4-400 high level modulated

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W4NEQ:
I'm considering a grid-driven 4-400 class-c rf stage, high level modulated.   The advantages I see are ease of drive, and likely no neut if I keep the screen at RF ground and add just a bit of resistive swamping at the grid.  Should be stable on 160 / 80 / 40.

Modulating a tetrode with the goal of _low audio distortion_ is where I ask for the collective wisdom.   

One way would appear to use a modulated B+ resistive divider for the screen voltage, and that's an option, but looks like I'll be burning 200 watts in heat to do this.

I know that CCA used a separate screen supply, with an 80 henry series choke to let the screen sort of float with modulation - I'm not precisely clear on the theory there, but they achieved excellent ( < 1.5% thd ) performance that way.

Seems like if memory serves, the RCA  BTA 1r1 used tetrodes, but I don't recall their strategy.

Or, possibly capacitively couple modulation directly to the screen ...   gotta make sure there's no phase shift relative to the plate ...

Chris

KM1H:
Nobody will hear the difference in distortion from 1.5 to 5% or a bit more. What you want is full modulation that will be heard and not buried in the carrier as so many are.

Build the best you can with whats available and let it rip and try to find a mod transformer that has the screen winding included which is most any tetrode/pentode Class C BC amp stage.

Carl

n2bc:
Hi Chris, the RCAS BTA-1R1 uses a pair of 4-400s in the PA modulated by another pair running in AB1. The PA and mod tubes share the same HV B+. The mod screens are fed by a separate 750V supply.

Chuck...K1KW:
Using a series choke in the screen lead works very well.  It's operation is as follows.  When the plate is modulated 100%,  the instantaneous plate voltage varies from zero to 2X, at the audio modulating rate.  The screen current with fixed voltage on it will vary inversely to the plate voltage.  (If you just put screen voltage on a tetrode without plate voltage, it will pull excessive current and burn out the screen very fast!)  What happens with the choke in series is that the choke presents an impedance at audio frequencies so that as the plate voltage drops and the screen current tries to increase, the series impedance of the choke prevents it and causes the screen voltage to also drop exactly in phase with the plate voltage.  It becomes self modulating.   A series resistor would also do the same thing but it consumes lots of power in the process, whereas a choke doesn't. 

I have found that to prevent overmodulation at higher audio frequencies, a resistor in parallel with the choke helps, typically 15 to 50 K.  Make it a power resistor (10 to 25W), it will consume significant audio power.  This happens because at higher audio frequencies the choke presents a higher impedance resulting in greater voltage drop on the screen.  The resistor smoothes out the frequency response very nicely.  The screen bypass caps will also help with this and to really fine tune it, sweep with an audio oscillator while looking at the modulated RF envelope and play with the resistor value.

I've done this on several rigs and is on my latest which is a pair of 4-400A's modulated by a pair of 3-500Z's.

Chuck K1KW

W4NEQ:
Do you recall how RCA modulated the screens?

Chris


Quote from: n2bc on August 06, 2012, 03:11:20 PM

Hi Chris, the RCAS BTA-1R1 uses a pair of 4-400s in the PA modulated by another pair running in AB1. The PA and mod tubes share the same HV B+. The mod screens are fed by a separate 750V supply.

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