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.
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!
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.