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PostPosted: 17 Mar 2020, 14:14 
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Hello,

I am working on an old Magnavox console style tube amp. I just finished rewiring the AC mains up to the power transformer.

When I turn on the amp it immediately blows the fuse. I chose a 3A fuse, based on reviews of what other folks seemed to be using. The amp had an automotive type fuse before, and I think it was rated at 12A (it is very difficult to make out, even with a magnifying glass, could be 2A maybe).

I did some basic continuity checks to look for shorts and have not found anything.

The primary winding of the power transformer measures 5 Ohms. It this were DC current then that would imply 24 Amps, which would explain the blown fuse. But I realize the winding in the transformer acts like an inductor on start up and provide additional impedance. I am not able to measure the impedance of the primary winding.

I am looking for some ideas on what to try next. How does one determine the correct fuse value? How do I know if my transformer is good?

Here are some pics:

Image
This is the fuse I am using.

Image
A picture of the power transformer.

Image
Stamped numbers on the transformer.

Image
Red is Hot, Blue is Neutral. Fuse is on the Hot side, switch is on the Neutral side.

I was pretty careful not to reverse the leads going into the primary winding of the power transformer, but what would happen if that were to occur. The voltage on the secondary winding would be reversed I think, but what effect does that have downstream? There is no way to tell the wires apart.

Thank you - Gustave


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PostPosted: 17 Mar 2020, 16:06 
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Hi,

Here are some thoughts for you,

When you power up a mains transformer there is no magnetic field so you get inrush current.
You can reduce this effect by fitting an inrush current limiter.
Then you can use a closer rated fuse to the running current.

The heaters on a tube amp draw more current when they are cold and the current goes down as they heat up.

The current on toroidal transformers is even worse at start up and can even weld power switches closed.

Check you have the correct mains voltage selected <<<or you will burn out the transformer
Having said all that the thing to do is disconnect any power from the secondary of the transformer and put the connections in a strip block so none of them are touching. Then power it up and measure the voltage out of each winding.

You should be able to run the system with something like a 6 amp fuse, NB the fuse needs to be time delay not fast blow and should not be automotive. <<<the insulation of the fuse and holder must have a withstand voltage greater than the applied mains.

As always its always at your own risk.

Regards
M. Gregg

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PostPosted: 17 Mar 2020, 16:14 
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5 ohms sounds too low for the primary winding. check the primary for shorts to ground. All wiring on the primary should be floating except for the earth ground. May also have a shorted death capacitor


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PostPosted: 17 Mar 2020, 16:22 
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It might be more help if you could tell us what Magnavox amp it is.
Or show some shots of the tubes and top of the chassis.

Have you disconnected the transformer ie you haven't got any wires crossed.
Ie you do have the primary winding connected to the mains?

Regards
M. Gregg

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PostPosted: 17 Mar 2020, 16:44 
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How come you have switched the neutral? :cold:

Try removing the supply wires and take a picture of the birds nest where the 120 ohm resistor is.

Looking at the amp there are lots of problems those hunts caps are probably leaking.
Just my opinion.
But you need to start again and just get the transformer working on its own without all the other stuff.
Sorry for the stark comment, if you build slowly and post pics people on here will be able to guide you.

The first thing is safety.
Then get the transformer running.(if you can't get the transformer working there is no point in going any further).
Then strip out the caps and recap.

Regards
M. Gregg

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PostPosted: 17 Mar 2020, 17:07 
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M. Gregg wrote:
How come you have switched the neutral? :cold:

Try removing the supply wires and take a picture of the birds nest where the 120 ohm resistor is.

Looking at the amp there are lots of problems those hunts caps are probably leaking.
Just my opinion.
But you need to start again and just get the transformer working on its own without all the other stuff.
Sorry for the stark comment, if you build slowly and post pics people on here will be able to guide you.

The first thing is safety.
Then get the transformer running.(if you can't get the transformer working there is no point in going any further).
Then strip out the caps and recap.

Regards
M. Gregg


Thanks to you and Mike for your guidance. Much appreciated.

I had another post here just a bit ago which shows some pictures:
http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=6736

This is the very first thing I have done to this amp, getting my feet wet. I figured rewiring the mains should be simple enough! I guess not...

It is a bit of an old dog of an amp, I agree. But I figured it would be a good project to start on. Before I build an amp based on a proven design.

I wired the transformer back up exactly as it was originally. And the amp did work before I started this, quite well except for the new symptoms of hissing and popping.

I wired the switch into neutral based on a book I read. "Vacuum Tube Amp Basics" by EJ Jurich. The way I understood it was that since the wires leading to the switch are neutral, then if the switch is located a ways away, there will be less interference from the wires leading to it. But now as I write this I realize that it is current that causes interference. And the current should be the same, whether hot or neutral. In any case, I cannot think of any reason why it would matter whether the fuse and switch are on either the hot or neutral. The are both designed to cut the current.

