Do I have this Bias setup correct?

[dragonbat13]

This is a schematic I made of the AB165 balance bias amp I am working on. I added the bias adjustment to the factory bias balance setup.

I do not have the 100K Bias Pot protection resistor R4 installed yet. I want to make sure that I have R2 and RV1 set up correctly first. Should the wiper (Pin 2) go to R2 instead of Pin 1, with Pin 1 going to ground? Should R2 be between RV1 and ground? I cannot see any reason it would be wrong the way it is in the schematic as it sits now.

Also, is R4 installed correctly with regards to the schematic, pulling the power tubes into cut off if there is a failure in the bias adjustment circuit of R2 and RV1?

EDIT: I forgot to ask if there were any way to install a resistor to protect against a malfunction of RV2?

Anything else about this schematic that is incorrect?

Please understand this is only the second time I've used Kicad to draw out a schematic. I am learning as I go. Just being able to post a schematic of what I have done makes me happy with the program already.

[martin manning]

Anything else about this schematic that is incorrect?

I doodled around with SPICE and came up with a better bias/balance circuit. IMO the balance range is much too wide in the original, and if both bias and balance are included the bias range should be large, and the balance adjustment small. In your circuit it will be the opposite. In the circuit below, the bias voltage matches the original with the same input voltage and both pots centered. The extra 22u caps greatly reduce the ripple voltage as compared to the single reservoir design in the AB165. This circuit is a close relative of the bias/balance circuit I posted here for the Dumble SSS driver stages.

You need to make sure that all design requirements are met, the circuit functions properly, and that the components are sized correctly. KiCad can link with SPICE, so you can design and analyze the circuit and eliminate possible errors redrawing the schematic, which insures that the schematic from which a PCB is created is a working schematic.

[Ten Over]

This is a schematic I made of the AB165 balance bias amp I am working on. I added the bias adjustment to the factory bias balance setup.

I do not have the 100K Bias Pot protection resistor R4 installed yet. I want to make sure that I have R2 and RV1 set up correctly first. Should the wiper (Pin 2) go to R2 instead of Pin 1, with Pin 1 going to ground? Should R2 be between RV1 and ground? I cannot see any reason it would be wrong the way it is in the schematic as it sits now.

Also, is R4 installed correctly with regards to the schematic, pulling the power tubes into cut off if there is a failure in the bias adjustment circuit of R2 and RV1?

EDIT: I forgot to ask if there were any way to install a resistor to protect against a malfunction of RV2?

Anything else about this schematic that is incorrect?

You don't need R4 (100k), just tie pin 2 to pin 3 on RV1 (10k-L). The bias voltage will default to the coldest setting should the wiper fail and you will be able to finish the gig. If you want the amp to go into or nearly into cut-off, then leave the circuit as you have it and eliminate R4 (100k). The bias voltage will default to the peak voltage from the transformer upon wiper failure. Make sure C1 has a sufficient voltage rating for the wiper failure scenario.

A protection resistor for RV2 (10k-L) is shown in the attachment.

[Ten Over]

Here is a possible layout for an AB165:

[dragonbat13]

I appreciate the help. I believe I'll use the last layout/circuit since I think I have all the parts and it won't be much extra work.

Martin. Do you prefer building schematics on spice over KiCad? I'm going to dabble in spice also. Main thing I have been using KiCad for is to just build up a schematic so I can read the thing and share a schematic online. My next step is to figure out the library features. I need to update my thread in the PCB forum. I think I'm gonna download spice and try to link it over to KiCad to see what happens.

[martin manning]

Do you really want to drill a hole for another pot in a vintage amp chassis? You can easily add a bias level trimmer to the rectifier board, and if you rewire the balance pot to the circuit I posted you will get a wider bias range with a smaller balance adjust. All reversible if desired. You can even leave the existing resistor values in if you want, except maybe adjust the one feeding the diode, to center the bias range.

I use SPICE for most things, and bring KiCad in only If I will be having PCB's made.

[dragonbat13]

I already have a 10k bias resistor mounted to a bracket and installed, I'm just not sure if I have the parts. I know I don't have the capacitors. Matter of fact I may not have the parts for either setup.

Right now the amp is setup with my schematic, except for R4. When I get back to the amp at my other house I'll see what I can do. It all depends at this time. I got this idea and values from merlins book. I plan on changing over to your setup when I'm get the correct parts. I would much rather have the bias adjustment wider and the balance narrowed.

EDIT: I suppose "layout for the second one" was a little vague. I meant the schematic only. I gotta wait till I hit the bench again before I order more stuff. I've been on quite the roll at digikey.