How To Cathode Bias Amplifiers Using Ohm's Law

[@Steve-wz5pz]

EXCELLENT JOB!!! I have seen no clearer explanation of this process anywhere. Kudos!

[@cosmai23]

The best explanation and demonstration I’ve seen for cathode bias. Great job.

[@snakebite8925]

Great video. The proces is rather simple but often very difficult explained. This video is so simple and clear that everybody can understand this calculation bias proces. Chapeau good teacher!

[@paulgraham1999]

Excellent Video Nathan. A big help with my current amp repair

[@mishmael1]

Excellent !!! I am looking forward to watching all of your videos. One question. How were you able to safely probe your amp with two hands while the power was on ?

[@thierrycaudron3266]

Bonjour de la France je vous regarde souvent et vous comprend très bien parceque vous parlez lentement et mon langage n est pas parfait bravo respect courage pour la suite Thierry

[@abl9087]

Very nice and clear explanation. Thanks

[@toddmayer6859]

Thanks much for this instructional video Nathan! I have an Ampeg Jet J-12-A that I am restoring, so this video helped me finally understand the whole process. That 5% that needs to get subtracted baffles me a bit ... but I think most of us just do not want to have the pwr tubes biased too cold or too hot. I'd like the amp to be somewhat biased closer to hot. But I am seriously thinking of splitting the cathodes in case the pwr tubes are not balanced. I had read that I would have to double the resistor value. Is that correct? So if 200 ohm is good for combined cathodes, I'd have to have 400 ohm resistors (plus 50uf/50V caps) per cathode. Thanks again!

[@giulioluzzardi7632]

Great stuff. Show the viewers the " Ohms law Triangle" the VCR way? (Our Tutor would call it VIP triangle) makes it easy to remember and keeping electronics simple is the key to success. Thanks young Man. PS; let us hear the thing at-least, it's not everyday we get to even see a 53 Fender amp. 6L6s or 5881s?

[@davebollinger8863]

I got one for ya Uncke Doug. ET AL.... I have a blackface PR AA964 that went "Pop".. and "all the bass and tone went away." It was the 1ohm 1watt cathode current indicator, that we have all been installing for a long time now. What's up with that? Never heard of it.

[@alpal1966]

What a band! Lou Ann and nic curren....WOW

[@LuchoJ2110]

Hey Nate, do my understanding from this video is that if an amp is cathode bias and you are replacing the power tubes and you like the way it sounded you can just go ahead and replace them without dealing with the resistor? Thanks great video

[@nathanleyvas6667]

The resistor is the the only way to bias the power tubes. So if you replace the power tubes and leave the same resistor it is possible that the tubes will draw too much current. This can cause very short tube life and tube red plating. It’s possible that the existing resistor is the correct value but it is still necessary to check. The amp may sound good, but you may be cooking those tubes! On the other hand, it’s possible the tubes are biased cold and your amp could sound even better than it does now!

[@sglee4708]

Thank you for that awesome explanation great job it really helped

[@Mikexception]

Are you sure this way you are not getting combined powers and currents of plate and 2nd screen together?

[@TelmoGuitar]

To get power dissipation for each tube do you use plate voltage referred to ground or plate-cathode voltage both referred to ground? If you multiply plate voltage without subtracting cathode voltage aren’t you calculating power dissipation for both tube AND bias resistor?

[@snakebite8925]

You are right but it will makes the explanation more difficult to understand. The difference will be under 5% so imo negligible.

[@BrewerShettles]

Hi - going into a live amp - aren't you supposed to put one hand in your pocket to avoid electrocution?

[@Olegstuff21986]

He's probably already drained the filter caps (I hope). However, the two measurement cables have relatively long plastic handles that he's holding.

[@borg3575]

Plate voltage should be measured from plate to cathode and not to ground on cathode biased amps. It's a bit more precise for calculating plate dissipation. You are biased at about 92% which is fine for a cathode biased P-P.

[@doctorcircuit5338]

Great job but you might want to remind everyone to discharge the capacitors and use cation and a voltage limiter these can be deadly