External bias jack

[Dandrix]

Would anybody have advice for externally bias jack for a fender ab763 clone? how would you suggested they be wired shunt etc..??? your input wil be appreicated, thanks

[Jazz P Bass]

Most 'bias jacks' simply read the bias voltage.
Which does not help much with todays output tube parameters being all over the place.
A 1 ohm, 1/2 watt, 1% resistor from pin 8 of each tube to ground gives a good reading of the current draw of each tube.
You could then parallel each pin 8 to a jack.
Or copy Fender's HR Deluxe circuit.
But that only gives a reading of both tubes.
Not too sure what's up with the diode.
Protection if the resistor goes open?

[g1]

Not too sure what's up with the diode.
Protection if the resistor goes open?

Actually protection for the resistor (and pc board) if a power tube shorts. It would be a pain to have to change the resistor every time a power tube shorts.
Original "The Twin" was AFAIK the first common Fender with bias jacks. They did not include the diodes. There were many warranty claims involving burnt resistors, melted bias jacks, and smoked/burnt bias jack boards. Fender issued a service bulletin to tack on the diodes across the resistors in all units (warranty). The next version of the bias jack pcb had the diodes incorporated.

[Dandrix]

how about this idea?


Google Image Result for http://i23.photobucket.com/albums/b392/Structo/Dlite%252044%2520build/BiasPoints.jpg

[Jazz P Bass]

With the protection diodes as G One stated.

[daz]

how about this idea?


Google Image Result for http://i23.photobucket.com/albums/b392/Structo/Dlite%252044%2520build/BiasPoints.jpg

Thats how i did it, but i did one thing different to make it easier. I used 2 probe jacks instead of 3, one for ground and one for the resistors. I then used a toggle switch to select which tube's 1 ohm R the jack was attached to. This way you don't have to keep pulling the probe and putting it into the other jack and you can go back and fourth from one tube to the other instantly. Makes it easier and also because when you adjust one tube it can affect the other's current pull. I wish i'd have had this on my amps back when i was gigging. I can't help but wonder how many times my amps had one of those bad nites where they'd sound like crap and if it could have been due to lower mains voltage changing the bias. With this i could have quickly checked and changed my bias when i set up !