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In Reply to: FAQ? Yes! posted by mark on December 11, 2002 at 14:31:04:
OK, here it is again with a little embellishment:FINDING WEAK OR DEAD TUBES IN MA-2 AMPLIFIERS THROUGH SWAPPING
The symptom is inability to achieve zero offset voltage.
There may be a bad driver tube. Test the driver tubes first if you have access to a tube tester. If not, try replacing all the 6SN7s temporarily to see if the offset problem is cured.
The MA-2 has two banks of ten 6AS7 output tubes, one on each side of the chassis. If the amp cannot be adjusted to zero offset, there may be a dead tube in one bank, or you may have a statistically rare combination of mostly weak tubes in one bank and mostly strong tubes in the other. Recall that 6AS7s have fusible links and can be totally dead even though the filament lights up and looks normal. See the FAQ section on the Atma-Sphere web site (http://www.atma-sphere.com/visual.htm) for a pictorial discussion of the fusible link and how to spot bad tubes visually.
There is also variation in emission from tube to tube, so that in any population of tubes there will be some strong ones and some weak ones. Most tube testers available to hobbyists are not capable of distinguishing weak from strong 6AS7s (because these have such high plate current), but they can detect totally dead ones.
Tube swapping is quicker than testing all 20 tubes (Murphy's Law of Tube Testing: no matter what order you choose to test the tubes, the bad one, if present, will always be the last one in the group), but you run the risk of confusion or the chance of having a bunch of strong tubes in one bank that masks the presence of a dead one. Here is a systematic procedure to avoid confusion.
To start, go to the amp with zero offset and, without adjusting the offset control, measure the offset with one tube removed from one bank. Place the amp in standby to remove and replace the tubes, but do not shut down the filaments. It helps to have a digital voltmeter to measure the offset for this kind of testing, but you can use the meter on the chassis. Be sure to use a good pot holder! Note the polarity and magnitude of voltage offset caused by removing the tube. You may want to repeat this with several tubes to get a better statistic of the degree of offset caused by the average dead tube.
On the problem amp, make a note of which polarity of dc offset you have, and the magnitude. It is assumed that you have the offset control all the way over to one end of its travel.
There are several ways to proceed, depending on what you want to do. If all you want is to achieve zero offset, then a binary search pattern is the quickest. A binary search is to swap half of the tubes (see below for how to swap tubes, but shut down filaments for this degree of manipulation) and check for correction. If zero offset still cannot be obtained, swap half of the remaining tubes (three on each side, in this case), check again for correction, and if not corrected, then swap two of the remaining tubes. The last two unswapped tubes contain the problem if the offset still cannot be set to zero. Think Murphy's Law again at this point.
However, since all this is a nuisance, it makes sense to root out the weaker tubes and replace as many as necessary to get a good, healthy population. Swapping the tubes two at a time will allow you to do this.
Swap pairs of 6AS7s across the banks. For the sake of this discussion, imagine the tubes are numbered 1 to 10 in each bank, so we have L1-L10 for the left side bank and R1-R10 for the right side bank on one chassis. You would swap L1 and R1, and record how much the offset changes without adjusting the offset control. Your observations on the good amp will tell you which of these tubes is the weaker (or dead) one. If you adjust the offset control to see if you can achieve zero, then put it back. Put the tubes back in their original positions. Continue with all the pairs in this way. Your notes will now tell you which tube(s) to replace. How many to replace is an economic decision. There should be one or two really bad ones, that got you into this mess, but there may also be a few weaker ones.
Remember to burn the new tubes in for 48 hours on filaments only. It helps at this point, when you are eager to get on with listening, to have a supply of precooked spare tubes handy. Otherwise, you will be tempted to run the new tubes immediately, and have the risk of having to repeat all this in a few days!
Good luck and happy listening!
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Follow Ups
- Re: FAQ? Yes! - Al Sekela 20:11:49 12/11/02 (0)
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