Calculating cathode resistor for cathode biasing.

[jazbo8]

Also 35V on the cathode seems rather high?
It's often ok to run the tubes at a static plate dissipation approaching 100%, as it generally tends to reduce under signal, and also due to the inherent limiting / regulation that cathode bias provides.

35V is based on the value from the fixed bias Deluxe. Not sure what the advantage is for biasing the tubes hotter than 70%, is it done purely for the tone and/or to eliminate the possibility of crossover distortion when heavily driven?

[Chuck H]

is it done purely for the tone and/or to eliminate the possibility of crossover distortion when heavily driven?

Yes.

There's a couple of things that can be done with zeners to make the cathode bias limited and variable. I'll need to find my files though. I can't promise I'll be able to. More to follow.

As I read it, what Alan proposes is that there will be 400V between the plate and cathode. With 35V on the cathode that's a static Vp of 435. About as high as a Deluxe Reverb but with the relief of cathode bias. I think it's a bad idea for a couple of reasons. The idea of biasing as hot as is usually necessary for cathode bias with such high Vp is problematic and the voltage rise (bias cooling) at the cathode, if it pulls the current draw down to a safe range, will surely leave the circuit prone to crossover distortion when driven hard.

[jazbo8]

Did a quick review of the Aiken's article, and I believe the above quote mainly pertains to Class A or near Class A operation AND if the cathode resistor isn't bypassed. With a properly sized cathode bypass capacitor in place, you should not bias the tubes at 100% Pda_max, since it would result in excessive plate dissipation during a large part of the output cycle and also shift the load line well above the knee.

[Alan0354]

You need to subtract the cathode voltage from the plate voltage to get Vpk, so 400 - 35 = 365V, now re-run your calculations. Here is a handy calculator to save you the trouble in the future.

Thanks, I did specify it's 400V, that means the real plate voltage is about 440V and I already taking into consideration of the 35V on the cathode.

I follow Fender RI 65 Deluxe Reverb and verified. The voltage BETWEEN plate and cathode is 400V and when I adjust to 25mA/tube, the bias is about -35V. This is the number I am using for calculation as I have not build the cathode bias circuit yet. The reason I change to cathode bias is because I am going to do power scaling and I don't want to deal with the fixed bias.

[Alan0354]

Yes.

There's a couple of things that can be done with zeners to make the cathode bias limited and variable. I'll need to find my files though. I can't promise I'll be able to. More to follow.

As I read it, what Alan proposes is that there will be 400V between the plate and cathode. With 35V on the cathode that's a static Vp of 435. About as high as a Deluxe Reverb but with the relief of cathode bias. I think it's a bad idea for a couple of reasons. The idea of biasing as hot as is usually necessary for cathode bias with such high Vp is problematic and the voltage rise (bias cooling) at the cathode, if it pulls the current draw down to a safe range, will surely leave the circuit prone to crossover distortion when driven hard.

I thought it should be save to make the cathode bias having the same voltage as the fixed bias in Deluxe Reverb. Why is it different. What is bias cooling.

Why is it prone to crossover distortion?

I actually going to experimenting with putting zeners at the cathode in series with the cathode resistor so I can adapt to 6L6 also. Is that what you mean?

Thanks

[Chuck H]

I think that's an excellent idea. In fact I use a bias arrangement for EL84's that has a zener across the cathode resistor. I have the zener valued to act as cathode biased up to the moment the waveform starts to clip sharp. This way the amp acts like it's cathode biased, with the softer attack and mild compression, up to the point of hard clipping, when it tightens up and acts like a fixed bias amp. I like this a lot. But...

For a DR type amp (and existing DR's with high Vp) I thought it might be an idea to use the same circuit, but value the zener so that the amp acts like it's fixed all the time. I don't think the character of the DR would be improved by any bias shift, but the circuit would give the plates, typically abused in the DR amps, a little relief while still maintaining the basic sound and feel of the amp without the need to use a higher rated zener on the CT of the HV wind, changing the power transformer or only selecting tubes that are up to the challenge. Which may only get harder, who can tell?

Regarding bias shift and crossover distortion...

In a cathode biased amp the cathode resistor is in series with the tubes internal resistance, SO, any current moving through the tube also must pass through this additional resistor. As current increases plate voltage drops and voltage across the cathode resistor rises. Lower Vp and a larger negative relationship between the cathode and grid both correspond to a colder bias condition. Under these circumstances it's very likely that the bias will cool enough to fail in holding the crossover point above audible cutoff and crossover distortion is the result. This is especially true when high Vp is combined with cathode bias. Probably because the safe standing bias at idle doesn't allow much shift before going too cold.

But, as you can see, none of this applies if you strap a zener with a ZV of the standing cathode voltage across the cathode resistor because there's no bias shift. The bias is fixed. Fixed cathode bias.

[pdf64]

'I believe the above quote mainly pertains to Class A or near Class A operation AND if the cathode resistor isn't bypassed'

I can't see anything in the text that may lead to that understanding?
Pete

[Chuck H]

The reason I change to cathode bias is because I am going to do power scaling and I don't want to deal with the fixed bias.

