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In Reply to: Help with ampex 6973 monos posted by brent101 on April 21, 2016 at 14:26:39:
I also have a pair of the Ampex 6973 amps that I rebuilt. My units also have a 33K resistor between the 40uf and 20uf caps.
Have you checked the B+ voltages? If so are they within the specs given in the circuit diagram? Does the 33K burn out quickly?
To figure out what's wrong with your amp I would address your hum and resistor problems separately.
First figure out what's causing the 33K to overheat and burn-out. The reason is usually the circuit is drawing too much current across the resistor. That said it could be 1) the 20uf cap is leaking or a fault in the phase inverter (12AU7) circuit 2) the 10uf cap leaking or a fault in the input (ef86) circuit.
A preliminary way to determine a leaking cap is if it get warm. Unplug the amp and discharge all the electrolytic caps before checking temperature of the 20uf and 10uf caps.
To further check the caps, remove the + leg of the cap and use the cap test on your DMM (unplug the amp and discharge all the electrolytic caps first).
To determine if the fault is with the phase inverter disconnect B+2 and check if the 33K heats up.
To determine if the fault is with the input section disconnect B+3 and check if the 33K heats up.
Once you determined what if frying the 33K the hum may no longer be an issue.
Hope that helps!
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