thermal cutout

[Andy Le Blanc]

Anybody have any good or bad experiences with a thermal cut out in a PT.?

[VacuumVoodoo]

You should worry only if your PT is undersized for the application.
Thermal fuse is there to prevent fire hazard in case of prolonged overload. If it blows it does so because winding temperature rose above safe level (110degC in isolation class B and 130degC in class C PT). This can weaken interwinding isolation foil and lead to in internal shortcircuit-----> smoke at the very least. This why thermal fuses in PTs are non-resetable and you should never attempt replacing them.

The only PTs I came across that blew their thermal fuses were undersized ones in Classic 30 amps. Quite a few of them actually so I did the maths and had correct size replacemnt PTs custom made:

Correct size custom replacement on the left, original on the right. Same foot print.

[IMG:631:411]http://i26.photobucket.com/albums/c149/ ... ic30PT.jpg[/img]

[Structo]

Anybody have any good or bad experiences with a thermal cut out in a PT.?

No but I have seen quite a few posts where guys talk about yanking that thing out of there.
They don't always behave and could cause a power down at the most inopportune time.

[Andy Le Blanc]

I got lifted by an old tranny just the other day. I can personally vouch for the
rational to have one. That's exactly what happened, the inter-winding
isolation foil /material was shot and broke down, shorting, knocked me on my arse.
I can see why it could be an issue if it worked too well too.

Anybody know if its a UL required safety device?

[VacuumVoodoo]

I got lifted by an old tranny just the other day.

pervert.....

I can personally vouch for the
rational to have one. That's exactly what happened, the inter-winding
isolation foil /material was shot and broke down, shorting, knocked me on my arse.
I can see why it could be an issue if it worked too well too.

Anybody know if its a UL required safety device?

There are some instances where it's required and others where it's optional. It's detailed in IEC/UL61558 norm, don't have this one at hand now.
AFAIR it is not mandatory in an amp if all secondary windings have correctly dimensioned individual fuses and proper ventilation is available.
This will inevitably bring question whether to fuse bias supply winding. The answer is yes and that's where you can go wrong.
If that fuse blows (due to first filter cap or diode in bias supply going short circuit - seen this happen in a couple RI Fenders) it will take out power tubes with it unless the HT fuse is also correctly dimensioned so it blows withing seconds after bias voltage goes away.

[Andy Le Blanc]

Many amps are modeled on very old designs. You have death caps, those
transformerless little killers, un-grounded riggs where hum is addressed by flipping the plug.....

So.... lets say we got 140 ma. , 700v ct with a 50v bias tap, 4A 6.3v and a 3A 5v winding.

Where do go from here if all secondary windings are to have correctly
dimensioned individual fuses?

[VacuumVoodoo]

Andy, sorry for delay in answering.

Secondary windings should be protected against short circuits. Usually a fuse rated two-three times the nominal rated current draw is sufficient to protect against short circuit load.
Primary (mains) fuse should provide protection against about 25% power overload.
In your case your total nominal secondary power consumption is:

HV: 700V x 140mA= 98VA
H1: 6.3Vx4A=25.2VA
H2: 5Vx3A= 15VA

adds up to: 123.2VA + 25% overload gives 154VA.

Now, take into account that transformers are only about 95% efficient in energy transfer and you will get 162VA
Assuming worst case operating conditions at lowest mains voltage, in case of 120V nominal it will 10% down so 108V.
Current drawn at 108V mains and 25% overload is 162/108 =1.5A
At nominal power draw and nominal mains voltage current draw will be 123/120=1A.

So a 1.25A time delay (slow blow) fuse will be fine on the primary side.
Now you need to check if it will withstand inrush current... You can try to measure it (not a simple matter unless you have a very fast DMM with a peak hold function and an inductive current probe) or make a rough estimate by measuring DC resistance of the primary winding. Suppose in this example it is 3.5 Ohms.

Initial inrush current pulse will be at the very worst when your mains voltage happens to be 10% above nominal = 132V

Inrush = 132/3.5 = 38A !!! and it lasts at least one period of mains frequency: 8.3ms

Will the fuse survive this? You need to check the fuse data sheet and find out. You'll discover you will need at least a 3A fuse to be relatively sure it won't blow on turn on.
I however recommend to use the 1.25A fuse and a NTC resistor (inrush current limiter) on the primary side. That's what I do.

[VacuumVoodoo]

DP