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Tech: Valve amps & 'Y' speaker cables ???

GUEST,erbert 19 Jan 11 - 11:53 AM
Richard Bridge 19 Jan 11 - 11:58 AM
eddie1 20 Jan 11 - 02:17 AM
Joe Offer 20 Jan 11 - 02:47 AM
Mitch the Bass 20 Jan 11 - 04:04 AM
treewind 20 Jan 11 - 05:01 AM
The Fooles Troupe 20 Jan 11 - 05:49 AM
GUEST,BobL 20 Jan 11 - 06:57 AM
JohnInKansas 20 Jan 11 - 07:50 AM
pdq 20 Jan 11 - 10:31 AM
Richard Bridge 20 Jan 11 - 03:33 PM
Richard Bridge 20 Jan 11 - 03:37 PM
Leadfingers 20 Jan 11 - 08:37 PM
pdq 20 Jan 11 - 08:56 PM
Bernard 21 Jan 11 - 10:57 AM
GUEST,erbert 21 Jan 11 - 12:50 PM
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Subject: Tech: Valve amps & 'Y' speaker cables ???
From: GUEST,erbert
Date: 19 Jan 11 - 11:53 AM

I want to use a valve guitar combos internal 16 ohms speaker and a separate 16 ohms cab at the same time.

Please confirm if it is safe and ok to connect a valve guitar amp
to 2 X 16 ohms speakers
using a "Y" splitter speaker cable..

Should it be plugged into the single 8 Ohms speaker output ?

or the single 16 ohms speaker output ?

cheers...


I probably already know the correct answer if this means they'd be run in parallel,
but always have niggling anxiety
and doubting confidence about electric techie practicalities...


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Subject: RE: Tech: Valve amps & 'Y' speaker cables ???
From: Richard Bridge
Date: 19 Jan 11 - 11:58 AM

8


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Subject: RE: Tech: Valve amps & 'Y' speaker cables ???
From: eddie1
Date: 20 Jan 11 - 02:17 AM

I could probably have answered this question with no difficulty some 55 years ago. I used to play in a skiffle group and we had a regular Sturday night gig in a local youth club.
This involved carrying a huge radiogram (if you can remember them), with a 12-inch speaker, down the street to the club. The lead guitar (for the musically challenged this meant the one that played louder than the rest) was plugged into a socket using bare wires and matches. Ingenious use of sellotape (Scotch tape for US Mudcatters) allowed the mike to be linked into this. The mike itself came from a WWII tank and was on a stand that during the rest of the week supported a budgie's cage. Later two more speakers were added to the setup - one didn't even have a case! This was real hi-fi!

We did have the occasional shower of sparks and sometimes all the lights went out, which was fun but nobody was actually killed.

Nowadays everything is so much more complicated - or illegal!

Eddie


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Subject: RE: Tech: Valve amps & 'Y' speaker cables ???
From: Joe Offer
Date: 20 Jan 11 - 02:47 AM

I've often had questions about what speakers I can plug where into what amplifiers. If somebody can give us a basic understanding of this, it would be very helpful.
I suppose Richard is saying that if two 16-ohm speakers are wired in parallel, that halves the resistance and makes it 8 ohms. And I suppose they would make 32 ohms if wired in a series.
Or not. I'm mostly just guessing.

-Joe-


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Subject: RE: Tech: Valve amps & 'Y' speaker cables ???
From: Mitch the Bass
Date: 20 Jan 11 - 04:04 AM

The short answer is 2 speakers in parallel look like a speaker with half the impedance. Two speakers in series look like a speaker with twice the impedance.

2 x 16 ohms in parallel = 8 ohms
2 x 8 ohms in parallel = 4 ohms
4 x 16 ohms in parallel = 4 ohms
2 x 8 ohms in series = 16 ohms
2 x 4 ohms in series = 8 ohms

Chapter and verse on impedances in audio (including speakers and specifically valve amps near the end) at http://www.soundonsound.com/sos/jan03/articles/impedanceworkshop.asp

Mitch


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Subject: RE: Tech: Valve amps & 'Y' speaker cables ???
From: treewind
Date: 20 Jan 11 - 05:01 AM

Add to Mitch's summary: it is always safe to connect a speaker combination whose impedance is more that what the amplifier is rated for.

E.g.
Amp spec 8 ohms, speaker can be 8 or more e.g. 16
Amp spec 4 or 8, speaker can be 4 or more e.g. 8, 16

"Safe" in the sense that a transistor amplifier won't overheat because a higher impedance speaker means less current flows.