No worries with stark comments, that's part of being a newbie 8^)

I would seem strangely coincidental that my transformer failed as I was rewiring the mains. Hence my suspicion, that if all the wiring is Kosher, then I have selected too low a fuse value. I did also solder in a new octal base for the rectifier. I labeled all the wires and was very careful to wire things back the way they were. As always, that does not guarantee no mistakes were made :)

I realize that many of the capacitors and resistors are very old and could stand to be replaced. But the amp did work, so I am focussing as you said, on figuring out this transformer or power up issue.

Thank you both - Gustave


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PostPosted: 17 Mar 2020, 17:09 
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Forgot to add that I replaced the original death cap with a Y2 safety cap. The orange thing near the middle of the picture.


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PostPosted: 17 Mar 2020, 18:09 
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looking at your other post and comparing pics of your socket replacement (5U4 TUBE) your socket is miswired if the key way is the turned the same as the old one.
wires marked 2 and 4 are backward. Two of the wires should be much heavier (thicker) than the other. These are the filament wires and should go to pins 2 and 8. Going clockwise on the bottom of the socket pin 1 is the nearest the left side of the keyway. I can't make out the other two lables.

Two thinner wires go to pins 4 and 6. There will probably only be one additional wire on the tube coming from pin 2 which go to a filter capacitor. (can cap)

If you mixed up these wires thin ones to pins 2 and 8 you will have tried to lite the filament with your high voltage winding and cooked the transformer


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PostPosted: 17 Mar 2020, 18:43 
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mike567 wrote:
looking at your other post and comparing pics of your socket replacement (5U4 TUBE) your socket is miswired if the key way is the turned the same as the old one.
wires marked 2 and 4 are backward. Two of the wires should be much heavier (thicker) than the other. These are the filament wires and should go to pins 2 and 8. Going clockwise on the bottom of the socket pin 1 is the nearest the left side of the keyway. I can't make out the other two lables.

Two thinner wires go to pins 4 and 6. There will probably only be one additional wire on the tube coming from pin 2 which go to a filter capacitor. (can cap)

If you mixed up these wires thin ones to pins 2 and 8 you will have tried to lite the filament with your high voltage winding and cooked the transformer


Whoa. Ok. I printed a pin-out schematic for the socket to use as a guide. But that's no guarantee it's right. I will check, thanks.


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PostPosted: 18 Mar 2020, 13:28 
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Location: Vancouver Canada
You could used what is called "dim-bulb-tester". In short it is a 100 watt light bulb (incandescent) in series with the live side of the line cord from the wall outlet. It will light up brightly at first due to the inrush current (as mentioned above). Then it will get much dimmer. If it stays bright this means you probably have a short somewhere in the wiring. Look up dim bulb tester on google. They are very helpful in such cases.

Use a "slow-blow-fuse" in amps as they will not blow immediately due to inrush current of the transformer and it's load when first powered. The dim-bulb-tester will act in the same manner as a slow-blow-fuse. (slo-blo-fuse). Inrush current can be many many times the operating current of a device, up to 10 times as much thus blowing even a properly valued fuse (if fast blowing like your pic).

I add 2 switches in my tester. One in series for on / off, and the other in parallel across the bulb allowing the bulb to be shorted out after normal running operation is achieved. The tester will allow you to SEE short conditions at power-up, and after a few moments, shorting out the bulb, test the normal current draw while running, helping to determine a value of fuse during operation.

As for the switch being in the neutral side of the line rather than the live side of the line. I am not sure but I believe it was due to the age of the unit as back then AC plugs were not polarized and and a lot of radios and TV's etc had the line neutral side electrically connected to the chassis. Scary scary and hence most tech benches having an isolation transformer powering these devises when being worked on.

Today however we use polarized plugs which (if wired correctly) ensure the hot side of the AC line does NOT go to the chassis or the cct ground.

Properly wired,, the narrow ac plug pin (hot) goes to first a fuse, then the switch, then into the cct (trans) ensuring a switch turned off, or a blown fuse, will leave the device electrically dead and SAFE. The wide pin will then go to the other wire on the transformer. The 3rd wire (GND) is then connected to the metal chassis and is where you would solder the X2 cap between it and cct gnd.
In reality the neutral and gnd (3rd wire) are at the same potential connected together in/at the breaker box in your house. The 3rd wire is a second ground for safety allowing the ground to be independent of the neutral ground return path (which can still give a nasty jolt). It is just as easy to wire it up correctly. Also for audio equipment having a second (safety) ground is much quieter when connecting the cct gnd to electrical gnd rather than neutral.


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