From the KR Audio web site
DEBUNKING SOME POPULAR HiFi BUNKUM
MESH PLATES
Sometimes referred to as mesh bases (sic). Output triodes with plates formed from wire mesh material seem to be the latest hot item in tube HiFi.
Mesh plates are nothing new, in fact they were first introduced in the mid to late 1920s both in the USA and UK/Europe, however they were only ever used on low power (by present day standards) tubes.
Typical examples were the 227 in the USA, the MH4, ML4 range from MOV in the UK and 354V and 904V from Philips/Mullard. With regard to the 227 examples from the following manufacturers have been seen. KenRad, Majestic, Sonatron, RCA and Van Horne as well as tubes marked Fada and Philco which were probably rebrands made by other manufacturers. In the UK the MH4 and ML4 are seen with both Marconi and Osram labels. Western Electric made a 244 with a mesh plate. All these tubes have balloon shaped envelopes.
By the early 1930s the practice had been discontinued although the MH4 and ML4 are occasionally seen with an ST envelope and mesh anodes.
So why mesh plates today? Well the first answer has to be ease of manufacture.
It is obviously much simpler and therefore cheaper to form a sheet of mesh material into the required shape than solid metal. There is also a visible benefit in as much as the filament can be seen glowing inside the plate, which excites some HiFi enthusiasts.
The claim is that these mesh plate tubes sound better but there is absolutely no physical or electronic reason why this should be so. In fact using mesh plate construction on a power triode has to be a serious disadvantage for a number of reasons. Chief of these is plate dissipation.
In conventional power triodes most of the plate heat is got rid of by radiation through the glass envelope, very little heat is lost by conduction. Consequently, if the material is full of gaps as in the mesh plate it will run hotter than a solid plate of the same overall surface area when dissipating an equal amount of power. Furthermore if the plate runs hotter some of the excess heat is radiated back to the grid thereby heating that electrode with the resultant risk of grid secondary emission. It is interesting to note that none of the original power triode manufacturers ever used mesh plates. They are never seen on the original WE 300B, RCA 2A3 or MOV PX25.
Another disadvantage of the mesh plate must be the gaps themselves. The gaps are orders of magnitude larger than the electrons rushing towards the plate and inevitably some electrons will pass through the plate. Whilst the majority of them will be attracted back to the plate because of its high positive potential these will have taken longer to get there than the electrons captured by the plate first time. This phenomenon could give rise to some unusual transit time effects and although at audio frequencies transit time is not usually a problem it introduces another variable into the tube characteristics, which is undesirable.
The electrons that escape the influence of the plate will eventually reach the inside of the envelope and can build up a negative charge there with a resultant change in tube characteristics a problem avoided in tubes with solid plates.
Finally there is the question of thermal conductivity. The thermal conductivity of mesh material is significantly worse than that of a solid plate. This means that if the emission is not uniform within the tube over the length of the filament (and it often is not) certain areas of the plate will be required to dissipate more power than others and the poorer thermal conductivity of the mesh plate can result in hot spots with a resultant reduction in tube life. This is far less likely to occur in tubes with a solid plate.
To conclude, the mesh plate offers no benefits save to the tube manufacturer and in power tubes may result in undesirable side effects in tube performance and shortened life. There is no reason other than in the ear of the listener why tubes having mesh plates should sound better than those with sheet metal plates and the originators (WE, RCA and MOV) of power triodes never used them in their tubes.
BLUE GLOW
There seems to be a school of thought that imagines that a blue glow inside a power triode is a good thing. Some modern manufacturers of power tubes actually show publicity pictures of their tubes exhibiting a pronounced blue glow.
The blue glow can be caused either by high speed electrons striking the envelope causing the glass to fluoresce or by residual gas within the tube which is ionised by the electron stream. Neither of these effects is desirable but the latter is far more serious. For reasons stated in the comments on mesh plate tubes these can often exhibit the fluorescent glass effect and whilst it might look pretty in a darkened room it contributes nothing to the tubes performance.
