A recent acquaintance and newfound friend suggested I cover the tube versus transistor dilemma. Some Blog readers may even be saying why are you wasting my time with this subject.
With a degree of certainty if you were born after 1960 then likely you have not dabbled a lot with tubes. So, it is somewhat of a question of familiarity with the use of tubes that will position most blog readers. There are definite pros, cons and negatives to the use of tubes.
Your antenna really doesn't know if the fire in the wire is coming from a tube or transistor. When I use my several pieces of Collins gear the station at the other end does not know that it is a tube radio until I tell them. Now if it was a Swan tube radio or perhaps a homebrew SS using an LC Analog VFO likely the other station would know because of drift in the signal.
The real negative with the use of tubes is that high voltage is present and can kill you. In line with the power supply concern is the operation in the field. The POTA / SOTA guys would be unhappy as the portability issue is key to their operations. Tubes and portability are like serving rice and mangoes on the same plate.
There are so many cons with the use of tubes so let us start with the Pro side first. Tubes are around and often at great prices on eBay and at ham fests. There are many circuits that are well documented within old issues of CQ and QST.
Lest I forget the N4TRB website that has the RCA Ham Tips and the GE Ham News where the bill of fare was tube radios. Plus, Tubes have bragging rights when you say, "the rig on this end is a homebrew tube transceiver".
The "sound" both on receive and transmit has that tube warm glow to it. Why do you think serious audiophiles have tube amplifiers?
Last but not least is the personal technical achievement of successfully scratch building a tube radio.
The Cons for using tubes are many and perhaps I have missed some, but these stand out as technical challenges.
- The Power Supply has to have Safety features and supply multiple voltages such as Plate, Screen, Bias, Filament and DC control voltages (TR Relay). Power Transformers to supply such voltages are expensive.
- A Tube Tester is a must have tool.
- Specialty Fabrication Tools like Greenlee Chassis Punches, Metal Saws, Electric Drills and Aviation Tin Snips are standard in the tool box.
- A developed skill at metal working and circuit layout way beyond glue down Manhattan pads is requisite.
- The Resistors and Capacitors must have higher voltage and wattage ratings. These parts tend to be physically large and not a good place for SMD. Read also tube electrolytic caps like 100uF at 450 VDC are not 2 for $1.
- Many special inductors are needed like RF Chokes capable of passing 300 Ma. Shielded metal encased IF Transformers are hard to find as are ceramic coil forms. Typical Pi Network Coils (Air Dux) are expensive and not as easy to calculate inductance's such as you would have with T-68-6 powdered iron core.
- Metering is often required so the front panel suddenly got a lot more crowded.
- If some solid-state devices are used with the tubes -- there may be issues like voltage spikes that find their way into the circuitry. I had this problem initially with a Si5351 LO in a Tube radio. Scroll down to the You Tube Video.
- The equivalent tube device to the ADE-1 or SBL-1 is the RCA 7360. You will have to sell your 1st born to purchase one of these. There are subs such as the 6AR8 and 6JH8 but nothing like a 7360.
- Tools like LT Spice have limited applicability with vacuum tubes.
- Most solid-state circuits are broad band whereas tubes circuits tend to be tuned. That said I have a tube CW transmitter that uses a broad band tuned network following a 6AG7 oscillator.
- Neutralization of the Final Amp stage is required and involves a bit of black magic. Often the plate and screen voltages must be disconnected from the final tube to do this step. You know you are neutralized when the plate dip and maximum power output occur at the same spot on the Plate meter. (Remember our earlier note about metering.)
- Regulated plate voltages are needed on tubes used in the VFO and BFO. Your tube radio might have an OD3/VR150 or OA2 or OB2 to supply that regulated voltage. Now for a shocker and that is the Collins KWM-1 and KWM-2 used no voltage regulator tube on the PTO's. There was a service bulletin mod for adding a 150V Zener regulator to the KWM-2's used in Vietnam. I know from personal experience that at night in Chu Lai it could get down to 60F and during the day 120F -- that is a 60-degree swing!
- Tubes are not immune from being smoked so tune up (the dip and load process) must be done quickly.
- Heat is significant. Vacuum Tube radios in winter are a great shack warmer but not so good in summer. Often fans are standard on some radios like the Yaesu FT-101 and that only has three tubes.
For me the above list is not a huge blinking yellow caution sign but just another day in the office. You ask why would you even consider using tubes? My answer because I can.
73's
Pete N6QW