Correct methodology for biasing class AB1 modulator?

[KC9KEP]

Could someone please help to understand more about this topic or kindly direct me to an appropriate resource?  (The ARRL handbooks don’t go into much detail about this.)

I’ve recently constructed an AM modulator using 807 tubes.  The original design calls for a -22.5V “hearing aid battery” to be used to bias the tubes.  I had used (2) 9VDC and (3) 1.5VDC batteries in series to generate a bias of about -22.5 VDC.

However, my power transformer provides a higher plate and screen voltage than what the original design had called for.  Also the original design suggests that the bias be set so that the cathodes indicate about 50 mA of current draw at idle (The cathode current is monitored by measuring across a 50 Ohm cathode resistor.)

Although my output waveform looks very nice on the scope, the 807’s are running hot and I was seeing some plate redness.  And, measuring the voltage across the cathode resistor, I had calculated about 125 mA of current draw.
So I reasoned that if the plate and screen voltages are higher than specified in the article, the bias should probably be more negative as well.

So, I tried (4) 9VDC batteries to yield a bias voltage closer to -36VDC in an attempt to bring the cathode current down to the specified 50 mA.

At -36VDC bias, the cathode dropped perfectly to 50 mA and I thought all was good.  But .. looking at the modulated wave on the scope, the wave shape doesn’t look as sinusoidal as it did with the -22VDC bias .. the rise and fall of the waveform looks more “straight” than curved.

So, then I reasoned that although I’ve “corrected” the cathode current to 50 mA, this may not be the appropriate bias for my higher-than-called-for plate and screen voltages.

Now, I am wondering how this balancing act should be correctly performed.

Should I simply add an adjustable bias supply that allows me to adjust the bias somewhere between “too hot” and “poor linearity” while monitoring output on the scope?  (Maybe trapezoid test?)   Or is there a specific method or tables indicating how to setup the operating bias for a push pull class AB1 amplifier?  Maybe the idle current needs to be higher than 50 mA when running the modulator at higher than specified plate/screen voltages?

(Note, to check the performance of my modulator, I have been connecting it to my 6146B transmitter; modulator transformer in series with the 6146B plate supply, and to 6146B screen via dropping resistor as indicated in the construction article.  I have been using 1 KHz audio test signal.  I monitor transmitter output into a 50 Ohm “Cantenna” load via a current monitor, a small torroidal transformer.)

Thanks all!
--KC9KEP

[W4NEQ]

There are many on the forum with more expert knowledge on running 807s, but here is the basic idea ...

AB1 indicates biasing to produce idle plate current somewhere between class B (no idle or very little) to class A - so much idle that the negative part of the waveform does not cut off conduction.

It's a trade off between cleanliness, output, and tube life.

With a pair of 807s, assume maximum plate current of 200ma.  For a modulator, somewhere around 10 to 40? ma idle sounds about right.  If true AB1, then you will want to monitor grid current to reduce drive when it starts to flow.

In general, adjust parameters for best linearity (clean sine wave reproduction).   Need to also check screen current to make sure that's not excessive. You should not show any color on the plates at idle, and maybe just a bit at maximum output.

Chris

[ke7trp]

ITs normal for them to show color.  They shoud be cherry.  The RCA manual shows 30 to 50 ma of idle current depending on the plate voltage. I doubt any slight variance will have any effect at all. It does not on my rig here.

here is the pdf. Look up the plate volts in the manual for two tubes, then set bias to matching MA.   Should be good to go.

C

[WBear2GCR]

I'd not use batteries for negative bias on power tubes.
If the batteries get weak, the plate current will climb excessively.
I'd prefer a small negative supply and a pot or two for adjustment.

What did you say the B+ voltage is?

With 50 ohms on the cathodes there is some "cathode bias", in addition to the
fixed bias. Some folks like this arrangement.

There is some benefit to running AB2 over AB1 as you can squeeze out more power and swing from
the same tubes.

                                   

[WD5JKO]

Alfred,

    The 6L6 variants (807/1625) can do a very good job as a class ab1 modulator.

Keep in mind that maximum output in class ab1 is when the peak grid swing matches the bias voltage. So with a bias of -22v, the grid swing must peak 22v, or 44v peak to peak with respect to ground. As the bias goes up, so does the required grid drive to achieve the same output. So my bet is that some (but not all) of the distortion you are seeing is also present at the grids from the audio driver stage.

The Hi-Fi audio guys like to bias tubes so that the standing plate dissipation (no audio) is just under the rated plate dissipation of the tubes. This results in a lower bias, lower drive reguirement, and lowest intermodulation distortion.

A bias battery has a finite life, and if used infrequently, you will eventually find a situation of the red plated tube when the bias is low or absent. Going to fixed bias with one or two pots has already been suggested, and I'd say an excellent idea. However if you rather not do that, consider cathode bias with a twist...

The cathode bias is to set the idle current to 20-25 watts Pd per tube (that could be 40ma per tube at 500v B+, or 33ma per tube at 600V B+). So lets say that works out to be -33V bias. Now take a 5 watt zener diode, maybe 34-36V (could be three 12V zeners in series) and bridge that across that cathode resistor. Then bypass the cathode to ground with a cap, maybe 220 mfd 50 v. So when you drive the amplifier, the cathode current will rise, as does the cathode voltage (but it will be slow due to the capacitor). Soon the zener will clamp, and now the amp will act like it has fixed bias.

So without a battery, and without a fixed bias supply, you can have fixed bias like performance. One concern is if the plate supply goes on and off with PTT is the cathode bypass capacitor must charge up each time, and therefore there will be a plate current surge each time you key the amplifier. There are ways around this issue.

Jim
WD5JKO

[KA2DZT]

Higher than normal plate and screen voltage might be your problem.  Don't think 807's should be running red.

What plate and screen voltage are you using??  I ran 807's with 700 volts plate and 300 volts on the screens.

You need to build a small bias supply, about -50 volts that you can adjust the idle current, which should be about 50-60 ma.

If you are using fix bias you don't need the cathode resistor.

Fred

[w7fox]

I'm with Fred.  I also run 807's at 700 on the plate and 300 on the screen, with no plate color.  If you are running AB1, the bias supply is very simple because you never draw current from it. Just a pot across a neg. 50 volt supply will do.

73,
Chris

[WBear2GCR]

The schematic shows drive via an interstage transformer. So, this circuit is indeed capable of AB2 operation, assuming the driver tube(s) have enough swing and current available. Afaik, there is never current drawn from the bias supply in AB2 operation, rather the current is drawn via the driver, in this case because of the DC coupling of the interstage transformer.

I'd say definitely run in AB2 if possible.

Switch to a small bias supply: little transformer with a secondary of <100vac, diodes, caps.

Regardless, a cathode resistor should always be bypassed for this situation, imo.