What Audio Research power amps have you owned or admired?
- Thread starter Jay_S
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HI All,
Yeah, my D250 has individual tube bias pots, All 16 of them!! and 65 mA is the magic number. I have not noticed bias changing very much over time. I probably ought to check since it's been about a year.
Sparky
Yeah, my D250 has individual tube bias pots, All 16 of them!! and 65 mA is the magic number. I have not noticed bias changing very much over time. I probably ought to check since it's been about a year.
Sparky
Hi Sparky,
If the bias of a tube at 65mA did not change though the course of a year or 2, does that mean that the tubes are still strong or as strong as the first few months?
HI jadis,
Sorry I took so long to reply. I've been vacationing in glorious Northern California. I had no computer with me and I felt I had left some vital body part behind. Have you ever experienced an old-growth Redwood forrest? Man, it's like going to church. Spectacular! And spiritual.
In general, the answer to your question is yes. If the bias is stable, the tube is stable. But, tube conductance is only one of a number of measures that indicate the health of a tube but it is a good one. When I see the bias values starting to vary in the short term, I become very suspicious of the tube.
Different than this is long term tube aging. There will be a long, slow shift in the bias current as the tube goes through its normal life cycle. This should not be a cause for concern until the tube reaches the end of its life. Then the bias changes will transition to much quicker changes.
The only really certain way to determine the health of a tube is with a good tube tester. Back in the day, every repair shop could test tubes. Owners of fine equipment, if they were smart, would have their tubes tested about once a year. But now it is more difficult because most repair shops don't have tube testers. This is ironic since tubes have made such a strong comeback. Owners of tube equipment are now forced to use indirect methods and do some good guessing. Usually, this works OK but it's not ideal.
I'm lucky because I have 3 mutual conductance tube testers. Each is a different brand and were designed at different times. I do not have to rely on guess work. All owners of equipment that use a large quanity of tubes should have a tube tester and know how to use it. But, this is impossible because general purpose, high quality tube testers are no longer being made. Your only option is buy a used tester which is do-able. Be prepared to pay big bucks for the good ones.
Even if you get a good tube tester, your problems may not be over. You need to make sure you can actually test the tubes you have. For example, the 6550 was a modern tube design, one of the latest tubes to come on the market before tubes folded. Testers older than the 6550 (most of the testers) do not have set-up information for the 6550. The work around I used is to test the 6550 as if it were a KT88. They are very similar.
Only 1 out of my 3 testers have 6550 test information.
Another technique I use for both my personal tubes and those that belonged to my customers is to set up a data base where the test results for each tube can be entered and tracked. By doing this, a record of tube aging can be seen. This is a very valuable tool.
Sparky
Sorry I took so long to reply. I've been vacationing in glorious Northern California. I had no computer with me and I felt I had left some vital body part behind. Have you ever experienced an old-growth Redwood forrest? Man, it's like going to church. Spectacular! And spiritual.
In general, the answer to your question is yes. If the bias is stable, the tube is stable. But, tube conductance is only one of a number of measures that indicate the health of a tube but it is a good one. When I see the bias values starting to vary in the short term, I become very suspicious of the tube.
Different than this is long term tube aging. There will be a long, slow shift in the bias current as the tube goes through its normal life cycle. This should not be a cause for concern until the tube reaches the end of its life. Then the bias changes will transition to much quicker changes.
The only really certain way to determine the health of a tube is with a good tube tester. Back in the day, every repair shop could test tubes. Owners of fine equipment, if they were smart, would have their tubes tested about once a year. But now it is more difficult because most repair shops don't have tube testers. This is ironic since tubes have made such a strong comeback. Owners of tube equipment are now forced to use indirect methods and do some good guessing. Usually, this works OK but it's not ideal.
I'm lucky because I have 3 mutual conductance tube testers. Each is a different brand and were designed at different times. I do not have to rely on guess work. All owners of equipment that use a large quanity of tubes should have a tube tester and know how to use it. But, this is impossible because general purpose, high quality tube testers are no longer being made. Your only option is buy a used tester which is do-able. Be prepared to pay big bucks for the good ones.
Even if you get a good tube tester, your problems may not be over. You need to make sure you can actually test the tubes you have. For example, the 6550 was a modern tube design, one of the latest tubes to come on the market before tubes folded. Testers older than the 6550 (most of the testers) do not have set-up information for the 6550. The work around I used is to test the 6550 as if it were a KT88. They are very similar.
Only 1 out of my 3 testers have 6550 test information.
Another technique I use for both my personal tubes and those that belonged to my customers is to set up a data base where the test results for each tube can be entered and tracked. By doing this, a record of tube aging can be seen. This is a very valuable tool.
Sparky
Hi Sparky,
No problem and I can see you enjoyed your vacation.
Thanks so much for the information about tube life and testers. That's a good read.
No problem and I can see you enjoyed your vacation.
Thanks so much for the information about tube life and testers. That's a good read.
Jadis,
Triggered by your interesting questions I decided to look for some information on tube life.
