Fender Pro Jr blowing fuses, output tube sparks

I'm working on a Fender Pro Jr (rev c) that came in with a blown fuse. I replaced the fuse and switched it on. One of the 6BQ5 (EL34) power tubes turned bright red and I switched it off. I tried swapping the two power tubes and I switched it back on, and this time neither tube began to glow red. However, after a few minutes of testing it out the fuse blew again. So I replaced the fuse, and switched it back on only to have sparks inside the EL34.

Pretty much all the voltages are reading high, like 379VDC at B+ instead of 319VDC.

Any help would be appreciated

http://ampwares.com/schematics/pro_jr.pdf
 
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turk 182

Member
2012-10-26 3:03 pm
you are running this on a DBT for testing i hope...with the high B+ i'd check the main bridge rectifier diodes. -14 for bias is correct according to the schematic, sparks in the output tube would be a grid short so you'll definitely need to change that.



and i am a Fender authorized service tech...
 

32860

Disabled Account
2009-12-06 7:13 am
Did you check for continuity of the primary windings of the output transformer? An interruption in one (or both) of the primary halves could explain the high B+ and the sparking in a 6BQ5. If no anode current can run in a 6BQ5, the screengrid current probably gets to high.

If you take out the 6BQ5's you can check for continuity by measuring resistance between pin 7 (anode) of the tube socket and the centertap of the output transformer.

Greetings, Robert
 
you are running this on a DBT for testing i hope...with the high B+ i'd check the main bridge rectifier diodes. -14 for bias is correct according to the schematic, sparks in the output tube would be a grid short so you'll definitely need to change that.



and i am a Fender authorized service tech...
And what do you think would cause excessive HT voltage with a faulty diode?
If a diode goes short, it will take out the HT fuse, if open there will be no HT. I am intrigued and await for your explanation.
 
dc with an ac pulse on it,no?
Technically not feasible as the only way to get AC pulses on the HT is with a short circuit diode and that would blow the HT fuse or possibly with an efficiency diode, (which in this case is not used, collecting AC spikes created by flyback on the output transformer). In the same way why we used a BY228 efficiency diode in old fashioned television flyback circuits in an attempt to reduce power consumption. Philips G8 chassis for instance, luckily long gone and has never been used in audio circuits
*****

The mains can be +20% or -10% so add 20% to the recommended HT allow for inaccurate meter reading and that is the answer to the higher than expected HT voltage.
 

turk 182

Member
2012-10-26 3:03 pm
well with no HT fuses in the Pro Jr (there's only one on the primary side) and not knowing just what's in there for a fuse at the moment i made an erroneous conclusion as to what could have been contributing to the high B+.


by "efficiency diode" do you mean perhaps fast recovery?
 
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With a short circuit in the bridge circuit, the fuse or mains transformer will blow.

No, not fast recovery. It was a diode strapped between the HT on the line output transformer and the main smoothing capacitor. When the HT rises above the set voltage, the diode would conduct the voltage back to the main smoothing capacitor meaning better regulation hence "efficiency diode".


On this amplifier, replace the output valves, (EL84,s) check the screen grid feed resistors for damage, repair/replace as required and it will work again.

Very old school.