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
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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That isn't very useful to me, I'm trying to obtain information. If you don't have any, that's fine, but I already know how to send something away to be fixed
-10.9 volts is recommended for the bias. -14 off load is fine. NOT -50!!!
Check the screen grid feed 100R resistors, R24 &R25, and replace the EL84s for a matched pair.
Check the screen grid feed 100R resistors, R24 &R25, and replace the EL84s for a matched pair.
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 i am a Fender authorized service tech...
I did not check the schematic but read somewhere EL34.. therefore my info about the -50 bias. EL84 is a different story. Did not see that until now.
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
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
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.
I would remove the output tubes and check voltages at their sockets
BTW what an awful way to get the bias voltage, isn't it ?
BTW what an awful way to get the bias voltage, isn't it ?
Your first post described things that happen when a tube is bad. Nothing since has changed my feeling.
First pull the power tubes and check that there is B+ on both screen and plate pins of each socket. And bias on grid pins. Once that is right, and yes voltages will read high that way, try a NEW set of EL84s.
First pull the power tubes and check that there is B+ on both screen and plate pins of each socket. And bias on grid pins. Once that is right, and yes voltages will read high that way, try a NEW set of EL84s.
Are you using the amp with a limited current supply like a filamentary lamp in series with the line cord?
Are you loading the amp with a resistor of the same resistance and power rating as the speaker? If not, output tube amps will spark because of high voltage spikes on signal.
Are you loading the amp with a resistor of the same resistance and power rating as the speaker? If not, output tube amps will spark because of high voltage spikes on signal.
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
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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.
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?
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.
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.