Saturday, August 27, 2011

One Nuvistor on 40m CW

"Tube manufacturers have unveiled, in recent weeks, drastically new concepts and techniques aimed to keep them in the race with the transistor industry. Smaller than a thimble, more rugged and efficient than present tube designs, and particularly suited for mechanised production, the 'Nuvistor' represents a radical departure in the electronic-tube concept."
April 1959, Electronics Design

Some time ago I salvaged a pair of RCA type 8056 Nuvistors from the vertical amplifier board of a defunct Tektronix TV waveform monitor. These are medium-mu triodes, specially designed for low anode voltages.

Last week I decided to place one of my Nuvistors on the 40m CW band. The elements of this single triode are relay-switched between the receiver and transmitter cicuits. My setup is a vacuum tube implementation of W2UW's, now classic, "FET-1" design. G3XBM made a spendid job of Glenn's original circuit; the results of which may be seen here.

The circuitry that I settled on dates to the 1920's. For example, the regenerative receiver is a standard, tickler-feedback affair. I can just detect my 1uVrms weak-signal generator through a 20dB attenuator with this receiver. I suspect that a 0.5uVrms signal would provide comfortable copy under quiet band conditions. Out of curiosity, I learned the receiver will continue to function as a self-oscillating detector with as little as 5Vdc on the anode (for this test the feedback coupling was raised to four turns). 

I found the crystal-controlled Colpitts configuration produced a slightly higher transmitter efficiency as compared with a Miller-type layout. Although the Miller circuit has the advantage of a grounded-cathode (switching would only be required at the grid and anode) I decided to use the Colpitts arrangement. With 24Vdc on the anode the RF output power is 24mW. 

My receiver and transmitter circuits are shown below. I didn't bother to include the receive/transmit switching details in my drawing. I use a pair of 12V-coil DPDT sub-miniature relays (the coils are wired in series for 24V operation). One pole each is used for switching the grid, cathode and anode. The fourth pole switches the antenna between the receiver and the transmitter. A hand-thrown SPST switch activates the relay coils to enable operation of the transmiter. Of course an 8PDT manual switch would achieve the same end with considerably less current consumption on the 24Vdc supply line.

Also not shown in my drawing is the Nuvistor heater supply connections (the 8056 requires 6.3V at 135mA). I notice a mistake in my receiver schematic; C4 is shown as a fixed-value capacitor when it should be drawn as a variable capacitor. In fact, this is the "regeneration" control.

Component List

C1: 5pF
C2, C3: 100pF
C4: Improperly shown as fixed; should be ~15 to 100pF variable
C5: 18nF
C6: 470uF @ 50V
C7: 20pf
C8: 150pF
C9, C11: 0.1uF
C10: 75pF
C12, C14: 390pF
C13: 680pF
CV1: 5 to 25pF (main tuning)
R1: 1MegOhm
R2: 6.2k
T1: 3.5 to 5.5uH, 16 turn primary, 1-turn coupling
T2: Same as above, except 3-turn secondary
L1,L2: 1mH RF choke
L3, L4: 1.4uH
X1: 40m quartz crystal
HP: High-impedance headphones
V1: RCA Nuvistor, type 8056

I had my first QSO with this radio shortly after completing the final connection, when VE3DTJ/3 answered my CQ from his lake house north of Toronto; a distance of 303m/488km. Barry gave me a 579 and we kept a ragchew going for nearly twenty minutes.

When I came up to the house for lunch I sent a message to the online QRP-L group, telling of my operation. VE3DJX answered my call that same afternoon. Jim reported that I was 539 in Smiths Falls, Ontario (170m/274km). Despite my weak signal, Jim appeared to have a fairly good copy for the duration of our ten minute QSO.

I had an email message that evening from WA9ETW. Mark said that he had roughly a 20% copy of my signal in Monticello, Wisconsin (846m/1362km); not good enough to reply on the air, but he did manage to copy my complete callsign. 

Two receivers in the Reverse Beacon Network made a number of captures of my signal in the two days that I operated last week. K3MM is located in Damascus, Maryland (402m/646km). WZ7I is located in Pipersville, Pennsylvania (280m/451km).

I'll close with an excerpt from a letter that W2UW sent to Roger, G3XBM.

By the way, you might be interested in knowing that I have made 451 QSOs with my little rig. I have not met my goal of working all of the states east of the Mississippi river. HI I still have 4 to go (Fla., Ala., Geo., and Miss.)....

I am 81 yrs. old and still think radio is "magic". Hi
Glenn, W2UW

That is simply wonderful!


  1. may I ask you a photo of this construction?

  2. Hello Piter,

    Unfortunately, I neglected to snap a picture of this radio. It had been constructed on a plastic protoboard and taken to pieces when I moved on to the next project. I imagine the chalked drawings that one occasionally finds on sidewalks tend to last longer than most of my radio projects. :-)