PT Open Primary

[Thisismyname]

A friend recently received an amp I built for him, an AB763 Bandmaster with a switchable T-Wreck 3rd stage, cathode/fixed bias switch, NFB switch, presence and resonance controls. This was built inside a 1995 Blues Deluxe using the stock power transformer. Previously, the stock amp had power supply resistors burn up.

I had played it loud for around 10 hours before I gave it to him. Everything worked fine. He had only played it a few hours really quiet when the power cut out.

I found the power transformer primary to be open, fuse is still good. What should I check to find out if the transformer was the problem or the symptom?

[sluckey]

Order a new PT. Build a current limiter light bulb while waiting. 60W to 90W light bulb.

[Thisismyname]

Order a new PT. Build a current limiter light bulb while waiting. 60W to 90W light bulb.

Thanks for the reply.

I already have a limiter. So the test will be to determine the location of the fault, right?
If the bulb stays bright, it's the circuity, if it dims, no excessive current draw and the old PT likely just died on it's own?

[Latole]

Order a new PT. Build a current limiter light bulb while waiting. 60W to 90W light bulb.

Thanks for the reply.

I already have a limiter. So the test will be to determine the location of the fault, right?
If the bulb stays bright, it's the circuity, if it dims, no excessive current draw and the old PT likely just died on it's own?

In a homemade amp with a lot of modifications one has the right to suspect a construction and/or design error. Originally a Bandmaster would require more current (IMO) than a Blues Deluxe whose PT would be appropriate?

And if you ask how a bulb limiter works, I'm more afraid of errors in the Bandmaster

[Thisismyname]

Order a new PT. Build a current limiter light bulb while waiting. 60W to 90W light bulb.

Thanks for the reply.

I already have a limiter. So the test will be to determine the location of the fault, right?
If the bulb stays bright, it's the circuity, if it dims, no excessive current draw and the old PT likely just died on it's own?

In a homemade amp with a lot of modifications one has the right to suspect a construction and/or design error. Originally a Bandmaster would require more current (IMO) than a Blues Deluxe whose PT would be appropriate?

And if you ask how a bulb limiter works, I'm more afraid of errors in the Bandmaster

I know how a limited works, I was just double checking to see if that's what test Sluckey was implying I do. I am very skeptical about executing anything I do, which leads me to do a lot of research beforehand with a lot of second-guessing what I think is right. I'm still very open to the possibility of a mistake I made. That's not something I ever shy away from amd I always encourage people to point them out.

I forgot to mention that it's a single channel bandmaster with no tremolo.

[Latole]

It is very difficult to repair at distance without having the amp in front of us.

[Thisismyname]

It is very difficult to repair at distance without having the amp in front of us.

I'll post updates as I find out more.

[sluckey]

Looks like the Blues Deluxe PT should be adequate for a Bandmaster, but you'll have to use a FWB and maybe a special bias supply.

Power transformers usually don't just fail for no reason. Failure is usually due to some other issue that causes high current to be drawn from the PT. That high current could be caused by a failure (or wiring error) in the filament circuit, or the B+ circuit, or output circuit. May not be a dead short. Just something causing excess current. Usually not too difficult to find. An ohm meter can be very helpful. So can your eyes and nose. Hopefully you can find the issue without powering up.

The current limiter light bulb should allow you to power up without risking your new PT if you use the recommended size light bulb.

[Thisismyname]

Looks like the Blues Deluxe PT should be adequate for a Bandmaster, but you'll have to use a FWB and maybe a special bias supply.

Power transformers usually don't just fail for no reason. Failure is usually due to some other issue that causes high current to be drawn from the PT. That high current could be caused by a failure (or wiring error) in the filament circuit, or the B+ circuit, or output circuit. May not be a dead short. Just something causing excess current. Usually not too difficult to find. An ohm meter can be very helpful. So can your eyes and nose. Hopefully you can find the issue without powering up.

The current limiter light bulb should allow you to power up without risking your new PT if you use the recommended size light bulb.

