As for the fan circuit..... which is also the tube filament power circuit.... and part of the +16V power supply.....
Q17 is regulating the voltage across power amp V1 filament to 12V. By doing that, it also regulates the current in the power amp V2 filament to the proper constant amount. All 12AX7 are made, or should be, to have a given filament current with little variation about 150mA at 12V.
TS1 provides a voltage that rises with temperature. At low temperatures, the output of IC1B is high, which gives Q16 more input voltage, and produces more current in it due to the emitter resistors R65, R66.
The current coming to Q16 is regulated by the tubes and Q17, so there is a fixed amount. Any current in Q16 is not available for the fan, so it slows down.
At higher temperatures, the output of IC1B (an inverting amplifier) comes down, reducing the drive to Q16, allowing more current to go to the fan, which speeds up.
If the fan operates too fast, then either: the voltage at IC1B output is too high (is it good? is it's input OK?), or Q16 cannot bypass enough current (maybe there is too much flowing?).
There should be only about 150mA available, if Q17 is working right (check for the 12V on V1 filament). IC1B can supply considerable current, so Q16 only needs a gain of 15 to 30 to operate correctly.
Since the +16V is supplied from the same circuit, check if it is OK.... IC1B could draw so much current that it pulls down the 16V, especially if it isn't working right, or the circuitry around Q17 is not working right.....
There's a long list of possibles, but some voltage checks will likely offer a good clue to the issue..
A problem with switching gives a clue that the 16V supply may be not so good.... the JFET switches need a full swing on preamp IC3A to operate properly.
Yes I USED TO work for Ampeg...but I haven't forgotten everything.
Last edited by Jerrold Tiers : 12-20-2012 at 08:40 PM.