Bias pot value theory

[tubeswell]

You choose the pot (wired as a variable resistor) that, in combination with the series resistance in the leg of the bias voltage divider, gives you the range above and below the nominal bias voltage you are aiming for, so you can adjust it. This usually ends up being a 25k pot or a 50k pot. The bigger the pot, the more-sensitive/harder it is to adjust. Anyway, that's my theory and I'm sticking to it.

[roberto]

Is there a particular bias schem you're thinking of?

95% of fixed bias schemes are simply:
- bias tap
- reverse diode
- capacitor
- resistor
- capacitor
- pot + resistor to ground.

Could you break that down a little bit more for me, please?

http://www.ampbooks.com/home/amplifier- ... excursion/

[Reeltarded]

Reeltarded, I may be realtarded, but you are hard to follow sometimes! So you think that's correct that a bias circuit that normally has a 25K pot should be split into two 50K pots?

OMG don't listen to me then! I try not to, but the voice in my head is louder than the voice in my head. umm.. In your head.

Wow. I am hard to follow. Run away!

As said above, the bias resistor value is changed when you add a second trimmer because you are voltage dividing the network? So the same voltage needs to be available at the tail of each trimmer as it was with a single trimmer.

Ok, yes. Run away. I barely understood myself. I like ice cream.

[VacuumVoodoo]

You are splitting current when you parallel branches in a circuit. Kirchoff #1.

[Gaz]

Could you break that down a little bit more for me, please?

http://www.ampbooks.com/home/amplifier- ... excursion/[/quote]

Ok, but I was just confused about what your comment had to do with the RC time constant in the bias supply. I understand the relationship between the coupling caps and grid leaks.

[roberto]

Read that page until the end, not only the graph.

[renshen1957]

For example, here's a bias circuit I've used before. I arrived the values by simply tweaking them. Honestly, I didn't know how to calculate the decoupling caps on the wipers, so I just guessed

I've observed that the caps take about 4 seconds to charge from power on, but I'd really like to know how to calculate the RC time constant, so I can see how high of pot values I can get away with.

Hi Gaz,

I only have two 10uf 160V e-caps, 100k 1W resistors (I don't use single Xformer bias winding), and a 100V 1W zener diode for the bias supply), the reason, I take my bias from both sides of the B+ lines (I don't like half wave bias) and omit the pair of E-caps connected to the pots as shown in you drawing. Never have a problem with RC time constant.

As to RC time constant, I have been know in 4 tube power amps to give a separate signal cap to each tube (at half the original value) and a separate grid leak resistor (at double the value) to each tube along with the separate bias pot. This gives better overall performance. And yes, I am heavily influenced by the TUT series by Kevin O'Connor. changed my way as how I look at amps.

There are other ways to approach the four tubes 1 bias pot to four tubes four bias pots

Best Regards,

Steve

[Gaz]

Thanks, I too got the simple dual bias from TUT, but never tried the full-wave and zener. The only reason I added the decoupling caps on the wipers was because my PPIMV (type that replaces the grid leaks with a dual pot) was not completely silencing the amp when all the way down. Adding the caps fixed it. I don't know why is worked, but I sure do have my own little theories!