It has a FWB rectifier. The bias circuit is the stock Blues Dlx. I had to lower the resistor between the bias pot and ground to get proper bias. The tubes aren't well matched, but in the relm of usability according to some forum information I looked into about it.
They are some Sovtek 6L6WGCs that I'm told were in another, less used amp since the 80s. I had some microphonic new JJs that I ended up not using for it. The Sovteks sounded better too.

Thanks for the replies guys.

[Thisismyname]

Looks like the Blues Deluxe PT should be adequate for a Bandmaster, but you'll have to use a FWB and maybe a special bias supply.

Power transformers usually don't just fail for no reason. Failure is usually due to some other issue that causes high current to be drawn from the PT. That high current could be caused by a failure (or wiring error) in the filament circuit, or the B+ circuit, or output circuit. May not be a dead short. Just something causing excess current. Usually not too difficult to find. An ohm meter can be very helpful. So can your eyes and nose. Hopefully you can find the issue without powering up.

The current limiter light bulb should allow you to power up without risking your new PT if you use the recommended size light bulb.

How could a power transformer primary blow without the fuse blowing?

The amp has a 3A fast-blow fuse, as required by the Blues Dlx power section, the fuse holder is not shorted across itself or to ground.

I also checked the DC resistance of the secondaries against a similar PT's datasheet DC resistance (stock PT datasheet excludes this) and the secondary measurements are within the tolerance of the similar transformer DC resistance. The datasheet didn't have a resistance for the bias supply, but I measured 5.1 ohms.

None of the primary or secondaries wires are shorted to the transformer shell, nor are any secondaries shorted to each other.

A visual inspection revealed no burnt components. Nothing in the amp or power transformer has a bad smell. I think I remember a faint smell when I first opened the amp up, but it was far from strong or nasty.

I'm not sure why the primary would blow but not the fuse? It's the right fuse for the power supply, the fuse was in the primary circuit, but the primary failed open without the fuse blowing. I thought fuses were supposed to blow at a current load lower than the upper limit of what the transformer is designed to safely operate at. That didn't happen.

What fault conditions can explain this?

[sluckey]

Fuses aren't fool proof. But, maybe the primary didn't open because of excessive current. Maybe there's a broken wire on the primary winding, possibly where a primary lead connects to the tiny primary winding wire.

Regardless of what caused the primary winding to open, you need a new PT, unless you get lucky and are able to repair the winding. You have nothing to lose by opening the PT and carefully peeling back some paper to expose the primary lead to actual winding solder connections. Worth the effort IMO.

[Thisismyname]

Fuses aren't fool proof. But, maybe the primary didn't open because of excessive current. Maybe there's a broken wire on the primary winding, possibly where a primary lead connects to the tiny primary winding wire.

Regardless of what caused the primary winding to open, you need a new PT, unless you get lucky and are able to repair the winding. You have nothing to lose by opening the PT and carefully peeling back some paper to expose the primary lead to actual winding solder connections. Worth the effort IMO.

 Thanks for the reply. I have the new PT and will replace it this week. I'll use my current limiter of course.

 I'm worried about the amp running fine after the installation. As you mentioned, isolated PT primary fails are rare, and I think the odds of it happening to me and also as the first problem with any of my builds is very slim.
 Crazy coincidences definitely happen, but I'm really hoping this problem will stare me in the face instead of being a mysterious gremlin that may or may not exist.
 For that reason, I will check the old transformer to see how the primary failed 👍
 Hopefully that and the condition of the amp after the installation will result in a clear cause or direction.

Thanks again.

[Latole]

My assuption ;

A slight permanent short circuit ( in the amp) but too small to blow the fuse but enough to heat the transformer windings, and with time, could be the cause of this failure by destroying the insulation of the windings.

[tubeswell]

None of the primary or secondaries wires are shorted to the transformer shell…


I think I remember a faint smell when I first opened the amp up, but it was far from strong or nasty.

I'm not sure why the primary would blow but not the fuse?

What fault conditions can explain this?

Previously, the stock amp had power supply resistors burn up.

