Running a radio without speakers

with tr amps its harmless, do what you like with the vol ctrl. With most valve sets its harmless, but perhaps not all. Sometimes valve amps were fitted with spark gaps on the output transformer HT side.

NT

It is best to run any amplifier with a matched load. If you turn up the volume, but not to the clipping point, there should not be any damage. But, running any amplifier with no load is not something I would recommend.

--

JANA _____

What if the volume control is kept at the minimum?

This wouldn't be a problem with a tube set iiuc, right?, but I'm not so sure about transistors, if, say, the volume were accidentally turned high. (No I didn't do that, but I was wondering how much effort I should expend just so I can turn the set on and measure some voltages.)

If you are inclined to email me for some reason, remove NOPSAM :-)

I second this. I've built and rebuilt several transistor amps, from 60W to

500W, and running without a load is not a problem. The final stage, the current amplification, only starts pulling current when there is something to dump it into.

On Tue, 26 Dec 2006 00:13:43 -0800, meow2222 Has Frothed:

Some vintage valve guitar amps had a shunt prong across the 1/4" spkr output jack when the plug wasn't in it. IIRC Marshall amps had a warning tag that to operate the amp without a load or with a resistive attenuator would void the warranty. I know that operating most any class AB1 valve amp is risky to the OT.

--
Pierre Salinger Memorial Hook, Line & Sinker, June 2004

COOSN-266-06-25794

I had an old amp that would spark across the spark gap with no load connected. But whats the mechanism for that? No load cant take any Vs higher than normal run voltage, unless theres some kind of LC thing going on in the op tf, and no load means no damping.

NT

Have you ever seen the output tubes arc over internally when there is no load? The spikes exceed the maximum plate voltage, and you have major problems. it an trip the power supply, damage the tubes, or cause a complete meltdown of the tubes. Its quite a sight when a broadcast transmitter loses its load, and the plate relays don't trip out.

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

On Tue, 26 Dec 2006 07:19:09 -0800, meow2222 Has Frothed:

Not sure, I just trusted the word of my elders that damage could occur. Maybe some of the deeper thinkers in alt.guitar.amps could explain exactly what mechanism(s) is/are involved in damaging a AB amp with no load. Now that you've (sparked) my curiosity, I'm eager to know.

--
Pierre Salinger Memorial Hook, Line & Sinker, June 2004

COOSN-266-06-25794

The voltage can rise to the point that the insulation in the output transformer windings breaks down and arcs over, destroying the transformer.

With transistor amplifiers, they are all some form of emitter-followers (common-collector) and they can run safely with no output load.

Tube amplifiers can produce voltage spikes when unloaded.

I see thread has already jumped the gap to the guitar group, lets hope it sparks someone's curiosity there too. Perhaps someone's resistance to contributing will break down, and they'll tell us something that resonates with us all.

NT

--
Tube e or not tube e

also:

"This varies from amp to amp. Some amps have no problem at all with being unloaded, as long as no signal is being passed. Others (certain Boogies, for instance) will break into oscillation when the impedance of the feedback loop changes when the speakers are disconnected. If your amp is wired so that a switching jack is used to throw a short across the amp's output when the plug is removed, then everything will be fine. If you have a standby switch, it's always a good idea to put the amp on standby when removing or connecting speaker cords."

The speaker presents a ?load? on an amplifier, just as a ?flywheel? presents a load on an internal combustion engine.

"SPEAKERS & OHMS!!!! Everything you need to know about SPEAKERS & OHMS!!!

Ohms: A value of electrical resistance. The lower the number, the less resistance. The higher the number, the greater the resistance.

Speakers are rated at certain ohms (or impedance), most commonly, 4, 8, and 16.

Your amplifier is also rated at a certain impedance, or has multiple taps for speaker configurations. It is important that you match these numbers up.

EXAMPLE --- 8 ohm cabinet--->8 ohm jack on amp EXAMPLE --- 4 ohm cabinet--->4 ohm jack on amp EXAMPLE --- 16 ohm cabinet--->16 ohm jack on amp

It is typically safe to mismatch one step higher

EXAMPLE --- 8 ohm cabinet--->4 ohm jack on amp. EXAMPLE --- 16 ohm cabinet--->8 ohm jack on amp.

Now for the do-not do's.

NEVER EVER hook up a cabinet with a lower ohm rating than the head.

EXAMPLE ---4 ohm cabinet--->8 ohm jack on amp...THIS IS A NO-NO!!!

