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When is a 6as7 not good? What measurement makes it not good anymore? Driver tube end of life is easy to determine, but power tubes aren't to me. Besides a shorted section. If sections are not shorted, can a 6as7 still be bad? Audibley bad? Is plate voltage a good indicator of whether a tube is toast? For instance, if the two sections of a 6as7 are dramatically different, does that result in poor performance?
Follow Ups:
> When is a 6as7 not good?Let's start with the easy ones:
- Internal shorts
- One or more dead filaments
- One or more blown internal links
- Loss of vacuum (gassing)
- Fractured envelope> What measurement makes it not good anymore?
Without getting into the really wooly world of tube testing (and tube testers) there are two main quality factors:
- Emission (cathode)
- TransconductanceFor most small-signal & power tubes, transconductance is the benchmark of quality. The 6AS7 is a special case however, due to its' high transconductance and low Mu. This combination of characteristics creates a problem for transconductance tests; most testers (even the Hickoks..) don't have enough resolution or accuracy (mainly on the control side, not the meter side) to give a meaningful transconductance figure for a 6AS7.
If we need to test an otherwise working 6AS7 at the factory, it's done on a 1940's era Philco emission-type tube tester. This tester has a simple pass/fail readout - if in doubt, we compare meter readings to a known-good (new) tube. This approach has never failed us.
Bill
Thanks for the reply, Bill. For those that don't have access to what you have in the factory bench, it's more difficult isn't it?Maybe I can rephrase my question. If I measure the 6as7 plate voltages directly in my amps, and get readings that look OK (like 50 - 80 mV), does that mean my tubes are perfectly OK? Or it is possible that even with the measurements tubes may in fact not be good (transconductance)? The thing is my tubes basically all measure ok but I don't think the performance of my amps is good and the only thing I can think of (short of something amiss with circuit components) that would cause this is the output tubes. My driver tubes were all recently tested with a good tester and are excellant. By process of elimination I suspect power tubes. But the plate voltage measurements don't tell me thats the problem. And before I spend cash retubing I'd like to know if its possible that something like transconductance could be off in a tube but plate voltage measures ok. I hope what I'm asking is clear, and there is a yes/no answer!
Bryan
> Thanks for the reply, Bill. For those that don't have access to what you have in the factory bench, it's more difficult isn't it?If by 'the factory bench' you mean the old Philco, then I guess the answer is yes. This said, a simple emission-type tube tester (like the Philco) can be had for $25 or so at a Hamfest, or on eBay. Noting special about it, just make sure that the 6AS7 / 6080 is listed in the tube data for your tester - these tubes require 2.5A @ 6.3V for the filaments, and some cheap testers just don't have enough power supply for them.
When referring to 'plate voltage', it seems that you are actually referring to cathode current, derived from voltages measured across the cathode resistors? In this case, 50-80mA per section is pretty much nominal, and gives a good indication of the tube's DC characteristics.
Yes, it is quite possible for a tube to show good emission and DC characteristics, and still have poor transconductance. However, this is less likely with a simple triode than with a more complex (multi-grid) tube, such as a pentode.
I just had an idea; suddenly, I'm getting the itch to do a little bench work on this.. if I come up with any interesting results, I'll post them. For now, it's just an idea. I'll see how it pans out.
> > Yes, it is quite possible for a tube to show good emission and DC characteristics, and still have poor transconductance.If one were to have a few tubes that do have poor transconductance in their M60, how would that manifest itself in what I hear? Would it sound pretty bad? Would it sound like noise? Distortion? Screwed right up?
THanks,
Bryan
Bryan, Since you have 8 X 6AS7 sections per phase (two per tube), it seems unlikely to me that one or two bad sections would seriously affect your listening pleasure. More than once I have noticed that my own MA240s sound just a tad "off" but still excellent and then found out that the cause was due to blown fuses on one or up to three out of six of my output tubes. In other words, it's surprising how subtle the differences are when one or a few output tubes are not doing anything at all, let alone just losing transconductance. All of this is to say that I would take a hard look at your input and driver stages. Check the coupling caps too.
Thanks for the advice, Lew. I'm going to mail off my 6as7s to a friend and he'll have a listen to them and we'll see if he thinks they sound bad. I have experienced sections go on me and heard what that sounds like, but this is different. Could be what I'm hearing has nothing to do with the power tubes. If thats the case, I don't know what I'm going to do. I can't troubleshoot myself, and I don't want to ship my amps from Canada to the US.
> I'm going to mail off my 6as7s to a friend and he'll have a listen to them and we'll see if he thinks they sound bad.As several others have mentioned, you would need a set of desperately bagged-out tubes to notice any real shift in character. You might notice a loss of dynamics & overall power, but to get a change in tonality, mushy distortion, etc.. things would have to be well beyond just worn-out.
I tend to agree; there is likely something else going on here, if the amps are sounding truly 'bad'.
