Modulation Transformer Calculation

Discussion in 'Technical' started by WD5JKO, Jun 30, 2014.

  1. WD5JKO

    WD5JKO Member

    Some years ago I wrote on this subject. If you look at the connection diagrams for any multi-match modulation transformer, it is apparent that for any given connection choice, there will be a variety of primary and secondary impedances supported. Therefore I make the point that turns ratio is the primary concern before looking at the impedances.

    Let me try to make an example to see if this helps. You built a home brew transmitter using a pair of 6146's that you want to plate modulate. Your power supply provides 600V, and is beefy enough to supply both the RF amplifier, and the modulator.

    So let me start this process for the plate modulator. You want the ability to modulate fully, and with a little extra headroom. So lets say you want to be capable of 110% on positive peaks.

    So to get to 110%, multiply 1.1 X 600 = 660v. The modulated B+ goes from 600 to 600 +660 = 1260v peak at 110% modulation. On the other way down it will go to -60v (over mod!).

    For modulators you have a pile of 807's, so lets explore them. You also want high quality audio, so you want to bypass the interstage transformer idea, and instead you want to build a phase inverter, and R-C couple the audio to the 807 grids. This means class Ab1 operation since you cannot provide any grid current. With class Ab1, the plate voltage minimum occurs when the peak AF grid voltage matches the grid bias amplitude. If the bias were set to -33v, and peak AF swing brings the G1 to 0v. You cannot go positive since grid current would flow.

    Next look up the 807, and look at the plate characteristic graph. At zero bias, the plate voltage saturation is about 50v, but that is the hard clip point. Back off from that, and lets say the plate voltage minimum is 150 volts. So this means the 807 plate can each swing from B+ (600v) to 150v (Eb min) to 1050v (Eb (max), or +/- 450v. Since this is Push Pull, one 807 at Eb min of 150, and the other will be at Eb max of 1050. So we make 900v peak across the primary of the modulation transformer.

    We said earlier that we need 660 v peak across the secondary for 110% modulation, so the voltage ratio is 900/660 = 1.36:1. The voltage ratio is the same as the turns ratio.

    So lets say the 6146's run at 600v @ 200ma. The RL = 600/.2 = 3000 ohms.
    With a secondary load impedance of 3000 ohms, and a mod tranny with a 1.4:1 turns ratio, the primary plate to plate load impedance is:

    1.4^2 X 3000 = 6000 ohms.

    It looks like a pair of 807's will do fine. But what if it worked out to 3500 ohms instead. that is too low for those 807's, but two pair in push pull parallel would be fine.

    If you concentrate on impedances only, you could end up matching impedances when the turns ratio is way off. This could limit the maximum modulation percentage to something below 100%.

    Jim
    Wd5JKO
     
  2. W5HRO

    W5HRO Administrator

    Modulation transformers simply provide an air gap between the final modulator and the RF amplifier. The only thing that matters is the transformer's ratio. Always go by the ratio and not the impedance.
     
  3. WD5JKO

    WD5JKO Member

    For years now AM'ers that want to build a plate modulated rig with a push pull modulator have had to scrounge for a decreasing supply of old modulation transformers. One exception is the "big-iron" tube type broadcast transmitters that are being scrapped. One recent development with smaller AM Ham modulations is from Hammond Transformers that is increasingly offering the Peter Dahl line of transformers. This is cool...not cheap, but cool. A brand new Viking 500 modulation transformer for $390. This ran a pair of 811's in push pull modulating a single 4-400 RF tube.

    http://pwdahl.com/PWD_HamCatalog.html

    At the above link there are 6 mod transformers for sale. Here are a few:

    COLLINS KW-1 MODULATION XFMR
    PP 810 MODULATORS TO PARALLEL 4-250A'S
    CORE) EI-175 EE-STACK STYLE #) 80 E-I LAMINATED CORE
    PR) 1800/2400 VCT, 18000/11600 OHMS PP
    S1) 1768 VAC @ 283 MA CCS (6250 OHMS RFPA)
    DM) HT = 6.400 WT = 7.000 DT = 7.000 MD = 6.812 MW = 4.750
    PRICE) CALL or e-mail Plus S&H FOB Buffalo, NY WEIGHT) 35 LBS


    COLLINS 30K MODULATION XFMR
    P/N 677-0316-00, 200W,+/- 2DB 100-5000HZ
    CORE) EI-150 X 2.5" 29GM6X STYLE #) 07 E-I LAMINATED CORE
    PR) 2450 VCT @ 175 MA (32K OHMS)
    S1) 1770 VAC @ .150ADC & .113AAC(16.7K OHM)
    S2) 248 VAC @ .050ADC
    DM) HT = 6.062 WT = 3.750 DT = 4.390 MD = 3.500 MW = 3.000
    PRICE) $595.00 Plus S&H FOB Buffalo, NY WEIGHT) 12 LBS


    JOHNSON VIKING 500 MOD
    VIKING 500 MODULATION TRANSFORMER
    CORE) EI-212 X 3 STYLE #) 04 E-I LAMINATED CORE
    PR) 23000 OHMS P-P 2400 VCT (200-3500 HZ)
    S1) 8000 OHMS (2000VDC @ 250MA)1414VAC 250W
    DM) HT = 6.375 WT = 5.313 DT = 5.678 MD = 4.250 MW = 4.250
    PRICE) $390.00 Plus S&H FOB Buffalo, NY WEIGHT) 29 LBS


    THORDARSON P/N T-11M77
    125-300 WATT MULTI-MATCH MODULATION XFMR
    CORE) EI LAMS POTTED CASE STYLE #) 07 E-I LAMINATED CORE
    PR) PP 811A'S @ 1000 VDC
    S1) SINGLE 813 @ 2000 VDC @ 225 MA DC
    DM) HT = 7.500 WT = 6.000 DT = 7.250 MD = 5.875 MW = 5.375
    PRICE) CALL or e-mail Plus S&H FOB Buffalo, NY WEIGHT) 26 LBS

    Jim
    Wd5JKO