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Johnson Viking II Modification #2 |
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| Another Viking II followed me home the other day. I checked with Antique Electronic supply for another P-T157 interstage audio transformer and I found that they were no longer available. So I ordered the model P-T156 since it's the same as the P-T157 but without the nickel core.
I performed the speech amp modifications listed in modification #1 and replaced T3 with the P-T156 transformer. I ran frequency response curves using the new transformer and found that the P-T156 is no better than the original Viking Ii transformer! I went back to the original transformer. This modification shows how to use the existing interstage transformer. Its quite simple really. First perform the speech amplifier modifications described in modification #1 on tubes V1 and V2. Then remove C7, the .001 capacitor across the secondary of T3. Replace this capacitor with a 10K 1 watt resistor. The 10K shunting resistor flattens out the response curve so that audio is flat from 500 Hz to 15 kHz. Audio is 3 db down at 300 Hz. With this modification the modulator is driven a bit heavier than in modification #1. 100% modulation can be achieved at 100 watts out. I did not regulate the screen voltage in this modification since R13 was still good. I did replace C8 0.5 uF with a 22 uF at 450V. C32 and C46 (0.01 uF) were replaced with 0.001 uF at 6KV capacitors. I have received many good audio reports with this modification. Mod #1 has better response on the low end and lower audio distortion. If you have a good transformer replacement transformer for T3, use mod #1.
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