Wem Dominator 45 Bass impedence question

[Christophano]

Hey Basschatters,

I wondered if I could pick your brains!?!

I'm in the process of putting a shopping list together to do a bit of a restoration of my old WEM Dominator 45 Bass. It's a 15" combo with 4x el84's and 3x ecc83's.

The old Fane speaker looks and sounds a bit naff and I was going to replace it, it doesn't say whether it's a 8 or 16 ohm speaker and I can't find any solid conformation to say which one it is. I also don't have the means to test the speakers impedance.

Anyone got any idea what I should purchase?

Thanks in advance for anyone's advice.

[bumnote]

[quote name='Christophano' post='1048717' date='Dec 5 2010, 08:57 PM']Hey Basschatters,

I wondered if I could pick your brains!?!

I'm in the process of putting a shopping list together to do a bit of a restoration of my old WEM Dominator 45 Bass. It's a 15" combo with 4x el84's and 3x ecc83's.

The old Fane speaker looks and sounds a bit naff and I was going to replace it, it doesn't say whether it's a 8 or 16 ohm speaker and I can't find any solid conformation to say which one it is. I also don't have the means to test the speakers impedance.

Anyone got any idea what I should purchase?

Thanks in advance for anyone's advice.[/quote]

If you are restoring a vintage amp have you considered reconing the loudspeaker which will help to keep its vintage character

[url="http://www.wembleyloudspeaker.com/"]http://www.wembleyloudspeaker.com/[/url]

can do it

[icastle]

[quote name='Christophano' post='1048717' date='Dec 5 2010, 08:57 PM']Hey Basschatters,

I wondered if I could pick your brains!?!

I'm in the process of putting a shopping list together to do a bit of a restoration of my old WEM Dominator 45 Bass. It's a 15" combo with 4x el84's and 3x ecc83's.

The old Fane speaker looks and sounds a bit naff and I was going to replace it, it doesn't say whether it's a 8 or 16 ohm speaker and I can't find any solid conformation to say which one it is. I also don't have the means to test the speakers impedance.

Anyone got any idea what I should purchase?

Thanks in advance for anyone's advice.[/quote]

Thats a pretty tough question.
Back when the WEM Dominator 45 was made, Fane had a ridiculously large product range.
If I had to make a guess, I'd go for 8Ω, but it is only a guess.
Your best bet really would be to either buy a cheapy multimeter (even Maplin do one for £5!) or borrow one off a mate.
Disconnect one of the wires going to the speaker, set the multimeter to the Ω setting, shove the meter probes onto the two speaker terminals and see what the meter says! Simples

[Ghost_Bass]

[quote name='icastle' post='1048747' date='Dec 5 2010, 09:23 PM']Disconnect one of the wires going to the speaker, set the multimeter to the Ω setting, shove the meter probes onto the two speaker terminals and see what the meter says! Simples [/quote]

Don't forget to multiply that reading for 1,3! I had a similar question a few months back regarding a EVM15L i had laying around wich the multimeter read at 6Ω until someone told me i had to do that multiplication (don't remember why) but it turned out to be an 8Ω speaker like the makers specs said.

Edited December 6, 2010 by Ghost_Bass

[bumnote]

[quote name='icastle' post='1048747' date='Dec 5 2010, 09:23 PM']Thats a pretty tough question.
Back when the WEM Dominator 45 was made, Fane had a ridiculously large product range.
If I had to make a guess, I'd go for 8Ω, but it is only a guess.
Your best bet really would be to either buy a cheapy multimeter (even Maplin do one for £5!) or borrow one off a mate.
Disconnect one of the wires going to the speaker, set the multimeter to the Ω setting, shove the meter probes onto the two speaker terminals and see what the meter says! Simples [/quote]

That wont give you an accurate measure as you will be measuring resistance which is not the same as impedence

[Bigwan]

If you know wiring and are confident enough you could open the amp up and have a nosey at the output transformer - it may say what tap the speaker is wired to.

[Slipperydick]

As said, DC ohms is not impeadence, so you cant really trust it.

IIRC Fane used to sell speakers marked 8-15 ohms. Would make sense to ring Watkins Electric Music, I found em really helpful. 020 8679 5575

[Christophano]

Thanks for the advice chaps, it all seems good.

I did hook the dominator up to my 8ohm 4x10 cab and it sounded pretty rippin', not to say that this is the right impedance though!...

I'll have a look at the output transformer to see if it can give me a clue, then ring watkins if I don't find anything.

[Phil Starr]

Just so you are clear resistance and impedance are not the same but the impedance is always a little above the DC resistance depending upon the exact form of the voice coil. In practice an 8 ohm speaker usually measures around 6.3 ohms and a 4 ohm speaker will be half this. It will probably be 8 ohms. The Fane speakers of those days had great efficiency but like many speakers of that era high power melted the glue in the voice coil and distorted the former so they tended not to be reliable long term. If Watkins can't help then tell us what you want to achieve and the internal volume of the cabinet and we can suggest a replacement.

Are you looking for the best sound possible or to restore the original sound?

