Zenith 8-5-463 black dial console. Didn't need; couldn't resist!

[init4fun]

Originally Posted by Electronic M

I've had the flat plate ones short on me in an EICO audio signal generator (both the one in it and it's replacement that tested good before installation, but not after). Never tried the X ones, but I'd assume they were better than the flat plates since the later 6X4 and 6AC4 used and those and were considered more reliable than the 6X5.

Of course your correct here , the ones with the "X" shaped plates are the preferred ones to use if your going to use a 6X5 , as you rightly say the 6X4 and 6CA4 being the later model 7 and 9 pin replacement for what was the final improvement of the 6X5 design , the X shaped plates . In that transition they must have done something about improving the heater to cathode insulation , the way I always found the "flat plate" ones to be shorted . The 6Z4 , formerly known as 84 , was the first of these type of dual diodes and it was never pushed very hard with respect to it's H/K breakdown voltage since it was designed for the first vibrator power supply car radios (that replaced dynomotor powered ones in the early 1930s) and these were under 200 volt applications . The 6Z4/84 was the old large 6 pin base used before the Octals came to be , and when the Octals were introduced the 0Z4 was introduced along with them for car radio rectifier service , as we all know it's a Gas filled full wave rectifier that needed no heater at all . The trouble for the 6X5 started , in my opinion , when it was coupled to transformer supplies with 350 or more volts being pushed through a design that originally would have seen barely 200 .....