tube fanatic Posted August 5, 2015 Share Posted August 5, 2015 A commonly cited reason for avoiding tube type audio equipment is having to deal with replacements due to failure. In 1953, RCA's tube division published one of the finest articles on tube longevity which I've come across (see below- it starts on page 14). Although written in the context of industrial reliability, there are many lessons to be learned which are applicable to the equipment we use. This can be an ongoing discussion as we talk about ways to avoid tube failure in the context of the RCA article. http://frank.pocnet.net/other/AWV_Radiotronics/Radiotronics_1953/1953_01_AWV_Radiotronics_18_01.pdfI'll start by saying that excessive filament voltage is a major culprit. In vintage equipment, such as our favorite Scotts, Fishers, and so on, we often see a recommended range of operating line voltage from 105-125. When powered at the high end of that range, the tube filaments are often exposed to voltages in excess of their intended value (I have found many vintage units which, when fed 120-122V put 7+V on tubes designed for 6.3V). Even some modern equipment, using unregulated filament supplies, can show the same over voltage. So, one way to get things under control is to take a measurement (questions about how to do that in particular pieces of equipment can be posted in the "test bench" section) and, if found to be too high, employ a variac (a variable transformer which allows the line voltage to be set over a very wide range- they are not very expensive and are readily available commercially) to set the line voltage to a value which puts the filament voltage at the proper level. Another possible route to failure is the filament turn-on current. Like incandescent light bulbs, tube filaments draw much more current at initial turn-on than when fully heated. In equipment which does not employ soft-start circuitry, the variac can be used to ramp up the voltage over, say, 15-20 seconds to give the filaments a chance to heat slowly.I hope this gets things going. This can be a valuable discussion for anyone interested in tube type audio equipment. Maynard Quote Link to comment Share on other sites More sharing options...
Jim Posted August 5, 2015 Share Posted August 5, 2015 Does a variac mess with sound quality? And what brand would you recommend? Quote Link to comment Share on other sites More sharing options...
minermark Posted August 6, 2015 Share Posted August 6, 2015 I run imports, they would be original from china KT88s among many others. Never had a problem with quality/longevity these are still the ones that came with the units, knock on wood. Someday maybe i should buy some backups, kind of like a "Prepper" eh? Do have spare QSC1700 units at the ready just in case. Quote Link to comment Share on other sites More sharing options...
derrickdj1 Posted August 6, 2015 Share Posted August 6, 2015 run imports, they would be original from china KT88s among many others. What type of amp are you running? I don't think this applies to tubes used in the preamp or preamp section of a hybird amp? 1 Quote Link to comment Share on other sites More sharing options...
minermark Posted August 6, 2015 Share Posted August 6, 2015 run imports, they would be original from china KT88s among many others. What type of amp are you running? I don't think this applies to tubes used in the preamp or preamp section of a hybird amp? Quote Link to comment Share on other sites More sharing options...
Guest thesloth Posted August 6, 2015 Share Posted August 6, 2015 Source resistance of the transformer winding will dictate how much inrush current there will be. If you find that the wall voltage where you are is high and it is causing issues with your tube amp I think there are other options besides a variac. For the heaters you can add some series resistance to drop the voltage, this will also make the transformer happier by lowering the inrush current. 2R2 or 3R3 can get you there. For high B+ you can add a Zener in series with the center tap of the HT secondaries. Quote Link to comment Share on other sites More sharing options...
LarryC Posted August 6, 2015 Share Posted August 6, 2015 Another possible route to failure is the filament turn-on current. Like incandescent light bulbs, tube filaments draw much more current at initial turn-on than when fully heated. In equipment which does not employ soft-start circuitry, the variac can be used to ramp up the voltage over, say, 15-20 seconds to give the filaments a chance to heat slowly. I hope this gets things going. This can be a valuable discussion for anyone interested in tube type audio equipment. Maynard Like This Quote MultiQuote Report #2 Jim Forum Veteran Members 842 posts LocationNJ OFFLINE Posted 05 August 2015 - 06:23 PM Does a variac mess with sound quality? And what brand would you recommend? I can't name you a brand of variac, but Jud Barber's Joule Electra OTL amplifiers use variacs in their power supply -- see the black knob on top of the smaller unit in my avatar. Jud says to slowly increase the filament voltage readout to 64 over about a 15 sec. period. It's all very manual -- no thermistors or auto-biasing (?) in Jud's amps. But, Jud is a perfectionist in his own way, who eliminates such things in the interest of a purer sound quality from his amps and preamps. I never heard him comment on it, but would be a little surprised if he thought a variac degraded the sound in his amplifiers. Quote Link to comment Share on other sites More sharing options...
Guest thesloth Posted August 6, 2015 Share Posted August 6, 2015 (edited) A variac designed for the desired transmission frequency (USA is 60Hz) and is rated correctly for power so the core doesn't saturate shouldn't make an audible difference. Edited August 6, 2015 by thesloth Quote Link to comment Share on other sites More sharing options...