You will cause your output transformer to overheat, and eventually, it will blow.

NEVER - Turn on a tube amp without a speaker load attached to it. This can cause an output transformer to blow in a short time.

The only instance where it would be safe to turn on a tube amp without a load would be on amps that have a standby switch (Class AB typically) This would only be for visual diagnostics (tube heater check, bulb check, etc) BUT, DO NOT take it off of standby. While it's in standby, only the tube heaters are working, and will not cause damage to the output transformer.

Also, NEVER use instrument cable to hook up a speaker cabinet. The speaker jack can put out a high enough voltage to burn the small wire in instrument cable, thus leaving the amp without a load, and frying the output transformer. ALWAYS use a good heavy speaker cable.

General Speaker Info

Sensitivity - This number indicates how efficient the speaker works. Generally, this number will be between 89 and 105. If you replace a speaker with one of a higher sensitivity rating, you will also increase the apparent volume.

Generally speaking, an increase of +3dB will double the overall loudness of the amp.

Wattage - How much the speaker can handle. Smaller, combo amps usually have a speaker rating close to the rating of the amp. If when cranked, you don't like the sound of the speaker breaking up, then you might look into changing it out with one of a higher wattage. Some like the way a speaker sounds while being pushed to it's limits, and some don't. It's all in your presonal taste.

Wiring up multiple speakers.

When wiring speakers, there are three ways to do it

SERIES: one wire from the input jack to the "+" terminal of one speaker, from the "-" terminal of the same speaker to the "+" of the next speaker, last wire from the "-" terminal to the input jack. When wiring speakers in series, multiply the ohms by the number of speakers used. Also, when one speaker blows, it will cut power to the other speaker as well, leaving both speakers not working.

EXAMPLE: jack------"+ '8ohm' -" ----"+ '8ohm' -" ---jack=16ohms

8x2 = 16

PARALLEL: One wire from the input jack to the "+" terminals of both speakers. Other wire from the input jack to the "-" terminals of both speakers. When wiring speakers in parallel, divide the ohms by the number of speakers used. In this case, when a speaker blows, the remaining speaker will still work.

EXAMPLE: jack----"+ '8ohm'"------------ "+'8ohm'" jack----" - '8ohm'"-------------"- '8ohm'" = 4 ohms

8/2 = 4

This is also the way you will want to hook up multiple cabinets to an amp.

EXAMPLE: (2) 8 ohm cabinets ---->4 ohm jack on amp. EXAMPLE: (2) 16 ohm cabinets--->8 ohm jack on amp.

SERIES-PARALLEL:

This is the most confusing. The total speaker load will equal the ohms of one speaker. Most commonly used on speaker cabinets that contain 4 speakers. This theory only works with 4 speakers of the same (ohm) rating. In this case, if a speaker blows, it will take out it's series counterpart, leaving two speakers working, and two not working, hence doubling the impedance.

Starting at the upper left speaker, you will run one wire from the input jack to the "+" of that speaker, then from the same terminal to the "+" of the speaker below it. Then run a wire from the input jack to the "-" of the upper right speaker, then from the same terminal to the "-" of the speaker below it. Finally, connect the "-" of the upper left speaker to the "+" of the upper right speaker, and the "-" of the lower left speaker to the "+" of the lower right speaker.

Wiring Multiple Cabinets

Sometimes, it's inevitable that we need to run multiple cabs of different impedances. Well, here's how you figure out how to solve that problem.

EXAMPLE:

The amp has 4, 8, and 16 ohm taps.

Cabinet A = 8 ohms Cabinet B = 16 ohms

In this case,

1/8 +1/16 = 3/16

Divide the back number by the front number to get the impedance.

16/3 = 5.33

You would want to run both cabinets from the 4 ohm taps, or daisy chain them if (2) 4 ohms taps aren't available. Remember, it's safe to run a slightly higher mismatch.

...And just as a reference

(2) 8 ohm cabs = 4 ohms (2) 4 ohms cabs = 2 ohms - *typically unsafe* (2) 16 ohms cabs = 8 ohms

(1) 8 ohm cab + (1) 4 ohm cab = 2.66 ohms - *typically unsafe* (1) 4 ohm cab + (1) 16 ohm cab = 3.2 ohms - *somewhat unsafe*

The two above examples would typically be unsafe to run at, although, the very bottom example would be better to run, since it's closer to 4 ohms. This would get by in a pinch on some occasions.

