Calculating cathode resistor for cathode biasing.

[Chuck H]

Maybe. This has been discussed some over at the AX84 site. Using caps like 1000uf and 2200uf. I never looked into it. Is it because the cap is so large that it can never charge? What about the current raising DC voltage across the resistor? Can anyone shed some tech on this?

[Gingertube]

For Cathode "Auto" bias then the maximum dissipation is at idle. That means that you can safely bias the tubes "hotter".

One of the additional reasons for the 70% bias recommendation is that in Guitar Amp designs the max Rg1 value is routinely abused (to keep the load on the phase splitter low), reducing the idle dissipation to 70% allows you to use up to X2 the max Rg1 value, and this is typically what IS used.

Max Rg1 for cathode bias is typically X5 what it is for fixed bias because there is an opposing mechanism to bias shifts cause by grid current, when the grid is at a slightly poitive voltage due to grid current flowing through Rg1.

So with Cathode bias then feel free to bias at 90% of rated dissipation, some even say use 100% but that is tempting fate.

Cheers,
Ian

[Gingertube]

Chuck,
You usually find this under blocking distortion discussions. The cap will charge according to the average volatge at the cathode. Some use the arguement that the idle current flows through the bias resistor and then signal current flows thru' the bypass cap. That is too simplistic but the effect of average current under signal being higher than idle (that is once you push past the Class A to Class B transition) then the voltage at the cathode does rise effectively increasing the bias. Discharging that extra voltage will depend upon the cap value and how often and how hard you drive the peaks.

I once tried the HiFi method of biasing using a current source in place of the bias resistor. I abandonned that coz. it had the worst overload recovery I've ever seen. At least with a resistor then the increased voltage will discharge thru' that resistor a bit faster. With The CCS bias amp (EL84s) I found I could easily pump the normal +12V at the cathode up to 2X that (ie +24V) with continuous overdrive (actually a continuous sine wave at high level).

I've always believed that this is the main reason people peferred fixed bias for guitar amps.

So the "bias shift" with the bypass cap will depend upon the how hard and how often the output tubes go into the Class B area (one tube shut off and the other at more than 2X the idle current).

The old HiFi texts say specifically that for Class A no bypass cap is required but for Class AB they are essential for low distortion. Of-course low distortion is not usually our design brief for a guitar amp.

Cheers,
Ian

[jazbo8]

For Cathode "Auto" bias then the maximum dissipation is at idle.

I'm still not 100% clear, this again presumes Class A operation, doesn't it?

[Gingertube]

Ah! is my face Red - Yes it assumes Class A
Did anyone link this article yet??
Idle Current Biasing - Why 70 percent?

Cheers,
Ian

[jazbo8]

Thanks for the link, I had to re-read that article again, but in it, Aiken did not touch upon the cathode bypass capacitor, but it is well known that the capacitor value changes the feel of the amplifier, i.e., small value - loose, large value - stiff. So I believe the "above 70% cathode bias" rule should be apply with some caveat.

[Alan0354]

Guys, After thinking about the design with power scaling in mind, I decided to go back to fix bias using opamp. I don't want to deal with change of bias current when I lower the +B. It's not hard to design tracking grid bias voltage with +B so I know I am optimizing the operating point at any +B voltage.

Thanks

[Alan0354]

Alan,

I've posted this a few other places but repeat it here .

Powerscaled 6V6 ( a quad) with fixed/cathode bias switching on each pair and and triode/pentode switching on each pair.

For your possible interest here are my power amp set to work notes:

Set to Work of the Power Amp.

Recall that I am using Hammond 370FX Power Tranny, 4 x 6V6G (ST Shape) Output Tubes and a 50W Marshall Output Transformer(Raa = 3K4).

Measured B+ unloaded (no tubes) => +378 Volts
Measured Raw Bias (no tubes) => -77 Volts

Set Power Scale Control to 10
Set Compression (SAG) Control to 0
Set Fixed Bias
Set pentode Mode

A quick calc for 70% of 12 Watts Anode dissipation gives 22mA per tube.

Fit the 4 x 6V6G Output Tubes and selected 12AX7 PI tube (selected for matched triode sections). Actually the best I had was a CV4004 so used that.

Set the 4 bias contrls for 20mA per tube as a "start point".
Remeasure (loaded) B+, now at +349 Volts
Redo the calc above and get 24mA per tube.
Allow to warm up for 10 minutes and then set bias pots for 24mA per tube.

Checks:
B+ = 345 Volts
Bias Current/Voltages
24.0mA @ -27.1V
24.0mA @ -26.9V
24.0mA @ -26.6V
24.0mA @ -26.7V

Switched to Triode Mode - each tube idle current drops about 0.3 to 0.4 mA
Switch back to Pentode Mode

Set Power Scale Control for B+ = 250V
Checks:
B+ = 250V
Bias Current/Voltages
16.5mA @ -19.5V
16.9mA @ -19.3V
16.6mA @ -19.1V
16.6mA @ -19.1V

Set Power Scale Control for B+ =150V
Checks
B+ = 150V
Bias Current and Voltages
9.8mA @ -11.3V
10.4mA @ -11.2V
9.7mA @ -11.1V
10.0mA @ -11.1V

Set Power Scale Control back to 10 => B+ back to +345 Volts

Wind Compression Control from 0 to 10 => B+ drops to +105V
This looks a bit "drastic" but actually sounded OK.

Set Compression back to 0 and switch to Cathode Bias (each 6V6 pair has a common, unbypassed 470R 5W).

Checks:
B+ = 343V
Bias Currents / Voltages
Pair#1 Cathode Volts = +23.9V
25.9mA , 25.3mA tube currents
Pair #2 Cathode Volts = + 23.7V
25.2mA, 25.4mA tube currents

Cheers,
Ian

Looks like the bias current goes approx. proportionally down as you lower the +B, How does it sound when the bias current decreases at lower +B?

From the 6V6 data sheet, if using fix bias and scaling the +B and grid proportionally down, the plate current increases. Am I reading it wrong?

Cathode bias sure is a lot simpler for power scaling if it sound good when scale down. Do you worry about current being too low when you scale down the voltage?

What is sag control?

[jazbo8]

In case anyone was wondering, here is the Fisher cathode bias adjustment method mentioned earlier by JoeM:

[Chuck H]

Well, That's just a big ol' rheostat. I've used rheostats to dial in cathode bias circuits because it can be a PITA swapping big resistors in and out. Once I've established a value I like for tone and performance I put in a fixed resistor.

[JoeM]

No it's not a rheostat. Some of the voltage off the divider adjusts at the tube grids (i.e. via the pot wiper), placing more or less + voltage to adjust the bias.

[Chuck H]

Ah, right. Took a quick glance and saw it wrong.



But a big ol' rheostat would work too

[JoeM]

Yes, that's the way I've seen done frequently too, with a big rheostat in the cathode circuit. The Fisher circuit doesn't require a high current pot.

[Alan0354]

Well, That's just a big ol' rheostat. I've used rheostats to dial in cathode bias circuits because it can be a PITA swapping big resistors in and out. Once I've established a value I like for tone and performance I put in a fixed resistor.

I think it did not show the cathode resistor, the pot divider just tap off the voltage of the cathode.

[frus]

So with Cathode bias then feel free to bias at 90% of rated dissipation, some even say use 100% but that is tempting fate.

consider this schematic, EL84's are happily cooking at 15W at idle

http://www.blueguitar.org/new/schem/vox/ac301960.jpg