Valve ("tube" in the USA) amplifiers are a bit different as they tend to limit current safely, so you are less likely to get damage if you connect a speaker of too low an impedance. With any mismatch, however, you'll not get the amplifier's maximum possible power.

Joe: "if two 16-ohm speakers are wired in parallel, that halves the resistance and makes it 8 ohms. And I suppose they would make 32 ohms if wired in a series."
Absolutely correct.


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Subject: RE: Tech: Valve amps & 'Y' speaker cables ???
From: The Fooles Troupe
Date: 20 Jan 11 - 05:49 AM

A subtle thing many do not know, is that if you connect two DIFFERENT speakers in series (simplest case is both with same impedance, but if they differ in impedance, one can overload one of them - that's another story) you get a pseudo stereo spread.

This is because if the two different speakers have differing frequency spectrum responses, you get an apparent pseudo stereo spread, as each speaker is slightly louder than the other at differing frequencies.


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Subject: RE: Tech: Valve amps & 'Y' speaker cables ???
From: GUEST,BobL
Date: 20 Jan 11 - 06:57 AM

I take it we talking about an amp and speaker in a box, with the internal speaker permanently connected, and a separate impedance-selectable output for an external speaker.

Quite simply, the impedance plugged into this output should match (or be more than) the value marked on the socket, so you can plug a single 16-ohm speaker into the 16-ohm socket, or two paralleled 16-ohm speakers into the 8-ohm socket. Or two 4-ohm speakers in series into the 8-ohm socket, come to that. The impedance of the internal speaker shouldn't come into it.


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Subject: RE: Tech: Valve amps & 'Y' speaker cables ???
From: JohnInKansas
Date: 20 Jan 11 - 07:50 AM

it is always safe to connect a speaker combination whose impedance is more that what the amplifier is rated for.

One should never say "always."

One way of looking at it is that valve (tube) electronics are voltage devices. The amplifier produces a voltage, and the load (speaker) determines how much current that voltage will push out. If the load is a higher resistance, less current flows, so the power (the thing that makes smoke) will be low, and likely within the capacity of the amp. If the load is low resistance, lots of current (and smoke) likely will come out of the amp.

Transistor devices mostly produce a current, and the load you connect determines what voltage that current will produce. If the load is too small a resistance, the current is still limited by the amp, but the voltage may be low. The speakers will receive little power, but the output transistors are unlikely to be overloaded in conventional circuits. If the load (speakers) are too high a resistance, direct coupled output transistors will still attempt to push the designed current through them, and the voltage will go "to the limit" that the power supply in the amp can supply. With good design, the power section of the amp will limit how high the output transistors can go, but designs aren't always completely optimised. The damage probably will be in the amp rather than to the speakers, but the end result is still somewhat likely to include smoke.

The sound made by an overdriven transistor often is a bit like a kernel of popcorn going off, but I don't know of any edible products resulting from it.

"Modern" transistor amps usually have an isolator of some sort - often output transformers - and sometimes other "limiters" like zener regulated voltage clamps, so that the better designed ones will survive almost anything you can connect to them; but it's best to observe the ratings if you want decent performance and long healthy life for your hardware.

If you anticipate connecting "combinations" of speakers to an amp designed to drive a single channel, there are various "pads" available that allow the amp to see it's rated load while the speakers see (approximately) their expected inputs - within limits. These were better known when all the amps had hot glass innards, but if you can find someone who knows what you're talking about you probably can still find them.

Very old versions of the "Ham Book" (Radio Operators' Handbook, or something like that) suggested a "ladder connection" that can sometimes be used quite effectively to connect "many identical loads" to a single driver. I don't know whether anyone else who heard of the configuration is still on the outside (of the asylum) and I'd have to do some digging to see if enough survives in the gray matter to offer much more than the name, but Google might find a "ladder network" or a "T-Pad" for you, with explanations.

John


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Subject: RE: Tech: Valve amps & 'Y' speaker cables ???
From: pdq
Date: 20 Jan 11 - 10:31 AM

Threre are actually three things that can change when your amp's load that is not optimum:

    power
   
    frequency response

    feedback ratio

Many solid amps that draw feedback from the output will have much more feedback when loaded higher than optimum. Since feedback is used to get published distortion specs as low as possible, more feedback can cause instability or, at worst, oscillation, which can destroy speakers or the amp or both. Loading a solid state amp with higher than recommended loads is not adviseable unless you know the circuit.

Optimum load for both tube and solid state will give the most power and cause the output devices (solid state) to dissipate the least power possible.

Putting to low a load on a tube amp usually results in poorer frequency response which is important in very high quality stereo systems but not so important in a guitar amp.

