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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!!
If you are putting new production tubes into sockets in vintage/older gear, keep in mind that most modern tubes have slightly smaller pins than the original old stock tubes did. So you may have to re-tension (or worst case even replace) the original sockets that can't make good contact with the smaller pins.
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 and Tung-Sol 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. 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...
7. Variance in the brightness of the tube heater is normal and not an issue. It is important if you use tubes that you learn to differentiate between the orange glow of the tube heater located right in the center of the tube and the reddish glow of the large metal internal tube plate. The picture below shows one of a set of tubes with a reddish-orange tube plate. That is a problem for sure!
Now look at the next picture below. It shows some variance in the brightness/glow of the tube heaters both at the top of the internal tube structure and at the bottom of that structure. This is entirely normal and it is the result of small variances in production. It is nothing to be concerned about at all.
The amount that the heater protrudes outside the cathode sleeve varies from tube to tube - and if one protrudes more than the other at the top it will be brighter. It can also happen at the bottom of the tube; it just depends on if the heater is protruding more (just like happens at the top). Don't worry! It is not a malfunction and is no reason for concern at all.
I just wanted to share a few things that maybe will help people struggling to understand why they had "problems" with their tubes. One of the points mentioned above is almost always the case!
Edits: 08/05/14 11/01/17 11/02/17 07/17/18 05/01/20 12/07/20Follow Ups:
Great guidance and advice. Let me add that if you're rebuilding a vintage tubed amp, replace all the sockets. We've rebuilt a bunch at this point and have found a number of sockets that either would not clean and retension, or failed after we delivered the amp (after extensive and fun testing).
Therefore, now when we do a premium rebuild, we replace all tube sockets. 60 years is enough.
+1 Here. Thanks for all you've done for this hobby/obsession Jim!
Yes, right you can add a blue glow
Thanks. Not too late, and yes, it was the socket/tube connection. However, I have been listening of late to a Schitt Fulla DAC that I like very much. In fact, I purchased a second one for use as the DAC and preamp (with volume control) at the front end of my main system. It sits just before a McIntosh 240 tube amplifier, and I think it sounds great.
I'm a little late to this party but very helpful info. Thanks Jim.
Jonesy
"I know just enough to get into trouble. But not enough to get out of it."
Are there other symptoms of poor tube pin contact other than noise,like loss of highs or lows,dull sound?What does the typical poor contact noise sound like?
Thanks Jim!
Good info Jim. Newbee question. Having never used Deoxit before do I just give the tube socket a blast and then use the pipe cleaner or would you spray the pipe cleaner with the Deoxit and then use it on the socket? In other words, will this stuff be a problem if it gets elsewear?
Paul
Jim:
Is the De Oxit D5 also okay to use to clean the bias pots on a tube amp?
Sorry to butt in, but... I use Caig Fader Lube. It's less likely to dry out any lubricant in the pot. And less likely to dissolve any carbon contact material.
Why would D5 dissolve carbon contact material? It will flush out loose material (as it should), but if the material is loose it's not helping the pot anyway.
I have used D5 quite often as a cleaner for pots. I often do follow up with Faderlube or one of its earlier incarnations from Caig, but certainly not every time. Caig describes D5 as "Superior Cleaner, Enhancer & Lubricant for Metal Electrical Connections". That certainly is not something that will harm a pot.
I've used D5 to clean many a pot - with zero problems.
...and here some errors and myths about tubes - in german unfortunately...
http://www.elektronikinfo.de/strom/roehrenirrtum.htm
N.
The times, they are changing...
I'm curious what you would see in such an application, Jim, instead of a socket. I've heard of guys soldering tubes in place, but is there another alternative? What did the military do with critical location tubes? I've seen the spring hold-down type sockets. Were there other methods?
Thank you for a great post. I'm glad Rod is placing it in the Asylum's FAQ section.
That's not what I meant - a tube in a hi-fi amp may be an important connection to us, but a connection in a critical application such as a signaling system, in an aircraft, etc. would NOT use the same type of connectors we use for tubes.
