PR Clone blown Tube, now blows Fuse

[RacerXYZ]

Dear all, I've build a TAD Princeton Reverb Clone (Blackface 14 Kit). It is my first amp build, i finished it last summer. All went well, just one incomplete solder joint that caused crackling noise. Found it via chopsticking. Otherwise the amp had no hum and only a little noise (maybe because of all carbon comp resistors?).
Now to the actual problem. I was playing at "bedroom" level (around 3 on the vol) when the amp suddenly quit working. No loud noises, it just went off. I had a look and the fuse was blown. Had a look at the tubes and to found V4 lost its top  The getter completely white. How can something like this happen? The tubes had metal covers.

The problem is now, that the amp is blowing the fuse without any tubes. Are they educated guesses on what a broken vibrato/pi tube can have damaged inside the amp?

Thanks in advance for your advices! I can add pictures if needed.
Please excuse spelling mistakes, i am German.

cheers, Martin

[sluckey]

I doubt that busted tube is related to the fuse blowing issue. Fuse blowing with no tubes narrows the problem area considerably. There's somethin amiss between the fuse and the PT, or the PT itself, or filament string wiring or PT secondary wiring. Disconnect all PT secondary wires. Don't let any touch each other or the chassis. Does the fuse still blow?

[shooter]

since the fuse is blowing you got a short, they are pretty easy to isolate,  Disconnect the output of rectifier, check, fuse blow?
disconnect the PS secondary from everything, fuse blow?

[shooter]

I waited to long 

[pdf64]

Now is a good time to use your light bulb limiter; makes it way easier to trace the fault and takes all the stress of repeated exposure to fault current away from the amp.
And saves on fuses 

[RacerXYZ]

Thank you all for your help and advices. Build myselft a lightbulb limiter.
I disconected all secondaries from the power transformer after each other. Bulb is still glowing 

I looked at the geofx article about PT problems. Here are the measurements:
PT primaries:
green/black to black 15 ohm
green/black to chassis: open
black to chassis: open

secondaries:
red (pin 4) to red/yellow (centertap): 39 ohm
red (pin6) to red/yellow: 127 ohm
red to red 121 ohm

yellow to yellow (5V winding): 0,2 ohm
green to green (6,3V heater): 0,1 ohm
each green to green/yellow: 0,1 ohm

model of the PT is 125P1B from TAD
So does this look like a fried PT?

[sluckey]

red (pin 4) to red/yellow (centertap): 39 ohm
red (pin6) to red/yellow: 127 ohm

These two readings should be about the same. This indicates a bad PT.

[RacerXYZ]

Damn, the most expensive part...Well will order a new one.
Can the blown tube cause this PT problem? Or are there places where I should look before i install a new one. The amp was running without problems before, at home level and at practice room level.

[sluckey]

Can the blown tube cause this PT problem? Or are there places where I should look before i install a new one.

Busted tube did not cause this. Busted glass is usually the result of rough handling, mechanical shock, dropping the amp, etc.

I would look for a low resistance reading on one or more of the filter caps before powering up again. And initially power up with the rectifier tube pulled. Check for proper AC voltages on all the secondary wires. If OK, then plug in the rectifier tube and power up using the lamp limiter.

[PRR]

>>     red (pin 4) to red/yellow (centertap): 39 ohm
>>    red (pin6) to red/yellow: 127 ohm
> These two readings should be about the same.

Yeah, but, he also has 121 from end to end. Which does not add-up. I'm thinking measurement error, maybe not digging through tarnish on terminals.

[RacerXYZ]

Measured again, see attached posts. Cleaned and renewed the solder at the end, although they looked shiny before. Same results, plus minus some ohms.

[trobbins]

Always worth taking extra care to confirm a bad transformer, and confirm it can't be fixed without buying a new transformer.

If there is an insulation breakdown between secondary layers, including between layers that are one each half of the HV secondary, then that quirky result is plausible.

If you can't easily identify a bad part, or location where shorting occurred on the secondary side, and can get a replacement transformer, then well worth adding some fuse protection on the secondary, and work out a safe test that shows up any lurking bad part.

[tubeswell]

Seems like your PT could be bad. When you fire it up with the bulb limiter, is the measured VAC on each 1/2 of the HT winding the same? If not, then you would have a winding short. 

[RacerXYZ]

I have around 20 and 10V AC respectively with a 60 W lightbulb measured to ground.
So seems everyone is right. Will definitely order a new one.
Measured the filter can as well...all around 17 Mohm against ground.

Concerning the secondary fuse (reading into it at the moment), which amp rating should it have?

[RacerXYZ]

Quick update:
Installation of a new PT was a success. The amp is up und running again.
Big thank you to everyone for their help and advices!