Tube amp no load/nonimal load/infinite load

"Abraham"

** Drivel.

The primary has a very high impedance and so the tubes see virtually no load. Plate voltages swing over the full range with no current changes.

** Wrong.

** Wrong terminology - a short is simply called a short. ** More drivel.

With the secondary shorted, the tubes see a very low load impedance resulting is high current flow with little change in plate voltage. Results in very high plate dissipation.

** Yep.

** If you switch off the current flow in an inductor, you get a back emf spike.

Sometimes no load and over-driving the tubes does this.

Similarly with an inductive load.

.... Phil

(Note: I know circuits, but I never diddled much with tube amps. So this is more theoretical with a dash of practical experience from switching supplies and relays and such).

That's no more likely to happen without a load as with. The average current through the tubes is set by the tubes; the transformer primary always "tries" to have zero _average_ volts DC across it (neglecting resistance).

Uh -- what?

The tubes pull current, the transformer primary responds with a voltage. If the secondary is shorted, then the AC voltage across the primary goes down (because the short is reflected back). Depending on the output impedance of the tubes you may or may not pull too much current (I don't have the practical experience to say whether this is common or not).

For a real and positive output impedance the output-shorted case will always cause the amp to put out more current, while the output-open case will always cause the amp to put out more voltage.

The tubes will pull current as the grid voltage goes up. The plate voltage will go to some minimum (it depends on the tube and the circuit). Then the audio signal on the grid will swing low, and the plate transformer (being an inductor) will try to force the plate voltage high. It'll force that plate voltage as high as it needs to in order to push the current through -- if that's too high for tube or transformer, it lets the smoke out.

Dunno whether it's usually the tube or transformer that goes first. I do know you don't want to do that...

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com

"Tim Wescott"

** So better shut the f*ck up.

Your drivel only adds to the noise.

Accompanying the range of knowledge, there is a range of designs, from good to terrible. Keep this in mind, too.

No load: As the voltage swings around (presumably, under control of the feedback loop, if any), little current is used, which means little grid voltage is required. Loop gain is high, and the parasitic resonances of the OPT will be accentuated. (A properly designed amp will accommodate this change in loading without affecting frequency response, gain or stability.)

As physics go, the transformer's inductance is the same as it has always been; magnetizing current will dominate the total current draw, but this is far less than nominal load current, at least for (volts / freq) below saturation (which is usually designed to occur around 20-60Hz at full power, lower at less power).

The conditions under which voltage spikes can be produced are in clipping, where the output tubes are overdriven. This is possible in any load condition, but with no load to dampen the spikes, the voltages can become particularly high. The physics behind this phenomenon are, even though the amp may be attempting to control the output voltage (by feedback), this feedback is effectively delayed by the transformer's leakage inductance, which acts as a series inductor between primary and secondary. When a tube turns off while carrying current (magnetizing current, in this case), the primary voltage leads the secondary.

Under short circuit conditions, loop gain is minimized (indeed, zero), so all the gain goes into driving the output tube grids, pushing them into cutoff sooner and pushing current way up. Meanwhile, since voltage isn't saturating, current *stays* high, so the average current draw and power dissipation are much greater than under proper load conditions.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

"Abraham"  wrote in message 
news:ebb7fb9b-0308-4cda-88d0-7c2a558f8e6d@googlegroups.com...
There seems to be a lot of wrong reasoning when it comes to the load on a 
tube amp.

Anyone with real knowledge, both theoretical and experimental, mind setting 
me straight?

My understanding of it is this:

No load:

Without any load, e.g., infinite impedance on the secondary, the output 
transformer acts like a simple inductor(as if there were no secondary 
winding).

The inductor, will charge up and eventually act like somewhat of a short. 
Hence larger than expected currents will flow through both the primary 
winding and tube when the tube conducts. Class A and class B amps will have 
a slightly different response but both may or may not be ruined.


Infinite load:

With an infinite load, e.g., the secondary is shorted, the output 
transformer acts much differently. The shorted secondary allows a back emf 
to counter the driving voltage of the primary somewhat canceling out some of 
the current. The effect is to reduce the maximum current in the no load 
case.

"Tim Williams = top posting autistic s*****ad.

** Why don't you shut the FUCK up ??

YOU have neve has any dealings with tube amplifiers - have you ?

OTOH, I have two examples in the workshop right now.

SO PISS OFFF !!!!!

Take your meds.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com

They need to up his dosage of cyanide.

** Shame there is no medication for grossly autistic, narcissistic fuckheads like you - Tim.

But a nice dose of rat bait will do in place.