As everyone has said, two 8 ohm speakers should paralleled and put on the 4 ohm tap. Performance is still affected by efficiency, since the more efficient unit will play louder than the other speaker.


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Subject: RE: Tech: Valve amps & 'Y' speaker cables ???
From: Richard Bridge
Date: 20 Jan 11 - 03:33 PM

Do not - repeat do not - assume that you can run valve amps safely into too high an impedance. They do not like it. I am not wholly sure why. But you can often hear it - after a while, when nothing is being played, you can hear them produce a periodic "pop" like a dud capacitor discharging. They will often tolerate too low a load impedance better. I suspect it is because valves are a voltage amplifier not a current amplifier so it is easy to produce excessive internal voltages into high impedances - but I don't absolutely know.

Transistors - safe into high impedances, dead into low ones!


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Subject: RE: Tech: Valve amps & 'Y' speaker cables ???
From: Richard Bridge
Date: 20 Jan 11 - 03:37 PM

PS - resistance in series of A, B, &C, etc - A+B+C=D

Resistance in parallel of A, B, C etc 1/A + 1/B + 1/C =1/D.

Complex loads that include capacitance or inductance are much more difficult to work out because it depends on frequency.


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Subject: RE: Tech: Valve amps & 'Y' speaker cables ???
From: Leadfingers
Date: 20 Jan 11 - 08:37 PM

Its a long time since I played with Electronic theory - but , as I see it , IF the amp has a built in speaker AND a choice of 8 Ohm or 16 Ohm output sockets , a Y Splitter feeding two 16 Ohm speakers - WITH the assumption that they are Identical speakers - run from th 8 Ohm socket , you will effectively have a Three Speaker system that should sound OK !


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Subject: RE: Tech: Valve amps & 'Y' speaker cables ???
From: pdq
Date: 20 Jan 11 - 08:56 PM

Again, all amps should be loaded correctly.

There is no way to be certain that a solid state or tube amp can drive a load that is higher impedance than optimum and be safe. The reason is that the feedbak ratio goes up and that can lead to intability. The "pop" sound is a temporary oscillation called a "birdie" by sound men and radio engineers. Full oscillation will cause damage your speakers on both solid state and tube amps, and can destroy the output devices of solid state amps.

Only putting too low a load on valve amps is safe, and that is to maybe a factor of four, but it causes a large increase in distortion.


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Subject: RE: Tech: Valve amps & 'Y' speaker cables ???
From: Bernard
Date: 21 Jan 11 - 10:57 AM

It's worth noting, too, that Ohms Law alone is not an accurate way of determining the output power of an amlifier into a given load.

Other factors, such as the actual voltage an amplifier is capable of producing at a given impedance, can seriously affect the power.

As an example, it is possible that an amplifier channel rated at 500 watts into 4 ohms may produce as much as 350 watts into 8 ohms, and not the 250 watts that Ohms Law would suggest. However, two identical 8 ohm loudspeakers would share the 500 watts at 4 ohms and demand 250 watts each.

When in doubt, consult the manufacturer's specification! If you don't have it, then be very cautious!

Being silly about impedances on a valve amplifier can result in burned out valve bases, too.


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Subject: RE: Tech: Valve amps & 'Y' speaker cables ???
From: GUEST,erbert
Date: 21 Jan 11 - 12:50 PM

Thanks for the replies and advice.

The combo amp is factory configured so that the internal speaker is disconnected
if using any of the rear extension speaker outputs.
So that's why I need a 'workaroud' alternative option.

Now I've had a chance to look closer inside the combo,
I'm happily surprised to find the on-board 16 Ohms speaker is not hardwired to the circuit
as I presumed,
but connected by it's own dedicated internal jack socket
hidden away behind the valves in a dark corner out of sight under the chassis.
So I reckon I can get away without need to buy or make up a "Y" splitter lead.
The external 16 Ohms cab has input and output jacks;
so I should be able to run a lead from the combo's rear 8 Ohms output to the extension cab,
then disconnect the combo's internal 16 Ohms speaker, and plug it into the passive output of the speaker cab
using a male to female jack extension speaker lead.

Instant mini stack !!!!

1x10" open back 5 watt valve combo sat on top of matching 1x12" 50 watt sealed back cab.


Should be interesting;

[reasonable quality OEM speakers, so no available tech specs for 'efficiency']
.. I'm guessing the smaller brighter toned 10" speaker [unknown wattage, but at least sufficent for purpose]
will begin to crunch nicely
while the 50 watt 12" cab remains clean and beefy..

.. and I can connect up a speaker emulated power attenuator/line-out -> DI box to the frontline mixer whenever necessary...


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