what can cause a blown pt?

[Stevem]

Since your amp was working there are only a couple of things that could have happened.
1)since you built it from a kit some of your. Heater wiring has shorted.
This can be checked by pulling all the tubes, un soldering the the heater wiring, the two green wires,and the resistance checking for shorts like across pins 2 and 7 of the output tubes, and then from 2and 7 to ground.
If no low resistance and or shorts are found then the problem started due to a shorted output tube, or just a defective power transformer.
2) Some of these transformers are not of the best manufacture and it could have been that one of the output tubes shorting out screen to heater as the do many times was enough to take out the 6.3 volt winding on the power transformer, or the PT failed all on its own.
If you unsolder the 6.3volt leads and have all the tubes out,than install a 1/4 amp fast blow fuse , that fuse should not blow if you power up the amp, if it does the power transformer is bad, but you still may have a shorted out 6v6 that started the whole mess!
If that .250 amp fuse does not blow and you read over 12vac across the green wires, red older the back in place and recheck again for shorts.
If this checks out carefully check the output tube sockets for burn/ arcing marks between pins 2 and 3, as this could be the source of the short.
If you want to double check for this type of short, unsolder each red wire from the recto tube socket,install all the tubes, put the 2 amp fuse back in and fire up the amp.
If both of your output tubes are good and the output tube sockets are hood the fuse will hold.
One of the main things you need to do before any of the test outs that have been posted for you is to confirm that both of you output tubes are in fact good!

[Phil_S]

AC voltage should be a little on the high side because there is no load.

so,how much is this "little"

Maybe 5% to 20%. Probably 10% to 15%.

[alex40c]

thank you very much guys!!i checked the heater wiring in the sockets and everything is fine. The two green wires coming out the PT have zero resistance.i'm doing the 2nd step tomorrow,cos i don't have such fuses right now and i'll post the results.Thank you very much again

[rdjones]

thank you very much guys!!i checked the heater wiring in the sockets and everything is fine. The two green wires coming out the PT have zero resistance.i'm doing the 2nd step tomorrow,cos i don't have such fuses right now and i'll post the results.Thank you very much again

Find a 1 amp circuit breaker and solder on some clip leads to clip across the fuse socket so you don't keep wasting fuses.


rd

[rp]

Will a HT fuse prevent damage to the PT if a power tube shorts? If so, why do so many amp designers omit the HT fuse?

I really think it's not savings but that they tend to pop on boot or switching the standby. The customer would think there's something wrong w/ the amp when it's just normal stress on the fuse, but it would be an annoyance for him and look bad for the company.

Find a 1 amp circuit breaker and solder on some clip leads to clip across the fuse socket so you don't keep wasting fuses.
rd

ooo ooo ooo, very interested! I've counted down my remaining fuses and my money troubleshooting, got a Mouser link for one of these things?

[rdjones]

Find a 1 amp circuit breaker and solder on some clip leads to clip across the fuse socket so you don't keep wasting fuses.
rd

ooo ooo ooo, very interested! I've counted down my remaining fuses and my money troubleshooting, got a Mouser link for one of these things?

Here's the kind I have, was once a Fender/Sunn replacement part.
Angela had them at one time, I can't find it on their site now.
http://www.ebay.com/itm/400071257727

This kind might be easier to find and easier to deal with.
http://www.ebay.com/itm/261384283628

Just make sure to heat shrink or otherwise properly insulate the connection to the breaker body, and use clips with a good insulated cover.
1A or 2A is plenty for troubleshooting most amps.

rd

[alex40c]

Find a 1 amp circuit breaker and solder on some clip leads to clip across the fuse socket so you don't keep wasting fuses.

Can you explain it a little further, please? English is not my language and i admit i understand nothing of this, sorry!

[Stevem]

You will likly have to order up a 1 amp circuit breaker and 1 amp fast blow type glass fuses can be had even in assortment packs from a auto parts or hardware store, heck sometimes even large food stores.

[alex40c]

yeah, i got this, but where exactly do i put this?

[Reeltarded]

In place of the normal mains fuse holder.

[Diablo1]

Since no one mentioned it, it's possible your PT failed because of a poor quality PT. If it worked for a year, and everything else in the amp is still good, and there are no shorts in the amp, and there is no excessive current draws on the secondary side of the PT, then I would conclude the PT was marginal quality from the start. They could have gotten a batch of transformer wire with a poor insulating coating. When that insulation breaks down, it's adios transformer.

[alex40c]

If you unsolder the 6.3volt leads and have all the tubes out,than install a 1/4 amp fast blow fuse , that fuse should not blow if you power up the amp, if it does the power transformer is bad, but you still may have a shorted out 6v6 that started the whole mess!

i've done it and the fuse went bad.so the diagnosis after all these checks is 100% PT, right?I'm gonna buy new tubes anyway...

In place of the normal mains fuse holder.

thanks!