The AM Forum

I was given a Johnson Viking Challenger the other day. When I opened it up I was surprised to see that the 6DQ6A finals had been replaced with 6146 tubes. Also the 6DS5 oscillator tube had been replaced with a 6AU6. Is this a common modification for the Challenger or other similar rigs?

Over the life cycle of the Johnson Challenger, Johnson changed the design of the oscillator. You can find them floating around either with a 6AU6 in the oscillator or with a 6DS5 in the oscillator. They also issued separate manuals for each version.

Could be previous owner wanted to move from a sweep tube operation to a beam power tube operation.

6DQ6 Substitution analysis: https://ozvalveamps.org/6dq6substitution.html

For more information, you could also ask Mr Google: "replacing a pair of 6dq6 tubes with 6146s"

Thanks Pete. Its good to know that EFJ produced a version with the 6AU6 oscillator. The schematic in the manual that came with it shows the 6DS5 with a hand written note 6AU6 by it. Thanks for the 6146 sub info as well.

It wasn't just a direct substitute. There was also wiring and component changes.

As Pete indicated, this was a common tube substitution for the Challenger since the 6DQ6 was a rather "puny" sweep tube. The 6DQ5 would have been a better sub but its profile may have been too tall for the cabinet; I can't recall if that was the case for not using the more power-sensitive 6DQ5.

In any case, tube substitutions require component changes to the respective circuitry.

If you want better audio, you can use this circuit below to enhance the audio. The original value for R21 was ridiculous, IMHO, as was C22, and resulted in SSB-sounding audio.

Phil - AC0OB