The presence of residual gas in the tube is however to be avoided at all costs. When the gas molecules are ionised by the electron stream giving rise to the blue glow, the negative ions are attracted to the plate and become part of the plate current. The positive ions, which are the gas nuclei are the heavy ions and these are attracted to the cathode which they strike with some force and will eventually damage the active coating causing the tube to lose its emission. Tubes with the highest degree of vacuum and therefore no blue glow are likely to have the longest life.
THE RECTIFIER MYTHS
Vacuum tube rectifiers have three major parameters and some secondary considerations. The main parameters are peak inverse voltage rating, maximum rectified current and internal resistance. Generally speaking the rectifier for any particular amplifier will be selected on the basis of the first two, i.e. what voltage and current are required.
The myths are that some rectifiers produce a better sound than others.
How can this be?
In a properly designed Class A amplifier the output valves draw a constant current from the HT supply regardless of the output power, likewise the low power stages should have their HT supplies adequately decoupled and filtered from the main HT. Therefore the load on the HT supply should be constant and stability of HT voltage against load current should not need to be considered.
Provided that there is adequate filtering between the rectifier and the amplifier to remove all the supply frequency components from the HT and sufficient audio frequency decoupling so that the HT supply presents a very low impedance over the audio frequency range the amplifier itself should not be able to see the rectifier. Whatever type is used therefore should make no difference whatsoever to the sound of the amplifier.
There are other parameters to be considered when choosing a rectifier such as the maximum peak repetitive cathode current, which will determine the type of supply frequency filter and the value of the reservoir capacitor. The resistance of the high voltage windings of the supply transformer will also need to be taken into consideration. Whether the tube is directly or indirectly heated will affect the time taken for the HT supply to reach its operating value.
It should be noted that the above comments apply strictly to amplifiers with pure Class A output stages. For Class AB or Class B amplifiers in which the load current varies with the level of sound output the design and considerations for the high voltage supply are far more complex and under those circumstances the choice of rectifier could have some bearing on the sound quality of the amplifier.
DEBUNKING SOME POPULAR HiFi BUNKUM
MESH PLATES
Sometimes referred to as mesh bases (sic). Output triodes with plates formed from wire mesh material seem to be the latest hot item in tube HiFi.
Mesh plates are nothing new, in fact they were first introduced in the mid to late 1920s both in the USA and UK/Europe, however they were only ever used on low power (by present day standards) tubes.
Typical examples were the 227 in the USA, the MH4, ML4 range from MOV in the UK and 354V and 904V from Philips/Mullard. With regard to the 227 examples from the following manufacturers have been seen. KenRad, Majestic, Sonatron, RCA and Van Horne as well as tubes marked Fada and Philco which were probably rebrands made by other manufacturers. In the UK the MH4 and ML4 are seen with both Marconi and Osram labels. Western Electric made a 244 with a mesh plate. All these tubes have balloon shaped envelopes.
By the early 1930s the practice had been discontinued although the MH4 and ML4 are occasionally seen with an ST envelope and mesh anodes.
So why mesh plates today? Well the first answer has to be ease of manufacture.
It is obviously much simpler and therefore cheaper to form a sheet of mesh material into the required shape than solid metal. There is also a visible benefit in as much as the filament can be seen glowing inside the plate, which excites some HiFi enthusiasts.
The claim is that these mesh plate tubes sound better but there is absolutely no physical or electronic reason why this should be so. In fact using mesh plate construction on a power triode has to be a serious disadvantage for a number of reasons. Chief of these is plate dissipation.
In conventional power triodes most of the plate heat is got rid of by radiation through the glass envelope, very little heat is lost by conduction. Consequently, if the material is full of gaps as in the mesh plate it will run hotter than a solid plate of the same overall surface area when dissipating an equal amount of power. Furthermore if the plate runs hotter some of the excess heat is radiated back to the grid thereby heating that electrode with the resultant risk of grid secondary emission. It is interesting to note that none of the original power triode manufacturers ever used mesh plates. They are never seen on the original WE 300B, RCA 2A3 or MOV PX25.
