Tube rig "tuning" question

Discussion in '"Boat Anchor" & Classic Equipment' started by AA4OO, Feb 11, 2018.

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  1. AA4OO

    AA4OO XML Subscriber QRZ Page

    I'm new to tube rigs.

    Sorry if this is a "basic" question but

    Can someone explain to me the purpose of the preselector, final tune and load levers to peak the RF output?

    My "transistorized" rigs simply transmit into a 50 Ohm load. I use an external impedance matching device if I my antenna is not 50 Ohm.

    Why were tube rigs designed with all these variable capacitors? I understand (I think) that it is doing a bit of impedance matching like a "tuner" normally would, but is there something special about tube finals that differ from a transistorized final to require all this matching?

    Just trying to learn...

    Richard AA4OO
     
  2. KP4SX

    KP4SX Subscriber QRZ Page

    They covered a range outside of simply 50 ohms fixed. Most could easily tune into loads of 2 or 3:1 swr with no problem. 50-600 ohms was a typical spec.
    Trying to load outside of this range with an external tuner gets a bit more complicated and that's why they invented dummy loads :) Get the rig tuned first and then do the tuner.
    That's the short version. Im sure others will explain why in more detail.
     
  3. AA4OO

    AA4OO XML Subscriber QRZ Page

    Ok. So I think you're saying that, the tube rig finals don't have a fixed 50 ohms output. Depending on output frequency, there will be different output impedance. So the variable caps are there to match the radio's own variable load to the expected 50 ohm antenna.

    Is that understanding correct?

    I just found this article today and it was helpful to me.

    https://www.w8ji.com/tuned_input_circuit.htm
     
    Last edited: Feb 11, 2018
  4. KP4SX

    KP4SX Subscriber QRZ Page

    Basically correct. The ratios between amps/volts and subsequent impedance are totally different in a tube circuit compared to a solid state one. The actual output impedance of a tube amp may be several thousand ohms and the tuning controls act much like an antenna tuner to facilitate a match.
    The article referenced is about tuned inputs although many of the same principles apply.
     
  5. G3YRO

    G3YRO Ham Member QRZ Page

    It's not just about impedance matching . . .

    It's the fact that rigs of that era actually TUNED the input (of the receiver) and output (of the transmitter to a particular frequency, rather than make it broadband, which most later rigs do.

    Although less convenient for the operator who wants to keep changing frequency, it actually gives better performance, both on transmit and receive.

    The Preselector control (which Kenwood rigs incorrectly labelled Drive) not only resonates the Transmitter Driver stage and the Receiver front end, it also resonates the Tx and Rx Mixer stages. This wasn't just done on all-valve rigs . . . it was also done on rigs that were 90% transistorised.

    And the better rigs didn't use variable capacitors for these tuned stages - they used ganged variable inductors.

    An added advantage - as you realise - is that most Valve PAs (inc Linear Amplifiers) can match antenna impedances from about 30 to 600 ohms, without the need for an ATU.

    Roger G3YRO
     
    W4NNF likes this.
  6. N2EY

    N2EY Ham Member QRZ Page

    I'll assume you're talking about the HW-101 you recently restored.

    The "preselector" on that rig tunes the tuned circuits used for the receiver front end (RF amplifier and first mixer) when receiving, and the transmitter low-level stages (transmitter second mixer and driver) when transmitting.

    The "tune" and "load" controls adjust the variable capacitors of the final amplifier pi-network, to effect a match between the high-impedance output of the 6146 tubes and the 50 ohm impedance (more or less) of the antenna. They function only on transmit.

    Their design is many decades newer than that of the HW-101, too. And the HW-101 and similar rigs don't have that many adjustments (see below).

    They are VERY different.

    1) Tube circuits generally operate at much higher impedances. A pair of 6146s operating at 750 volts/240 mA (plate voltage and current) will have an output impedance of about 1600 ohms, which needs to be matched to the ~50 ohm antenna. That's much, much higher than the impedance solid-state final transistors operate at, and the matching techniques are very different. The circuits used in your transistor rigs just won't work.

    2) The use of the adjustable pi-network in the HW-101 and similar rigs means the antenna does not have to be exactly 50 ohms resistive in order to work. Such rigs can typically handle SWR of about 2 to 1, and sometimes more, without the need for any sort of external "tuner" or matching device.

    3) It is possible to design tube rigs that do not have the above tuning controls. The Central Electronics 100V, 200V transmitters and 600L amplifier are examples from the 1950s. However, such designs require additional parts, which add to the size, complexity and cost. Also the flexibility of being able to match loads other than 50 ohms is lost. So, such designs did not reach high popularity.

    Simplicity and cost considerations drove many of the design decisions when the HW-101 and similar were designed. That HW-101 cost $249.95 when first introduced in 1970. That's $1604.09 in 2017 dollars - and for that price you did not get the CW filter, microphone, or power supply - and you had to build the thing!

    73 de Jim, N2EY
     
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  7. AA4OO

    AA4OO XML Subscriber QRZ Page

    Thanks to all for the explanations. I'm learning a lot.
     
    N2EY likes this.
  8. W2WDX

    W2WDX Ham Member QRZ Page

    You can think of the tune and load as being similar to a built-in or external antenna tuner. While not precisely the same or for the same reasons, the effect is the same. "Preselector" is somewhat different. In some transceivers, the receiver and transmitter sections share some common circuitry that need to be tuned to the specific operating frequency. In more modern radios, the band selector switch handles this function, where circuits are set to "pre-tuned" sections that do the same thing, in some cases digitally. Much like how an auto-tuner selects certain tuned circuits based on the band selector on the transceiver. A "preselector" control puts this function the hands of the operator literally and allows one to fine tune to the specific frequency, not just a range of frequencies.
     
  9. AI3V

    AI3V Ham Member QRZ Page

    Transistors , like the usual pair of bipolar devices outputting 100 watts on hf have a impedance much lower than 50 ohms, the typical pair of 6146 tubes have a much higher impedance than 50.

    Antenna systems are rarely 50 ohms exactly.

    In either case, you need a impedance matching unit.

    It's cheap and easy to match, with a fixed tuned circuit the transistor to 50 ohms, its much more expensive to add adjustable impedance matching.

    It's cheap and easy to match the high impedance of the tube to 50 ohms with a fixed tuned circuit, it's much more expensive to add adjustable impedance matching.

    You have allways needed some sort of impedance matching unit between your tx and antenna, it made no sense to the old timers to make a fixed tuned tube amp and seperate adjustable impedance matching unit.

    Then came transistor radios.

    They were a lot more expensive than tube rigs at the start.

    So, manufacturers left out the adjustable impedance matching unit to keep the cost if a 100watt solid state rig close to the price of a 100 Watt tube rig.

    And the marketing team got everybody to believe that leaving out the necessary impedance matching unit (which now is sold as a costly option) was a good idea.

    After all, it's a bit odd that everybody's "no tune hf transmitter" needs a "antenna tuner"

    :)

    Rege
     
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  10. KD2ACO

    KD2ACO Platinum Subscriber Life Member Platinum Subscriber QRZ Page

    So true! :p
    The funny thing is that your radio could be a tiny little thang (like a Yaesu 857) yet it may need a much bigger box to match the antenna.
     
    Last edited: Feb 19, 2018

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