Backwards modulation swing on bird meter

[crambone]

So I was watching a video on YouTube and a guy showed a Tram D201A that had a 3 position power switch I believe it was 4, 10, 20 watts. On 4 watts it looked great on the bird but on 10 or 20 when he keyed and modulated the meter went backwards. I’m assuming it’s not good for that to happen! What would cause that? 

[ytradio]

In AM mode a radio needs about 4 times peek power above carrier power to make 100% modulation. If it does not, you can see the backward swing. In side band you will not have this happen, so the higher power levels on the radio you are speaking of are probably for running in SSB mode.

[crambone]

Ok that makes sense thank you

[oldgeezer]

Anytime you see backward modulation, the transmitter has run out of headroom to produce forward modulation. Only way to restore forward modulation is to reduce the carrier (dead key) power. Yoy want a peak of four times the dead key power for 100% modulation. Example is 4 watts dead key swings to 16 watts PEP out for 100% modulation. High modulation is louder on the receive end.

Same issue can exist on a RF amplifier. Tubes are better with 4 watts in vs SS (pill) amps that many want only a 2 watt dead key. Too high carrier (dead key) power in an amplifier can result in a loss of modulation, An example is 50 watts deadkey only swinging to 100 watts. You want 50 watts dead key swing to 200 watts PEP out.

BTW- only a peak reading watt meter will indicate peak power out. A standard watt meter may only show 100 watts out when the amp is producing 200 watts peak power out.

[Sandbagger]

Sept 12, 2019 23:46:11 GMT -5 crambone said:

So I was watching a video on YouTube and a guy showed a Tram D201A that had a 3 position power switch I believe it was 4, 10, 20 watts. On 4 watts it looked great on the bird but on 10 or 20 when he keyed and modulated the meter went backwards. I’m assuming it’s not good for that to happen! What would cause that? 

This is what happens when screwdriver "techs" try to engineer something without fully understanding what they are doing. Yes, jumping out the plate limiting resistors will greatly increase output power, but a stronger carrier requires a much stronger modulator to fully modulate the increased carrier. In this case, that part of the "design" wasn't taken care of. And that's part of the problem you run into when using peak reading wattmeters. They give you a false sense of security, because nearly all modulation will swing forward, whether it's balanced or not. Using an average reading meter like a Bird is actually a better indication of whether or not you are transmitting a balanced symmetrically modulated signal. If the meter stays steady or shows a slight forward swing, you can be reasonably assured that you are getting the proper modulation balance between positive and negative peaks. If the meter swings backward, then you don't have enough positive modulation to fully 100% modulate your signal. At that point you either need to back down the carrier until it no longer backswings, or you have to increase the level of modulation to once again fully modulate your signal. Trying to get double the modulator power is not an easy task, so it's easier to just back the carrier down. If you want more power, then get an external amplifier. That way you aren't straining the radio.

[air1]

All the old books and later many internet sites showed how to boost the AM power for many radios but apparently whomever wrote up those mods never owned a scope. Most radios can't do more that 5 or 6 watts before the modulation starts to fall off. On the Tram the switch is for Am only. It's just switching R643 and R642 in or out of the circuit to increase voltage to the final tube. On SSB the full voltage is always applied as both resistors are bypassed.

[cbrown]

Sept 13, 2019 13:34:44 GMT -5 Sandbagger said:

If you want more power, then get an external amplifier. That way you aren't straining the radio.

Best advice ever for anyone looking for more power output.