Understandable, BUT, doesn't that mean that trying to idealize is moot? Suppose you end up with a 415R cathode resistor for a pair of 6V6's at 400Vp. Having been very careful and asking a lot of questions to make sure you have the right value resistor for a given plate voltage. Ok, now lets turn the voltage down. WAIT!?! Wha-wahaaat!?! Oh noooooo, now my resistor is the wrong value

Unless I'm mistaken I'm going to bet that the amps that work best with power scaling that relies on cathode bias for self adjusting don't push the limit on acceptable plate voltage and so can be biased generously into class A for a standing idle at full voltage. I don't think self bias/cathode bias as a means for bias control for power scaling is going to work well on an amp that needs to idle at 70% dissipation to start. I could be wrong, but if so, why do we even need to speculate a cathode resistor value. If I'm wrong it's a one size fits all (I guess that size would be 250 ohms for a pair and 470 ohms for a single tube as precedented by Fender on countless amps). No. What I think is happening with power scaling and self bias is that the amp needs to start in a place that allows a broader margin of less wrongness The DR circuit isn't going to do that.

[pdf64]

I agree, as a starting point 440V seems rather too high of a VB+ for 6V6 in cathode bias.
Suggest a max of 320V for EL84, 360V for 6V6, 400V for 6L6GC / EL34.
Above those levels and methods to control bias shift need to be introduced, to avoid a weedy tone when cranked (due to bias shift).
Pete

[Gingertube]

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

[Alan0354]

I think that's an excellent idea. In fact I use a bias arrangement for EL84's that has a zener across the cathode resistor. I have the zener valued to act as cathode biased up to the moment the waveform starts to clip sharp. This way the amp acts like it's cathode biased, with the softer attack and mild compression, up to the point of hard clipping, when it tightens up and acts like a fixed bias amp. I like this a lot. But...

For a DR type amp (and existing DR's with high Vp) I thought it might be an idea to use the same circuit, but value the zener so that the amp acts like it's fixed all the time. I don't think the character of the DR would be improved by any bias shift, but the circuit would give the plates, typically abused in the DR amps, a little relief while still maintaining the basic sound and feel of the amp without the need to use a higher rated zener on the CT of the HV wind, changing the power transformer or only selecting tubes that are up to the challenge. Which may only get harder, who can tell?

Regarding bias shift and crossover distortion...

In a cathode biased amp the cathode resistor is in series with the tubes internal resistance, SO, any current moving through the tube also must pass through this additional resistor. As current increases plate voltage drops and voltage across the cathode resistor rises. Lower Vp and a larger negative relationship between the cathode and grid both correspond to a colder bias condition. Under these circumstances it's very likely that the bias will cool enough to fail in holding the crossover point above audible cutoff and crossover distortion is the result. This is especially true when high Vp is combined with cathode bias. Probably because the safe standing bias at idle doesn't allow much shift before going too cold.

But, as you can see, none of this applies if you strap a zener with a ZV of the standing cathode voltage across the cathode resistor because there's no bias shift. The bias is fixed. Fixed cathode bias.

Wow, I did not expect this can get complicated. I want to verify this on cathode bias.

1) If bias to say 70%, if you hit it with large signal, the voltage on the cathode bypass cap gets charged up. Higher cathode voltage cause the bias to go down to the point you see crossover distortion when one tube turns off and the other one completely takes over?

2) You put a zener in parallel to prevent the cathode from rising too high and cause cross over distortion?

That's brilliant!!! You're the man!!!

I was just going to put zener in series because 6L6 need more cathode voltage, so I put say a 5V zener to take up part of the voltage and the rest through increase of the bias current.

Takes me a long time to come back because I was reading the Aiken amp article before I even talk.

[Alan0354]

Thanks guys, keep the post coming. I am slow in responding, but I am reading everyone's post. I need time to read, digest and think. I just finished reading the Aiken article. I still need to read Ian's post.

I thought London power use cathode bias to avoid scaling the grid bias voltage. I did power scaling in the 70s using variac but using separate supply for filament and preamp. Then just put the variac on the power transformer to control the plate voltage of the power tube and PI tube and the grid voltage. It worked very well.

[Chuck H]

1) If bias to say 70%, if you hit it with large signal, the voltage on the cathode bypass cap gets charged up. Higher cathode voltage cause the bias to go down to the point you see crossover distortion when one tube turns off and the other one completely takes over?

To be clear, I never said anything about a bypass capacitor. The DC voltage on the cathode rises with current because it's a resistance in series with the current path. Considering the size of any reasonable bypass cap parallel to a resistor rated in ohms (not k's of ohms) I think the time constant for voltage loading can be ignored WRT the bypass cap.

Not sure how the parallel zener will work with power scaling. Since the standing voltage on the cathode should only go down with any decrease in plate voltage I can't see a problem.

[Gingertube]

The cathode bias / fixed bias switching.
I did not use a bypass cap on the 470R. With a bypass cap it sounded so close to the fixed bias that the switching would have been a waste of time/space.

IN fixed bias mode you need the tracking bias regulator.

Cheers,
Ian

[jazbo8]

'I believe the above quote mainly pertains to Class A or near Class A operation AND if the cathode resistor isn't bypassed'

I can't see anything in the text that may lead to that understanding?
Pete

Yup I need better glasses... he was talking about Class AB cathode biased amps, but my point remains - IF Rk is bypassed with a large enough capacitor, the cathode voltage will remain stable thus causing no bias shift...