[Thisismyname]

None of the primary or secondaries wires are shorted to the transformer shell…


I think I remember a faint smell when I first opened the amp up, but it was far from strong or nasty.

I'm not sure why the primary would blow but not the fuse?

What fault conditions can explain this?

Previously, the stock amp had power supply resistors burn up.

I'm not entirely sure what your implying. The only thing that remains of the stock Blues Dlx is the choke, the power transformer, and the PT primary circuit. My tests found the primary circuit isn't shorting across itself or to ground. Maybe a bad power cable?

Do you think the burned resistors from the stock Blues Dlx damaged the PT enough to shorten it's lifespan?

[Thisismyname]

My assuption ;

A slight permanent short circuit ( in the amp) but too small to blow the fuse but enough to heat the transformer windings, and with time, could be the cause of this failure by destroying the insulation of the windings.

Apart from a visual inspection, is there anything I can do to test this possiblity?

[Latole]

A burn resistor is more like an open circuit ( not short circuit ) It can't damage a PT

[Thisismyname]

I pulled the transformer apart a bit. The primary leads have continuity from the tip of the wires to the spot right before they go into the interleavings. So the break occured inside the transformer and is probably the result of excessive current.

I will inspect every millimeter of the amp and make sure everything is clean, separated from other parts of the circuit, and has a good solder joint. As vain as I am about my solder joints, I'll still assume a solder joint could contribute to an issue. I'll install the new power transformer and power it up with my current limiter.

Do you think a small partial short could go undetected by a current limiter?

[Latole]

I pulled the transformer apart a bit. The primary leads have continuity from the tip of the wires to the spot right before they go into the interleavings. So the break occured inside the transformer and is probably the result of excessive current.

I will inspect every millimeter of the amp and make sure everything is clean, separated from other parts of the circuit, and has a good solder joint. As vain as I am about my solder joints, I'll still assume a solder joint could contribute to an issue. I'll install the new power transformer and power it up with my current limiter.

Do you think a small partial short could go undetected by a current limiter?

1- "I'll still assume a solder joint could contribute to an issue."
     - No, a colder joint is an open circuit wich is safe for PT, not a short circuit wich may damaged PT

2- "Do you think a small partial short could go undetected by a current limiter ? "
     - No, it can't.

[Thisismyname]

I guess I thought if it didn't blow the fuse, it may not be strong enough to significantly show on a limiter. Thanks for the reply.

[Latole]

I guess I thought if it didn't blow the fuse, it may not be strong enough to significantly show on a limiter. Thanks for the reply.

Right !

[PRR]

My 2 cents is on corrosion. Even if it has not been near the sea.

[Thisismyname]

My 2 cents is on corrosion. Even if it has not been near the sea.

Corrosion within the transformer or somewhere in the rest of the amp? The transformer is about 28 years old and the amp circuitry is only about 4 months old.

[tubeswell]

None of the primary or secondaries wires are shorted to the transformer shell…


I think I remember a faint smell when I first opened the amp up, but it was far from strong or nasty.

I'm not sure why the primary would blow but not the fuse?

What fault conditions can explain this?

Previously, the stock amp had power supply resistors burn up.

I'm not entirely sure what your implying. The only thing that remains of the stock Blues Dlx is the choke, the power transformer, and the PT primary circuit. My tests found the primary circuit isn't shorting across itself or to ground. Maybe a bad power cable?

Do you think the burned resistors from the stock Blues Dlx damaged the PT enough to shorten it's lifespan?

Power supply resistors only burn if too much current is dumped through them. This happens when there’s a short to ground on one side of the PS resistor for whatever reason. The BD and Hotrod are well known for dodgy filter caps (and other issues). If a filter cap shorts, there will be 300-400 volts across the PS resistor at that node, which will make it burn and quickly fail short if it’s not rated to take such voltage. A short to ground on the PS rail can over stress the PT. If the mains fuse is correctly rated, it should blow well before the PT catches fire. If someone used the wrong fuse, it would stuff the PT, and you get the frying bacon smell. Once you’ve had that smell, you shouldn’t trust the PT . YMMV

[Thisismyname]

Back from vacation, power transformer installed, powering up, testing voltages. Everything tests normal, but I noticed an indication of a short that has me stumped. I also included a schematic.

