Donnie, testing today with an R-4, which has only one 6HS6 -- the pre-mixer.
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I tested 5 tubes in the socket: 6HS6, 6AH6, 6AU6, 6BA6, and the Russian 6J5P. There was no difference in the AGC set point using any of the tubes -- the bias was stable (with no antenna connected and RF gain fully CW).
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On 40m, the calibrator was about 10dB over 9 with the HS6, 5dB over with all others except the BA6. The 6BA6 was about S8.
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So in summary, perhaps a 5dB or so loss in gain using any other tube than the 6HS6 as the pre-mixer. As noted previously, there was no difference in signal strength with 6AH6/6J5P as the first mixer instead of 6HS6 in another receiver.
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Dave
