Biasing Tube Amps 

Excellent and very concise video. Thank you

Great vid thanks.

Thanks for the informative video. What does it usually mean if the bias is drifting a lot? The bias on my two rock studio pro 35 amp test point varied from 0.054v to 0.074v during a gig after I set it at the recommended 0.064 initially! Also the test point feels soft with the multimeter probe almost like I’m stabbing it into play dough!! I’m so confused!! The tubes are new and quality matched tested ones so unlikely to be the cause.

At 10:11 your schematic shows trans secondary output as DC voltage? No rectifier?? Educate me please :)

SONICTONE< you didn't show how you measured or calculated the plates voltage drop? once your got the plates voltage drop * bias current = tubes power dissipation, how do you compute to get the tubes power dissipation?

How about this -20 volt....and I need -13 volts...show me the circuits to get negative voltage.

Don't forget about the infamous Fender hybrid fixed/cathode bias circuits, introduced around 1968 and quickly abandoned. I happen to own 2 of those beauties, a 1968 Bassman (AC568) and a 1969 Bandmaster Reverb (AA768). Both had the bias balance (output tube matching) pots, so I added a bias trim pot and use the output transformer resistance method to bias them.

I’m still learning so I have a question, isn’t the B+ voltage inclusive of the -voltage on the bias? As an example, the figures of the voltage be B+ and - bias voltage =512 volts in your example. I’m not saying I’m correct, genuinely interested to know. Thanks for the video and am learning a lot from them.

I don’t think your calculations for cathode bias are correct. Plate voltage is with respect to cathode, not to ground, so plate dissipation Pd = (Va-Vk) * Ia.

There is no mystery about why is the cathode current higher than Plate current. As soo nas you have beaming quattrode or pentode you ill have this "phenomenon". In triodes they will have the same value. You got it already? Yes it is the screen current. Screen takes its own few mili Amperes current. So plus come from the plate from the output transformer and screen gets it's plus voltage from screen resistor or in the case of ultra linear tube amplifiers from output transformers screen tap. In both cases screens have usually smaller voltage than plate. On the tube you have only one connection to negative side and that is the cathode so that is the only place to go for both currents. So Cathode current is Plate current plus screen current. Usually screen currents are a few mili Amperes (3-5 for common output tubes). Also as soon as you do change bias current to new value you need to measure plate voltage that will correspond with new situation. You can't just measure palte voltage and think it is going to stay the same no matter what plate current we draw. It might be sort of logical to think like that because plate currents are mili ampers and that should not make much difference right? Wrong. 5 mA more will drop plate voltage considerably. It will lover it down. When you are lowering the plate current your palte voltage will go up. So you always have to measure both values. To be honest 60 percent is all you need nowadays with the fixed bias amplifiers. Your working currents will reach the maximum value with maximum signal even with 60 percent value. 60 percent is not sounding worse either. Especially not in cleaner amplifiers. Fixed bias amplifiers are more efficient and therefore it is easier and more clever to make a big amplifier with fixed bias power stage. Plus with NFB those big amplifiers will stay usable and not turn into the unplayable monsters. One thing is true we are talking about percentage of the dissipation power and not about the mili Amperes. Though we have to obey the tube datasheet and not overstep the maximum plate current. For example in some nice amplifiers designers use lover voltage overall. They do not care about how to use full power of the tube they are after the sound it self. So for example they do not go with 450V for 6L6GC but they opt to go with 320V. If that would be a cathode bias amplifier to reach 90 percent of maximum plate dissipation you might have to go more than maximum plate current. Then they do leave it at lower biassing current and do not achieve 90 percent. But if that sounds good who cares?? Who cares is it 20 or 30 or 40 watt amplifier? They are after the tone and not the power in such amplifiers. Plus if that turns out to be no NFB cathode bias amplifier with big output tubes and not that much power still it is going to sound bigger and louder than actual Watt numbers would make you think. Think VoxAC30. Not many people were complaining it is not loud enough for what it is. There is no shame in bringing your amplifier for tube change and inspection and biasing to proper tube amp tech. Amplifiers contain lethal dose of DC Voltage. Voltage that stays present in the filter caps even when the amplifier is off. Most of the amplifiers have discharging resistors but those take time to drop the voltage to safe levels. Plus there is a benefit to it. Proper tech will check everything and find if something is lose or broken and your amplifier will be back as new again. Ask me how I know? Well people brought me amplifiers with one issue and most of the times there are more issues. Like one amplifier came to me for tube change and biassing and checkup and I found out that presence cap have had broken hookup wire. Presence can't work without that cap. And that was in really good built boutique amplifier. Once owner got it back he called me after his first gig and asked WTH have I doen to it and how come it sounds so much better now :) Well new tubes, fixed presence and overall cleaning and check up and reflow of few spots does that. Just make sure that if you own really valuable amplifier you do not go to random guy. Often you see bad parts and parts that lower the value of the amplifier. Also if he will not call you after the initial checkup and before he fixes what he thinks needs to be done without consulting you about costs and what needs to be done I would avoid those people. Some people just assume you got money and you want everything done so they just go and do their stuff and charge you after it. You need good communication and pictures and even videos.

Only an armature would trust an amps factory bias test points. Real pro's use a bias meter taking the results directly from the tube. I have proven time and time again the bias testing circuits build in amps are GROSSLY incorrect.