Poseidon/MarkIII startup problems
Posted: Sat Mar 10, 2012 11:31 am
Hello all!
I'm new to the forum and new to tube tech. First off, allow me to thank all of the knowledgeable folks here who have educated me thus far as a learker. I have acquired a pair of Dynaco MarkIII's and, with careful research, have restored/upgraded with Poseidon boards and SDS Labs power supply capacitor boards, and ALL NEW parts. The only remaining original parts are the chassis and transformers.
At first startup, both amps suffer the same symptoms:
(1) I have no bias voltage at front pre-amp socket test point... further inspection shows no voltage at pins 1/8 of output sockets. I read 10ohms through either test point
(2) Voltages don't check out:
Power transformer 2nd's are about 400VAC each leg for HV, and heater voltages are about 3VAC per leg.
Voltage at bias diode look ok at about 56VAC cathode and -76vdc anode.
Output sockets pin 5 or 6 are about -54vdc
Posiedon:
H= -60vdc
F&G= -59.8vdc or so once balanced
So... my questions are as follows:
(1) Could the power tranny voltages be sagging due to the series cl-80 thermistor on the mains? 2-3 volt drop leaves about 115v-116v measured at the primaries.
(2) What might cause the absence of voltage at output tubes pin 1/8?
These conditions occur with or without tubes (rectifiers have yet to be installed) and are currently present in both amps... which leads me to believe that I may have made a mistake, or possibly have overlooked something.
Many thanx in advance for any help with this matter, and best wishes to all,
Bob
I'm new to the forum and new to tube tech. First off, allow me to thank all of the knowledgeable folks here who have educated me thus far as a learker. I have acquired a pair of Dynaco MarkIII's and, with careful research, have restored/upgraded with Poseidon boards and SDS Labs power supply capacitor boards, and ALL NEW parts. The only remaining original parts are the chassis and transformers.
At first startup, both amps suffer the same symptoms:
(1) I have no bias voltage at front pre-amp socket test point... further inspection shows no voltage at pins 1/8 of output sockets. I read 10ohms through either test point
(2) Voltages don't check out:
Power transformer 2nd's are about 400VAC each leg for HV, and heater voltages are about 3VAC per leg.
Voltage at bias diode look ok at about 56VAC cathode and -76vdc anode.
Output sockets pin 5 or 6 are about -54vdc
Posiedon:
H= -60vdc
F&G= -59.8vdc or so once balanced
So... my questions are as follows:
(1) Could the power tranny voltages be sagging due to the series cl-80 thermistor on the mains? 2-3 volt drop leaves about 115v-116v measured at the primaries.
(2) What might cause the absence of voltage at output tubes pin 1/8?
These conditions occur with or without tubes (rectifiers have yet to be installed) and are currently present in both amps... which leads me to believe that I may have made a mistake, or possibly have overlooked something.
Many thanx in advance for any help with this matter, and best wishes to all,
Bob