Thursday, January 11, 2018
Why You Need A Tube Tester
I've been around this line of work for a good ten years or so, and if you count the radios, since 1983. Here are images of the three different tube testers I use in amp repair, and each one has a specific purpose.
The Sencore is used only for testing ten pin base tubes such as compactrons like the 6C10 and 6K11 plus the 7868 power tube. It is a basic emission tester.
The Hickok 532 is a mutual conductance tube tester, and the one I use is older than I am, having a build date of 1946. Nonetheless is is a durable piece of kit, and if it ever fails me I have another on standby-even though it was not modified for 9 pin Noval base tubes so I'll have to do that if push comes to shove.
These two testers have one feature which makes them indispensable, and that is a shorts test. They both have neon lights which will illuminate brightly if a shorted tube is encountered.
What does this mean to you? Simple. A shorted tube will render your amp inoperable and if it is a power tube can cause serious damage to associated components.
Both testers will tell you if the tube or grid circuits are open. Either it won't illuminate, or no results will display.
A lot of folks say "Well. The only real test of a tube is how it performs and sounds in your amp." That's true as far as it goes, but the purpose of using a proper tester can save you a lot of work, and it can prevent damage from occurring that can run up the repair bill.
As an example today a Marshall JCM2000 DSL 100w amp came in today, with no output. I knew the amp because it doesn't get used much and I'd marked the tubes with the date when I repaired it two years ago.
If I didn't have a tube tester chances are I'd still be down in the shop pulling my hair out trying to figure out what had failed, and it would be running the customer's repair bill up as I hunted and pecked and hoped to get lucky.
As it turned out it took five minutes to find a preamp tube with an open filament which was all it needed to get it back running again.
Now. The third tester is a Maxi Matcher II made by the fine folks in Seattle at Maxi Matcher which just goes to show there are more things of value that come out of that fine city besides coffee and big airplanes. It tests a certain range of power tubes for mutual conductance and plate current and it allows the operator to match tubes-which makes for a nice quiet and well balanced performance.
And, if it detects an overcurrent situation-a short-a LED illuminates and you get an overcurrent shutdown, thus preserving your tube tester.
In fact, while matching some brand new tubes the other day, it detected a brand new tube which was shorted, and that prevented an expensive failure and some rework I would not get paid to do.
This was not the first time, either. A customer brought in an amp for service and he provided tubes he'd obtained from an eBay vendor-always a risky proposition. On the Maxi Matcher they went and they were not matched at all-there was a 15 ma split which was unacceptable. So more postage back and forth was required, and a careful reading of the guy's ad on fleabay revealed he did not have any idea what he was talking about when he advertised the tubes as "matched".
Now. You do not have to spend a lot of dough here. A reasonably good quality emission tester like the Sencore, or a Knight KG1, or an Eico 625 and many many others can be had for less than fifty bucks.
If it sorts out one bad tube before it causes damage you're money ahead. And that is something the people who insist "the only true test is in your amp" do not seem to be able to comprehend.