As I said before, I decided perform the DC testing of the tube assembly for the source Selector Unit in the clamping fixture. But just as I started testing, I ran into a little problem.

First off, the problem was not in assembling the test setup. That went without a hitch. Here’s how it looked just prior to starting testing.

I pigtailed some longer leads onto the main power transformer and tied it into the tube assembly. Then I wired in a high power variable resistance box to help determine the correct value for the Rd dropping resistor necessary to obtain the nominal 300V B+ voltage.

The initial checkout without any tubes mounted went ok. The high voltages were as expected, the filament voltages were acceptable, and the power for the meter driver board and indicator was correct. The indicator dimmer behaved exactly as expected, the meter backlights worked well, and the power indicator on the meter driver board lit. Once the tubes were installed, the filaments all powered up, and the rectifier started providing high voltage. This is where I ran into a slight issue.

The first thing I did was begin to dial up the dropping resistance to try and drive the B+ voltage down to 300V. And that’s when I topped out the resistance. The box in the photo has two 1KΩ rheostats. So I stopped, wired the two in series, and tried again. And again it wasn’t enough.

I didn’t fully appreciate how much dropping resistance was going to be required. The reason is that the preamp only draws a total of about 15mA. Since voltage drop is current times resistance, and the rectifier drop was minimal, I need much more resistance than I had originally anticipated. So I did what any good Engineer would do, I improvised. Here is what the test setup look like then.

In this photo I have added another resistance box both with both 1kΩ and 500Ω rheostats. All wired in series with the other resistance box.

In the end I determined that a standard 2.74kΩ 1% resistor should give approximately 302v for B+. Due to the low current level, the power dissipation in the resistor is only about 617mW. However, when digging through my resistors I discovered that the highest rated 2.74kΩ resistor I had was 1/2W. So I placed an order for some resistors. But they wont arrive until early next week so, until then I’m at a standstill.

I could have performed all the rest of the DC checkout with the resistances boxes but I didn’t see the point. Once I get Rd installed, I’ll have to check all the DC voltages anyway. So I decided to save the rest of the DC checkout until my resistors arrive.

As always, questions and comments are welcome.