When I rub the tip of my multimeter probe across the plate wire/plate resistor turret of the 2nd gain stage/TS makeup stage, it produces a static noise a few inches away in the middle of the amp up by the channel switch area. Haven't located the actual source. My current limiter shows a very small surge of current corresponding to the static noise.

The second indication of a short is when I rub the tip of my probe across the turret that connects the PI input cap, grid leak resistor, and PI grid wire, my limiter shows a much bigger current surge. Power tube voltage decreases too, but current roughly doubles.

But there isn't any static noise rubbing this turret. If I rub the turret that connects the PI grid leaks and cathode resistor, I don't get a current surge. I also don't get this current surge if I rub the turret on the upstream side of the PI input cap.

I'm not sure why this test produces current surge. I'm also not sure where the static noise is coming from. I only looked at a few spots.
 If I knew troubleshooting like this wouldn't stress/blow the new power transformer, I'd spend more time with it. Since I don't know this for sure, does anyone have any advice or guidance to share?

Thanks for any and all help.

Here is the schematic. The red dot shows where the first test (static noise, small current surge) was taken, the green dot shows where the 2nd test (no static noise, large current surge) was taken.

[Latole]

Here's your schematic;

[Thisismyname]

I'm just going to start replacing things, starting with the channel switch. Maybe it got too hot while soldering, melted a bit internally, and drew in solder where it shouldn't have gone. Idk

[mresistor]

So, it looks like this has a switch the switches between fixed and cathode bias,   what does this switch physically look like ?

[Thisismyname]

It's hard to get a good picture of the actual switch.

[Thisismyname]

But it's one of these.

[tubeswell]

The way the DPDT bias change switch is shown in the schematic looks wrong.


When the amp is in cathode bias mode, you want the 220k grid leak resistors grounded and nothing shorting the cathode resistor to ground.


When the amp is in fixed bias mode, you want the 220k grid leak resistors to go to the -ve bias supply voltage source, and the cathodes connected to ground.

[Thisismyname]

The way the DPDT bias change switch is shown in the schematic looks wrong.


When the amp is in cathode bias mode, you want the 220k grid leak resistors grounded and nothing shorting the cathode resistor to ground.


When the amp is in fixed bias mode, you want the 220k grid leak resistors to go to the -ve bias supply voltage source, and the cathodes connected to ground.

I noticed my schematic is wrong; the bottom right terminal is supposed to be unused, while the terminal above it is connected to ground. I messed up transferring my handwritten schematic to a digit one.

The switch is wired correctly in the amp for both biases.

[Thisismyname]

  So I bypassed the channel switch (instead of replacing it) and the problem remains; a static/crackling noise on the DC side of the PI input cap that occurs when I scrape something metal across the turret. The noise is around the power supply, not the turret I'm scraping. I'm usually my meter probes with the other lead to ground.

There is also an excessive current draw through my limiter when I poke or scrape something metal across the turret on the downstream side of the PI input cap. None of this happens when using a chopstick or when moving wires.

Both of these symptoms go away when I remove the PI tube. Different tube, same symptoms.

Perhaps the smoking gun to someone more knowledgeable; when I first powered up the amp, the PI cathodes/grids etc didn't have a DC ground reference because I accidentally wired the 4.7k shunt resistor to the top end of the presence pot blocking cap instead of the bottom end, which is connected to ground.

I noticed this when measuring the PI grid voltage and saw it was 250-300VDC (I don't remember exactly.) I fixed the wiring mistake, replaced the tube out of ignorant paranoia, and forgot about it 'til now. The amp worked totally fine for about 15 hours before the primary blew. The correct fuse did not blow.

Does this reveal something useful in fixing the amp?

