Output Tube Bias Using a Micro-controller

[kward]

To be clear, I wasn't exactly serious about my previous comment of doing EFB digitally. I think it could be done, but its hard to beat the simplicity (and reliability) of Dave's existing circuit.

 

[knockbill]

This just strikes me as an overly complicated way to make things less reliable for no real benefit. Unless you can improve both, it just seems to me that its a solution in search of a problem.

Kinda like fixing XP with W10??!! Sorry I'm on a computer hating binge!

 

[peterh]

I would go with a controller for 2 different functions :
1- on demand ( press button or whatever ) adjust bias to predefined value
2 - at all other times : watch current through tubes, and if problems turn off B+ and display a meaningful message
on a display what the problem is.
In addition Turn on B+ 30s after power on, to prevent surges.

 

[BinaryMike]

2 - at all other times : watch current through tubes, and if problems turn off B+ and display a meaningful message on a display what the problem is.

Text display may be well beyond the OP's stated goals, but certainly one or more trouble indicator LEDs should be included.

In addition to monitoring cathode current, it should be simple to monitor the commanded grid bias voltage. Grid bias that's way too low or too high is a trouble sign even when the target cathode current is still achieved, because the power tube is likely near failure.

 

[LexDM3]

Well... As someone who works with Arduinos a lot, I can see the value in this approach. But I also think that the more stuff you put in there, the more there is to screw up. And part of that is when the designer/programmer finds out after the fact that there was one set of conditions that he/she never considered when designing and building it. Ask me how I know about screw-ups like that...

 

[primosounds]

I just hit a wall a few months ago,repairing a Sonic Frontiers Power 2 which is PPP KT88 x 4 and 3 6dj8 for input/splitter per channel. It has a very "clever" LED indicator for easy biasing of the output tubes. There is a LED next to each power tube and when the proper bias level is achieved by adjusting the pot the led switches from red to green. This is great for the user except when the system doesn't work. I went thru the whole bias system to try and repair the one tube that the bias light did not switch color. It finally comes down to the IC which is part of the system. It has to be the IC being defective after spending many hours checking and ruling out all the other components. Why not change the IC? Besides questionable availability, i was not about to disassemble everything again and reinstall an IC which i may ruin when i resolder the new one into the circuit board. To me an overly complex circuit for a simple voltage adjustment . All that so a guy can have a gee whiz moment when the led goes from red to green. Forget that! I will take a test point and my Fluke meter every time!

 

[triode17]

This just strikes me as an overly complicated way to make things less reliable for no real benefit.

yes, I'm good at raining on people's parades, but reliability and benefit have to be primary concerns for any sort of modification. Unless you can improve both, it just seems to me that its a solution in search of a problem.

I agree, this seems like overkill. Simpler is better. Take a look at Joe Curcio's auto bias circuit. BCS OUTPUT TUBE BIAS CONTROL SYSTEM. All you need is an Op Amp to act as an error amp x 4.