Silverface Pro Reverb Ultra linear with a power transformer mystery that is winding me up

[christarak]

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

[Enzo]

You blow a fuse even with a bulb limiter? That would be odd, a dead short on an amp generally results in the bulb glowing brightly. Unless you are using something huge like a 300 watt bulb.

ANy chance the new cap in the bias supply is backwards? Remember, in a negative supply, the POSITIVE end goes to ground.

I am watching the election returns, getting scared, and sucking up a majority of my 750ml of vodka. SO far it goes well with orange juice, and with pink grapefruit juice.

[christarak]

You blow a fuse even with a bulb limiter? That would be odd, a dead short on an amp generally results in the bulb glowing brightly. Unless you are using something huge like a 300 watt bulb.

Any chance the new cap in the bias supply is backwards? Remember, in a negative supply, the POSITIVE end goes to ground.

I am watching the election returns, getting scared, and sucking up a majority of my 750ml of vodka. SO far it goes well with orange juice, and with pink grapefruit juice.

The Vodka Method sounds perfect, Enzo. Its 7pm here but way too late for you to be up.
My bulb is 100w, but the tripping of the main circuit breaker is instant. The power is shut down before the bulb limiter knows what's hit it.
The new bias caps are + going to ground.

I think the problem has to do with the HT Centre tap and the bias not having its own independent winding...but I can't make the link.

[pdf64]

It would be a good idea to sketch out what you’ve got.

[Jon Snell]

If you have an RCD trip in your house then it has probably got a leak to earth and 30mA won't light any lamp.
Look for a neutral to earth leak. Has it still got the death cap installed? If it has, take it out and throw it away.

[christarak]

It would be a good idea to sketch out what you’ve got.

Good idea, PDF64. When you compare what should be in this amp to this replacement transformer, you can see the secondaries are not the same.

The dead short happens when I connect the Centre Tap. Could it be that the bridge rectifier being earthed and the Centre Tap being earthed is creating short across the HT winding? But if I don't earth the Centre tap then the Bias has no reference to ground. I wonder what would happen if converted the 4 diode bridge rectifier to 2 x 1N5408 with centre tap to ground? That would also give a ground reference for the bias tap.

What do you think? Is it appropriate that I mod a SF Pro because someone put it the wrong transformer, or should I ask the customer to stump up for the right transformer?

[christarak]

If you have an RCD trip in your house then it has probably got a leak to earth and 30mA won't light any lamp.
Look for a neutral to earth leak. Has it still got the death cap installed? If it has, take it out and throw it away.

Hi Jon. An excellent thought on the death cap, but alas the amp has already had its Capectomy. The RCD has kept quiet on all amps except for this one, and a Mesa Boogie (don't get me started on Mesa amps!). Its definitely inside this Pro Reverb. I might try converting the 4 diode rectifier to 2 diodes and grounding the centre tap in the morning. That should give my RCD a good test!

[pdf64]

Yikes, the -ve output of the old full wave bridge rectifier MUST BE disconnected when converting to 2 phase rectification!
As it is, the rectifier is shorting the HT winding, no wonder (and thank goodness) something trips

[christarak]

Yikes, the -ve output of the old full wave bridge rectifier MUST BE disconnected when converting to 2 phase rectification!

Excellent! Then I should be able to connect the Centre tap without shorting, and give the bias winding a path to ground, too.

While you're there PDF, can you suggest a reason why in the original schematic it shows the centre tap being connected the mid point on the B+ totem connection? I can't see why. It seems like its putting a DC offset of 260VDC on the HT AC windings? Am I looking at this arse-about?

[Jon Snell]

"While you're there PDF, can you suggest a reason why in the original schematic it shows the centre tap being connected the mid point on the B+ totem connection? I can't see why. It seems like its putting a DC offset of 260VDC on the HT AC windings? Am I looking at this arse-about?"
That is because it is a voltage doubler.
Here is a simple doubler circuit;

[vintagekiki]

Yikes, the -ve output of the old full wave bridge rectifier MUST BE disconnected when converting to 2 phase rectification!

[J M Fahey]

Excellent! Then I should be able to connect the Centre tap without shorting, and give the bias winding a path to ground, too.

While you're there PDF, can you suggest a reason why in the original schematic it shows the centre tap being connected the mid point on the B+ totem connection? I can't see why. It seems like its putting a DC offset of 260VDC on the HT AC windings? Am I looking at this arse-about?

You are using the WRONG circuit with your transformer.

You have a PT for a "full wave TWO diode" rectifier, center tap connected TO GROUND.

You must NOT use a bridge rectifier with it which will give you CRAZY HIGH voltages everywhere, only POSITIVE bias voltages, etc.

FORGET that schematic you showed.

Some kind Forum Member will certainly dig and post the power supply schematic for your build using that transformer.

Which also, given the extremely high HT voltages involved, it must certainly expect a tube rectifier, also hinted at by the presence of a 5V winding, and which of course will use a full wave rectifier system.

Just wire everything the proper way, grounds and all, including a tube rectifier, and all will work fine.

Oh, and go kiss that hardworking circuit breaker, it saved your amp from exploding and/or burning your house down.

[pdf64]

The use of that rectifier with a Vac input of 750V will have tried to put 1kV on to the HT caps. Everything that may have got a dose of that is now suspect, best to replaced. eg diodes, snubber caps, HT caps etc.

[glebert]

What do you think? Is it appropriate that I mod a SF Pro because someone put it the wrong transformer, or should I ask the customer to stump up for the right transformer?

This sounds like a question your customer needs to answer.

[vintagekiki]

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?

https://www.amplifiedparts.com/products/transformer-fender-replacement-power-325-0-325-v-200-ma
^{Transformer - Fender® Replacement, Power, 325-0-325 V, 200 mA}


1)
The transformer 022798 is designed to work with a tube rectifier.
If instead of a tube rectifier used semiconductor diode rectified DC voltage will be higher by 50 - 100V