For years this section of my website listed thermionic valves for sale and I still have hundreds for Sale ~ They are mainly British or US made Vintage parts which are tested to the Manufacturers or AVO 163 specification before shipping ~ They are either New Old Stock or lightly used and may have logos rubbed due to handling and may not be in the manufacturers original boxes but they are all good quality items

Refreshing the listings and trying to keep them in line with ebay and other crazy prices was a thankless task so I now ask ~ If you need a particular valve please contact me and I will check what I have and may be able to match the type and style of the one you need to replace if you send clear pictures of the valves you want to match

~ When Correctly Tested ~ VALVES DO NOT TEST STRONG ~

When measured using the correct method and reference data valves should not be described as testing strong or more than 100% ~ This suggests that the valve is conducting more anode current I_a than it should for the specified negative control grid voltage V_g1 and indicates that the static gain is out of specification

At higher anode current I_a than specified the gain gm will also be higher but this greater than 100% mutual conduction may not be achieved in practice with auto–bias or even desirable for some designs ~ Valves sold as testing more than 130% should be considered out of spec especially if I_a >100% with gm <100%

According to most manufacturer and valve meter data a correctly measured gm of as low as –50% is still considered GOOD

There are valve test conditions other than I_a and gm that also need to be tested which include cathode to heater insulation CH and positive grid current due to GAS or an imperfect vacuum ~ These measurements need to be made after some time when the valve is fully hot and are often overlooked so when purchasing from places like ebay you should check that they have been correctly measured

The AVO 163 Valve Characteristic Meter provides for measuring the screen grid current I_g2 and its gain gm_g2 ~ This measurement often proves very useful and can highlight problems with modern copies of classic valves like the PM Components retro look KT66 where the g2 current and gm_g2 are almost non existent due to bad alignment between grids g1 and g2 which prevents these and some other tetrodes KT66 from performing correctly ~ especially in circuits like the QUAD II and some 'Ultra Linear' designs but they are often okay when triode connected

When testing valves like a CV5112 that have high gm >10 it is often difficult to get meaningful readings with most valve testers and some give over optimistic results ~ For high gm valves and also other valves the most meaningful test is the gm measured at a fixed and stable I_a ~ When the specified I_a is achieved at a low V_g1 as occurs with high gm valves ~ the V_g1 value is not important and in practice these valves should be run with constant anode current and in their original use designs like telephone repeater amplifers thay were

For an explaination of the codes used by some valve manufacturers see here

Dynamic testing and comparison of valves using a TEK 576 curve–tracer

Testing with a Tektronix 576 or similar will often show differences in valves that are not obvious from the single point I_a and gm testing available with a standard valve tester — It may also show that many modern equivalent valves do not follow the characteristics of the original items which may explain reported differences between NOS valves and some modern replacements

See also