Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: duke of earl on March 08, 2011, 08:23:59 pm

Title: Cathode Bias AB763?
Post by: duke of earl on March 08, 2011, 08:23:59 pm: Any experience good or bad on a fixed/cathode bias switch in this circuit with 6L6's and if the cathode bias resistor needs a bypass capacitor?
Title: Re: Cathode Bias AB763?
Post by: chocopower on March 09, 2011, 07:11:13 am: i have done to a 2x6L6 clon of a twin reverb. See the attachment.

I install a swicht for the cathode bypass cap, but it was always coneccted.

Sound between cathode/fixed bias is not day and night, but is an easy and cool option.
Title: Re: Cathode Bias AB763?
Post by: phsyconoodler on March 09, 2011, 10:22:59 am: i cathode biased one of my V-Verb amps and it sounds softer and definitely breaks up earlier.I liked it.I also used a separate cap/resistor for each power tube and have a switch to lift the bypass caps that effective drops the output for small clubs.Very nice.
Title: Re: Cathode Bias AB763?
Post by: duke of earl on March 09, 2011, 09:30:29 pm: Chocopower,

The 50uf cap, is it an electrolytic? I have a 500 ohm 25 watt resistor but am unsure how to go about selecting a capacitor value. Phsyconoodler, what resistor and cap did you settle on?
Title: Re: Cathode Bias AB763?
Post by: Pleximan on March 10, 2011, 09:19:25 am: Effective cathode impedance for pentodes is
the Cathode resistor in parallel with 1/gm
Most 6L6 data sheets give 5.1mA/V value for transconductance

   1
Rk // 5.1mV/A

To choose the frequency response you can start with a frequency well below
82 hz for full bass response (lowest note on guitar with standard tuning)

Ck= cathode cap, Zk =cathode impedance, F = frequency

F= 1
   2piCkZk

Or you can rearrange the formula to find the value of Ck

Ck = 1
   2piFZk

Cause a push pull output doesn’t have as much cathode degeneration
as a SE design the effect is not that great for push pull.

The value of the cathode resistor and cap will also depend on what plate, screen voltage
You intend to run the tubes at?

Maybe I’m reading your post wrong?
Okay you want to a build 6L6 push pull amp around
a 500 ohm 25 watt resistor since you already have this part.

25 watts is a bit over kill but can still use it just fine.
You should be able to run the 6L6s at maximum screen & plate voltages
with a 500 ohm resistor.

Using a frequency of 20hz using the above formula Ck calcs to around 56uf so you can use the next common value 47uf or 50uf etc and yes use a electrolytic
Title: Re: Cathode Bias AB763?
Post by: chocopower on March 10, 2011, 10:23:38 am: Quote from: duke of earl on March 09, 2011, 09:30:29 pm
Chocopower,

The 50uf cap, is it an electrolytic? I have a 500 ohm 25 watt resistor but am unsure how to go about selecting a capacitor value. Phsyconoodler, what resistor and cap did you settle on?

As Plexi said. Yes. The cap is electrolitic. I forgot the "+" simbol.
And for the resistor, i´m using a 25w aluminium resistor. And again, as Plexi said, is overkill, but there are a lot of space inside the chasis and those resistor are not expensive.
(http://www.talonix.com/images/RH25.jpg)
Title: Re: Cathode Bias AB763?
Post by: duke of earl on March 10, 2011, 05:40:46 pm: Thanks fellas. I have a 50/100 electrolytic I will try.
Title: Re: Cathode Bias AB763?
Post by: chocopower on March 10, 2011, 06:20:40 pm: try the splited resistor/cap, with switching option for the caps. Is a GREAT volumen reducction method.