[MRCA] 6AK5/5654

[Ray Fantini]

I have recently been doing lots of work on the APR-4Y receiver, have one that I picked up in Dayton last year and after removing all the modifications and returning the receiver back to a somewhat stock configuration have to say that it's a different animal then its distant relative the APR-4 or older APR-1 The APR-4Y with the multiband CV253/ALR head is a good ten to fifteen dB more sensitive with the ability to pick up signals down to three to five microvolts in comparison to the older 4 that required thirty to forty microvolts with the other advantages being that the Y supports both AM and FM, has a surprisingly flat and broad IF system with calibrated gain control and uses modern style seven pin tubes, and that where the problem is. They used 6AK5 or 5654 for all the IF stages and considering that this must be one of the first modern seven pin tubes produced it has a fairly good response and noise figure but between the receiver and a APA-10 companion pan adapter the two of them use about twenty 5654/6AK5 tubes and I have discovered that about half including almost all of them that were in the APA-10 are dead. The filaments are all good but the emission is way down below where it should be. I have a TV-10 tube tester that I tend to believe and any of the new tubes I had pulled from stock have all tested good. So the question is how do I account for the high failure rate of this tube? I can speculate that maybe they were cooked by the previous owners who installed stupid high levels of B+ in the radios and the 6AK5 being rated for only 185 volts on the plate don't like being run at 250 volts but not one hundred percent on that. One thing I did do is when converting the radio back for AC operation that the B+ was in the design level of 185 Vdc that results in the plates of the tubes being in the 175 to 180 Vdc range but am starting to wonder if it is natural for the 6AK5/5654 to also have a short life expectancy? When I started the APR-4Y project I had a surplus of NOS 5654 and now to the point of looking for more, wonder if I will have to plan on buying a bunch?

Ray F
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