Testing tubes without a tube tester

shakti

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I am matching up a number of old EL34s I have amassed through different amps over the years. Matching them for idle current draw is easy enough by trial and error.

But I suspect some of them are more tired than others. Do you have any tips how to weed out or sort them by usage? One thought I had was to monitor their current draw while playing (either with a plug-in bias tool or on an amp with 1 ohm bias read-off resistors), but is this any good indication about their health?

Wish I could get a tube tester but for my use I don’t see it as a good investment yet.
 

Matthews Guitars

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Whatever are you talking about? There's about forty tube testers on ebay right now listing for under 100 dollars!

You can get a decent tube testers for the price of a good pair of power tubes. Or a really good one for the price of a set of four EL34s.

Get a B&K or a Hickock, and you'll definitely have a good one. But most any decent tube tester will get the job done for common tube types.
 

shakti

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I am in the dark about this but was under the impression that a good tube tester was considerably more expensive than that. What can I expect from tube testers in that price range?

Bear in mind that I don’t need to test for shorts, I already know these tubes are working in the amps I use them in. I’ve matched up some quads reasonably well, but then I don’t know if two tubes are strong whereas two tubes are really weak or whatever.

I am not averse to spending that amount if it gets me a really good tester, but I don’t want an expensive paper weight. Case in point; that Orange tube tester which was around $300 here in Norway but is apparently a total joke.
 

Weapons Man

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I am in the dark about this but was under the impression that a good tube tester was considerably more expensive than that. What can I expect from tube testers in that price range?

Bear in mind that I don’t need to test for shorts, I already know these tubes are working in the amps I use them in. I’ve matched up some quads reasonably well, but then I don’t know if two tubes are strong whereas two tubes are really weak or whatever.

I am not averse to spending that amount if it gets me a really good tester, but I don’t want an expensive paper weight. Case in point; that Orange tube tester which was around $300 here in Norway but is apparently a total joke.
I have a Knight tube tester and it works well. I bought it several years ago on Ebay from a guy who had it recalibrated. It also had all the instructions included with the unit. Avoid those where the seller indicates that he doesn't know if the unit works or not. You can go on Youtube and see the different types being used and see which one you want and then wait for one to pop up on ebay or another site like that.
 

FleshOnGear

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I am in the dark about this but was under the impression that a good tube tester was considerably more expensive than that. What can I expect from tube testers in that price range?

Bear in mind that I don’t need to test for shorts, I already know these tubes are working in the amps I use them in. I’ve matched up some quads reasonably well, but then I don’t know if two tubes are strong whereas two tubes are really weak or whatever.

I am not averse to spending that amount if it gets me a really good tester, but I don’t want an expensive paper weight. Case in point; that Orange tube tester which was around $300 here in Norway but is apparently a total joke.
I was also under the impression that testers that actually test transconductance are not especially cheap, but ones that only test emissions are affordable.
 

Pete Farrington

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A fixed bias valve guitar amp with 1ohm cathode resistors, in conjunction with a light bulb limiter to protect from shorted / arcing valves, make a great valve tester.
My understanding is that basic emission testers are a bad thing, the zero bias test condition using up big chunks of valve life with every test.
 

Matthews Guitars

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Get a B&K 606. They're cheap enough and they're transconductance testers.

Get one that's been recently serviced, if possible.
 

StingRay85

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Like Pete said. Start testing tubes with a bulb limited to pick out the bad ones that are shorted. After that is sorted, remove the bulb limiter (it lowers overall voltage to your amp including the heaters), and use idle current as reference. Take voltage measurements of the plates, screens, and negative bias supply. If you're looking for a good reference, a 400V plate voltage would make the perfect testing amp, if you put the negative bias supply to -36V, your tube should idle between 20 and 50 mA. Typically the higher the better. Everything above 30 mA is surely a decent tube. Only weak tubes with current draw in the range of 15-25 mA benefit from a transconductance measurement.

I use a Maxi Matcher 2 but it's pricey, 1K and it only does octal power tubes and EL84. Let's say I justified the purchase by collecting more than 300 EL34
 

XTRXTR

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I'd follow Pete's and your own practice you have been doing already. When a tube goes out then find another in your stash of tubes. Build a dual bias circuit for a 50 watt and a quad bias for 100 watt. Even with a 100 watt only check two at a time until you find four within +/- a few mA of each other at 65% dissipation and you should be fine.
 

Pete Farrington

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My method to pair valves is to check that they both idle, and peak, roughly around the same cathode current.
Use a scope across the 1ohm cathode current sensing resistor to determine the latter.
 

XTRXTR

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Testing the peak value as well is fine, gets you more balance etc but then, you need to be sure PI output is in balance including grid stoppers and coupling caps. Not saying any of that is wrong. You would also need a scope or DMM that will show peak hold to compare.

I like to go with keep it simple thinking, let the amp show its uniqueness and character. Sometimes you find gold by not chasing perfection though you can find more experience by doing so.

All manner of learning is a path.☯️🌄♾️🧐
 

Pete Farrington

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:shrug:Testing the peak value as well is fine, gets you more balance etc but then, you need to be sure PI output is in balance
I’m not checking the peak current for reasons of balance (gm) particularly; within the linear range, I’m not bothered about linearity, ie minimising power amp distortion :)
Also I don’t think similar gm necessarily equates to similar peak current.
Rather I’m checking to make sure that the upper and lower clipping points will be reasonably close.
Then I can select a higher or lower power pair, or a pair that will clip asymmetrically.
including grid stoppers and coupling caps
With grid stoppers, I don’t see they need to be particularly close in value? Differences should only affect the sharpness of the grid clipping and the extreme high frequency bandwidth.
And if the OT primary impedance is high, eg JTM45, grid clipping won’t be particularly audible anyway, as the output stage will hit anode current cut off at a lower signal level than grid clipping will occur.

Likewise at the extreme low frequencies for coupling cap value balance.

DMM that will show peak hold
Do you know of any models that provide peak measurements? I’d be interested if so.
As far as I can make out, the ‘min max’ or ‘fast min max’ modes on my Fluke 189 are in regard of the short term rms value.
Maybe I’ve got that wrong though
 
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