Robyn 123b modulation pot

[tulmkr]

I have a couple of robyn 123b tube radios that I have been trying to get to talk a little better. One of the things Im trying to improve is of course the audio output. On this radio there is a modulation pot that can be adjusted, however while adjusting the pot, and looking at the audio through my scope, I am seeing only a very, very small increase, or decrease in audio output. Looking at the pot, I can see that there are two resistors, one goes to chassis ground, and the other to the modulation tube. The value of the resistor to ground is 150 ohm, and the value of the other is 47k. Would changing the value of one, or both resistors increase or decrease the output value of the modulation pot? If so, would I use a higher or lower value resistor? would changing the resistor only change the baseline value, or would it result in an actual increase of the audio output?

[MDYoungblood]

Does the "Range Expand" switch in the "ON" position give it an increase in modulation. Also check both V5 (12AX7) and V6 (6BQ5). If I remember they were not big talking radios but a D104 would help a lot but it took it's toll on the tubes.

3's

Greg

[tulmkr]

Yes the range expand works as it should. I replaced the 12ax7 with a mullard 12at7, and it made a nice improvement in the sound. Both the modulation tube, and the final... both 6bq5's were also changed with nos vintage mullards, as well as all of the other tubes that I had mullards for. The pot shows very little increase or decrease with the range expander on or off. I am using an amplified d-104 tug-8. I know it was never a big talker, but I think I can get a little more out of it. Secret cb mag has a mod to basically volt the final by tying the modulation tube into it. I tried it on the crappy robyn I had, but after awile one of the big caps blew up on it.