My apologies for the basic question but I am trying to understand the basic process of the power supply in a tube radio. I am new to tube radio restoration and am trying to understand as much theory as possible. I understand the process of AC voltage being applied to the cathode/filament of a basic rectifier tube (say a number 80 tube) and that a positive AC (?) voltage applied to the plates speeds up the flow of 'electrons' from the cathode to the plates but when I look at any schematic for a superhet radio, it shows what appears to be the DC voltage coming off the 'cathode' and not the plates of the rectifier. If electrons are flowing from cathode to the plate how does DC current end up on the cathode?

What am I missing? I have spent considerable time on the net and looking at old radio/tube books but I just can't seem to fathom how on earth the DC voltage appears on the cathode when all the theory suggests electrons moving away from the cathode? Regards, Angelo

It's a matter of current flow convention.

If you consider the symbol for a semiconductor diode, the arrow "points" in the direction of conventional current flow (i.e. from positive to negative) -- if you like the positive portion of AC cycle flows through the semiconductor and emerges at the cathode side, thus the DC positive is connected to the cathode side of the diode where the positive direction current emerges.

So, too, with valve rectifiers. The positive direction current flows from plate to cathode (which in the case of the 80 is the filament), thus the DC positive emerges at the cathode.

I hope that helps.

So it is possible to have both AC and DC voltage on the filament/cathode? Is 'electron' flow different to 'current' flow?

Electron flow is inversely proportional to current flow.

When theory of current flow was first proposed it was assumed that current consisted of positive charges flowing from positive to negative and this became the convention.

The discovery of the electron proved that actual current is carried from negative to positive.

However the convention remains - the power of positive thinking.

Thanks. So I should with a multimeter be able to measure the 6volt AC on the filament as well as say 300v DC?

Whilst the cathode of a directly heated rectifier is itself connected to two circuits, they remain seperated by the fact that the negative side of the HT circuit does not go back through the valve but straight to the centre tap of the HT winding on the power transformer. There is no current flow between the heater supply and the HT supply.

With regard to current flow, there will always be a debate on whether current flow equates to electron flow or not. As far as I recall, the British position on this (and therefore the Australian position) is that current flows from + to -. This was taught to us at tech in the electrical and electronics trade courses and I believe nothing has changed in this regard.

I think the best way of proving this is to look at electrolysis. Two dissimilar metals placed together, as part of a building for example, will see one metal corrode and the other experience a buildup of mineral deposits. The metal that loses mass is always the positively charged one. The same principle applies in electroplating. The same also applies in your hot water system and in marine applications where the current flow from sacrificial anodes is inversely proportional to the flow of electrons.

So it is possible to have both AC and DC voltage on the filament/cathode? Is 'electron' flow different to 'current' flow?

Yes, the filament's AC heating voltage is present as well as the pulsating DC that emerges from the rectifying function.

The rectifier is followed by a filter circuit of capacitor+resistor+capacitor or capacitor+inductor+capacitor to eliminate the pulses or ripples on the DC voltage which would otherwise be heard as hum in the output.

As for what really happens it's as well to remember that it's free electrons in conductors that allow current to flow through them when a potential difference is applied, and it's free electrons that leave a filament when it's heated. So, when considering conventional current flow, as a compromise, just imagine that electrons carry a positive charge and the current will flow from positive to negative as the early pioneers imagined.

One of the significant traps when measuring mixed voltages, is the ability of the measuring instrument to diferentiate between AC & DC.

There are digital & some analogue meters that will read DC on an AC Range and digital that will measure AC on the DC range. Especially if there is nothing to block the DC component.

An IF valve will have say 250VDC on its Plate and several volts of AC in the form of Radio Frequency. Transistors and diodes within the meter, may well be able to rectify the RF & that will cause an error.

An anologue meter like the aged AVO 7x will not measure DC on an AC range as there is a transformer involved and DC will not pass through.

Due in part to frequency & moments of inertia the mechanical meter movement (D'Arsonaval) will not measure AC: It needs DC.

DC can ride on an AC rail an AC can travel with AC if it is of a significantly different frequency. Reflexed tubes amplify both audio & RF in the same tube.

All a big learning curve.

Marc

There are digital & some analogue meters that will read DC on an AC Range

Yes, I fell for that momentarily just yesterday with my el cheapo digital meter.

Apart from the meter that is in the VTC valve & circuit tester. I do keep a couple of analogue meters.

The AVO I spent money on getting the movement repaired. Anyone who tries to set a DC generator in an old vehicle with a sensitive DVM, is in for some drama and are likely doomed to failure.

Their output is filthy (brushes). (Spikes & RF) AVO is also good for loading circuits. to proove capacitive induction.

Anyone with a 7X check the paper (oil) caps. I have had to replace mine's, one is common to the 400V DC Range. It showed me it was leaking.

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A point with valves we apply positive to the plate and the electron flow is considered to be too the cathode.

The reality with the diode & magnets, is that like charges repel.

The electron cloud at the cathode / (directly heated fiament) is negative therefore only positive going current will pass. Negative current will be repelled.

The Solid state diode is similar in that the positive substrate will repel positive going current.

An issue that presented with valves is that if you used filaments with AC. You got "Hum" at half the AC frequency, as the valve rectified it .

That's why we ended up with heater tubes, which still do get "Hum modulation" in some.

Marc

Back in the old days they didn't know about electrons. They could only tell that something was flowing when lightning happens, or later on when light bulbs turned on. They couldn't tell which direction, so they guessed and got it wrong. This before vacuum tubes were developed, like that CRT like tube with the iron cross inside casting a shadow on a phoshper screen. This error is way too deeply entrenched in the world of electronics today to ever be fixed. Every electrical engineering student gets puzzeled by this... Today with PNP transistors we can talk about "holes" flowing in the same direction as the current goes....

Edison had the light bulb glowing long before Boyle described the electron in the very late 1800's and later they realised that globes went black as that which was being thrown off was actually a charged particle.

A lot has changed since then and it led to the development of the electron tube.

Marc