resistors, R15 & R14)
https://nvhrbiblio.nl/schema/Grundig_80U.pdf
Lawrence.
