Dumb valve amp questions?

Grumpyrocker · July 2021

Possibly a very stupid question. But one that's puzzled me. It would be madness and dangerous to have a cable in the home with. a regular 13amp plug on each end. I remember seeing on at high school once and when one of the teachers saw it in use went mental at someone for it even existing.

So how come we use speaker cables with amplifierss with a male connector at each end. Aren't we connecting it to a lot of power? After all a 100w amplifier is a powerful thing - how much power is coming down that cable to the speakers? Surely one could be easily shocked by a speaker cable.

Or if not much power is coming down those cables to the speakers then where is all that power going? Two hefty transformers in a 100w head - all that electricity must be going somewhere?

And on that subject, why two transformers in a valve amp? Can't we make one do all the work?
y.
Thanks for anyone taking time to ask these baby questions.

Edit: To answer myself slightly. Of course I just realise 100W amplifier head sounds crazy powerful. But that's like an old lightbulb isn't it. So it isn't that much power.

springhead · July 2021

A loudspeaker is a coil and a magnet. It needs current through the coil to make it (and hence the cone) move in the magnetic field. So it's a high current low voltage device. You won't get a shock off a dangly speaker lead from the amp. It's also the most dangerous scenario for the amp - turning it up loud with no speaker plugged in. It can generate a high back emf that can kill the output transformer. At which point there is nothing at all on the dangly lead and you have a big repair bill.

Valve's are high voltage low current devices. The opposite of what a loudspeaker needs. So a second transformer is required to step down the high voltage low current of the output valves into the low voltage high current need to move the speaker. This is in addition to the power transformer that provides the high voltage for the valves, plus their heaters and maybe a bias winding too.

The mains plug 'double ender' is supposedly an old farmers trick for if there is a power cut. One end into a generator, other end into a socket in the farmhouse. Sorted!

ICBM · July 2021

Yes, a 100W amp can produce enough voltage at the speaker output to be potentially dangerous, especially at higher impedances (voltage goes up with impedance). In fact, in the 1970s, Marshall 100W amps made for export to Canada had the 16-ohm output setting disabled exactly for this reason, because Canadian regulations at the time prohibited it - the voltage can be over 50V.

However, operating a valve amp with no speaker connected at the sort of volume setting where that voltage can be generated is likely to be quite a brief exercise...

The two transformers are the power transformer (PT) which converts the incoming mains power into voltages the amp needs internally (hundreds of volts for the signal part of the circuit), and the output transformer (OT) which converts those high voltages back to lower ones that the speaker needs. Solid-state amps don’t need the OT because transistors operate at voltages low enough to be directly connected to the speaker.

You’re absolutely right that if a speaker isn’t connected to the OT, the energy can’t just go nowhere - it then causes very high voltages to be developed in the winding which can arc through the insulation and destroy it. (Or if you’re lucky, it may arc in the valves instead and blow those.)

And really, you’re quite right that 1/4” telephone plugs (what they were originally designed for) are a very poor choice for speaker connections! But they’ve become an industry standard and it’s hard to change that now - at least for amps with power ratings below about 300W. Most modern bass and PA amps of higher power than that use Speakon connectors, which were purpose-designed for higher voltage and current levels, don’t have exposed contacts, and lock into place.

These aren’t stupid questions by the way

.

Grumpyrocker · July 2021

Thanks for the answers.

So all the internal stuff like valves and stuff - they are using more power than the speakers are?

springhead · July 2021

Grumpyrocker said:

Thanks for the answers.

So all the internal stuff like valves and stuff - they are using more power than the speakers are?

Yes. An amplifier isn't 100% efficient. More like 40-50% for a push pull amp. They get warm - that's power from the mains being turned into heat, not driving the speaker. Plus of course valves have to be heated in order to work, more 'lost' power.

Grumpyrocker · July 2021

Yup I get the inefficiency. But the actual power requirements for the valves to work is more than speakers?

ICBM · July 2021

Grumpyrocker said:

Yup I get the inefficiency. But the actual power requirements for the valves to work is more than speakers?

Yes. Valves are also inefficient because they require filament (heater) power just to work, even before they conduct signal current. That’s the orange glow you see inside the valves when the amp is on, even on standby (which turns off the signal voltage usually).

There are other losses in the circuitry too. A valve amp is often no more than about 20% efficient in terms of input power to speaker power - low-powered ones worse than high-powered, because the filament power is a more significant fraction. A modern switch-mode/Class D audio amp can be up to about 80% efficient, possibly even higher.

Grumpyrocker · July 2021

Thanks. Making more sense to me now.

ecc83 · July 2021

Yes, the voltages from high power amps are a potential (no pun intended!) danger but I have never heard of anyone being killed by a speaker output. The aforementioned 16 Ohm, 100 watts produces 113V peak to peak into the load but much more into an open circuit so it could kill but music signals, even heavily distorted guitar, are 'non-continuous' so it would likely just be a bit of a jolt depending on how thin and damp your skin was!