Tube life is determined mainly by the deterioration of cathode emission. The usually measured parameter trans-conductance, also known as mutual conductance, is affected by the regular decrease of the cathode emission but the relation between these parameters is not trivial and is tube dependent, so unless you have a lot of experience and records to compare (as Karma does ), it is very difficult to estimate tube life with a reasonable accuracy from typical tube measurements.
The problem was studied by the military, who were very concerned about tube life, but their reports are not easy to find. However, as tube reliability and lifetime were of great importance to communications the UK Post Office also conducted a lot of research on this subject. I have found two papers :
The life of oxide cathodes in modern receiving valves (1951)
The relation between cathode emission, cathode resistance and mutual conductance in receiving valves (1956)
that are easily accessible and have plenty of interesting information on this subject.
I hope to go through them during my holiday - specially now that my students project of a 6550s and 6H30 Labview controlled tube tester is almost finished!
Triggered by your interesting questions I decided to look for some information on tube life.
Tube life is determined mainly by the deterioration of cathode emission. The usually measured parameter trans-conductance, also known as mutual conductance, is affected by the regular decrease of the cathode emission but the relation between these parameters is not trivial and is tube dependent, so unless you have a lot of experience and records to compare (as Karma does
), it is very difficult to estimate tube life with a reasonable accuracy from typical tube measurements. The problem was studied by the military, who were very concerned about tube life, but their reports are not easy to find. However, as tube reliability and lifetime were of great importance to communications the UK Post Office also conducted a lot of research on this subject. I have found two papers :
The life of oxide cathodes in modern receiving valves (1951)
The relation between cathode emission, cathode resistance and mutual conductance in receiving valves (1956)
that are easily accessible and have plenty of interesting information on this subject.
I hope to go through them during my holiday - specially now that my students project of a 6550s and 6H30 Labview controlled tube tester is almost finished!
Thanks for the insights as well, Microstrip. And talking about the 6H30 tube, I always wonder what sound ARC amps and phono stages would have sound like had they used the 6922 variety specially when 'tube rolled' since I heard the 6H30 tube does not have a direct tube substitute type.
Cathode deterioration is a very quirky process, quite nonlinear, and to test you need to place the tube under load. Those old tube testers Sparky described did a much better job than the last of the breed (the dime-store testers). Dynamic testers (the equivalent of a transistor curve tracer, another thing you rarely see outside a fab or specialized test house these days) have pretty much faded away except for very specialized applications (I bet ARC has a few!)
I have a few gov't reports in my files, but they are buried deep in the pseudo-tame black hole masquerading as a basement storage room... There were some tube handbooks from companies like RCA that held a wealth of "lost knowledge". I just reached over to grab mine, but that shelf is now filled with woodworking books (another fun, expensive hobby for which I have no time ). Undoubtedly now a victim of that durn black hole. There is quite a bit of info on tubes in several textbooks from various undergrad and grad classes I took way back when, and my Reference Data for Radio Engineers handbook (Sams/ITT, 5th ed, was a bright shiny replacement for my old one when I first bought it back in the 70's) has a lot of information about tubes and tube circuits.
I have a few gov't reports in my files, but they are buried deep in the pseudo-tame black hole masquerading as a basement storage room... There were some tube handbooks from companies like RCA that held a wealth of "lost knowledge". I just reached over to grab mine, but that shelf is now filled with woodworking books (another fun, expensive hobby for which I have no time
). Undoubtedly now a victim of that durn black hole. There is quite a bit of info on tubes in several textbooks from various undergrad and grad classes I took way back when, and my Reference Data for Radio Engineers handbook (Sams/ITT, 5th ed, was a bright shiny replacement for my old one when I first bought it back in the 70's) has a lot of information about tubes and tube circuits.
Cathode deterioration is a very quirky process, quite nonlinear, and to test you need to place the tube under load. Those old tube testers Sparky described did a much better job than the last of the breed (the dime-store testers). Dynamic testers (the equivalent of a transistor curve tracer, another thing you rarely see outside a fab or specialized test house these days) have pretty much faded away except for very specialized applications (I bet ARC has a few!)
I have a few gov't reports in my files, but they are buried deep in the pseudo-tame black hole masquerading as a basement storage room... There were some tube handbooks from companies like RCA that held a wealth of "lost knowledge". I just reached over to grab mine, but that shelf is now filled with woodworking books (another fun, expensive hobby for which I have no time
Thanks Don,
I have some of the RCA books - a very used Receiving Tube Manual is on my desk - and happily my library still has the US Navy tube courses, full of useful information about tubes. Unhappily, all I could find about tube life was an indication that tubes that measure on the red part of the meter should be replaced.
The tester I referred is a real dynamic tester - we cheated as we emulated some of the circuits using a National Instruments myDAQ usb unit.
Did you hear about cold cathode testing (testing using the filament at a lower temperature than nominal)? I am curious about this aspect.
