what can cause a blown pt?

[alex40c]

Hi, I'm about to change a PT in my 5e3, because of a blown one. So, before i put in the new one, i'd like to ask if it is anything that can cause a blown pt, so i could check it in my amp.thank you!

[Stevem]

Alot of amps like yours only have a fuse in the AC side of the amps wiring and alot of these fuses are of the slow blow type which can place a big strain on the PT during overload time.
I have repaired tons of vintage and non vintage amps that had shorted output tubes and or leaky and shorted power supply filters, and after the owner replaced the fuse 3 or 4 times with the same blow the fuse result the extended total overload time for the PT was too much and the PT shorted out, finito!
This may be the likly case for why yours is toast, but some times its just age that gets them.
Back when your amp was made the wire used to wind the transformers was just coated with a lacqure paint to isulate the wires form each other, with heat and age and vibrations a small spot of this can where winds touch and flake off and bingo, its short time.

Before you ploop in a new PT you need to change out the power supply filters if they are original, or even if the are not the orignal paper cased ones, but over 15 years old they should go.
Next you need to test all the tubes for shorts.
For added protection for the new PT you can add a inlline fuse holder in the OTs center tap, load this with a 1 amp fast blow type fuse and you will sleep better at night!

[armillary]

Are you sure the PT is shot? Have you checked the fuse? While checking the fuse, is it 2A? Make sure it's 2A and no larger. Have you had all the tubes tested to make sure they are good? Are you running 6L6's in it? It's only designed for 6V6's.

Things that can cause a PT to go bad are shorted tubes, shorted electrolytic capacitors and shorts in the wiring. All of these things cause more current to pass through the PT than it was designed to handle, and it overheats and either shorts inside or stops conducting (goes open).

A 5E3 that blows a PT deserves a re-cap and checkup before replacing the PT. It's a classic.

[alex40c]

thank you both for your replies.i'm using 6v6s and a 2A fuse.it's a weber kit. I've checked with new fuses and still it blows them instantly. i've also checked with the tubes pulled off,still the same.i've checked the wiring for incontinuity and it seems fine.so, maybe a silly and moron question,sorry about that,but, how do i check a tube for a short?

[martin manning]

Has this amp worked properly in the past or are you still trying to get it working for the first time?

[Phil_S]

You may have a wiring short, an error or just unintended that is causing the fuse to blow. I suggest backing up a little here for some simple diagnostics on the PT.

Disconnect the PT secondary high voltage. Pull the tubes and unsolder the center tap. Leave the filament wiring connected. It's a Weber kit so I'll assume you are using a copper cap rectifier in a tube socket [1]. Pull the rectifier, which disconnects the high voltage secondary. Use clips on your meter. Connect the meter leads to the high voltage leads at the socket pins. Set the meter for AC voltage. Now, turn the power on and see if a) the fuse holds, and b) if the voltage reading is reasonably consistent with what's expected. AC voltage should be a little on the high side because there is no load.

[1] If you are using a tube rectifier, disconnect the 5V filament supply and insulate the leads.

If the fuse blows, disconnect and insulate the filament winding(s) and try again.

Tell us what happens and report the high voltage reading.

[alex40c]

the amp was running smoothly for something less than a year.ok, i'll do this and i'll post here,thank you!

[alex40c]

Phil, just a last question before i check.do you mean to unsolder the OT center tap or the PT one that is grounded?sorry for asking maybe silly question, i've never done such troubleshooting again

[Phil_S]

Phil, just a last question before i check.do you mean to unsolder the OT center tap or the PT one that is grounded?sorry for asking maybe silly question, i've never done such troubleshooting again

Yes, unsolder the CT on the PT from it's ground connection. We are testing the PT. We do this so that the chassis and anything else attached to it doesn't interfere. You can do the diagnostics. We all learn this one step at a time. I was once where you are now.

[martin manning]

It's not necessary to unsolder the PT CT, just remove the rectifier from its socket, power up. If the fuse holds, measure AC voltage from pins 4 and 6 (the two red leads from the PT) to ground.

[Structo]

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?

[alex40c]

So, I did as you said and initially the fuse went blown in a sec. Then i unsoldered the fils and the amp powered on "normally". I measured 125vac in both pins. What does this mean? thank you again!

[martin manning]

You should be seeing ~340VAC from each transformer red lead (rectifier socket pins 4 and 6) to ground.

When you say you disconnected the "fils" you mean the two green (6.3VAC) leads, correct? If so, that means there is a short in the filament wiring.

With power off, and the green filament leads still disconnected, check the resistance from any tube socket's filament connection to ground. You should see infinite resistance (no continuity to ground) with all tubes in. If there is continuity to ground, remove the tubes and check again (a shorted tube might be the cause). Then with all tubes out, measure the resistance across any tube sockets filament connections (a power tube socket's pins 2 and 7, e.g.). Again you should see infinite resistance.

Where are you located?

[Stevem]

Yes, a HT fuse will be the way to go, the only thing It can not help with is if the output tubes heaters themselves short which is kinda rare.
Most times the tubes screen grid winding breaks and a short from the screen supply B+ to heater takes place.'This will pop a HT fuse.
Most amps do not have them just due to the added production cost.
Some well designed high power amps that have 4 power tubes or more use a resistor in the plate section that will go open with excess current and just drop that tube out, but still let the amp play on.

[alex40c]

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

so,how much is this "little"
i checked with and without tubes and the resistance is to infinity,except for pins that are connected to the ground or a cap etc.filaments to ground resistance is infinite.However, at V1 and V2 there is resistance almost everywhere between the pins (except from filaments).is it reasonable?Moreover, the is a 5Kohm resistance at V3 an V4 at pins 3 & 4
i'm located in greece