Fuck off.

Well, OK, then...

I diddle with 'em every day. I've built a few for some of the guitarists you may have heard on your radio or TV, in fact. I repaired three of them today, and will do two or three more tomorrow, and every day thereafter. Been doing that for decades. So I'll just tell you what happens; you rocket scientists can figure out why, ok? ;-)

No load - will often cause what's called high-tension flyback. This will result in an arc, either inside the tube base between pins for the anode and the filament supply (in most common octal power tubes used in guitar amps) or outside the tube on the bottom, where it will burn the face of the socket and the bottom of the tube. Sometimes the arc will occur on the rear of the socket. There will be a carbon track produced, and this track will cause many more arcs to occur, even if the amp is subsequently properly loaded. In many cases, the tube will be instantly destroyed, blowing the high tension fuse. Some amps have clamp diodes from anode to ground; one or more of these will short when flyback occurs, blowing the HT fuse. It's fifty-fifty whether the fuse will blow before the tube packs up. Since the spike will jump into the filament circuit, preamp/driver tubes can also be destroyed, and the output and even the power transformer can be damaged. If the amp has a filament hum balance pot or fixed hum balance resistors, these will also be burnt up. Being a repair technician, I love it when this happens. Especially if the idiot who owns the amp wraps the HT or mains fuse in aluminum foil. Aluminum foil paid for my house. ;-)

Infinite load - (I suppose that's wankerspeak for a short, yes?) Works the power tubes fairly hard, since they see the secondary winding plus the short as a very low impedance load...but usually nothing much happens in the amount of time it takes the muso to figure out something's wrong. Once in a great while a tube packs up, and once in a great great while an output transformer can bite the dust, but neither of those is common. In fact, many guitar amps have switching output jacks which short the amp's output to ground when no plug is inserted, as a safety feature to keep the amp from being run unloaded.

And that's the way it is, boys.

Lord Valve Old Tube Fart

Why don't they include some clamp diodes or MOVs or something? Seems silly to explode an amp just because the load got disconnected.

--

John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com   

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME  analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators

...

Because clamp diodes short when they protect, and must be replaced - it's a pain in the ass. MOVs work - I've seen amps with them across the OPT primary - but they sound horrid when they do their trick, and most people opt to have them removed. In fact, most protective schemes sound like ass in guitar amps, and musicians won't put up with that. The more primitive the circuit, the better the musos like it - this is why old Fender tweed amps from the fifties sell for thousands of dollars. They *sound* good. Reliability is secondary.

LV

"John Larkin"

** Because in most cases they are simply not needed. It is only particular topologies, tube types and output transformer designs that have an issue.

Some amp makers routinely include diode stacks from plates to ground on PP output stages - which usually works well.

However, there are traps for the unwary in adding such stacks to other amps as HT fuses blowing or simply operating a standby switch can generate a large " common mode " voltage transient and cause even 5kV stacks to fail short.

MOVs are generally not acceptable across OP tranny windings due to their high capacitance and tendency to fail when asked to do their job repeatedly.

A simple Zobel across the secondary also works well, but has an audible effect on the sound of many guitar amps.

** The amp does not " explode " nor is it JUST because the output is unloaded.

Dickhead.

.... Phil

When building ham AM transmitters half a century ago, it was a common practice to put a spark gap across the output transformer primary, mainly to protect the hefty transformer. The tubes (valves) used had usually anode connections at the top of the bulb (plate cap), so a socket flashover was not an issue.

The power levels were some tens to some hundreds of watts. For

100% anode (and screen) AM modulation, the audio power must be half of the Class C RF amplifier input power. This led to the common construction of having two tubes in the RF amplifier and another similar pair in the Class B modulator.

--

Tauno Voipio

He's right Phil, something needs adjusting.

"Dennis"

** Fuck off, troll lover.

"Tauno Voipio"

** A few guitar amps used beam tubes with top caps, notably early Ampeg SVTs used 6146Bs - a VHF power tube.

A few others used the 6DQ6B or even 6CM5 - TV horizontal deflection tubes.

No arc over problems, but a real hazard for owners not used to the idea that top caps may carry lethal voltages.

.... Phil

The only reply necessary.

--
I'm never going to grow up.

** Do I have a fan ??

.... Phil

Don't insult me, insult Lord Valve. He described the carnage that can result from an unloaded tube amp.

The real reason that tube amps can blow up like this is that most audio stuff is designed by amateur hacks.

--

John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com   

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME  analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators