An interesting short article on a very important component.
A valveholder is a simple-looking component, and yet it needs care in design. There are good valve-holders and bad ones, and it is difficult to tell from the external appearance whether a particular specimen is one or the other.
It may at first be thought that a valve holder is merely four spaced sockets held in position by a chunk of insulating material.
This is not quite true. In a good valveholder the first essential is that the sockets should be accurately spaced so that they grip the valve pins and ensure a, satisfactory electrical contact.
There are two main types of valveholder. The more common are those designed to obviate microphonic troubles. Some of you may not be absolutely clear as to what microphonic noises really are.
Well, if you look at the internals of an ordinary valve you will notice that the elements are rather delicately supported on metal pillars and it is evident from a casual inspection of some types that a small shock would tend to produce a vibration of these elements inside the valve.
lf a valve (of a certain type) is subjected to a series of small mechanical shocks the positions of the elements tend to change in relation to each other, and there is a corresponding variation in plate current which is amplified up and produces a howl in the loud speaker. This is one explanation.
The cause of the trouble is usually the detector valve, and it is therefore highly advisable to use a sprung type valveholder for this stage. Other valves can also give rise to microphonic effects and since sprung valve-holders are both cheap and easily obtained it is common practice to use them in all the stages.
They are not essential in the last stage, nor in many cases in the first LF stage. Rigid valve-holders can be employed if they are available.
There is quite a lot in the design of the spring supports in the case of the anti-microphonic types. The chief function of the springs is to damp out the multitudinous small shocks, or in other words, to alter the vibration period. if the springs are too rigid they will not do this satisfactorily, and manufacturers have given considerable time and thought towards producing something that really does eliminate these microphonic effects.
Then, again, there is the question of dielectric losses. The chunk of solid material which supports the grid and filament sockets of a valveholder is, in the case of an HF stage, directly across the tuned circuit, and in consequence any losses will reduce the efficiency of the set.
Now on the short wave-lengths, and even on the medium waves these losses can become serious if the valveholder should be one of an inferior quality. The tuning will be flattened and the amplification cut down. So you see that a component in which a poor composition is employed may very easily impair the working of your receiver. The moral is, donbt buy cheap foreign components. Choose those of reputable make.
While on the subject of valve-holders, there is one point which sometimes causes confusion in the minds of constructors. It is the question of SG valve connections. A number of readers write in from time to time pointing out that our diagrams show the wrong wiring to the screening grid and anode of the HF valve.
A Final Point
Actually these diagrams are perfectly correct, and these readers are not aware that the screening grid of an SG valve goes to the plate pin on the valve base, the true anode of the valve going to the terminal on the top of the bulb.
Recently the special AC all-mains valve has been becoming increasingly popular, and manufacturers have turned their attention to five-pin holders. These are mostly of the rigid type, since the robust nature of the AC valve does not call for a sprung type valveholder.
Here we have the Precision, Lotus, Magnum, Formo and Godwinex ordinary four-socket valve-holders. For AC valves the five-socket holders should be used. See also Benjamin.