[mresistor]

It might be helpful to take some decent pictures of the entire circuit, turret board, ps and preamp parts, photos showing wiring to tube sockets.  and post them.

[nandrewjackson]

Are you meaning that, with the amp powered up and running with speaker connected, when you scrape a probe on some socket terminals you hear something out of the speaker?


Isn't that completely normal operation?


 The current surge visible through lightbulb limiter when doing that is simply the current draw difference between idle and producing output sound.


Everything I've ever built, while getting live voltage readings, would make a sound at the speaker while hitting socket terminals with the DMM probes.

I'd agree with someone's earlier reply about "corrosion" on the 30 year old transformer. It may have had a slight slight manufacturing defect and it finally went open circuit.


If you are hearing a noise without power tubes installed, I wouldn't worry about it, unless it's making sparks or smells. I've had transformers that hummed like a fan motor, ghosts in the machine, a glitch in the matrix. All other things being equal, if the noise isn't coming from the speakers, but inside the chassis only when probing around with a metal stick, stop probing around with a metal stick.


🎸🎸🎸✝️✝️✝️✔️✔️✔️✅✅✅😎😎😎

[Thisismyname]

Are you meaning that, with the amp powered up and running with speaker connected, when you scrape a probe on some socket terminals you hear something out of the speaker?


Isn't that completely normal operation?


 The current surge visible through lightbulb limiter when doing that is simply the current draw difference between idle and producing output sound.


Everything I've ever built, while getting live voltage readings, would make a sound at the speaker while hitting socket terminals with the DMM probes.

I'd agree with someone's earlier reply about "corrosion" on the 30 year old transformer. It may have had a slight slight manufacturing defect and it finally went open circuit.


If you are hearing a noise without power tubes installed, I wouldn't worry about it, unless it's making sparks or smells. I've had transformers that hummed like a fan motor, ghosts in the machine, a glitch in the matrix. All other things being equal, if the noise isn't coming from the speakers, but inside the chassis only when probing around with a metal stick, stop probing around with a metal stick.


🎸🎸🎸✝️✝️✝️✔️✔️✔️✅✅✅😎😎😎

Thanks for the reply. I was using a dummy load, so I was not hearing it through the speaker.

Current surge caused by amplifying the normal popping of probe contact makes sense. I didn't think of that.

I've just been so worried because I never found the source of the problem. I didn't want to believe that I won the lottery and had a PT fail on it's own, which I'm told is very rare.

Going by Hoffman's Law, "An amp will work correctly if it's wired correctly", I think the amp should be fine if what I'm describing is normal. I'm very certain it's wired correctly, I did a very thorough inspection recently. I will just start it up, play it, and carry on. Hopefully I won't be back to this topic.

I want my friend to have confidence in his amp and not be worried about it dying again, something that will be in the back of my mind for a long time.

What's your opinion on the ethics of me telling my friend/the customer a white lie that I found the actual problem and replaced the damaged part instead of saying the old transformer likely died on it's own?

I wouldn't charge him for this non-existent part "because it failed due to my wiring mistake."

[mresistor]

In order for your friend to have confidence in the amp it would be prudent if you also did.  You might consider taking it off the limiter and playing it at moderate volume through a speaker and see how it does, monitoring cathode current for abnormally high value..  within reach of the off switch. A speaker would be prefered to a dummy load., this would be where a Weber attenuator would be helpful.

[Thisismyname]

In order for your friend to have confidence in the amp it would be prudent if you also did.  You might consider taking it off the limiter and playing it at moderate volume through a speaker and see how it does, monitoring cathode current for abnormally high value..  within reach of the off switch. A speaker would be prefered to a dummy load., this would be where a Weber attenuator would be helpful.

It's interesting you mention high cathode current. I started it up and noticed red plating on cathode bias mode. Cathode current was 250mA and climbing. I found the cathode shorted to ground. I disconnected the cathode bias circuit from the switch and measured the expected 250 ohms to ground. That led me to discover that the bias mode switch was shorted on both sides, 0 ohms on the cathode bias side, and about 9K on the fixed bias side. It was supposedly good quality switch. It is TUV and UL approved.

