B&K 500 Tube Tester Dual Triode Mod

[pfgelectronics]

I have gotten much help here over the years but never had cause to join, or post, but I finally have something that others might find useful so I thought I would share it. Anyone who has used the B&K Model 500 tube tester is probably aware that one of it's major shortcomings is that it tests dual triodes such as the 12AX7 in parallel. There is no means to test each section's output separately to determine balance between the triodes, such as for a phase inverter. After looking at the manual for their later 707 model, the answer became clear, put in a switch. I installed a simple SPDT toggle switch between the plate supply/meter and the plate terminals on socket 9A. This is exactly how they did it in the 707 and it works perfectly on the 500.

Some things to keep in mind are that you will need to flip the switch to check both plates for the other tests as well, and the fact that the power feeds from socket to socket so you will need to carry through from switch to the next socket. Finally the switch should be rated for 250VDC as the plate circuit is 205VDC, most commonly available are not DC rated at all or only to 12V.  Now I am going to install an 0n/0n/0n type DPDT switch to allow the other tests to work as designed by having both triodes connected in parallel in the center position. The switch currently in mine is only 250VAC rated, but I wasn't going to spring for a $50 switch until I knew the concept would work. The one I have ordered is Honeywell part number 32NT91-10.

With this addition, and perhaps wiring up a socket to handle EL84s, I think the B&K 500 becomes a great option for the casual user, especially considering that they can be found for ~$100 at the time of this posting.