Importance of Voltage Drop?

[jordan86]

Saw this video recently on another thread while doing some research on bias. One line that stuck out to me was very strange, at around 2:20. Bill talks about setting the bias based on the voltage drop, as if to say, it should be set to accommodate a particular number. Is that just the nature of the specific steps pertaining to the Blues Jr circuit?  Just seemed like a factor I have never heard discussed anywhere.  My limited reading/learning has always been more of a plate dissipation approach (plate voltage, plate current). Obviously the voltage drop is a helpful figure to know in determining that...

Side note: I just recently heard Billm passed away. Very sad. Just want to go on record as to how much I appreciated his site and input on amp mods.  I did put the TO20 that he and David Allen designed in my recent Princeton build.

[jjasilli]

One way to measure bias is with the Output Transformer Resistance Method:
 https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm#:~:text=Subtract%20the%20plate%20voltage%20from,voltage%20drops%20for%20both%20tubes.&text=Turn%20off%20the%20amp%20and,to%20get%20the%20bias%20current.


Uncle Doug in his YouTube videos also prefers this method.  It's similar to measuring the voltage drop across the cathode resistor, or a bias sense resistor; but is done on the "other end" of the circuit.  Unlike those methods, the Output Transformer Resistance Method measures only plate current; i.e., it excludes screen current.


However, in this case you don't have to measure the OT resistance.  About 2:40 Billm points out that the desired voltage drop is listed in the BJr instructions.  So you can compare your voltage drop to that.

[jordan86]

Yes, I’ve watched uncle doug’s video and read rob’s article. That’s how I’ve been calculating bias, or maybe more accurately said, my plate dissipation. So I guess I am not asking the question properly...

It seems to make more sense to set bias based on plate dissipation? Using Doug/Rob’s method. Or would it be better to just set my meter up and adjust bias based on the voltage drop and call it good? It seems that’s what BillM was suggesting. That just is confusing to me. Only accounts for 1 of several important measurements/variables. 

I guess if this video is specific a BJ circuit where plate voltage and the other factors are accounted for already, suggesting that the voltage drop be set to “x” would just be bill’s shortcode for dialing in the best bias setting, yes? Meaning, a specific voltage drop is not a universal way to dial in that particular OT on a different amp.

[jjasilli]

I guess if this video is specific a BJ circuit where plate voltage and the other factors are accounted for already, suggesting that the voltage drop be set to “x” would just be bill’s shortcode for dialing in the best bias setting, yes? Meaning, a specific voltage drop is not a universal way to dial in that particular OT on a different amp.

Yes, that is totally correct. 

Further, a pre-ordained voltage, or voltage drop, is probably not optimal anywhere.  Tubes vary; trannies vary; B+ voltages vary; etc.  The manufacturer's specs, bias calculators, etc., will get you to a good place, within a proper range of bias settings.  Then I play a guitar through the amp, and fine-tune the bias setting. 

[sluckey]

BillM has done a LOT of work with the BJ. He has already done all the calculations for proper bias based on plate dissipation. The things he didn't mention in that BJ specific video are the resistance of the OT plate winding and the plate voltage. Knowing the OT plate winding means all he has to do is measure that voltage drop across the OT plate to center tap winding. Dividing the "voltage drop" by that resistance yields the plate current. Multiply that by the plate voltage and you have the plate idle dissipation. BillM has all that info and he built a table that allows you to look up your desired dissipation and that shows the voltage drop necessary for that dissipation. Easy peasy.

You have to remember that a big part of BillM disciples are not tech savvy. He has developed this video with these people in mind so they can set the bias correctly (in his opinion) without ever knowing what they did.

While watching the video, monitor the voltage drop. Tweak this trim pot for a particular voltage drop. Wala! You have just biased your BJ for proper operation. That's all you need to know. Now set the knobs to 10 and play it like you stole it!   

[jordan86]

That makes way more sense. It’s a simplified process for a specific circuit and specific components. I just wanted to make sure I wasn’t missing some magic idea about specific OT’s sounding best at a specific drop value. Goes against everything I’ve learned so far.

Will continue on as normal. Thanks guys!!!!

[sluckey]

If you have a bj and have done all the BillM mods, including the OT upgrade, then blindly follow that video to properly bias that bj. If you want to understand what you did and be able to apply that procedure toward biasing any amp, hang out on Hoffman's forum or any other tech savvy forums and ask questions.

BTW, I'm not trying to take anything away from BillM. He was more than capable of explaining in depth what he was doing. He certainly became a legend among BJ owners.