 Do you think the switch failed or something caused the switch to fail?

[mresistor]

Hummm  I was thinking this earlier today - that if there is switching modes in the circuit one has to evaluate the problem in each mode. 

[mresistor]

Well  I don't know about that particular switch but I think I might use a switch that is more robust  or higher quality  and replace that one.  My thought was that whenever we are using switching on the cathode in an amplifier and subsequently the amp exhibits a higher than normal current situation then the switching should be looked at as a possible cause of the problem.  I would also make sure my wiring was 100% correct.

https://www.tubesandmore.com/products/switch-carling-mini-toggle-dpdt-2-position-solder-lugs-flatted

[mresistor]

I guess you could use a switch with an off position if you wanted a cool standby LOL  kidding of course

[Thisismyname]

Well  I don't know about that particular switch but I think I might use a switch that is more robust  or higher quality  and replace that one.  My thought was that whenever we are using switching on the cathode in an amplifier and subsequently the amp exhibits a higher than normal current situation then the switching should be looked at as a possible cause of the problem.  I would also make sure my wiring was 100% correct.

https://www.tubesandmore.com/products/switch-carling-mini-toggle-dpdt-2-position-solder-lugs-flatted

The customer was actually using the cathode bias mode when it failed. Do you think maybe the switch slowly failed/shorted and that's what caused enough current to stress the transformer over time but not blow the fuse. Someone suggested that might have been the case.

Like maybe the switch was subpar and the insulation slowly broke down? Could it be that simple?

I'll order that switch and replace the overdrive channel switch too for good measure.

[tubeswell]

...I want my friend to have confidence in his amp and not be worried about it dying again, something that will be in the back of my mind for a long time.

What's your opinion on the ethics of me telling my friend/the customer a white lie that I found the actual problem and replaced the damaged part instead of saying the old transformer likely died on it's own?

I wouldn't charge him for this non-existent part "because it failed due to my wiring mistake."

Better to be straight up and honest IMO. Trying to appear to be honest while not being entirely honest could lead you down all kinds of dark paths (Breaking Bad). Whereas if you're totally honest, there's nothing to hide and nothing to find out.  Whereas right here, well you've already put it on record, so I vote for being honest. Jimminy Cricket signing off.

[pdf64]

  …
There is also an excessive current draw through my limiter when I poke or scrape something metal across the turret on the downstream side of the PI input cap. None of this happens when using a chopstick or when moving wires.
…

It may be that probing those points temporarily created a really bad lead dress scenario, allowing a strong positive feedback loop to form sufficient for free running oscillation to start.
Did you scope the amp output during these tests?

It seems the PT probably failed due to a bad switch; that’s fine, these things happen.
Hint - I like HT fuses, they can help prevent big expensive things like PTs becoming collateral damage

[Thisismyname]

  …
There is also an excessive current draw through my limiter when I poke or scrape something metal across the turret on the downstream side of the PI input cap. None of this happens when using a chopstick or when moving wires.
…

It may be that probing those points temporarily created a really bad lead dress scenario, allowing a strong positive feedback loop to form sufficient for free running oscillation to start.
Did you scope the amp output during these tests?

It seems the PT probably failed due to a bad switch; that’s fine, these things happen.
Hint - I like HT fuses, they can help prevent big expensive things like PTs becoming collateral damage

I did not scope it. Also, for some reason, I didn't see your hint about HT fuses until I quoted you. Did you edit that part out or something or is it just a glitch?

[pdf64]

Sorry, I edited it. I know it’s not ideal but it seems preferable to several posts in succession.

[mresistor]

HT fusing is a good idea and not hard to implement. Just have to have the room and the parts.

[pdf64]

The Hammond replacement PT HT winding is max rated for 250mA.
With a cap input FWB rectifier, DC must be limited to about half that (please could someone check that in PSUD2?).
The point being that the operating conditions for cathode bias need to take account of that, ie the output valves can’t be idling much above about 22W anode dissipation.