Greetings. I have found another hole in my knowledge that I hope you can help with. I am working on a ProReverb that a previous tech has given up on.
Problem: My house Residual Current Circuit Breaker will shut off quicker than you can say “What the F…” when I turn power on, with the Standby switch in Standby (plate voltage OFF ). The fault is so quick than my bulb current limiter can even raise a flicker.
It looks like the original transformer has been replaced with a NSC022798. There is no separate winding for the Bias, only a tap off the secondary, so both the HT and Bias must share the common ground reference of the HT centre tap. With no connections the PT windings seem to be on spec for this transformer. The values are refenced to the centre tap. The transformer is OK, but can this transformer do the job?
Red (373V) White (0V) Blue (53.9V) Red (376.6)
NSC022798T_FenderTX.pdf
I have a couple of questions:
KNOWLEDGE Question:
I understand that a bridge rectifier does not need a centre tap on the HT secondary of the Power Transformer, but I have noticed that some ultra-linear Fenders (as in this SF ProReverb) that do have a centre tap, don’t ground it. Instead the centre tap connects midway at the HT secondary at 520v÷2. Why is that?
The PROBLEM. (The HT wires are taped off and not connected. The heater is connected and looking healthy at 6.5VAC with no power valves installed.
Clue 1: With the Bias winding not connected and no Centre Tap connection there is no circuit breaker tripping
Clue 2: With the Bias winding connected and no Centre Tap connection = trip the circuit breaker. However, lifting the (replaced by me) bias filter caps = No circuit breaker tripping. Huh? I checked the unfiltered bias voltage, and it was at huge Negative Holy Shit volts. I assume because it had no reference to earth, it was floating very high.
Clue 3: Easy, me thinks, and I ground the HT centre tap with nothing else connected (other than heater) = Bam! Trip circuit breaker.
Clue 4: I connected the Centre Tap to the HT midpoint shown in the schematic with nothing else connected = Bam! Trip circuit breaker.
Any suggestions would be warmly received. I know all my American colleagues are glued to their TVs watching the election results roll in. Even here in Australia, I am on the edge of my seat (yes, sitting upside down, as Enzo would say) watching the results come in. I have a new expression: "if the USA catches a cold, Australia gets COVID".
NSC022798T_FenderTX.pdf
Problem: My house Residual Current Circuit Breaker will shut off quicker than you can say “What the F…” when I turn power on, with the Standby switch in Standby (plate voltage OFF ). The fault is so quick than my bulb current limiter can even raise a flicker.
It looks like the original transformer has been replaced with a NSC022798. There is no separate winding for the Bias, only a tap off the secondary, so both the HT and Bias must share the common ground reference of the HT centre tap. With no connections the PT windings seem to be on spec for this transformer. The values are refenced to the centre tap. The transformer is OK, but can this transformer do the job?
Red (373V) White (0V) Blue (53.9V) Red (376.6)
NSC022798T_FenderTX.pdf
I have a couple of questions:
KNOWLEDGE Question:
I understand that a bridge rectifier does not need a centre tap on the HT secondary of the Power Transformer, but I have noticed that some ultra-linear Fenders (as in this SF ProReverb) that do have a centre tap, don’t ground it. Instead the centre tap connects midway at the HT secondary at 520v÷2. Why is that?
The PROBLEM. (The HT wires are taped off and not connected. The heater is connected and looking healthy at 6.5VAC with no power valves installed.
Clue 1: With the Bias winding not connected and no Centre Tap connection there is no circuit breaker tripping
Clue 2: With the Bias winding connected and no Centre Tap connection = trip the circuit breaker. However, lifting the (replaced by me) bias filter caps = No circuit breaker tripping. Huh? I checked the unfiltered bias voltage, and it was at huge Negative Holy Shit volts. I assume because it had no reference to earth, it was floating very high.
Clue 3: Easy, me thinks, and I ground the HT centre tap with nothing else connected (other than heater) = Bam! Trip circuit breaker.
Clue 4: I connected the Centre Tap to the HT midpoint shown in the schematic with nothing else connected = Bam! Trip circuit breaker.
Any suggestions would be warmly received. I know all my American colleagues are glued to their TVs watching the election results roll in. Even here in Australia, I am on the edge of my seat (yes, sitting upside down, as Enzo would say) watching the results come in. I have a new expression: "if the USA catches a cold, Australia gets COVID".
NSC022798T_FenderTX.pdf
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