VVR bias question

[iknowjohnny]

Lets say your amps tubes are biased at 42 ma. Now you add VVR with bias tracking to the amp. When you turn it down is the bias supposed to stay the same as it was (42 ma in this example) no matter what power level the B+ is at? I assumed bias tracking was to do just that, but i'm wondering if the bias needs to be different at different B+ voltages and that tracking actually changes the bias to the correct current for different B+ levels.

[Richie]

""""When you turn it down is the bias supposed to stay the same as it was (42 ma in this example) no matter what power level the B+ is at?""""

No.. it should track to what theB+ voltage is going to the tubes.

if your using the VVR that is for this application,then yes the bias changes as the control is turned..the voltage and bias,track together.
The cathode bias VVR ,it will also change as less voltage is applied to the output tube..
Also if useing the bias tracking VVR..be sure you have it wired correctly..and it should work fine. I've installed a few of these ,and they have all worked fantastic,without problems.

[Colossal]

Lets say your amps tubes are biased at 42 ma. Now you add VVR with bias tracking to the amp. When you turn it down is the bias supposed to stay the same as it was (42 ma in this example) no matter what power level the B+ is at? I assumed bias tracking was to do just that, but i'm wondering if the bias needs to be different at different B+ voltages and that tracking actually changes the bias to the correct current for different B+ levels.

The bias does not stay the same. Your assumption is correct. The bias needs to and does track with an increase or decrease in system voltage.

[iknowjohnny]

Ok, so lets say for the sake of argument you reduced the B+ on a 50 watt marshall to 100V using a non bias tracking VVR circuit. how would you determine where to set the bias? This is just a theoretical that i'm asking to try and understand things.

[Colossal]

Ok, so lets say for the sake of argument you reduced the B+ on a 50 watt marshall to 100V using a non bias tracking VVR circuit. how would you determine where to set the bias? This is just a theoretical that i'm asking to try and understand things.

Is the 50w Marshall-in-question a fixed or cathode bias amp? If it is cathode bias, then turning down the power to 100V will automatically adjust the bias since the cathode bias is just a resistor (or a resistor paralleled with a cap). If it's a fixed bias amp, then you must track the bias voltage along with the B+ voltage drop. You have to get the right VVR for the job. If your amp is cathode biased, you could always get the fixed bias version and just not use the bias tracking circuit. If you ever decide to change over to fixed bias, then you have the tracking capability right there.

VVRs are easy to install and hats-off to Dana/UR12 who has done a great job on both the board and the documentation. You will also need to add an additional cap and resistor at two places in your amp if you add a VVR.

[UR12]

Lets say your amps tubes are biased at 42 ma. Now you add VVR with bias tracking to the amp. When you turn it down is the bias supposed to stay the same as it was (42 ma in this example) no matter what power level the B+ is at? I assumed bias tracking was to do just that, but i'm wondering if the bias needs to be different at different B+ voltages and that tracking actually changes the bias to the correct current for different B+ levels.

One thing people try and do is to equate the bias current with the bias voltage. This is apples and oranges. The VVR with bias tracking actually adjusts the bias voltage to the tubes which in turn causes the tube to draw less current at idle as you turn down the b+. Think of it this way on a fixed bias amp. If you have a 500v B+ and let say you have the bias voltage set at -40v in order to bias your tubes at 42ma. This would give you the same parameters as if you didn't have the VVR installed. As you turn down the b+ you bias voltage will also drop linearly. In other words if you drop your b+ by 10% then you drop your bias voltage by 10%. So in this senario your b+ would be at 450v (500- 10% or 50v = 450v) and your bias voltage would also drop from -40v down to -36v (-40v - 10% or 4v=-36v. If you drop your B+ down to 100v then that would be down to 20% of your full B+ voltage so your bias should also be down to 20% of it's original -40v or -8v. The current may or may not track linearly as this has more to do with the tube and it is really nothing to worry about as long as you get the current set to where you want it with the b+ at full.

[jaysg]

Ok, so lets say for the sake of argument you reduced the B+ on a 50 watt marshall to 100V using a non bias tracking VVR circuit. how would you determine where to set the bias? This is just a theoretical that i'm asking to try and understand things.

One of the tools you should pick up at Duncan Munro's site, is an Excel spreadsheet....something like 'load.xls'. It allows you to play around with B+ and tube types and it coughs out suggested bias voltages. You can get a feel for what's going on without having to understand what's really involved up front.

You should download the tone stack and power supply apps too.