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Post subject: Blurb on some tube problems
Posted: Wed Oct 23, 2013 6:46 pm
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From old buddy, Jim McShane:

1. The #1 cause of noise problems with tubes is poor contact between the tube pins and the socket. There is 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 do not make good contact, or in a few cases, bad solder joints on the socket itself.

Tube sockets - even good ones - are not long-term, consistently reliable connectors.

Do yourself a favor - clean the tube pins and sockets every time you change tubes. A bottle of DeOxit D5 or GN5 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, GN5 is good to 425F - so do not lose sleep over it because of something you read on the "Amplifier Hysteria" forum. Moreover, it does not gum up either. Spray some in a paper cup and let it evaporate - you will be able to see for yourself.

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

Honest - only a few percentage of the tubes I get back for noise are truly 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 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 KT-88 amp? It is more than just a heater current increase that is an issue. I have had a couple cases where excessive grid circuit resistance causes unstable bias. The KT-120 grid circuit max resistance is 51K Ohms. If your amp is above that, there is a risk in using the KT-120. Just like the EH 7591As in amps that used original US made 7591A, 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. It does not take much of an over current event to damage a tube or shorten its life. Adjust the bias UP to the spec - and be sure to thoroughly warm-up the amp - not just the tubes, but also 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. If you let up the pressure on the hose and more water flows - that is 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. 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 did not even CARE what your tube tester said! A little harsh - sure, but is reasonable. 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. Often sellers match output tube for transconductance. However, it is imperative that if you need matched tubes, then they must match in current passed - 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. Using a DVM as a voltmeter we do not 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.

However, keep in mind that not only the plate current, but also the screen current flows through the tube cathode to ground (of course triodes have no screens, so this doesn't apply). We will ignore situations where the amp pushes the grid to positive.

That 50 mA you just read above in our example. That is both the plate current and the screen current combined (unless the tube is a triode). The combined current is what is causing the voltage drop read 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 is important since too much dissipation (measured in watts) is a tube killer.

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

a. You will NEVER find perfectly matched tubes. 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.


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 usually the case!


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Post subject: Re: Blurb on some tube problems
Posted: Wed Oct 23, 2013 6:52 pm
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+1000

Jim definitely knows tubes.

I've dealt with him personally and I've yet to meet a more knowledgeable technician or theoretician.

Arjay

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Post subject: Re: Blurb on some tube problems
Posted: Wed Oct 23, 2013 9:18 pm
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Solid info,"Din"!!! Not familiar with Mr. McShane, or his business. But, I've learned a lot from Gregg Levy, at Hi Test, over the years!!! Art

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Post subject: Re: Blurb on some tube problems
Posted: Wed Oct 23, 2013 9:48 pm
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Good stuff. 8)

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Post subject: Re: Blurb on some tube problems
Posted: Thu Oct 24, 2013 2:46 am
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BMW WRITE ;

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!

5. Most tube testers do not test power or other high voltage/high current tubes at a high enough voltage to be very meaningful.
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+ 1000 EVERYBODY MUST READ THAT

These are few of big mistake people do on any amp forum.

Which is quite normal for those who have not been trained as a technician in electronic


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Post subject: Re: Blurb on some tube problems
Posted: Thu Oct 24, 2013 4:52 am
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I know its counter to how the Fender Forums work...... but this one needs to be a sticky. Even if Brad just cut and pasted Beemer's post and put it up in the Announcement section.

I think I'll print this one off for future reference.


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Post subject: Re: Blurb on some tube problems
Posted: Thu Oct 24, 2013 11:19 pm
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Quote:
...The #1 cause of noise problems with tubes is poor contact between the tube pins and the socket...


I doubt very seriously that it's #1, over just plain bad tubes. If I have a noisy tube, I try that tube in other amps before deeming it bad. I rarely have to clean or re-tension tube sockets, compared to the number of noisy tubes that I find, and I really hate to throw away any tubes.

Dirty/corroded tube sockets seems to be the latest "trendy" thing. :roll:

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Post subject: Re: Blurb on some tube problems
Posted: Fri Oct 25, 2013 1:42 am
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shimmilou wrote:
Quote:
...The #1 cause of noise problems with tubes is poor contact between the tube pins and the socket...


I doubt very seriously that it's #1, over just plain bad tubes. If I have a noisy tube, I try that tube in other amps before deeming it bad. I rarely have to clean or re-tension tube sockets, compared to the number of noisy tubes that I find, and I really hate to throw away any tubes.

Dirty/corroded tube sockets seems to be the latest "trendy" thing. :roll:


+1 .

IMO this happened mostly on used amp wich stay unplay some years.


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Post subject: Re: Blurb on some tube problems
Posted: Fri Oct 25, 2013 1:57 am
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shimmilou wrote:
Quote:
...The #1 cause of noise problems with tubes is poor contact between the tube pins and the socket...


I doubt very seriously that it's #1, over just plain bad tubes. If I have a noisy tube, I try that tube in other amps before deeming it bad. I rarely have to clean or re-tension tube sockets, compared to the number of noisy tubes that I find, and I really hate to throw away any tubes.

Dirty/corroded tube sockets seems to be the latest "trendy" thing. :roll:


Jim's #1 hypothesis is supported by a number of different possible causes, not merely "dirty/corroded" sockets. And his primary field of expertise and interest re vacuum tubes lies with old tube-fired hi-fi gear and tube ham radios, both of which fall victim to many of the syndromes he describes due to aging and routine use. Taken as a whole, his advice is sound and tube-gear enthusiasts ignore it to their detriment.

Arjay

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Post subject: Re: Blurb on some tube problems
Posted: Fri Oct 25, 2013 4:17 am
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I think the first ditty is geared toward average amp owner, who knows little about tubes. Dealers often push new tubes, new tubes, new tubes to solve all problems. Really, in my experience (esp with older amps), many "issues" are solved by cleaning contact surfaces and applying a good contact cleaner.

This does a couple of things, even if it doesn't solve noise problems. 1.) Helps to remove oxidation. 2.) Puts a nice coating of lubricant, than preserve the surface and makes insertion and removal easier.

Kinda like K-Y for the amp. :lol: :lol: :lol:


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Post subject: Re: Blurb on some tube problems
Posted: Fri Oct 25, 2013 11:44 am
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BMW2002Ti wrote:
I think the first ditty is geared toward average amp owner, who knows little about tubes...


Well, that must be why I can't swallow it. :wink: In fact, much of that seems to be "geared toward those that know little about tubes". For example, who here even looks at the grid voltage on output tubes when setting bias? No one that knows about tubes, unless there is a problem. Who here looks at "total tube dissipation"? No one that knows about tubes, they aren't rated for "total" anyway, you set the bias for Plate dissipation. You see I sub p, but when do you ever see I sub t? :lol: The only reason that we check Cathode current is because it is safer than measuring at the Plate, and it's close enough to be useful for setting bias. Those that know little about tubes aren't going to be able to adjust their screen grid current, which is rarely, if ever, necessary anyway.

All I'm saying is that much of this info could be worded much better, so as not to appear so misleading. :idea:

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