MK3 R49

[vanlifeson]

Can someone tell me what purpose R49 serves in a mark three side band transmitter? I have read that it’s for AM dead key, and I have read that it is for audio. It appears that it is in the audio circuit from the schematic. I have seen some mark three transmitters that have R49 jumped to lower the Resistance. what does lowering the resistance of this resistor do? Thank you and is it harmful? Because like I said I have come across some transmitters that have had this done to it
Thanks. Just trying to learn what some people are doing to these radios.

[2600]

R49 is there so you won't have a ten or twelve=Watt carrier.

A radio meant for AM only is designed for a carrier power around 4 Watts, with full power-supply voltage feeding to it.

A radio meant for SSB needs to handle an audio "whistle" into the mike. A pure audio tone modulating a sideband radio creates a RF carrier. A twelve-Watt carrier.

So when you take this SSB transmitter and feed a carrier into it for running AM, this is what the carrier power would be. Same as a sideband whistle.

The solution for this is done in more than one way, but they all boil down to one thing. Cut the power supply voltage to the final. A transistor radio cuts it down to both final and driver stages. A tube radio modulates only the final.

A fixed resistor is used in the tube-type SSB CB radios. Most 23-channel solid-state radios used a resistor, same as the Mark 3 SSB transmitter does. Changing that resistor changes the AM carrier power. There is always an upper limit, determined by the audio power available from the modulator stage.

The design of most 40-channel SSB CB radios uses a power transistor as a variable resistor to set the AM carrier. A tiny trimpot controls the transistor.

But this is what R49 does. Drops the 320-Volt (or so) B+ down to 200 Volts or less to feed into the secondary of the modulation transformer.

The schematic found online shows R49 as a "4 Watt" resistor. Every factory resistor I have ever seen for R49 in that model is a 10 Watt wirewound resistor.

73

[vanlifeson]

Thank you for the reply 2600! Now, is lowering the resistance of our 49 bad for the modulation transformer?(and does altering R49 actually increase modulation?) Recently picked up a transmitter and the person who had it said it has been like that forever. Maybe if you are too low with the resistance you could blow up the modulation transformer? Also, isn’t there another one of those power resistors on the final tube that people strap as well? I certainly do not want to damage the modulation transformer. Is that a very good possibility that altering R49 will damage the modulation transformer? Thank you so much

[2600]

Um, okay. The max carrier power you can fully modulate is determined by the raw, brute audio power you feed into the modulation transformer. Simple rule is for every Watt the final pulls out of the power supply, you need a half Watt of audio to reach 100 percent modulation. Naturally if you want more than 100 percent, you would also need more audio. This two-to-one ratio holds pretty well.

Increase the carrier, and the audio power needed to fully modulate it increases along with it.

The Mark 3 will run out of audio power when the carrier reaches 5 or sometimes 6 Watts. A weak audio tube or old filter caps may reduce that quite a bit.

I wouldn't worry about the modulation transformer. Don't see them go bad very often. There are more flavors of 'secret sauce' out there for this radio. Every component in it has been tweaked by someone along the way. Some tweaks are repeatable, and will make a difference you can hear. Other tweaks do little or nothing. And some of them do nothing good.

Wearing out audio tubes prematurely is what some of them will do, for sure.

73

[vanlifeson]

Feb 1, 2022 0:51:43 GMT -5 2600 said:

Um, okay. The max carrier power you can fully modulate is determined by the raw, brute audio power you feed into the modulation transformer. Simple rule is for every Watt the final pulls out of the power supply, you need a half Watt of audio to reach 100 percent modulation. Naturally if you want more than 100 percent, you would also need more audio. This two-to-one ratio holds pretty well.

Increase the carrier, and the audio power needed to fully modulate it increases along with it.

The Mark 3 will run out of audio power when the carrier reaches 5 or sometimes 6 Watts. A weak audio tube or old filter caps may reduce that quite a bit.

I wouldn't worry about the modulation transformer. Don't see them go bad very often. There are more flavors of 'secret sauce' out there for this radio. Every component in it has been tweaked by someone along the way. Some tweaks are repeatable, and will make a difference you can hear. Other tweaks do little or nothing. And some of them do nothing good.

Wearing out audio tubes prematurely is what some of them will do, for sure.

73

Great info thanks! Can you expound on your comment that some tweaks being repeatable and making a difference that you can hear?