Grid Leak Bias & PI ID

[Tonewood]

Trying to learn more about the following-

1: What is the name of this inverter? I would like to look it up & try to understand it better. If you can easily explain it here, please feel free!

2: I kinda get how grid leak bias works but have no idea how to DESIGN one. If anyone knows of a good source of info on designing a grid leak bias ckt please let me know. Also feel free to pipe up with any info you think would be useful. It seems like the bias moves around following the input voltage. Does that seem correct? If so, I can't really replace it with a cathode bias & get the same results??

Thanks!

[nashvillebill]

I, too, would like to hear more about grid leak bias, I also have an old amp where the input gain stage has grid leak bias. Mine has a 12BF6 low-mu tube, with a 1 M grid leak resistor, and they are running the plate at really low voltage. Merlin's book mentions grid leak bias but quickly moves on, dismissing the concept as too problematic. Kevin O'Connor mentions it as a mu-amplifier.

I am tempted to change to conventional cathode bias and raise the plate voltage, but don't have a good grasp of the theory yet either.

[loudthud]

I believe that phase inverter is called the "Paraphase" inverter. I've never seen one where the used a pentode to get the gain and a triode to invert the phase but there is no reason why not to do it that way.

I've never run across much info on grid leak bias but have tried it a couple of times. Changing the grid leak resistor from 5Meg to 10Meg really didn't change the plate voltage as much as I thought it would. When the plate voltage gets low enough, grid current starts to flow.

One time I was trying to build a phase inverter with a 12AT7 that had only 100V on the plates. Grid current was flowing and because of the different impedances on each grid, there was an offset that didn't make any sense to me at the time. Shoulda checked the grid voltages

[Gtr_tech]

I believe that phase inverter is called the "Paraphase" inverter. I've never seen one where the used a pentode to get the gain and a triode to invert the phase but there is no reason why not to do it that way.

:

Pentode driver with triode concertina splitter? Very common especially in Hi Fi designs of the 50s-60s. 7199 tubes were usually used here. Sunn, Dynaco, etc.

I assume you meant the pentode doing the splitting?

[loudthud]

Pentode driver with triode concertina splitter? Very common especially in Hi Fi designs of the 50s-60s. 7199 tubes were usually used here. Sunn, Dynaco, etc.

I assume you meant the pentode doing the splitting?

No, the triode inverts the lower side via a voltage divider somewhat like the 5C3 except grid leak bias is used. Feedback is brought back to the cathode of the pentode.