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.)
This is a detailed follow-up to my previous posts.
First, the usual method of doing this sort of thing is to look up the recommended operating values for the tubes in use and design around them. However, there isn't much direct info on 807s in AB1.
So here are some general rules:
1) The no-signal plate and screen power inputs should not exceed the maximum values for the tube type. For the a pair of 807s, you have a total of 60 watts plate dissipation and 7 watts screen dissipation. In practice it's a very good idea not to push the bottles that hard, particularly in an AM modulator, where the duty cycle is much higher than in a CW or SSB transmitter.
So let's say 50 watts plate dissipation and 5 watts screen dissipation - absolute maximum
2) The plate current will rise dramatically with screen voltage.
3) The screen voltage should be reasonably well regulated
4) Once you deviate from the original design, you probably have some design and modification work to do.
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.
Which is way beyond tube ratings! If you have, say, 700 plate volts, 125 mA plate current is 109 watts dissipation!
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.
That's because there are probably other things that need changing. - screen voltage, plate impedance, etc.
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?
There is a specific method. Fortunately it's pretty easy.
Maybe the idle current needs to be higher than 50 mA when running the modulator at higher than specified plate/screen voltages?
NO!! Just the opposite!
Remember Rule 1 - you must not exceed the rated plate and screen dissipation! If anything, you want LESS plate current at higher plate voltage!
(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 )
The problem there is that the modulated stage may not be a good matched load, and may not be linear. At this stage of the game, you'd be better off making up a dummy load of equivalent resistance using power resistors. For example, if the 6146B is running 600 volts at 120 mA, (72 watts in) the load resistance should be 600/.12 = 5000 ohms. The modulator needs to produce half the power of the stage input - 36 watts - so you need about a 40 watt resistor. Better yet, a combination of smaller resistors.
The scope patterns are not the best indicator of linearity until you have done the basic stuff.
Here's how I'd proceed:
1) Find out the actual B+ voltages your supply is putting out. The design values were 600 volts and 230 volts, but obviously you're getting more. You need to know how much more.
2) If the low B+ is 300 volts or less, and the high B+ is 750 volts or less, you're in luck - the original circuit is OK. All you need to do is adjust the bias and the plate load impedance.
3) In the original design, they ran the tubes at 600 volts and 50 mA. That's 30 watts static plate-and-screen dissipation - a good number to remember. Note that the 50 ohm resistor measures combined plate and screen current, not just plate current.
The best info I could find for 807s in AB1 is the following (values are for TWO tubes):
Condition 1: 600 plate volts, 300 screen volts, 34 volts grid bias, 36 mA resting plate current, 0.6 mA resting screen current, 140 mA maximum plate current, 16 mA maximum screen current, 56 watts maximum output
Condition 2: 750 plate volts, 300 screen volts, 35 volts grid bias, 30 mA resting plate current, 0.6 mA resting screen current, 140 mA maximum plate current, 16 mA maximum screen current, 72 watts maximum output
Other operating conditions may be interpolated. Note how these values involve higher voltages and lower currents.
The original design called for a plate-to-plate load impedance of about 12,500 ohms. This is for 600 volts on the plates, 250 on the screens, 50 mA idling cathode current and 100 mA full signal. With higher voltage and lower current, the load impedance will be higher. Selection of taps on the mod transformer is the way to obtain a match between the modulator and the modulated stage. The transformer data sheet tells how.
Hope this helps
73 de Jim, N2EY