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"Tube Problems" That Aren't Tube Problems

I have been deluged lately with people asking about issues with their tube gear that they have decided for whatever reason must be the tubes. Occasionally it is, but at least 75% of the time in my experience it is not. So I wanted to make a few points and pass along a few tips to help people out.

1. The #1 cause of noise problems with tubes is poor contact between the tube pins and the socket. There are a wide variety of possible causes, dirty pins and/or sockets (in the case of a noisy new tube the socket is the PRIME suspect), socket contacts that have been stretched out or worn out and don't make good contact, or in a few cases, bad solder joints on the socket itself.

Tube sockets - even really good ones - are not sophisticated, high reliability connectors. You won't see tube sockets in critical applications because they simply are not reliable enough.

Do yourself a favor - clean the tube pins and sockets every time you change tubes. A bottle of DeOxit D5 and a handful of good old fashioned pipe cleaners works great for octals and other large pin sockets; for the miniatures, D5 and the little tiny dental brushes for cleaning between your teeth (another drug store item) do a solid job. BTW, D5 is good to 400F - so don't lose sleep over it because of something you read on the "Hi-Fi Hysteria" forum. And it doesn't gum up either. Spray some in a paper cup and let it evaporate - you'll be able to see for yourself.

Sometimes you swap a tube and the new tube is okay. so the tube you removed must be bad, right? NO! Sometimes the scraping action of removing and reinstalling a tube in that socket is enough to temporarily restore contact - and fool you into thinking you have a bad tube! KEEP YOUR SOCKETS CLEAN!!

Honest - only a few % of the tubes I get back for noise are bad. Most go on to play fine in another socket.

2. Those metal cans around tubes are shields, and they are there to prevent external fields from inducing noise in a tube. Do you really think a manufacturer would spend the money to put them on there unless they were worth it? Will the tube run a bit warmer with it in place? Possibly, but be smart about removing shields!

3. Want to use KT-120s in your amp? It's more than just a heater current increase that is an issue. I have now had a couple cases where excessive grid circuit resistance causes unstable bias. The KT-120 has a grid circuit resistance maximum of 51K Ohms. If your amp is above that there is a risk in using the KT-120. Just like the EH 7591As, some KT-120s will be okay with excess resistance - but some will not.

4. Don't fry those expensive new power tubes! If your amp has adjustable bias turn the bias current DOWN significantly before plugging in new tubes. The bias that was correct for the older worn tubes may not be for the fresh new ones. And it doesn't take much of an overcurrent event to damage a tube or shorten its life. Adjust the bias UP to the spec - and be sure the amp is thoroughly warmed - not just the tubes, but the internal parts as well. The whole amp should be warm which takes a minimum of half an hour in my experience.

Keep one last thing in mind - often you see a voltage "spec" on a schematic at the control grid of a tube. DO NOT TRY TO SET THE VOLTAGE AT THE GRID TO THAT SPEC! The negative voltage at the tube grid should be whatever value is required to set the tube's cathode current to spec. If the tube is passing the correct amount of current and the voltage at the grid is -20 volts then the hell with the "spec" of -22.5 volts or whatever it is. Making the grid voltage more negative with respect to the tube's cathode is the equivalent of stepping down harder on a garden hose to restrict the flow of water. And if you let up the pressure on the hose and more water flows - that's the same thing that happens if the tube grid is less negative!!

5. Most tube testers do not test power or other high voltage/high current tubes at a high enough voltage to be very meaningful. And matching of current is pretty much out of the question for all but the few "lab grade" testers that can apply high voltages at reasonable current levels to the tube. What your tube tester says with 150 volts on the plate will differ significantly from what my rig says at 465 volts! Our old pal Steve Melkisethian (proprietor of Angela Instruments) used to say he he didn't even CARE what your tube tester said! A little harsh - sure, but not all that unreasonable either. Remember - a tube's test results depends on the conditions under which it is tested!

6. Last for today - other components besides the tubes themselves can make your matched tubes seem less well matched. Power tubes may be matched for transconductance, but it is imperative that if you need matched tubes that they are matched for current - some say plate current, I say CATHODE current. Why cathode current? Because between the cathode and ground of many (most?) power tubes you will find a resistor in the circuit - low value, 10 Ohms is common - and that resistor's purpose is to make it possible for you to measure the voltage across that resistor and use Ohm's Law to calculate the current. Measuring the current with an ammeter requires breaking into the circuit and inserting the current meter. But with a voltmeter we don't have to break into the circuit, we can measure the voltage across the resistor and know the current flow through it. For instance, if you read .5 volts across a 10 Ohm resistor you can compute the current flow by dividing the voltage by the resistance. .5 volts divided by 10 Ohms = .05 amps, 50 milliamps.

But keep in mind that not only the plate current, but the screen current flows through the tube cathode to ground (of course triodes have no screens, so this doesn't apply). We'll ignore situations where the grid is driven positive and current flows through it as well for now.

So that 50 ma. you just read above in our example? That's the plate current and the screen current combined (unless the tube is a triode). And that combined current is what is causing the voltage across the resistor. By knowing the total current - cathode current - and knowing the voltage across the tube from plate to cathode, we can easily calculate TOTAL power dissipation of the tube. That's really important since too much dissipation (measured in watts) is a tube killer.

Why would the readings be different on "matched tubes"?

a. The tubes are never perfectly matched - some tolerance exists
b. The screen and/or plate voltages are slightly different at the different tubes due to different resistances in the output transformers' primary windings.
c. The cathode resistors are never EXACTLY the same value.
d. Etc.

Or some combination of the above...

Anyway, I just wanted to share a few things and maybe this will help people struggling to understand why they had problems with their tubes. One of the 6 points above is almost always the case!

Edits: 08/05/14

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Topic - "Tube Problems" That Aren't Tube Problems - Jim McShane 15:51:50 08/29/13 (25)


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