Tube Rolling – Getting The Sound You Want

Valve or Tube Rolling is the process of trying out a number of valves in the same spot in an amplifier and selecting the one that sounds best to you. This can be very useful in optimizing the overall tone of the amplifier.

The appeal of valves, other than the ease with which they can be swapped out of a circuit (tube rolling), is that they often create a sound that seems more “relaxed” and “untiring” in many amplifier designs. It is hard to say why they sound so different to their solid state counterparts. Some experts claim that the distortion characteristic of audio valves (while typically higher in absolute terms than solid state) at even order harmonics vs odd order harmonics of solid state are easier on the ears (It is suggested that our ears are more sensitive to the odd order harmonic distortion which can show up in higher frequencies and can result in listener fatigue). Valves might sound better in a given circuit because of the design simplicity of the electronic circuit itself. In general valve circuits are fairly simple with relatively few components as compared to quite complex solid state circuits using a myriad of tiny components.

The same type of valve made by different manufacturers and in different variations from one manufacturer can have a definite impact on the actual sound of an amplifier. The most common and easiest audio valves to “roll” are the preamp tubes. Audio preamp valves are self-biasing and so generally no technical adjustments are required when they are installed.

On the other hand power audio valves will in most cases require resetting the bias after tube rolling. Also, you will want to change the whole set of power tubes at the same time. Some amplifiers have no bias adjustment. If you want to “roll” the tubes in these amplifiers, you order matched sets. Some amplifiers that use either 6L6 or EL34 tubes often have a switch to change the bias range between these tubes.

As you can see, tube rolling can give new life to an ordinary sounding amplifier. It is also the most logical way to try to find that custom or elusive sound you have been searching for.


Valve equipment runs on dangerous high-voltages and the tubes get very hot during operation. Equipment should always be switched off and unplugged from the wall power outlet whenever venturing inside the cabinet or chassis. It is also a really good idea to let valves cool for a few minutes before removing them – they can get very hot and can burn you.

Obviously some technical knowledge or advice is required before contemplating changes to the configuration of any electronic equipment. Some valves have subtle differences between them and what may appear to be a straight swap can become a nightmare for the unwary. Do your homework first!

If you feel uncomfortable trying this yourself, enlist the services of a competent technician who can physically inspect the amplifier and ensure that what you are proposing is safe and within the proper design parameters of the circuit! Remember there are voltages inside that can kill you. Even when the power is disconnected there can still be dangerous voltages around as energy remains stored in electrolytic capacitors for a time.

In the picture:

Some of the nicest we have are these E88CC’s / 6922 double triode audio valves with diamond bottom.
This 10,000+ hour tube is known as the HOLY-GRAIL for excellent performance, smooth highs, outstanding 3-D holographic soundstage, lots of open-air, neutral timbre, true to life un-colored sonic reproduction, magnificent inner resolution and detail, and overall perfect resolution and balanced sound. Improved dynamics, smooth frequency extension across the spectrum are immediately obvious, making the music sound much more accurate and real. A major improvement over other 6922 E88CC, these have been voted one of the “BEST” 6922’s ever made!

Valve Testing – Meaningful Measurements for Vacuum Tubes

I talked a little about measurements of valve “quality” in a previous post, but as the result of a few discussions about valve testing this week I think its worth a further post.

There certainly are a lot of numbers thrown around by vendors of vacuum tubes when they advertise valve testing results, and the challenge is to sort out what is a meaningful figure and what is not. It’s not always straightforward because there is no guarantee that the starting point is the same and different standards often seem to be applied. Not only that, but different valve testing equipment can and often does give significantly different results.

As an example, significant differences in measures of transconductance can arise due to many reasons, if a measurement is actually given at all.

  • Firstly the valve tester has to have been calibrated at some point in the not too distant past.
  • The tester has to be operated to given standards and in a consistent manner.
  • The original factory ratings and specified operating conditions (voltages, current, bias) for the tube should provide the benchmark for the results. These ratings are not hard to find.

For some folk a “tested good” valve is simply one that “goes” in the amp or radio, there is no actual measurement of anything against factory specifications here!

Alternatively, if a valve is simply quoted as x% good then what is that a measure of and against what standard?  For example –

Some testers use a red and green scale for bad/good or weak/strong. What is the exact value at the point of transition from good to bad and where did that number come from? Chances are it’s an arbitrary value determined by the manufacturer of the tube tester, based on around 60% or so of the tube manufacturers rating for the factory new tube. So what is a weak tube and what is a strong one and compared to what? Wouldn’t it be better to have an actual measurement against the manufacturers original rating? eBay is a great example where you can see valves offered measured at 105% or 120% etc but we are often not told what that is measured against. More often than not, it is not a comparison with factory new specification. If you think it’s confusing and at worst misleading then I have to agree!

In summary, I believe that you should know how these measures are determined and against what standard, before handing over your hard earned $! Remember the only stupid questions are those that are not asked. So, if in doubt, ask the vendor and they should be happy to explain further.