LAMBDA-10 TRX (under construction)

 This is a 10-11 meter TRX with only tubes. Semiconductors will be used only on the power supply, and the frequency display.

I will place a good block diagram of this rig as soon as I can find time to make one....

---------- There will be included a PIC PLL with LCD display on this unit   ---------------

Schematics available (some of them are preliminary, some are the final version....)

Gerber files available.....

As always, any help is appreciated... if you find bugs or you have suggestions, I will be glad to hear from you....





25-mar-08 - I made the PCB for the power supply, as you can see some pictures below taken from the 3D viewer of the KICAD board file.

Fig -1. In the picture above can be seen the huge caps used for filtering, the error amplifier, the 12V regulator IC heatsink (the IC can't be seen from here), and the driver of the output regulator.

Fig 2 - In the picture above you can see the big mosfet FS14-SM12 used for the series regulation of the 150V power supply.


Fig 3 - A view from below.

For now, no time to fill in more info....

I will place pictures of the actual board very soon.







First of all, any modulator to be used on an radio amateur transceiver does not need to have a good frequency response, indeed, if you could achieve the range of 300~3kHz with good gain, it is preferred, because you avoid "spreader" sidebands and splatter generation. Of course the THD must be kept as low as possible to avoid TVI. My concern here was to use the minimum number of tubes and run enough audio power. Also this amplifier is used to drive the speaker of the receiver.

Important: I never took a regular tube course on the college, and all I know I learned with friends, and hours & hours of reading my BOOKS and the Internet, so please apologize me if you find errors on the formulae application, and I will be glad to hear from you any suggestion/correction. Once again, my only intention here is to share knowledge.

Here follows the math I used to define the values for the several parts of the circuit. Please refer to the tube specs to ease understanding.

---- Triode section of the 6JZ8.

Most of the steps are done with the help of the curves given on the tube specs.

DC Analysis:

The first step is to define the plate load line of the tube, as seen below (the red line on Fig 4b), I used Ibmax = 11mA and Ebmax = 250V, giving a plate resistance RpDC (static) of:


For more deep math, see Refs. 1 & 3 below....

250/0.011=  22.727kOhms -> 22kOhms (Actual value of R10 & R11)

Worst case power dissipation of R10 & R11 (each one, in case of a shorten tube) = 22k * (11mA)*(11mA) = 2,662W. I used 3W resistors. Indeed, the real power dissipation is one half of this worse case value, because the actual Ib is 5.5mA, giving 1,33W of dissipation.

I inserted a 10k 5W resistor in series reducing the overall voltage and C11 capacitor is giving a better filter roll-off to decouple this stage.

The steady operating point of the triode will be (see yellow lines below on Fig 4b):

Eb = 125V and Ib = 5.5mA.

It can be calculated by:

Eb = Ebb - Ib/Rp

Eb = 250 - (0.0055*22k) ≈ 125V

The plate dissipation now is:

Eb*Ib = 125*0.0055 = 0.6875W = which is ok, because the maximum plate dissipation of the triode plate is 1W (see specs and Fig 4a). In Fig 4a can be seen the plate sissipation area and the area where the Plate resistor Rp (R10 & R11 for each tube) dissipates power.

Fig 4a - Dissipation of the triode section using the projected load line.

The internal resistance of the tube (Ri) will then be:

Ri = Eb/Ib = 125/0.0055 = 22.727kOhms

Fig 4b - Curve giving Plate-current-voltage characteristics of the triode section of the 6JZ8.

By tracing an interpolation on the grid curves, we can find -3.75V of operating point Vbias (see the green trace on Fig 4b).

Now, to operate the tube upon this point, it is necessary to place the proper biasing resistor, as mentioned on ref 2 (see bibliography below), because the use of the generated negative voltage on the cathode resistor, for low power applications, there are benefits, since it is self-compensating, temperature, and stable even with a derated tube.

Well, the cathode resistor Rk (R29 & R30) should then be:

Rk = -Vbias/Ib  ->  -(-3.75)/0.0055 = 681.81Ohms - I used 680 Ohms.

The power dissipation on this resistor will be: Rk*Ib*Ib = 680*5.5mA*5.5mA = 0,021W

Under this conditions the amplification factor can be achieved by the curves, as seen below:

   Fig 5 - Grid Voltage - Plate current characteristics of the 6JZ8 (triode section).

