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The limits of the “bucket method” for power tube matching

What is bucket matching?

If you receive a set of tubes that have exactly the same readings (rounded off to the nearest 100 for gm, for example) then you probably have a set of “bucket matched” tubes. By “bucket matching” I mean the practice of grouping tubes into rounded off groups or “buckets” like “57 mA and 5400 gm” (perhaps for a 6550 tube type). Tubes that test closer to 57 mA than they do to 56 mA or 58 mA fall in this bucket and likewise for gm. Some sources use even larger “buckets” than this with a single number referencing the whole group. When someone orders a matched set of tubes, the reseller just reaches into one of the buckets and grabs enough tubes to make the set.

Before I explain the downsides of this method, I should say that bucket matching makes sense for tube distributors due to the huge number of tubes they process. Passing along the distributor’s “factory matched” tubes also makes sense for many resellers, especially if their emphasis is on volume. The $1/tube upcharge resellers pay distributors for factory matched tubes may seem like a good deal when compared with owning, maintaining, and correctly using equipment to burn in, test, and match the tubes themselves. Besides the expediency factor, these matches generally work out okay most of the time. After all, most tube audio gear is usually pretty forgiving of poorly matched tubes.

But for those who want to minimize early tube failure and get the most out of their equipment, improvements can definitely be made. The benefits of more precisely matched power tubes include longer lasting and better sounding performance. Well matched tubes share the load equally and avoid uneven wear that can cause premature failure. Well matched tubes will also make the most of your amp's power and do a better job of canceling distortion and noise, especially in push-pull configurations.

Bucket matching drawbacks

Here are a few problems with the bucket matching method:

1. There are good questions to ask about the buckets themselves. What is the range of test results that is being divided into buckets? How many buckets does it take to cover the range? Are the tubes burned in before they are tested and put in their buckets? Do the buckets include both mA and gm scores? The answers to these questions vary depending on who sets up the system. In our experience, test results on new production power tubes can range from 65% to 160+% of their expected values. We reject tubes that test at the extreme ends of the spectrum because they are likely to create problems. Not everyone does this. For this reason, I’d be cautious of buckets that seem to represent one end of the spectrum or the other.

2. Bucket matching scores do not allow the end user to refine the matching by putting groups into sensible subsets. For example, if I purchased a set of 8 matched 6550 tubes that all have the scores 57 mA and 5400 gm on the boxes, I have no idea how to break those eight tubes down into two groups of 4 which is often what needs to happen for a stereo amplifier. More precise results allow the end user to make better decisions about installation.

3. The operating conditions under which tubes are matched using the bucket method is usually unknown. This matters because tubes are not always linear in their response to different operating conditions. For example, tubes that are matched with 250v on the plate, 250v on the screen, may not remain matched at 500v on the plate and 300v on the screen. Most of the tubes in the group may track pretty well with each other but often one or two of them will deviate significantly. Tubes that run higher than the rest of the set may draw more current than they can effectively dissipate, causing them to red plate. Using power tubes that are matched at the actual operating conditions in your tube equipment is the best way to prevent this failure.

Precision matching vs. bucket matching

If you want the most out of your tube audio gear, all you need to do is learn some basic information about its operating conditions. You do not need to be technically inclined to do this, simply take your amplifier to a trusted technician and find out the following information.

What is the plate voltage?

What is the screen voltage?

Compare these values with the standard matching specs in the descriptions for new production power tubes posted on our website. We base these test specs on typical operating conditions listed in tube manufacturer’s manuals. If your voltages match these, great! You can be confident that our matched set will be a great fit for the operating conditions in your equipment. If your voltages differ from the specs we use for standard testing and matching, you may want to consider our custom power tube matching service. Feel free to contact us if you are not sure.
TC Tubes specializes in precision testing and matching of vintage and new production vacuum tubes for guitar & bass players, audiophiles, and DIY tube audio enthusiasts. Our website features secure payment options and fast, convenient shipping around the world.

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