A lot of "band-aids" were applied to improve the basic tube socket, but none of the improvements could make our familiar connector as reliable as a Switchcraft EN3 or Amphenol Motiongrade or 97 series round connectors. That's why those sophisticated connectors exist.
That's the information I was looking for, Jim. Not so I could aquire them, simply for something to Google, as a lesson in how it was done.
Thanks!
I'm glad I could help!
I have a 10 year old tube amp that was having tube noise problems. It also gave off an electrical smell after running. I sent it to the manufacturer to be fixed. Not fun @75 lbs. and shipping across country. It came back fixed, but the same problems started occurring.
Next, I took it to a local shop. They replaced all the tubes. Again, it worked fine for a short time. The noise started again after a few months.
After doing research on this forum, I cleaned the sockets using Deoxit and pipe cleaners. Some of the pipe cleaners came out black. I also used a dental tool to tighten the pin connects. The problems are now gone. No noise, no smell.
I was new to tubes and learned my lesson! Thanks for all the help!
You made my day - thank YOU for sharing the story!!
I would add blue glow and light debris inside the tube.
Too bad this forum has no stickies.
Stickies are nice, but I find them annoying when they grow and you have to scroll past 10 or 12 to get to the new posts. However, I did add a direct link the the Tube section of the FAQ in the forum header.
-Rod
There's a lot more that can be added - such as blue glow - but I already ran on too long!
In response to 1973shovel above: The two letters "FL" were added to the tube designation to signify "Flying Leads". These leads would be soldered and there would be no standard "socket".
I had a friend who soldered the leads for the Westinghouse fire control system on the Phantom F-4. They were even used in some of the F-15's fire control systems.
Those tubes were 5703 triodes used in the tuning drive for the AIM7 air to air missile. Why were they still being used in the F15, because they were the most linear device around. I replaced many of those tubes during the 15 or so years on the F4 C/D/E and G.
Craig
Steve( Fender) and I are always stressing these issues almost daily between the two of us and I think you reiterating these things is going to send a clear message so that people take notice.
So many people ignore the basics.They own really nice tube gear and yet they don't invest in a simple but nice quality DVM to help them set the bias voltages. There is a nice Chinese made Fluke on Ebay for 28 dollars or they could go to Rat Shack and get one on sale but they need to get one.
I remember one guy a few years back had a Golden tube Se40 I rebuilt for him and he was using a 5.99 analog meter that he bought at the dollar store and then he wondered why he couldn't set his bias properly.He did get one after my insistence but I shouldn't have to to say anything because if people are going to have tube gear,it's a good idea to invest in a decent DVM.THis is especially true for people that roll tubes and cables a lot.
I am also glad you brought up the KT120s and explained that popping these in place of a KT88,doesn't mean an automatic power increase. In fact,quite the opposite can be true.
The Deuce and the Macs are honest.Keeping it real!!
This is worthy of the FAQ, thanks Jim.
Question: What causes a whistler?
I've got one that warms up and whistles for a few minutes and then is fine. Should I worry? There are 18 tubes in the chain, well, 9 in one side, so it's a bit of a pain to find the offender.
7308s on the input and 45s on the output with others in between.
-Rod
Hi Rod,
It's a pleasure to support my fellow inmates when I can!! Thank you for your kind words.
On your whistler - be sure the pins and sockets are really clean (I know, 9 sockets makes for a lot of work), and if it still does it it's likely tube internals at fault. It apparently takes a few minutes to thermally stabilize. As long as you don't mind it making noise during warmup don't worry about it. But if it is a hard to find tube I'd look for a spare ASAP - it may eventually start making noise all the time.
Yeah, I lived with a hissy noisy one for about a year beofre it was taking 15 minutes instead of a few to stabilize. I got lucky on that one as I swapped the rectifier and that was it. The amps are the easiest as those are assessable and I can swap those 3 back and forth in a few seconds.
But, it's probably about time to just bite the bullet and retube everything and do a good cleaning. I think I had the same ones in service for nearly 6 or 7 years. Luckily, I believe that I have more than enough spares, except for the 5881s perhaps.
-Rod
Definitely worthy!
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