Another Homebrew transmitter is being saved.

Discussion in 'Amplitude Modulation' started by N6YW, Feb 24, 2017.

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  1. N6YW

    N6YW Ham Member Volunteer Moderator QRZ Page

    I am picking up the iron for the project!
    Dennis is kindly supplying the RCA modulation transformer and modulation choke, plus
    some other goodies that will introduce some finite qualities to the new transmitter design.
    I am considering of going with a single 833A in the final into a Pi L, therefore coax output.
    Now, as much as I like OWL, my problem here is that my run to the feedpoint is too short
    without encountering problems at QRO. So, the self resonant fan dipole will be installed
    and coax fed. Having said that, I think it will be just fine and because I am planning on
    building this as a mono band transmitter. I can maximize it's performance and make it purr.
    Just thinking out loud as I haven't broken out the notebook or the Jones books.
    I am making progress though and the feel good aspect is awesome... rescuing anything like
    this from the landfill is pure satisfaction.
    The first order of business after the trip to RF Palooza @W7TFO is to drop the rack off for
    bead blasting and high gloss wrinkle powder coating.
    One item that WILL BE INCLUDED!!!! Windows for the modulators and final!
    Gotta have them!
    AF6LJ and N2EY like this.
  2. N2EY

    N2EY Premium Subscriber QRZ Page

    I take it the band will be 75?

    Hmm.....833A needs 25 to 50 watts drive (roughly, depending on operating conditions). A single 807 can probably do it, but you may want more reserve. Forced air on the 833A? MV rectifiers with their own window?

    Are you thinking of a stand-alone transmitter (VFO to antenna?)

    73 de Jim, N2EY
    N6YW likes this.
  3. N6YW

    N6YW Ham Member Volunteer Moderator QRZ Page

    All in one rack as stated previously.
    I will incorporate a VFO, or perhaps a stable HF rig packaged into the rack to look buzzardly.
    Then again, A DDS package readily available with a buffer would work just fine. Old & New works for me.
    N2EY likes this.
  4. N2EY

    N2EY Premium Subscriber QRZ Page

    Hmmm---if someone had a T-368 PTO assembly, or something similar......


    See also the article by W2YM "Stability With Simplicity", QST, October, 1960. Simple and stable VFO. Needs a buffer stage to build the output up to drive a Class C stage but that's a good idea anyway.

    73 de Jim, N2EY
  5. N6YW

    N6YW Ham Member Volunteer Moderator QRZ Page

    I have several T-368-E PTO assemblies because I own and operate a T-368-E. Seeing that's it a mono band transmitter,
    I could use crystals as I have a bunch of those too.
  6. N2EY

    N2EY Premium Subscriber QRZ Page

    Both are excellent options!

    I think many of us eagerly await developments.....and pictures....

    73 de Jim, N2EY
    N6YW likes this.
  7. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    I too prefer Pi-L net output circuits where possible.

    Here is a potential output stage using conservative values for tube longevity.


    1. ICAS with forced air for cooling,
    2. Power out = 350W carrier,
    3. Audio bandwidth determined by equilization,
    4. Pi-L net values determined by Wingfield Equations


    Attached Files:

    N6YW likes this.
  8. K4KYV

    K4KYV Subscriber QRZ Page

    The latter Gates BC-1 series uses a pair of 807s to drive a pair of 833As. An 807 would theoretically drive an 833 with reserve power, but in actual practice with all the circuit losses, they are marginal at best. That's why they have that tertiary winding on the modulation transformer to apply about 15% modulation to the RF drivers; that delivers additional driving power on positive modulation peaks when the PA tubes need it. In the older versions of the Gates BC-1 series, pre-1T, they used a single 813 to drive the 833As. The Raytheon RA-1000 uses a pair of 813s to drive the pair of 833s! No modulation applied to the driver stage.

    According to RCA, you need forced air on 833As only when operating them at something like 4000 volts on the plate. MV rectifiers behind a viewing window look pretty, but I don't think they are really necessary. The main purpose of the viewing window is so you can monitor the plate glow, which may indicate the tubes are over-dissipating. My Gates uses solid-state module drop-in replacements for HV and MV rectifiers. My HF-300 rig presently uses 4B32s, xenon-gas equivalents to 872As. I have some NOS 872A rectifiers and some 575A pulls that I may try some day.

    As for the VFO, I use the same T-368 exciter unit for all my rigs, plus the HF-300 rig is capable of xtal control. It is very stable once it warms up, and I let it run all the time to avoid stand-by cooldown drift between transmissions. Made that work by installing additional shielding and rf filtering so that the constant-running oscillator isn't audible in the receiver. I have a couple of DDS vfos, hamfest finds, but the PTO is stable enough that I have never tried to run them with the transmitters.
    N6YW and N2EY like this.
  9. AF6LJ

    AF6LJ Ham Member QRZ Page

    I like the DDS idea with a twist.
    Nixie tube readout...
    Old style knobs..
    A vacuum tube buffer with output metering.
    AC0OB and N6YW like this.
  10. N6YW

    N6YW Ham Member Volunteer Moderator QRZ Page

    I appreciate all of the input gentlemen. It's a wonderful resource to say the least.
    The idea of running just one 833A is based on the fact that I already have a QRO (Bauer 707) transmitter and it's backup being the T-368-E.
    However, building my very first AM transmitter is a challenge that I am very much looking forward to and building as
    a "loafer" medium powered unit is a logical choice. I will be running the 833A at 2500 volts. With the 810's running at design maximum,
    I should easily acquire good modulation to make the final sing. The RCA iron comes from the old RCA BT1-R I salvaged last year, sans
    the high voltage transformer which I will likely source from Dennis as well.
    One advantage I see in using the 810/833 approach is they are both Triodes, so no need for screen supplies. Less parts count and build time.
    Also, the 833A doesn't require a squirrel cage pressurized chassis like some designs. In fact, at 2500 vdc, it won't need a fan at all, although
    a small Rotron fan would be nice to circulate air out of the cabinet. I would like to keep it as quiet as possible which is my gripe with the T-3.
    To be able to use a quiet well built medium powered is something I look forward to. The pride of rolling your own is a peerless feeling.
    AC0OB and N2EY like this.

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