fender hot rod deluxe ot advice

[georage]

hey y'all i need some experienced advice. i ordered a new replacement output transformer for a fender hot rod deluxe. i think the new one is bad, but i may have jumped the gun...i havent installed it yet, but in a bench test i read dc resistance between center tap and the (output terminals?)(plate terminals?) the readings are 104ohms and 79 ohms.
the old tranformer reads 104ohms and open(because its bad). its over 20 percent different. is that ok?

[The Dude]

That's often how OT DCR reads, depending on how the transformer is wound. It's fine. DCR is different than impedance.

[georage]

of you say so. guess i owe an apology for jumping the gun

[Mick Bailey]

+1

Perfectly normal. The first half is wound, the centre tap taken off, then the second half wound over. As the diameter increases more wire is needed and DCR increases. The number of turns are the same, though - this is what matters more as the transformer operates on a turns ratio.

[georage]

that does make sense. well thanks guys.

[georage]

alright thanks for your advice guys. i installed the new ot without altogether removing the main board. that was pretty hard, but i really didnt want to stress that board or the ribbon cables. i had to make a 12inch long wrench to get at the nuts from under the board. if you dont mind let me explain what happened to this amp and maybe some of you can help me along, me please.

[georage]

so a friend tried to bias this amp using an (under-tube bias tool). he snapped off the aligment pin and presumably plugged the tube in with the wrong pin orientation....i then relaced the screen resistors and the diodes cr 4 and 5, then replaced with new 6l6gc and biased using the supplied bias test to around 60ma. the amp sounded great, we played for about 30mins and he took it home and played for 3x3 hour sessions. then the amp blew a fuse. he brought it back, i replaced the fuse and the amp made a hideous hum and blew the fuse....
i tested dc resistance on the ot primary and found one half to be open, the replaced the ot.....
so now i am ready to turn the amp back on, but, what should i check first? i dont want to blow this ot....
is there a procedure i should do to test for something i am ignorant of???

[Helmholtz]

Schematic says 60mV at TP 30. That means 30mA idle current per tube.

If you have a bulb limiter use it.

[georage]

i do use a bulb limiter, and appreciate that advice.
im always thinking too much, and, it confuses me why an open winding on an output tranformer would blow a fuse or make such a violent loud hum. i know its faulty, but still .. i better recheck that it was open and not shorted. could the sound be an arch in the windings?

[Helmholtz]

If an OT half-primary is open, the corresponding power tube is disconnected from its plate supply, essentialy taking the tube out of circuit.
This situation will not cause the mains fuse to blow nor is it likely to cause any further damage.

Main question is, what caused the winding to get open twice in a row.
An open OT primary winding is typically a consequence of excessive winding current.
As tubes were replaced, a shorting tube is unlikely.
The same effect as a shorting tube would have a shorted "protection" diode CR4 or CR5, i.e. first blowing the mains fuse and eventually melting the primary wire after the fuse was replaced.

[g1]

Main question is, what caused the winding to get open twice in a row.

My impression was that this is the first OT replacement, but we need clarification on that.

[Helmholtz]

My impression was that this is the first OT replacement, but we need clarification on that.

Yes, but I understand the original OT had to be replaced because one half-primary was open?

[georage]

there are only 2 output transformers...the old one that opened one half of its primary and the new one that is being installed now.

[georage]

no one knows why the original ot opened up in the first place. there was a possible event of misaligning a power tube in the socket, then a new set of power tubes and a tune up and bias, then thebold ot died after that. now the new ot is in and it is working.

[georage]

the repair went perfect and the amp biased to 57 mv on the bias test point and played for 30 minutes on a bulb limiter, then another 30 minutes on mains voltage with a bias re-ajust for the higher voltage. there were a few weird noises which were probably cell phone interference.
the amp drew about 50 watts at 103v(the limiter drops the voltage)....
i guess one more 30 minute play through and bias check and thats all i can do, right?