Vacuum Tube Transmitter Designs


A few basic guidelines should be followed even in simple minimal-component vacuum tube transmitters.

Common Point and Bypassing

The vacuum tube "pulls" on the high voltage like a variable resistor. The path for that "pull" is from cathode to anode, the grids just control the instantaneous resistance of that path. You can read more about how a tube (or transistor) drives a tank circuit on this page.

Because of the tube acting like a variable resistance in series with the tank input, the RF current's tube path is as follows:

Tank and tube currecnt paths

The PA tube cathode, unless intentionally floated at radio frequencies for RF drive or for RF feedback, should be solidly bypassed to a low impedance ground plane or a common point shared by C1 (the tank resonating or tuning capacitor). The red color path above should have minimum uncontrolled impedance, which means a layout with shortest possible and lowest impedance leads, including though ground.  

A typical power oscillator (PO) minimalist single tube transmitter of reasonably good compromise between complexity, cleanliness, repeatability, and reliability would have every component serve some critical function. The circuit in Colpitts (tapped capacitance) or Clapp (modified Colpitts) oscillator form, a reasonable circuit for PO use, would look like this:  

Simple 1 tube transmitter

Critical tube path is:

RF path Simple transmitter

We see the RF output current conversion from the DC supply is primarily through C6, back to C1 and C8 through the tube.

Component Function Power  Oscillator Colpitts  (or Clapp)

PO circuit simple transmitter















R3 sets the oscillating bias voltage developed by grid current. This tube has zero grid bias unless oscillating. If the tube is not oscillating, depending on screen and plate voltage, and if the cathode circuit is closed, the tube can exceed rated anode dissipation. Some fixed bias, either cathode or grid, would solve this issue. This circuit assumes damage from lack of oscillation is never a worry. If the tube is protective biased, it has to be biased at a safe dissipation but must not be near cutoff. If fixed bias is near cutoff the stage might not oscillate. R3 has a large effect on output power and crystal current. R3 is typically sized in the many ten's of thousands of ohms, somewhere between 47k and 470k is typical.  Less resistance is generally more power at the expense of crystal instabilitly and more sensitivity to loading and tuning.

C6 sets cathode RF voltage at point A for a given frequency range and tube cathode current. This capacitor should be as large as possible while still allowing oscillation on the highest band with the most sluggish crystal.
C5 limits current into the grid. It should be the lowest value possible while still allowing oscillation on the lowest frequency with the most sluggish crystal. 
C5 and 6 have a large effect on crystal current, but R3 ultimately limits current since it controls tube operating bias and peak currents.
The voltage of all grid and cathode side capacitors should be at least the dc anode voltage, otherwise a tube fault or open key can damage these components. C5 is the least critical for voltage.

C4 is the screen bypass. It should have a low impedance to the ground end of both C6 and C1. A .001 to .01 uF ceramic disk of more than twice the dc supply voltage is good enough.

C3 is non-critical and has minimal current. C3 along with R1 form a low pass filter, reducing RF flowing back to the power supply.

C8 is a coupling (dc blocking) capacitor. It keeps dc voltage out of the tank circuit. In most cases almost any value over a few dozen pF will work the same! The voltage rating and quality is important. This capacitor along with RFC1 protects your life, so make sure the voltage rating and quality is good. A 1kV ceramic disc of fairly large physical size, like a typical .01uF 1kV disk, is good.    

 Pierce Power Oscillator (PPO)  

Pierce Power Oscillator


In the circuit above, the high current RF tube path is in a loop from C1 andC6,  through the tube anode-cathode resistance, through C5, to the bottom of C1. The tank circulating currents are through C1, L1, C2, and back through ground to C1. The tank path, C1-L1-C2 and back to C1 ground is by far the highest RF current path.

R1 sets grid bias by Ig * R1 = Eg1. Bias determines tube power for a given screen and plate voltage, so it naturally has a large effect on crystal current.

C7 along with R2 determines crystal current. There is quite a bit of interaction between components, and some small shunt capacitance from screen to ground is sometimes needed to allow R2 to be in a safe range for the tube.

C5 must be at the tube pin to groundplane or common point, the path from cathode to C1 ground has to be fairly low impedance.

C4 is just blocking, and can actually be a fairly low capacitance value.

It does not hurt to have a resistance, even very high value (millions of ohms) across the crystal to keep any dc charge off the crystal.





Power Supplies

Power supplies are first, because everything depends on them. A transmitter's cleanliness and operating consistency can only be as good as the power supply permits.

Old power supplies were often not that good. Most supplies used filter chokes, which can be a very good thing with high vacuum rectifiers (reducing peak current). Unfortunately, input chokes were often the wrong size for the bleeder current, making regulation and peak currents worse than not even using a choke! There is nothing worse than carefully following a recipe and seeking out specific parts, just to have disastrous voltage regulation when done.

The primary advantage of a choke input filter is reduction of peak currents. By virtue of energy storage and release, a properly-sized choke provides a smoothly changing voltage boost throughout the low voltage part of the supply sine wave. Conversely, the same choke provides a bucking voltage near peaks. The mechanism behind this voltage bucking or boosting is an inductor's tendency to readjust voltages across the inductor as it tries to maintain constant current.

A properly sized input choke causes the rectifier to draw current throughout the entire conduction period of the input cycle. This current smoothing, almost appearing as a resistive load, extends in a closed loop from power mains all the way through the power transformer, rectifier system, and the first filter capacitor.

Capacitor input supplies can be very good for regulation, if high peak currents are properly dealt with. The only significant operational problems with higher power capacitor input supplies are caused by high peak currents, even while average currents are about the same as a choke input supply. A capacitor input supply draws all of the supply energy in a short period at the rising edge and just over the crest of each power line cycle. These high peak currents cause abnormal heating as they increase voltage drop in components. Capacitor input supplies can be particularly tough on vacuum tubes, and on old transformers using smaller copper sizes. To handle the apparent power factor (APF) of heavy charging currents at the leading edge of each sine wave voltage peak, capacitor input supplies should use a little larger copper winding size. The capacitor input supply, in essence, runs only on line voltage peaks. All load power is "taken off the peaks". This actually can flatten the peak of the line sine wave, causing line voltage to measure almost normal on meters even while the mains are sagging on peaks. 

An example of a good power supply is in the AL1200 (AL12 series) power supplies. I specifically designed the transformer for very tight coupling with minimal flux leakage, and the largest possible copper size that would fit in the core window. Because of the low ESR transformer, peak current is often limited just by the ESR of the power mains. With a really stiff mains, the AL12 series will hold voltage within 10% from no load to full load. The price for this voltage stability is very high peak current. I've measured about 40-50 amps peak current at the repeating line sine wave voltage crest leading edges, while average current over the entire cycle is only about 12 amps. This high peak-to-average current ratio clearly calls for increased copper size throughout the primary system. Capacitor input designs must minimize series resistance, or regulation falls apart.

In comparison, a well-designed choke input supply (with properly sized choke) has an APF (apparent power factor) of nearly unity. Current is drawn in smooth step with voltage over the entire sine wave, even the descending slope of the waveform! The smooth current window allows energy to be extracted from the mains over the entire cycle. This allows much smaller copper sizes, since the choke input supply requires far less attention to transformer and power line ESR (equivalent series resistance). With a properly sized input choke and good components, the power supply's line loading appears as a pure resistance.

The bottom line on this is the user has two choices:

1.) Live with the high currents, while maintaining good life and good regulation, by using low ESR (equivalent series resistance) transformers, solid state rectifiers, and  higher value filter capacitors

2.) Use a properly designed choke input with higher ESR components. "Properl