PA amps for use on distributed loads, factories, airports etc, use '100 Volt Line' operation and so are much more lethal , nearly 300V pk-pk and most of us who were in that game have copped a belt at some time, not fun in the rain! Saved again by the transient nature of the signal.

Worth noting that it is not just VALVE amps that are damaged by high or no load. It is the OP transformer that is the problem and a transistor amp with an OPT is actually more vulnerable than a vlave amp. Even for transistor amps sans traffs, try NOT to remove the load under drive.

Lastly, as ICBM says...NOT a silly question AT ALL!

Dave.

Grumpyrocker · July 2021

So I guess if valve amps were invented now, despite the lack of risks in practice, we'd probably have a safer cabling system. With unexposed contacts at the speaker end at least.

Rather than one where you could just use any old cable because it fits - not because it's the right cable to use.

ecc83 · July 2021

Grumpyrocker said:

So I guess if valve amps were invented now, despite the lack of risks in practice, we'd probably have a safer cabling system. With unexposed contacts at the speaker end at least.

Rather than one where you could just use any old cable because it fits - not because it's the right cable to use.

You would never get valve amps passed for use today Grumps! Maybe for top secret military use but not the civilian sector. Not just for their high voltages either, their shockingly low efficiency would dismiss them out of hand.

We accept many things just because they have been around 'forever'. I have read that if they discovered Aspirin today it would be prescription only. Can you imagine someone coming up with the IC engine in a world run by electricity?

Dave.

ICBM · July 2021

Grumpyrocker said:

So I guess if valve amps were invented now, despite the lack of risks in practice, we'd probably have a safer cabling system. With unexposed contacts at the speaker end at least.

Rather than one where you could just use any old cable because it fits - not because it's the right cable to use.

Yes, and you would almost certainly have some sort of interlock to prevent power being produced when no cabinet was connected. This is actually possible even when the disconnection is at the cabinet end - original Ampeg SVTs used a 4-core cable to the cabinet, the two additional cores operated an extra standby circuit in the amp, activated by a link between those two pins in the cabinet. You could do exactly the same with Speakons if you wanted.

Unless @ecc83 has just beaten me to it

, Blackstar use a similar sort of system except that it still uses 1/4" plugs - but there is a standby circuit in the amp activated by putting a plug into any of the speaker jacks, so it at least covers that end of the cable. Fender used to do it a cruder way by simply having a shorting switch in the speaker output jack - a valve amp is safer into a dead short than an open circuit, especially if only briefly.

Grumpyrocker · July 2021

Yes my Blackstar HT5 MK2 is perfectly happy to run without a speaker jack in. It's cuts the output safely. Can use headphone/line-out/XLR then.

Also doesn't have a standby switch - it goes into standby with no input jack in. This confused the hell out of me originally trying to use just the FX return with a modeller as I couldn't get any sound until I put a headphone adapter plug into the input.

ICBM · July 2021

ecc83 said:

You would never get valve amps passed for use today Grumps! Maybe for top secret military use but not the civilian sector. Not just for their high voltages either, their shockingly low efficiency would dismiss them out of hand.

There's an interesting question about this though - while it's certainly true that in terms of the ratio of output power to input power (what regulators would go by) valve amps are very inefficient compared to Class AB solid-state and especially Class D, how about in terms of real-world efficiency of output *volume* to input power? That might not be quite the same...

To pull some figures from the air - not exact but they will be in the right ballpark - if a 20W valve amp uses 90W, a 50W Class AB solid-state amp uses 100W, and a 100W SMPS/Class D amp uses 120W, but they're all the same perceived volume in a real musical situation, which is the most efficient?

ecc83 · July 2021

Hmm, taking 4x EL34 to pull a total of 72W (biased 75%) and 40W ish for the heaters we get a standing dissipation of 112W but, there are considerable losses in transformers, they get B hot! So I would guess a 100W V amp pulls close to 150W? A well designed class B* transistor amp will have a standing dissipation below 10W and if using an SMPSU be way lighter than a valve jobby and FAR less expensive to build. Even if you allow valves a 3 times volume advantage the transistor wins hands down for 'greeness'! Valves also only have that volume advantage at high levels where they beat speaker thermal compression and can enter distortion more gracefully.

If the Greens ever find out that some people are putting 50W into a load and using 100mW of it they will shoot them!

*Class B by Duggy Self's definition also see his views on amplifier efficiency.

Dave.

ecc83 · July 2021

Grumpyrocker said:

Yes my Blackstar HT5 MK2 is perfectly happy to run without a speaker jack in. It's cuts the output safely. Can use headphone/line-out/XLR then.

Also doesn't have a standby switch - it goes into standby with no input jack in. This confused the hell out of me originally trying to use just the FX return with a modeller as I couldn't get any sound until I put a headphone adapter plug into the input.

The valve is not actually "running without a load" it is biased hard off and the drive to it is gated off. Do not assume other amp brands can be run this way folks!

Dave.

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