Chicago Area Tube Tech

My strategy for testing 845s is to use my high voltage DC bench supply and switch mode power supply to heat the filaments. I expect 500V on the plate and -50V of bias to cause the 845 to draw about 90mA. If it draws substantially less than that, that's an indicator of low emissions.
From my tech in response:
That’s an emission test...and not very safe with all that voltage coming off a bench supply. Won’t tell you anything about leakage or shorts. The filament is 10 volts at 3 amps. That’s 300 watts of heat. Then there’s 500 volts on the plate. Imagine fumbling around with that. That’s lethal voltage. I’d have to make up a special test jig.
 
The problem with most testers (for any kind of power tube), is that they don't run them hard enough or at high enough voltage or current. Unless the tube is dead shorted cold, the chances of finding a thermally shorting tube are not very good.
I've had many power tubes test perfectly fine on a tester, only to go Chernobyl when put in an amp with 450V and pulling 30mA of current.
Paul's method does work, but does require a test set up to be specially made and well fused.
 
Rob, Shuguang Natural Sound 845T. They won't break the bank (at least not in the 845 world) and are fantastic tubes. The only issue is I don't think you'll be able to use the grill with the 845T given their much larger diameter than regular 845 tubes.

If you need a set, let me know.
 

paulbottlehead

Active Member
From my tech in response:
That’s an emission test...and not very safe with all that voltage coming off a bench supply. Won’t tell you anything about leakage or shorts. The filament is 10 volts at 3 amps. That’s 300 watts of heat. Then there’s 500 volts on the plate. Imagine fumbling around with that. That’s lethal voltage. I’d have to make up a special test jig.
Leakage and shorts are indicated by excessive current draw with this test. If you start turning up the high voltage supply and the current draw spikes, then the tube goes in the bin. Also 10V/3A is 30W of heat, not 300. This is what I use to heat up the 10V transmitter tubes (if you turn the voltage adjustment all the way down, it sits at about 10.4V)
Mean Well LRS-100-12 102W 12V 8.5A Single Output Switchable Power Supply | eBay

It is true that this type of setup isn't super safe, but if you have a 600V bench supply then one would assume that there's some awareness of how to properly use it.
image-20200427_143431.jpg
Here's a gassy 100TH on such a test setup. The HV supply and SMPS are on a power strip that gets unplugged when changing tubes, then all the connections are made, then the power strip can be plugged back in.
IMG_20200427_153230.jpg
Here's a good 100TH on the test setup.
 

paulbottlehead

Active Member
I do this once every 12-18 months. The problem with the enclosure is that I would need several different sockets, several different SMPS for filaments, lots of grid and plate cap connectors, and if I got it all figured out, then I'd find something that didn't fit (like the 833s I have awaiting me).
 
Ya, that does complicate things. I have to admit, i do the same thing about once a year or so, so i don't have mine in an enclosure either.
 
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