Apache TX-1 Modulation

[Robert & Linda McGraw K4TAX]

Recently went through the audio stages in the Apache TX-1 and made suggested changes to improve the audio.  Getting great audio reports but not satisfied with the results.

Basically I came across a condition which I question as being correct. First, I have not been able to determine if this is normal, or a design problem or a wiring error.  What I have observed is on modulation peaks the VR tubes for the screen of the 6146's will fire thus limiting the screen voltage to the sum of the two tubes.  Being OB2's the voltage is 210 volts.  The result is being unable to attain 100% or greater positive modulation.  Convenient was a OA2 on the spare parts shelf.  I replaced one of the OB2's with a OA2 and noted some improvement. However, still as positive modulation approached 100% the series OB2 and OA2 fired thus limiting the positive peaks.

Has anyone observed this condition?  If not, any ideas on what I am missing in my observations.

Your suggestions appreciated.

73
Bob, K4TAX

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