Playing With A 2E26

Discussion in 'Homebrew and Kit Projects' started by WA2FXM, Feb 16, 2017.

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  1. WA2FXM

    WA2FXM Ham Member QRZ Page

    This is my first attempt at hollow state designing. I'm learning a lot but I've hit a wall and need some help.

    I'm attempting to rebuild an old Stancor mobile AM transmitter into a 60 meter CW rig. It was designed for 10 meters with a 6V6 oscillator and 2E26 amplifier. I've removed the audio section and left most of the original component values intact. By replacing just the tank circuit coils I'm able to get the 6V6 going on 5 mHz and the 2E26 resonant with about 15 mA of plate current.

    The 2E26 is rated for 75mA at 400V, but using various values of impedance and capacitance in the final tank circuit and antenna coupling I haven't been able to get more than 40mA in the plate circuit. Using the original 35pF tank capacitor I wound a 30uH coil that resonanted fine, but there was no loading with just 2 turns on the antenna coupler coil. Most of the ancient tube books show just a a few turns on the coil and say coupling is done by trial and error. I kept adding turns and when I got to 9 I was able to load up to 40mA before the coupling capacitor maxed out.

    The 1936 ARRL Handbook has a formula for determining the best tank capacitance for maximun efficiency. Using this formula I rebuilt the tank circuit with 90pF and 10uH. Again, there's resonance with 15mA but no loading at all with 3 turns on the antenna coupling coil.

    Is it just a matter of adding turns to the coupling coil until it gets full load or should I be checking drive levels from the oscillator?

    Mark WA2FXM

    20170216_081306.jpg
     
    W7UUU likes this.
  2. N2EY

    N2EY Subscriber QRZ Page

    IMHO you should make it a pi-network output circuit. You may need to change out one or both capacitors but that's no big deal.
     
  3. KL7AJ

    KL7AJ Premium Subscriber QRZ Page

    The 2E26 is such a cute baby 6146. :)
     
    KV4PD, AF6LJ and K3XR like this.
  4. N2EY

    N2EY Subscriber QRZ Page

    In a way.

    The 2E26 predates the 6146 by several years.
     
  5. W7UUU

    W7UUU Platinum Subscriber Life Member Platinum Subscriber QRZ Page

    I was always told they are a direct substitute - aside from neutralization issues and power output of course.... is that true?

    Dave
    W7UUU
     
  6. AF6LJ

    AF6LJ Premium Subscriber QRZ Page

    It's a good tube and even has decent performance at VHF.
     
  7. N2EY

    N2EY Subscriber QRZ Page

    No, it's not true!

    The 2E26 can be used in 6146 rigs with modification - lower plate voltage, less plate current, etc. But it is not a direct replacement!

    The tube data sheets tell the story.

    73 de Jim, N2EY
     
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  8. K9STH

    K9STH Ham Member QRZ Page

    In the "goode olde dayes", like early 1970s, to get on 220 MHz for FM operation, we modified Motorola "G" transmitters. Those transmitters used a 2E26 as a doubler driver into a 6146 final amplifier. All that was required was to short out a turn on the plate coils of the driver and final amplifier. As such, the 2E26 became a tripler driver. However, the 6146 was not happy operating on the 220 MHz band. So, a 2E26 was also used in the final amplifier stage. The result was an output between 12-watts and 15-watts.

    The voltages, supplied by various Motorola power supplies, was well within the range of those for the 2E26 so no modification was needed.

    For receiving, we used Motorola Sensicon "A" receivers with the loaded cavity front ends. The tunable section of each cavity was stuffed full of aluminum foil which allowed the cavities to be tuned to the 220 MHz band. Even though modified, the receivers still met the specifications for high band operation.

    Such modifications might be considered crude. However, they worked well and allowed FM operation on the 220 MHz (now reduced to 222 MHz) band.

    Glen, K9STH
     
    AF6LJ likes this.
  9. WA2FXM

    WA2FXM Ham Member QRZ Page

    A pi-network is a possibility. I found a Pi Pi-L Calcualtor spreadsheet from G3SEK online. I'll have to dig through the junkbox and see what I can come up with. Thanks for the suggestion.

    Mark
    WA2FXM
     
  10. N2EY

    N2EY Subscriber QRZ Page

    You're welcome!

    There are all sorts of ways to calculate the correct pi-network values, from online calculators to graphical methods to charts to actual calculations. It's a good idea to do the calculations by two or more different methods to see how close the results are. Note that you need to know the tube operating conditions and the desired loaded Q of the network (12 is a good starting point).

    One of the simplest ways is described in QST article from the issue for December, 1971. I used the charts in that article and came up with the following values:

    2E26 operating at 400 plate volts and 75 plate mA, class C. This gives a tube load impedance of about 2600 ohms.

    Plate tuning capacitor: 150 pF (operating value, not maximum value)

    Inductor: 10 uH

    Loading capacitor: 800 pF (operating value, not maximum value)

    Since the plate voltage is only 400 volts, a capacitor with receiver-type spacing is more than adequate. Also, the required values can be had by a combination of fixed and variable capacitance.

    For example, the required 150 pF could be obtained using a 140 pF variable and a fixed capacitor of 70 to 100 pF in parallel. The 800 pF loading could be obtained using a dual-365 pF capacitor and a 300 to 400 pF fixed in parallel. Lots of other combinations possible too.

    The charts in the article don't include 60 meters, so I took the values given for 15 meters and multiplied them by four.

    73 es GL de Jim, N2EY
     

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