[disclaimer]

The information provided above is a general reference. In some circumstances, it may not be appropriate to mismatch impedances. Use common sense, and consult the manufacturer if possible. If you are in doubt, the safest bet is to find a cab that has a nominal rating of that of your amp. I am not responsible for damages caused from mismatching impedances.

[/disclaimer]

Very interesting FYI Thanks.

Guitar amps, as well as the typical table top radio, use pentodes for the output stage and pentodes tend to act like current sources.

Ignoring the finer points and noting ohms law, E=IR, if it's pulling 'X' current, essentially regardless of the load, then with 'no load' (which actually means 'infinite' load) the effective primary impedance (R) skyrockets and so does the generated E across the primary.

It's complicated by the topology. Negative feedback, for example, would tend to curtail that, or so it seems, because the secondary E would tend to cut off drive to the tube (we're 'here', E wise, so stop sending current). But there the problem is the 'infinite' load causes the amp to have incredibly high gain because it takes virtually no current to drive the 'infinite' load and with that much gain it'll likely oscillate, which will again cause high currents and potential arcing because it's out of phase with the negative feedback (that's how it decides what frequency to oscillate at).

A self shorting, when no plug inserted, speaker jack prevents the 'infinite' load situation but presents a very low impedance. And, again, looking at ohms law you see that the current through the primary will cause very little voltage drop, which means the tube sees essentially full B+ even at 'full current', so (in the presence of a signal) it'll quickly overheat, glow the plates, and melt the thing but that takes 'a little time' and is marginally safer than near instantaneous arcing.

So it looks like I almost had it backwards, and tube amps need more care about running without speakers. But iireadc, transistor amps can also have problems under some circumstances. So I will keep to my cautious ways at all times, unless there is an overwhelming reason not to.

Thanks to everyone. (I still have to finish reading thios thread too!)

If you are inclined to email me for some reason, remove NOPSAM :-)

Jeebus, folks, trim your frickin' replies.

"Mee too bye!" __ Steve .

A tr audio amp isn't going to have a problem with no speaker load, ever. And thats regardless of whether its low z output, as nearly all are, or current drive, as a very few are.

NT

It looks at first sight like an explanation, but I'm not sure it really is. Here's why. Whether an amp is driving max output or oscillating with no load, in either case the output pentodes can only deliver so much V swing to the output iron. The v limit is the same in either case. So why would one cause arcing and the other not? Why does oscillation create more primary V swing than audio?

I can think of 2 perhapses. One may be that due to valve impedance the pentode can produce more v swing when not loaded than when loaded. But still it takes around 300v to arc across a small gap in air, and I've seen quite a small amp arc across an air gap when unloaded, and while I dont know for sure, I wouldnt expect B+ to be over 300v on that one, more like 250.

2ndly there might be an LC resonance within the output iron, possibly in combination with some other capacitance, causing a voltage magnification. The frequency of that will pull the closed loop whole amp resonant frequency to it, and if the 2 are close enough one could get ac voltage above B+ on the output tfs.

Is there any truth in either of these 2?

NT

It is typically safer to mismatch one step *lower* on a tube amp.

EXAMPLE --- 8 ohm jack on amp--->4 ohm cabinet.

-Dave

No deep thinking required. The mechanism is inductive kickback in the primary of the output transformer.

Think of it this way. If there is no load on the map, then the secondary of the output transformer can't conduct any current. If it never conducts, then for purposes of mental exercise, it isn't even there. You could (in your mind) physically remove the secondary without changing the situation in question.

At that point, you are left with a pair of push-pull output tubes connected to the ends of an inductor - the output transformer primary. The inductor has a center tap which is connected to a power supply. When one of the tubes turns on, current begins to flow in it's half of the primary. The current builds up at a rate determined by the inductance of the primary. Once current is flowing, what happens if the tube suddenly drops out of conduction? (Think of an audio transient like a drum-snap). The result is that the current can't continue to flow, but the energy stored in the inductor's magnetic field has nowhere to go. The magnetic field collapses and produces a very high counter-emf (i.e., voltage spike).

This is the same mechanism that generates the high-voltage spark in a car ignition or a television flyback - allow current to flow, then suddenly cut it off.

By the way, this can happen in ANY amplifier, tube or solid-state, that uses an output transformer. In most practical cases, though, tube-based amps use an output transformer because tubes are high-impedance devices that have to be matched to the low-impedance load (the speakers). Transistor-based amps have an inherently low output impedance, and so do not require a matching transformer.

Bill