Re: Running an amplifier without load

No, they are not

No. A valve with an unloaded transformer winding in its anode can easily under some conditions [driven completely off] flick the anode voltage up thousands of volts and destroy the valve or output transformer due to flashover.

It essence the transformer acts like an ignition coil.

There is plenty of researchable information on line: you don't have to guess wrong.

The electrolyte is not always brown and easily visible. I once had a problem with an HP digital storage 'scope. After a lot of searching it turned out that an electrolytic capacitor had leaked colourless electrolyte onto the pcb where the surface resistance became low enough to change the division ratio of the voltage setting resistors in a voltage regulator. John

Agreed, not /fully/, but compared with fader-type linear potentiometers they might as well be. Most usually, faders are mounted facing up on synths or mixers. Anything which falls onto them can find its way onto the tracks. Granted, they might have a dust cover, but how effective these are I really don't know. See 8.13 onwards here:

Rotary pots - even those with a gap in the metal where the solder contacts are - would be mounted where the gap is on the other side to the spindle, and usually pointing down, behind a metal plate in a sealed box (I don't remember ever seeing a rotary potentiometer in an open-cased electronic device, but there may have been).

Modern rotary pots seem to be sealed. See two upper rotary pots here:

Indeed.

True, but I believe IJJ was referring to the practice back in the day, now largely forgotten, of using electromagnets rather than permanent ones to provide the field in loudspeakers (mainly in radios). This electromagnet also doubled as the smoothing choke for the HT supply. An additional humbucking coil was alsu required to cancel the field fluctuations caused by HT ripple current.

In article snipped-for-privacy@binnsroad.myzen.co.uk>, Graeme snipped-for-privacy@nospam.demon.co.uk> scribeth thus

Wont matter with solid state units. Use switch cleaner i think Toolstation have one do not use WD40!.

If its that age then have a look at replacing the main power supply capacitors at some 50 years old I'd expect they'd be well shot now!.

There will be a mains transformer and from that a rectifier bridge and connected to that will be the main power supply caps see if you can download a circuit diagram or manual from somewhere!...

You could take a few photos and post them somewhere a picture is worth a thousand words sometimes!

Which is still the issue.

Wjat happens in an ignition coil when the points open is what happens to an output transformer primary when the valve shuts off, if there is no load on the secondary. And that design was never used in anything approaching 'hi fi'

The leaking capacitor era, was caused by incorrectly formulated electrolyte with the wrong pH value. The liquid inside the capacitor in such cases, could eat through the metal shell of the capacitor, without any electrical bias at all applied to the capacitor. (I had a spare Antec power supply fail this way, and removed the casing top to verify. The +5V bank of caps failed.)

You could leave a new-in-box item on the shelf for two years, and the capacitors would be ruined by their own (incorrect) formulation.

This shows some pictures, of the colour to be expected, from capacitor failures in that specific era.

The capacitor I mentioned above was much older. The 'scope was made around 1985 so it was definitely not affected by the incorrect electrolyte problem. It is still in regular use. John

I find that just taking the knobs off the controls and runningor spraying some IPA down the shafts with vigorous twiddling gets rid of most crackling.

I will take you word for it on newer electronics, but there is a difference in the old radios I messed with: 1930s to 1960s vintage mostly.

First of all, an ignition coil is a step-up transformer with the two windings joined at one end which is the distributor and points connection*, an radio output transformer is very much a step-down transformer with two unconnected windings.

• there is also a capacitor in the distributor which is placed across the points. It is in series with the primary winding when the points open and it is shorted out when the points are closed. The interaction of inductance from the winding and the capacitor creates a very rapid series of sparks at the spark plug. If you ever disconnect the capacitor, the system won't run the engine properly.