Some of the better testers like the Hickock 539C had a cathode life test where it drops the cathode voltage by .5v if memory serves me correctly and it measures how much emission drops off. As I said before, in a perfect world, all vacuum tubes would die a natural death from cathode depletion. Many tubes die for other reasons or become unusable due to noise or microphonics. Other causes are micro leaks that cause the tube to gas up and shorts.
Happily! My system has 20 x 6h30. Can you imagine "rolling" them?
Hehe. I know how you feel. I had the same feeling when I had an ARC SP10MKII - 12 to 15 tubes I think. I sold it in 'fear' of the retubing cost. I should have been more brave then.
Since were talking about tubes and tube testers whats the best availble tube tester available in the market today.Is Amplitrex good enough compared to the vintage testers mentioned in the threads above?
tdh888
tdh888
Hehe. I know how you feel. I had the same feeling when I had an ARC SP10MKII - 12 to 15 tubes I think. I sold it in 'fear' of the retubing cost. I should have been more brave then.
I knew a reviewer who had the cj ART preamplifier and replaced the stock tubes with 10 Tele CCAs. You can calculate the cost
Ouch!
I think probably one of the best was the old Tektronixs curver tracer. Very few made and very, very expensive in its day. Exceedingly hard to find on the used market; Larry Smith, who used to design Perfectionist Audio Components preamplifier, used to have one. (old timers may remember his preamp--that I don't think is made any more
http://home.earthlink.net/~idos/One that I found personally useful but impossible to find on the used market was the George Kaye tube tester. Tested small signal tubes in a real Class A circuit.
The Amplitrex seems very good but I think it runs around $2500.
Another one that had caught my eye is the Maxi-matcher. The output tube tester in particular interests me!
http://www.maximatcher.com/
Interesting. Wonder if it will do all tubes given it was made in 57. For instance, my Hickok doesn't do 6550s but was able to figure out a setting with the help of a tube friend.
Yes, but I honestly remember very little about it. It was supposed to be the up and coming next best thing, but I think transistors started taking over about then and I am not sure the final determination. (Of course, it could have been researched for years before I noticed or heard about it.) There was quite a debate if it could really model the degradation, but it seemed to provide a reasonable approximation without stressing the tube (as much). I was working for a radar firm and it came up as a way to test tubes in-situ, with the idea of predicting failure while the tube was powered down and potentially switching to a back-up tube (which was also tested in-circuit while substantially powered off). Funny thing about pilots, they got very testy if the radar failed whilst tracking an incoming missile...
Aside: The old DW tester was great but a pain to use for the average Joe... I loved it, of course, just for the geekiness of it.
HI,
Yes, all the comments above are true. The old tube testers, Hickox in particular, were very good. I have seen them on ebay in a refurbished form. While expensive, they are not out of reach for someone who has managed to afford ARC.
I made a point of collecting all the tube manuals I could lay my hands on. I have a good collection. At one point VTL published a white paper that included some of the best information for tubes used in VTL equipment I have seen. These included types that were also commonly used by other manufacturers such as 6922's, KT88's, and 6550's. If you can find one I suggest you latch onto it.
All of this points to a problem. Moving into tube equipment involves more than simply buying an amplifier. One needs a support system. This used to not be a problem. But now it certainly can be. Many folks, new to tubes, are shocked to find that, to some extent, they are on their own. Tube equipment requires more TLC than, for example, Krell. It is also helpful to have a technical background. Many tube users are completely helpless when their expensive tube products bestow their inevitable problems.
I know I am biased (no pun!) but I think buying relatively inexpensive and unknown tube equipment is a gamble only the inexperienced are willing to take. Experience will teach that buying a well known name such as ARC, CJ, or VTL, a few others, will pay off in the long run because of the excellent support provided by those manufacturers. Also, these companies tend to produce very reliable designs. For many, the factory is the only reliable support system available.
Sparky
Yes, all the comments above are true. The old tube testers, Hickox in particular, were very good. I have seen them on ebay in a refurbished form. While expensive, they are not out of reach for someone who has managed to afford ARC.
I made a point of collecting all the tube manuals I could lay my hands on. I have a good collection. At one point VTL published a white paper that included some of the best information for tubes used in VTL equipment I have seen. These included types that were also commonly used by other manufacturers such as 6922's, KT88's, and 6550's. If you can find one I suggest you latch onto it.
All of this points to a problem. Moving into tube equipment involves more than simply buying an amplifier. One needs a support system. This used to not be a problem. But now it certainly can be. Many folks, new to tubes, are shocked to find that, to some extent, they are on their own. Tube equipment requires more TLC than, for example, Krell. It is also helpful to have a technical background. Many tube users are completely helpless when their expensive tube products bestow their inevitable problems.
I know I am biased (no pun!) but I think buying relatively inexpensive and unknown tube equipment is a gamble only the inexperienced are willing to take. Experience will teach that buying a well known name such as ARC, CJ, or VTL, a few others, will pay off in the long run because of the excellent support provided by those manufacturers. Also, these companies tend to produce very reliable designs. For many, the factory is the only reliable support system available.
Sparky
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