My new toy. Ultra modulation unit

[Gito]

Hi

I still believe that R1 value  is the impedance of the Rf Amplifier'

When we used an solid state amplifier as a modulator which has a low " P to P" impedance ,maybe 100 ohm to modulate a 807 transmitter which has 600 V B+,and load it to 100 ma .it has a 6 k ohm impedance.

So when R1 must have the P to P value ,R1 = 100 ohm.

 when the white cable connection at the Mod.Trafo,with negative modulation hits /goes down to  Zero ,

V1 conducts ,so a current flows ,from 600V (B+)  trough R1 to Zero volt,that means  the current is 600 V divided by 100 Ohm is 6 amp,

So R1 must dissipated  600 X 6 amp = 3600 Watt?


Gito

[Gito]

hi

  V3 is the limiter part of this unit,when the white cable connection at the mod trafo hits zero volts.it stop conducting ,so when the negative modulation peak is lower than zero ,it can not get trough to  the Rf final ,
but when there is no voltage at the  RF plate ,nasty things happened .

It's where V1 gets into action,when the voltage at the white cable connection gets below its B+ V1 begins to conduct,There is a current flow from B + to this connection

the current flows from B+ trough R1 to the connection.
if we put the tap in the middle of R1 ,the voltage is half  from  B+ to this connection.

So the RF plate voltage gets its B+ from this tap ,when the plate voltage is 1000 v,and  the downward swing  is  minus 100 volt, the plate voltage gets minus 50 volt,and the upward swing  (positive modulation) is 100 volt ,and the down ward modulation is only  minus 50 volt.

so when you modulate more than 100 %   ,say with a 2000 v sine wave,

than the downward modulation ,will gets its peak minus 1000volt( B+ - 2000v = minus 1000v),but since there is a B+ supply that gets to the RF final from the center tap  of R1,where the voltage is minus 1000 V.
in that way the voltage at the RF final never gets negative.(B+ - 1000V= 0 V),the B+ in this case is 1000 V.

So the upward modulation in these example is 2000 v,and the downward modulation is  only minus 1000 V.And the downward modulation never goes  , below zero V
of course ,its only an example,

I believed ,how this unit works.


Gito

[ke7trp]

The manual states, R1 should be RF Deck impedance. This is 6K ohms period.  I set Tap so the overall resistor R1 is 6K.

I then set the middle tap on R1 as the manual states,  Near the diode.. No action. I moved MIddle tap in one inch inrements all the way up to the other tap. No action.. I lose mod and cannot hit 100% as I move away from diode with middle tap.

I have unhooked the unit and now get 125 UP and 125 down. MUCh more PEP.  I dont believe UM unit is in working order. I am going to test caps and such over the next few days and try again.  Its been 50 years since it was used and it may just have a bad Black capacitor or electrolytic.

Clark

[Opcom]

doing some analysis on this unit's circuit, an interesting factoid was uncovered. On a given PA stage of 100W DC input at carrier level, equippped with a modulator of >1KW peak
power, and where the resistor tap was always adjusted so that with any given mod power of 100% or more (% deemed to be an additive voltage scale as seen on scope and therefore the SRQT of the power percentage. i.e. 100%=4x the peak power, 200% = 9X the peak power), the resistor tap is in the middle at 140% modulation (5.76 X the carrier level).

The only things changing were the audio power and the tap position required to keep the negative RF peak level at 10% of the positive RF peak level. It is amusing to note the near-complete waste of modulator energy during the negative going half of the modulation cycle. Not that it matters.

Normally I suppose the tap is to be set properly for where the modulator is maxing out, and at lower audio figures the data is different. It will be interestig to look into that and plot the currents and voltages over an audio cycle, but not now at 1 am.

[Gito]

Hi Clark
 
 I',m an old man, I overlook that you had the manual ( The Ultra Modulation System) posted on your article .

Reading it carefully,it stated that there' a voltage drop across this  diodes (866) ,it needs an additional voltage and there's also a voltage drop across the small resistance between the tap an resistor.

So the conclusion there's a" resistance "in this diodes ,so the plate voltage that gets to RF final is smaller(there's a voltage drop) than normal modulation,also  because this resistancei the modulation proses.the peak voltage is lower than normal modulation,

The modulator trafo turns ratio is the important part of the modulation ,it's not only the power of the modulator tube,that can create an high peak voltage.on the secondary part (transmitter part ) at the modulator trafo.
since the peak voltage at the primair modulator is limited  by its plate voltage.(modulator tube)

For example a  100 watt filament transformer, has an  10 volt output , a 1000 watt has the same 10 volt output

the voltage output  is not depended on the wattage of these transformers,but the turn ratio.
but of course with higher wattage we can used more tubes, but the voltage is the same.

When an high wattage modulator  that has more than enough power to drive an Transmitter with a modulator trafo not with the right turn ratio,even with full power it can not drive the transmitter for 100% modulation.
Because the peak voltage  at the transmitter can't reach the needed voltage for 100%  modulation

With a higher voltage at the 811 plate voltage  and the right turn ratio,we can get more peak voltages at the secondary (transmitter side ) modulator trafo.

Gito

[Gito]

Hi
Sorry,what i mean is an article from QST  October 1956 about " The Ultra Modulation System" as a PDF form 5610027(!) ,that Clark has attach  in His First "Question"

Reading this Articles,I think We don't need to have the "manual" to know how it works and how the schematic is.
The only thing that Confused me(for a while) is the connection of the white cable from T2.
The connection,it is written  to connect it "To Mod Amp Plate" giving the impression that it must be connected to the plate of the modulator tubes.

But actually it s must be connected to the RF final p;late circuit and becomes the B+ for the Final RF tube .

reading it carefully ,it is written...the value of R1 in ohms is the same as the modulating impedance of the RF amplifier.It CAN BE DETERMINED by dividing its plate current into the plate voltage.....(QST Oct 1956 page 29)

And sorry for my English,sometimes I did not write the right words to express my thinking.


Thanks


Gito

[Steve - WB3HUZ]

And sorry for my English,sometimes I did not write the right words to express my thinking.

We all suffer from that problem, even those of us who have English as a first language! 

[Gito]

Hi

Thanks Steve.
Carl,maybe the problem that You have in this unit is the 866 tube.I found an article more detailed from Hytron 866 it stated :

...(filament voltage)..the rated voltage of 2.5 volts under all operating conditions.The filament voltage,measured at the tube terminals,should not vary more than  plus or minus 5%,(0125 volt) from the rated value.

This tolerance should include the effects or regulation caused by transmitter-modulation load as well as the normal power supply regulation.
Less than the recommended filament voltage may cause a HIGH VOLTAGE DROP with consequent bombardment of  of the filament and eventual LOSS OF EMISSION.
Higher than the rated voltage will greatly shorten the life of the filament.

So maybe the tube (866) has lost it's emission or a drop in filament voltage.


Gito,