Setting cathode bias Push pull tube guitar amp easy method using a DCma meter Ex: Gibson BR-9

Here is a simple way to dial in the idle current of your output tubes. In this example, I am setting a pair of 6V6 tubes on the Gibson BR-9 amplifier. You can use this method for any cathode biased amp, including Class A. See diagram in video. If you want a copy, let me know. Can send a .jpg via e-mail. I have a complete series on Patreon covering repair of the Gibson model BR-9. Plus many other great tech tips. Take a look. / dlab_electronics

[@frankgeeraerts6243]

In French nous disons: " C'est beacoup trop ! ......."
Old nos 6V6 could be driven hard in guitar amplifiers in the past era , in that time period tubes were simple consumables , no longer today ......
As tubes ages ( differently ) ...........when using a shared cathode resistor often ends by redplating one of the tubes as the hotter vtube will runa way and the lesser going down just like balancing kids on a seesaw.
Nice review/upload , thanks.

[@jcool0122]

The one thing about measuring bias through the cathode is that the measurement also includes the screen current. Much more precise to calculate the plate current by calculating off the primary of the output transformer. Of course, the screen current is negligible and can be written off as an added buffer to make sure you're running your tubes under maximum. But as a technical note, the cathode current and plate current are not the same thing in a pentode or beam power tetrode.

[@miguelrichartmellado2834]

Some link where this system that you comment is explained ?

[@dughuff8825]

Came here to say the same thing! As you say, the screen current is very low but it's not zero. But having slightly colder biased tubes isn't going to cause any issue here so I guess the less potentially risky method is ok.. 🙂

[@miguelrichartmellado2834]

Thank you Terry.

[@giulioluzzardi7632]

Nice short-cut, thanks.

[@brentnearhood8874]

Thanks! Terry🔌

[@nathkrupa3463]

Nice video sir

[@danerickson1632]

I prefer adding 1ohm matched resistors on the Cathodes of both the tubes. Then across those 1ohm resistors 1mv=1ma and you can match the output tubes

[@Bleats_Sinodai]

Work smarter, not harder 🧠

[@stirlingschmidt6325]

Kewl!

[@TheDevice9]

How do you calculate the power rating of the cathode resistor? Is the amps times the volts on the resistor, or .060*19.5= 1.17 watts?

[@gingercat777]

Just by the size 😉

[@timburle4980]

that’s how you calculate the dissipation, but actual rating would probably be at least doubled.

[@pauldow1648]

Job site, seat of the pants tv repair.

[@waynegram8907]

D LAB, I think you're biasing the amplifiers incorrectly. Use a Current Meter to measure the maximum Power Supply current when strumming the guitar strings, What is the maximum power supply current? You have to factor this into the biasing formula which I never see you do in any of your video lessons.

[@dughuff8825]

Why would you need to factor in maximum supply current when setting the bias point of a tube? Do you mean that it should be taken into account where in the conduction curve the bias point is being set at?

[@waynegram8907]

@@dughuff8825 yes I am saying that you have to Factor that in because the power tube bias point of the conduction curve has a Conduction PLAEAU. When you strum the guitar lightly its AC biasing the power tubes GRID and when you strum the guitar harder its AC biasing the power tube GRID. On the Function Signal Generator you have to adjust the mV input to also AC biasing amplifier because the input signal level will also AC bias the power tubes into conduction. The harder and softer the input signal level the power supply current draw is going up and down which needs also to be factor into biasing amplifiers.