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Contact Form Issues


Between June and August 2019 I have had some problems with receiving messages sent via the website. Hopefully these are now resolved and I apologise for the inconvenience caused to anyone who has not received a response from me over this time.

Valves with Glowing Blue

From time to time I’m asked about the different colours that appear inside the warmed up valve, especially valves with glowing blue colours – are they an indication that all is well, or is there in fact a problem with the tube?

For purposes of this post I’ll break the types of emitted blue colour glow into 3 general categories, which were all best described by the manufacturers Sylvania back in the hey day of classic valve manufacturing. The following was written about valves with glowing blue by their manufacturing engineers at the time and still applies today –

“Blue Glows are not tube detriments per se. They are, however, suspects in the eyes of many receiving tube users for lack of a full understanding of their origins. There are several types of Blue Glow which can be described as follows:
MERCURY VAPOUR HAZE – is a blue-violet glow associated with those tube types which rely upon mercury vapour for proper operation. In such cases, the blue glow should be evident indicating proper operation. (Note that in common audio amplifier applications Mercury Vapour tubes are relatively rare these days)

FLUORESCENCE – this type of glow is usually violet in colour and most noticeable around the inside surface of the glass bulb. It is most pronounced on power tubes and is the product of electron bombardment of the glass taking place within the tube. It generally has no adverse affect upon receiver performance, and in fact, tubes displaying this phenomenon are particularly good with respect to gas content.

GAS – produces a blue haze, generally confined to the vicinity of the mount structure. The proper function of gas types such as thyratrons, voltage regul

ator and voltage reference tubes, requires the presence of this glow as an indication of proper tube operation. Some voltage regulators use neon instead of argon and as a result exhibit a pink-orange glow. It is, however, a distinct detriment in vacuum receiving types, where the presence of gas in large amounts can cause malfunction of the equipment.”

So there we have it. Most valves with glowing blue that you see are quite ok and nothing to be particularly concerned about in audio amplifier applications. Presence of a coloured haze in a receiving tube could however indicate a problem and can cause equipment failure.  In general the blue glows to watch closely for are those hovering around a wire or single element which could be an indication of air getting into the valve and these valves must be replaced. Air leaks can develop around the contact pins in the base of the valve or through a defect or small break in the tube itself.

It started with an Ekco

It was about 1965 when I first became hooked on all things radio and valve related. My grandparents owned a classic 1936 Art Deco style Ekco SW86 (of which I soon became the owner – but regrettably I no longer have this radio) like the one in the photo and I quickly discovered the joy of the medium and short wave bands and the electronic vacuum tube wizardry behind the radio dial.

Fast forward 50 years, add many hours with the soldering iron in hand constructing a variety of electronic gizmos on a hobby basis (mainly amateur radio related) and the mystery and passion is still there. Additionally the valve and equipment collection has grown.

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.

The Ubiquitous 12A*7 Family

Here are 5 audio pre-amp valves with just one letter in the nomenclature separating each one from the other. The one letter might be the difference in the name but they are very different in terms of characteristics in the circuit. They have quite different gain factors. They are all still in popular use in classic gear and new equipment where the vintage audio valves manufactured in UK, Europe or USA in the 50’s or 60’s are often greatly preferred over modern copies because of the way they sound.

Here’s the 12A*7 family together with alternative names for each audio valve. Sometimes the alternative tubes listed alongside each will have minor characteristic differences but in terms of gain this wont really be significant. Normally the alternative tube can be safely substituted for the 12A*7 but there can be exceptions – as should always be the case with any audio or radio valve substitution its best to check with a technician before proceeding and so avoid tears afterwards! Remember that your valve amp was originally designed for a specific tube so always get technical advice and exercise care when substituting valves.

12AX7 – ECC83, 7025, ECC803, E83CC, 6681
12AT7 – ECC81, 6201, 6679
12AY7 – 6072
12AV7 – 5965
12AU7 – ECC82, 5963, 5814, 6189
Now here’s how the gain factors compare if bench-marked against the tube with the highest gain, the 12AX7.
12AX7    100%
12AT7    60%
12AY7    45%
12AV7    41%
12AU7    19%
So in terms of gain factor it’s important to recognise what the effect of swapping between 12A*7 tubes will do within your application. It will result in either the over production or underproduction of signal which means too much drive (too loud with distortion) or not enough drive (too quiet).  We repeat again that swapping between these tubes should never be conducted without the  technical advice that it is OK to do so.
We carry a range of these classic tubes at various price points. Matched pairs always available ex stock. We will do a future blog about matching sections and matched pairs (two different things) – what’s the difference and why both might matter a lot in your audio application!

Meaningful Test Results for Valves

We are often asked what are the most meaningful test results when it comes to Vacuum Tubes or Valves?

That’s a good question as there seem to be as many different ways to report “results” as there are people selling valves. While it’s common enough practice to indicate the general state of a valve by using terms like “new, near new old stock (near NOS) or new old stock (NOS)” it is certainly not best practice to report valve test results in the same manner. Heres why – 

  • The first test on any valve is to identify any shorts and significant internal element leakage. We do this prior to testing the quality of the device because if these conditions exist the tube is essentially unserviceable and it is safely disposed of.
  • The next test will give us the best indication of the valves quality. Its the measure of Mutual Conductance (Gm) on all tubes other than voltage rectifiers and diodes. In addition a gas test is carried out (When testing rectifiers and diodes there is no such thing as a Gm test).
  • All valves (new or used) will exhibit variations in the observed Gm reading for a number of reasons. The brand, type and calibration reliability of the Valve Tester will also have a very significant influence and for this reason it is very important that the measurement is done using a recently calibrated machine. Some Valve Testers will only measure emission – this is a fairly unreliable test, it does not give the complete picture and it can also vary considerably between Testers.   
  • Many years ago standards were established and documented to determine the acceptable range of measurements for each tube. So if a particular tube has a rated minimum Gm of say 650 and we test it at say 870 that’s good. Its a quantitative measure having factual basis and is not a vague representation such as simply saying the quality of the valve is new, near NOS or NOS. 

When we buy a car we care about and usually take note of the mileage it has done – that’s quantitative data because it has been measured and recorded. The same applies to the measurements we make when testing valves.

We use the latest model Amplitrex 1000 Vacuum Tube Tester to check many of our valves, give us assurance of an accurate result and to provide a complete set of quantitative data in electronic and/or printed form. 

A Hand Crafted Vacuum Tube!

If you have not already seen this You Tube clip its really worth a look! Its amazing what some clever people can do.

Apparently this man’s name is Claude Paillard. He is a French amateur radio operator and his website is –