Fender Prosonic Bias Help

[Tubebender]

Has anyone ever biased a Fender prosonic before? I Have the Head version, not the combo.
I only have a Multimeter, and never tried bias probes.
I've biased my Carvin v3 head and Hotrod Deluxe, but I just don't know what's up with this amp, since it's Class A, Cathode biased as well as Class B.
I hate to use the moron method and slap a full set of JJs from eurotubes and call it "good".
Thanks
TB

[TweeDLX]

Has anyone ever biased a Fender prosonic before? I Have the Head version, not the combo.
I only have a Multimeter, and never tried bias probes.
I've biased my Carvin v3 head and Hotrod Deluxe, but I just don't know what's up with this amp, since it's Class A, Cathode biased as well as Class B.
I hate to use the moron method and slap a full set of JJs from eurotubes and call it "good".
Thanks
TB
TB,
Are you sure it's cathode biased? Cathode biased amps are usually "plug and play", but I see a bias trim pot on the inside when I look at the schematic. That simplifies things immensely. In a nutshell: you'll have to open up the chassis, measure current and plate voltage, adjust the trim pot.

Mike

[Tubebender]

TB,
Are you sure it's cathode biased? Cathode biased amps are usually "plug and play", but I see a bias trim pot on the inside when I look at the schematic. That simplifies things immensely. In a nutshell: you'll have to open up the chassis, measure current and plate voltage, adjust the trim pot.

Mike

Hey Mike,
Well I read when you run it on "A" mode, 30watt, it's cathode biased, then for class a/b and b, I dunno. I'm not an expert by any means when looking at a schematic, much less knowin what's what....

Would you happen to know if you can get a reading across all power tubes with a Mulitmeter, like on the HRDx? They have a test point on the pcb which you touch off and get a full reading.
I dont' have a way to measure the current and plate voltage without a bias probe,,,which I don't have at the moment. Just have a multimeter. Thanks

[TweeDLX]

Hey Mike,
Well I read when you run it on "A" mode, 30watt, it's cathode biased, then for class a/b and b, I dunno. I'm not an expert by any means when looking at a schematic, much less knowin what's what....

Would you happen to know if you can get a reading across all power tubes with a Mulitmeter, like on the HRDx? They have a test point on the pcb which you touch off and get a full reading.
I dont' have a way to measure the current and plate voltage without a bias probe,,,which I don't have at the moment. Just have a multimeter. Thanks
I couldn't see any jacks for testing the bias, but if you use your Multimeter, you can read current and voltage by placing one tip on the appropriate pin of the tube socket, and grounding the other against the chassis. Check the manual of your DMM for the correct settings, and if it's capable of doing this. Installing test jacks is a relatively simple process involving attaching 1ohm resistors to pin 8 of both power tubes and running them to ground & the jacks. I think Doug Hoffman has all the parts for this, and possibly an article on how to do it. Lord Valve has a great article on the bias process here: http://www.duncanamps.com/technical/lvbias.html
Good luck!

Mike

[Tubebender]

Thanks. I got tubes and a probe from Bob at eurotubes.