Bill
If you knew enough to measure the current through your tube sections ("50 to 80mA"), then perhaps we can talk you through checking the input stage. Obviously there is some danger involved due to proximity to high voltages, but if you keep one hand in your pocket at all times while taking measurements and do nothing else silly, then you would be OK, if you have a VOM. First, is the "problem" identical in both amplifiers? Have you checked out your preamp and/or other front end gear to be certain that nothing is wrong with them?
Hi Guys !!After using Atma-sphere OTL's for a number of years I think that I have become so spoiled to listening to a super transparent, smooth musical signal that when it changes for some reason we noticed it right away. I have chased my tail trying to find solutions to all sorts of noise and anomalys and when the day was done it was mostly due to bad line voltage. The noisy AC line can cause us to notice all sorts of things with these OTL's. I have noticed that when the AC line voltage is relatively clean that I hear a very smooth and dynamic presentation( ie. very late at night or early am like one or two in the morning). However, when the AC is dirty the signal becomes hashy and sluggish and sometimes hard and brittle. So, what's the solution ???? Well---pick your poison and buy or try the myriad of different types of power line conditioners. I have not bought one yet because the majority of the time my amps sound damn good. When I first bought my MP3 I had a annoying buzzing sound from my tweeter. I bought a couple of Quite line AC filters (the cheap little black boxes) that you plug in to your AC outlets. The buzz went away completely----so I came to the conclusion that allot of the problems we have is because of this dirty AC. I am going to buy a good isolation transformer and see if it it helps. Anyway---that's my 2 cents worth.
I also forgot to mention that all of the stuff( computers,halogen lights,digital components, etc)that we have plugged in at our houses really pollute our AC lines. I have noticed that even with dedicated lines the pollution still spills over into these lines. Maybe, it's the RF or EMI. I really don't know because I am not an EE. This really is apparent with Halogen and flourensent lights.
RF and EMI are issues, but one of the major problems caused by what we call 'electronic loads' (computers, audio equipment, flourescent lights) is distortion of the AC waveform.. When the AC waveform is a perfect sine, all is well, but when it becomes distorted, you not only have additional line noise to squash, but the *power factor* of the line falls off.It's very possible to read "120 volts" at the outlet, but if the power factor is off due to waveform distortion, the actual power that the line can deliver to the load can be considerably lower than expected.
Interesting to see that you feel dedicated lines are not the answer at least to AC noise generated in your own home. I have found with four dedicated 20-amp lines powering my audio system that my system sounds just about the same at any time of day, i.e., darn good. Have you checked to see whether the ground side of your dedicated AC line(s) is/are isolated from "house ground". My dedicated lines are grounded to separate copper poles imbedded in the ground (literally) outside my house, well away from house ground. Perhaps this makes a difference.The various methods for cleaning AC are all meritorious in their own ways, but one must be careful with large tube power amps, which consume power in big gulps, that the device, whatever it is, does not delimit the current available for the amplifier. Brian's problem seems more severe than that which we normally attribute to AC grundge. For one thing, he hasn't said that it's time-of-day dependent. If you get an isolation transformer, make sure it's industrial size. Overkill always makes us audiophools feel good.
Hi Lew,I do believe that dedicated AC lines and grounds are better than undedicated---However, they are not an end all answer to noisy AC. I do have dedicated grounds and I would probaly have more noise than I do now if I had the opposite.I would presume that every house is different in the amount of noise and grundge but I do know that late at night (10 pm to 2am) that my equipment sure sounds allot better--so the only conclusion is that the AC must be influencing my equipment.
Later
Jack
The 6AS7G being the beast it is, its rare for the tube to just fade away. It seems that the cathode coating starts falling off of the cathode as the tube ages. This is what causes the tube to arc over, and is often the best sign that the tube has gotten weaker. In later amps (made in the last 3 months only) the tube can actually survive this, but in older amplifiers usually the fuse link in the tube fails and that is the end of the tube.This being the case, there's not really been much need to test such tubes as they are obviously bad. Before they arc over, it is unlikely that the amplifier will exhibit any outright signs- no obvious distortion, noise or stuff like that. IF a tube is not lighting up, if it is glowing red, or there is an indigo glow deep in the middle of the tube, some attention is required, but otherwise there are no worries.
You say you don't think the performance of your amps is good. Does that mean they sounded good once but not now? Or does it mean they have never sounded good? I lack the equipment to check and match tubes, but was faced with the reality my M60's did not sound 'magical' any more, only just good. I bought new tubes and listened to the old ones as long as I could, then put in the new ones as a Christmas treat. There clearly was a difference.If your amps sounded really good at one time, it would do no harm to try new tubes. It would not be a waste of money because you'll need the new tubes sooner or later. Just my non-expert opinion.
Right now prevailing philosophy seems to be the amps are the test bed for the tubes: use statistical swapping to match them and playing music to determine quality. That philosophy seems to extend to the brand of tubes also. The more expensive Svetlanas are said to offer only better failure rates than the cheaper identical-looking Sovteks -not better sound or better matching.
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