Another disadvantage of the mesh plate must be the gaps themselves. The gaps are orders of magnitude larger than the electrons rushing towards the plate and inevitably some electrons will pass through the plate. Whilst the majority of them will be attracted back to the plate because of its high positive potential these will have taken longer to get there than the electrons captured by the plate first time. This phenomenon could give rise to some unusual transit time effects and although at audio frequencies transit time is not usually a problem it introduces another variable into the tube characteristics, which is undesirable.
The electrons that escape the influence of the plate will eventually reach the inside of the envelope and can build up a negative charge there with a resultant change in tube characteristics a problem avoided in tubes with solid plates.
Finally there is the question of thermal conductivity. The thermal conductivity of mesh material is significantly worse than that of a solid plate. This means that if the emission is not uniform within the tube over the length of the filament (and it often is not) certain areas of the plate will be required to dissipate more power than others and the poorer thermal conductivity of the mesh plate can result in hot spots with a resultant reduction in tube life. This is far less likely to occur in tubes with a solid plate.
To conclude, the mesh plate offers no benefits save to the tube manufacturer and in power tubes may result in undesirable side effects in tube performance and shortened life. There is no reason other than in the ear of the listener why tubes having mesh plates should sound better than those with sheet metal plates and the originators (WE, RCA and MOV) of power triodes never used them in their tubes.
BLUE GLOW
There seems to be a school of thought that imagines that a blue glow inside a power triode is a good thing. Some modern manufacturers of power tubes actually show publicity pictures of their tubes exhibiting a pronounced blue glow.
The blue glow can be caused either by high speed electrons striking the envelope causing the glass to fluoresce or by residual gas within the tube which is ionised by the electron stream. Neither of these effects is desirable but the latter is far more serious. For reasons stated in the comments on mesh plate tubes these can often exhibit the fluorescent glass effect and whilst it might look pretty in a darkened room it contributes nothing to the tubes performance.
The presence of residual gas in the tube is however to be avoided at all costs. When the gas molecules are ionised by the electron stream giving rise to the blue glow, the negative ions are attracted to the plate and become part of the plate current. The positive ions, which are the gas nuclei are the heavy ions and these are attracted to the cathode which they strike with some force and will eventually damage the active coating causing the tube to lose its emission. Tubes with the highest degree of vacuum and therefore no blue glow are likely to have the longest life.
THE RECTIFIER MYTHS
Vacuum tube rectifiers have three major parameters and some secondary considerations. The main parameters are peak inverse voltage rating, maximum rectified current and internal resistance. Generally speaking the rectifier for any particular amplifier will be selected on the basis of the first two, i.e. what voltage and current are required.
The myths are that some rectifiers produce a better sound than others.
How can this be?
In a properly designed Class A amplifier the output valves draw a constant current from the HT supply regardless of the output power, likewise the low power stages should have their HT supplies adequately decoupled and filtered from the main HT. Therefore the load on the HT supply should be constant and stability of HT voltage against load current should not need to be considered.
Provided that there is adequate filtering between the rectifier and the amplifier to remove all the supply frequency components from the HT and sufficient audio frequency decoupling so that the HT supply presents a very low impedance over the audio frequency range the amplifier itself should not be able to see the rectifier. Whatever type is used therefore should make no difference whatsoever to the sound of the amplifier.
There are other parameters to be considered when choosing a rectifier such as the maximum peak repetitive cathode current, which will determine the type of supply frequency filter and the value of the reservoir capacitor. The resistance of the high voltage windings of the supply transformer will also need to be taken into consideration. Whether the tube is directly or indirectly heated will affect the time taken for the HT supply to reach its operating value.
It should be noted that the above comments apply strictly to amplifiers with pure Class A output stages. For Class AB or Class B amplifiers in which the load current varies with the level of sound output the design and considerations for the high voltage supply are far more complex and under those circumstances the choice of rectifier could have some bearing on the sound quality of the amplifier.