Secondly if it is a valve radio of the type I have seen, the feed side of the output transformer is via a double diode rectifier valve and smoothing capacitor and I have never seen an anode feed voltage via that set-up to be more than the 350-0-350 volts output from the mains transformer. The idea is that via the transformer primary there is a steady HT+ DC feed to the anode so that it can have sufficient positive volts to operate, with the audio signal inserted via a negative voltage on the grid, and any output from that via that anode is just the superimposed audio signal into the transformer that is at audio frequencies which is all the loudspeaker or headphones responds to because the DC voltage doesn't create an output to the secondary. The primary winding has a DC resistance, so the voltage at the anode is going to be a bit less than the 350V or whatever other HT voltage the winding on the mains transformer is designed to give.

I didn't often run a radio without a loudspeaker connected, but I never damaged the output valve or output transformer on the occasions when I did. After all, running the output stage without a loudspeaker is no worse on the valve than running it with the signal grid earthed.

Forget tee secondary. An ignition coll will do a couple of hundred volts on the primary when the points open and an output transformer will do several kV.

The capacitor helps keep the primary voltage under control.

Until you switch the valve off hard with no load on the output transformer, at which point the output transformer no longer looks like a resistor but like an inductor.

Unless you touch the input and get a massive 50 Hz hum that drives the valve into overload.... I don't see why it is so hard to understand. Under normal linear conditions all is well, but switching the valve off with an inductor in its anode makes big volts...all that current with nowhere to go ...

"The best lack all conviction, while the worst Are full of passionate intensity."

The Second Coming. W. B. Yeats

A memory from my very distant past playing with home-brew valve audio outputs, typically a pentode with a transformer in the anode circuit, was sparks jumping across the transformer primary terminals when running with no load. I assumed this was the induction effect of the valve shutting off rapidly on the signal it was seeing, which was probably noise. The exact equivalent of the reason you always have a reverse-biased diode across a coil driven by a transistor, to catch the spike.

Anyway, I fixed it at the time by putting a capacitor across the primary, reducing the maximum rate of change of current in the unloaded transformer coil = inductor. I late noticed that some professional circuits had that too!

The valve and transformer by the way appeared not to be bothered by the high voltage sparks!

nib

And the step-down ratio of the transformer is quite large for low power amplifiers. The square of that ratio times the speaker resistance is approximately the resistive impedance presented to the valve(s). This is in parallel with the fairly large inductance presented by the transformer winding and core.

Remove the load, and the resistive component heads towards infinity, and the inductance now dominates. The valve flips between saturation and cutoff, and you cannot step change the current in an inductor, therefore the inductor voltage rises to try to maintain any current in it. The limit of the rise is often whatever flashes over first, valve, transformer, or air gap if you are lucky.

The old rule of thumb was that it was safe to open-circuit a transistor amp load or short-circuit a valve amp load, but not the other way around.

Some however have been. Hence the warning Its a bit like 'all snakes are deadly' Its not true but the only thing you need to know about snakes (or toadstools) is that *some* are deadly.

I remember the first electric shock I got, I had built a Meccano tram with overhead pylon to drive a big 12V electric motor. Running off my model train transformer.

It stalled and I tried to get it to move and in the process disconnected the motor from the 12V. The shock was massive. Probably around 400V.

It was about 15 years before I discovered 'why' in lectures

Yes. If you don't know what you are doing, that is a 'rules for fools' rule.

Of course part of my job then was to short out transistor amplifiers to make sure they could take it...

-- Those who want slavery should have the grace to name it by its proper name. They must face the full meaning of that which they are advocating or condoning; the full, exact, specific meaning of collectivism, of its logical implications, of the principles upon which it is based, and of the ultimate consequences to which these principles will lead. They must face it, then decide whether this is what they want or not.

Ayn Rand.

The curse of initialisms. India Pale Ale? (Yes I know, but that is the first IPA that came to mind.)

If an amplifier has a headphone socket it presumably has some system for removing signal from the speakers when the headphones are plugged in. Which probably makes it safe to run the amplifier without speakers. Or, for that matter, have speakers connected but not hear anything from them. The only possible problem is if the crackle is coming from a disconnected part of the circuit. But that is relatively unlikely.