FloridaBoy Posted August 9, 2015 Share Posted August 9, 2015 Directly heated rectifiers can be issue. You could install a standby switch or like me...just not worry about it. Replacing tubes just comes with the turf. Quote Link to comment Share on other sites More sharing options...
Thaddeus Smith Posted August 9, 2015 Share Posted August 9, 2015 this why i have an amp that uses $6 tubes. :D 1 Quote Link to comment Share on other sites More sharing options...
Dr Morbius Posted August 9, 2015 Share Posted August 9, 2015 May I ask, what amp uses $6 tubes? Quote Link to comment Share on other sites More sharing options...
Dr Morbius Posted August 9, 2015 Share Posted August 9, 2015 And what tubes may they be? Quote Link to comment Share on other sites More sharing options...
derrickdj1 Posted August 9, 2015 Share Posted August 9, 2015 I guess a good question is how long are some of the members tubes are lasting. I am on the same tubes for 43 years and counting. Quote Link to comment Share on other sites More sharing options...
Thaddeus Smith Posted August 9, 2015 Share Posted August 9, 2015 (edited) May I ask, what amp uses $6 tubes? https://community.klipsch.com/index.php?/topic/149147-fresh-off-the-bench/ output tubes: 6cm6 driver tubes: 6sq7 Edited August 9, 2015 by Thaddeus Smith Quote Link to comment Share on other sites More sharing options...
Thaddeus Smith Posted August 9, 2015 Share Posted August 9, 2015 whoops!!! $5.50 and $4.50, respectively. http://vacuumtubesinc.com/index.php/catalogsearch/result/?q=6cm6 http://vacuumtubesinc.com/index.php/catalogsearch/result/?q=6sq7+ Quote Link to comment Share on other sites More sharing options...
WMcD Posted August 9, 2015 Share Posted August 9, 2015 IIRC. The first trans-Atlantic telephone cable was laid in 1952. It needed "repeaters" or amplifiers along the cable and Bell Labs designed those using vacuum tubes. Even though transistors had been invented they were not considered to be as reliable as tubes which were well understood and could be designed for long life. To be fair, there was a built in redundancy in the tube-based repeaters so that the failure of one was not fatal. FWIW - and I have mentioned it before on the forum: Prior to that time Bell telephone provided trans-Atlantic telephone service via powerful radio transmitters (and receivers) located in Newfoundland and the British Islands. So if you see an old black and white movie with Americans talking to Europe via telephone, that is how it was done. Going really OT, but it is entertaining: The first trans-Atlantic telegraph cable was accomplished in 1865 or so. Of course there was no practical radio back then. It didn't work well but the technology was improved. So America and Europe have been in digital connection (Morse Code) for about 1.5 centuries. (It is an astonishing statement, but perfectly factual.) I also like to mention a point of language from old days in movies and novels. When people were sending a telegram across the USA, they would say, "sending a wire." But when sending a telegram to Europe, they would say, "sending a cable." I.e. one of the trans-Atlantic cables. The story of these is told in two books by A.C. Clarke. "Voice Across the Sea." (A quip on the support of Britain by the USA in Hands Across the Sea.) IIRC, most of that is contained in a follow-up "How the World was One." (A quip on the movie title, How the West was Won.) It goes into satellite technology and geo-synchronous satellites. The books don't go into tube technology but are fascinating reads. WMcD Quote Link to comment Share on other sites More sharing options...
Guest thesloth Posted August 10, 2015 Share Posted August 10, 2015 this why i have an amp that uses $6 tubes. :D There are so many tubes out there for cheap but most people are into "fad" tubes. With all this talk about SET around here I have decided to prototype one using some tubes that when wired as triodes are very similar to a 2A3 but will yield more power. I am trying for 10 watts @ 5% THD. El Cheapo 2A3 on steroids 1 Quote Link to comment Share on other sites More sharing options...
muel Posted August 10, 2015 Share Posted August 10, 2015 I started buying tubes as backups and to roll different makes and models less than 6 years ago. In that time I have only had two tubes go bad (a Japanese 5ar4 and a Western Electric 2c51) Craig took some power tubes out to save as backups when he was working on my Scott but other than that I've just never had a problem. At this rate I'm never going to run out of tubes. 1 Quote Link to comment Share on other sites More sharing options...
Don Richard Posted August 25, 2015 Share Posted August 25, 2015 The closer a tube, or anything for that matter, is run near it's maximum ratings the shorter it's life. I remember a ham transmitter I made with a 6L6 output stage that I was running really hard. The output tube was good for about a year. Quote Link to comment Share on other sites More sharing options...
MyOwn Posted October 16, 2015 Share Posted October 16, 2015 What is the longevity of the tubes in your current tube equipment? For instance....You put brand new tubes in your equipment January 1st 2012. Are they still working? If not when did you replace them? Quote Link to comment Share on other sites More sharing options...
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