Suppose we have an 1.2Vpp signal input, after tracing a proper curve on our Ib=125V, just the reflection of the curve can be given, on Ib. Of course, now we are talking about Transconductance (gm), because we will have a variation of voltage (dEbpp) generating a variation of current (dIbpp). So, lets do some math to see the average voltage gain (amplification factor or ):

gm = dIbpp/dVgpp  -> 0.003/1.2 = 2,5mA/V or 2500mhOs or 2,5mS (more appropriate). The specification says 2350S, or 2.35mS, which is quite close to what I found.

So the amplification factor will be defined by:

= -(dEb/dEg) for Ib is constant, the minus indicates the 180 phase shifting due to the commom cathode connection.  (see reference 1)

dEbpp = dIbpp*(Rp//Ri) = 0.003*(22k//22.727k) = 0.003*11,178 = 33,54Vpp

= (dEbpp/dEgpp) = (33,54/1.2) = 27 (with 180 phase shift)

or = gm*(Ri//RL)= 0,0025*(22k//22.727k)= 27,94 - real close to the values given on the curve.

Actually, on ref 3 there is another way to find the , which do not consider the fact that Eb is constant, actually it is right from the real world point of view, since the plate resistor (R10 & R11) will give a voltage drop under dynamic operation (When Ib drops due to the dynamic signal input on the control grid, Ib rises), so the point B indicated in purple is achieved, is the instantaneous lower plate voltage eb(t), which is approximately 100V on this example.

Now comes:

= (Eb - B)pp dVgpp = (125-100) / 1.2 = 20.83 - which is the real theoretical given on the tube specification.

In fact, on the real world, this value will decrease due to the load imposed by the input of the next stage, which will be in parallel with the Ri//Rp and the reactance given by the coupling capacitor (C12 & C13).

Also, the real operating point of the tube will be given on the purple line on Fig 4b, the dashed purple line indicates what is the current for a 0V signal in the control grid. Since the cathode resistor limits the excursion to 0V, so the grid will never be positive under normal operation. The maximum output ep+(t) will then be 160V, and the minimum excursion will then be ep-(t) = 90V, the maximum compliance (theoretical) will be:

ep+(t) - ep-(t) = 160Vp+ - 90Vp- = 70Vpp.

This purple line is called dynamic load line.

The control grid resistors calculation (See Ref 2 b):


Rg1 = control grid resistance [Ohms];

ΔIb = change in plate current permitted by maximum ratings of plate dissipation ;

ΔIc1 = change in control grid which is likely to occur or is permitted by the tube specs;

gm = transconductance [Siemens];

Rk = cathode resistor [Ohms];

RL = series plate resistor [Ohms];

= amplification factor



ΔIb = 11mA;

ΔIc1 = 1.5A (data taken on Ref 2c)

gm = 25mS;

Rk = 680 Ohms

RL = 22k

= 20


Rg1 = control grid resistance [Ohms];


 were found  grid









Further schematics descriptions and pictures....coming soon.....(I hope)



Here you can see the high grade tin box used to assemble the Oscillator Unit.

The oscillator is Hartley Type, very commom, not a big deal. Using the pentode section of a PCF80. The good points to remark are the high rate gearbox used for driving the tuning axis of the variable coil (with the large drum wheel used it will give real smooth tuning sensation), and the good NP0 caps used. The output is buffered through a cathode follower to drive low impedance loads and match with the output cable with minimum loss. I will make a good graph to see how the time affects the stability, since the power supply is well-regulated, as well as the heather (you can see the LM7809 at the left side of the 3rd picture above).



1st IF STRIP (10.7MHz mixer).



2nd IF STRIP (455kHz & AGC amp)






Ref 1-  TERMAN, Frederick Emmons. Radio Engineering; 1st Edition, 1932 - McGraw-Hill, New York; Ch. V, p.159-168; 

Ref 2 - LANGFORD-SMITH, F. Radiotron Designer's Handbook; 4th Edition,  1953 - Wireless Press (For Radio Corporation of America - RCA), Sydney, Australia;

        a)  Ch.12, Sect. 2, p.482;


Ref 3 - TERMAN, Frederick Emmons. Radio Engineers' Handbook; 1st Edition, 1943 - McGraw-Hill, New York; Sect. 4; p.296-298;