Home-made Valve tester

[Terry]

I know the grid does not draw any current, but the pot. does! (See diagram).
Bob

In addition to what Jeffrey said, there is no current flow through the slider and that is what causes a pot to go noisy.

[Sparks]

In my experience, carbon track pots. are not as good a wirewound ones, and in a number of reaction receivers that I have made in the past, I have noticed that the reaction no longer works after a while, until the pot. has been replaced, and there is no current flowing through them either! I know it will not make much sense to some, but I long-since gave up trying to make sense of what goes on in valve radios. When I had a 10K pot. in the grid bias circuit, it did not work as smoothly at the 5K one, and both were wirewound! Maybe the 10K was faulty - but I don't really know.
Bob

[Sparks]

Lawrence,
Just seen your question about the EL84. I have no idea, probably someone else will know though!
Bob

[ppppenguin]

Old WW pots can be horribly scratchy.

[Sparks]

The 5K one is fine, so not much point in changing it! Having an extra switch on the front doesn't bother me at all! In fact the more controls, meters and adjustmets there are on radio equipment, the more I like it, as I feel I am more in control of what is behind them. I was really dismayed to have my ship fitted with a main transmitter that only had an on off switch on it All the tuning was done by a tiny key pad on the receiver console. I dialled the frequency in and pressed a button, and it whirred, and clicked, and clunked for a few seconds, and when it stopped, it was all ready to go. But there always came a time when pressing the button produced nothing but silence, followed by the nightmare of getting it going again! When it comes to electronics, I only get pleasure from messing about with my Heath Robinson projects, the older the better. No doubt all my devices are flawed in some way, but it doesn't really matter!
Bob

[Sparks]

Here it is at the moment - Very rough indeed, but working great.    I haven't done antything on it today because it is freezing cold, with rain and sleet and any cutting work is done outside.    The four bolt ends under the left hand meter are holding the grid bias battery in position.
Bob

[Mike Watterson]

Question, does that mean that when it's pentode connected that the screen voltage is obtained from a different supply than the anode, if so why ?

Ideally the Screen is either from a fixed HT or decoupled with a capacitor. It's only the same as anode volts in Triodised mode.
If decoupled, then the average screen current (and thus voltage drop from HT via resistor) is constant.
The "high" screen voltage "hides" the dropping anode voltage from grid.  (Lies but suffices).

A beam electrode or a suppressor grid is needed (Pentode vs Tetrode) because at a low anode voltage the electrons might get attracted back from Anode to g2, causing the "kink" (Hence KT = Kinkless Tetrode, a Pentode done with electron lens, the "earthed" beam forming plates do same function as suppressor grid (suppressing secondary emission).

Triodised gives more power, none is wasted on the g2 / screen

[pwdrive]

In the context of basic valve testers a lot of pentodes are triode strapped for the test when the settings are given as Vg2 = Va, so far as I can make out.

Lawrence.

[pwdrive]

To save on a meter function you could obtain the grid bias voltage from a small mains transformer, rectifier & filter and possibly a zener diode for regulation, the set bias voltage pot could then be fitted with a scale and calibrated in volts.

Lawrence.

[Mike Watterson]

In the context of basic valve testers a lot of pentodes are triode strapped for the test when the settings are  given as Vg2 = Va, so far as I can make out.

Which is misleading. Better like the decent AVOs to be able to set Vg2 and Va separately. Gain of Pentode can be hugely dependent on Vg2.