[Christophano]

Thanks for that info, Phil.

I'm very green to this, but would like to learn as I find it all very interesting.

I would like to enhance the Dominator as much as possible really without ruining it, full of beef, with a hint of chardonnay.

I've been putting together a shopping list on tubeampdoctor.com New matched valves, pots and caps. Anything else you can suggest that I should look in to changing/upgrading?

[dincz]

[quote name='Christophano' post='1049048' date='Dec 6 2010, 10:48 AM']Anything else you can suggest that I should look in to changing/upgrading?[/quote]

It might be worth having a close look at the EL84 valve sockets. If there's any sign of previous arcing or burning (i.e. charcoal), it'll be worth having them replaced.

[Phil Starr]

[quote name='Christophano' post='1049048' date='Dec 6 2010, 09:48 AM']Thanks for that info, Phil.

I'm very green to this, but would like to learn as I find it all very interesting.

I would like to enhance the Dominator as much as possible really without ruining it, full of beef, with a hint of chardonnay.

I've been putting together a shopping list on tubeampdoctor.com New matched valves, pots and caps. Anything else you can suggest that I should look in to changing/upgrading?[/quote]
If you are doing major surgery you might want to replace some of the old carbon resistors though make sure you get ones that can handle the wattage.

[icastle]

[quote name='bumnote' post='1048978' date='Dec 6 2010, 06:47 AM']That wont give you an accurate measure as you will be measuring resistance which is not the same as impedence[/quote]

No it won't but it'll be close enough to determine what the impedance of a single unmarked speaker is.

[bumnote]

[quote name='icastle' post='1049540' date='Dec 6 2010, 04:42 PM']No it won't but it'll be close enough to determine what the impedance of a single unmarked speaker is.[/quote]




If an 8 ohm speaker reads at 6.3 ohms as per Phils post above, and without further clarification, the speaker could be identified under your method of either 4 or 8 ohm.

[umph]

schematic says 12ohms

[icastle]

[quote name='bumnote' post='1049733' date='Dec 6 2010, 07:17 PM']If an 8 ohm speaker reads at 6.3 ohms as per Phils post above, and without further clarification, the speaker could be identified under your method of either 4 or 8 ohm.[/quote]

Only if you round it down.

[Christophano]

[quote name='umph' post='1049767' date='Dec 6 2010, 07:49 PM']schematic says 12ohms[/quote]


Have you got a schematic for a Dominator 45 Bass? I'd love to get my hands on it if you have.... Could I ask if you would kindly send it to me please?

[Phil Starr]

Just in case some of the posts have confused you I'll repeat the ohms thing. The impedance of a speaker is nominal, an approximation because the reality is that the impedance varies with frequency. It is made up of the DC resistance of the coil plus an extra bit of resistance due to the inductance of the coil which varies with frequency. If you want to go on Eminence's or Fane's sites and look up the specs of the speakers they both give a chart showing how the impedance varies with frequency and they give the DC resistance of the coils.

The impedance of the speaker therefore is always higher than the DC resistance and multiplying the DC resistance by 1.3 is as good a rule of thumb as any. Since the impedance varies with frequency you can be happy that most amps will work fairly well and with no risk to the amp with a slight mis-match. You might not get every last watt out though. If the amp is looking for a 12ohm speaker (no longer available) then an 8 or 16 would both be acceptable.

Give us the dimensions of the cab and how much you might want to spend and we can make some suggestions.

[umph]

[quote name='Christophano' post='1049812' date='Dec 6 2010, 08:24 PM']Have you got a schematic for a Dominator 45 Bass? I'd love to get my hands on it if you have.... Could I ask if you would kindly send it to me please?[/quote]

havn't got the exact schematic on my computer, they're very similar to the dominater 50 though with a different speaker

[goingdownslow]

I recall WEM amplifiers having a minimum load of 6Ω and their cabs being 12Ω
In fact there are a pair of 12Ω column speakers on [url="http://cgi.ebay.co.uk/PAIR-LARGE-WEM-TOWER-SPEAKERS-12-OHMS-WORKING-/150528773511?pt=UK_ConElec_SpeakersPASystems_RL&hash=item230c36cd87"]eBay[/url]

Edited December 7, 2010 by goingdownslow

[Christophano]

Hmmm, interesting. So if it had a minimum load of 6 ohms an 8 ohm speaker could be alright. I've got a multimeter in the post so will find out for sure after i've had a play around.

Will post up my results!

[icastle]

[quote name='Christophano' post='1051636' date='Dec 8 2010, 01:32 PM']Hmmm, interesting. So if it had a minimum load of 6 ohms an 8 ohm speaker could be alright. I've got a multimeter in the post so will find out for sure after i've had a play around.[/quote]

Yep.

Modern 'off the shelf' replacement speakers of the size you are looking for only come in three impedances:
4Ω - well below the minimum impedance of the amp and carries a high risk of damaging it.
8Ω - nearest value to the 6Ω minimum impedance of the amp.
16Ω - would work fine but output would be reduced.

With the multimeter, as I said before - make sure at least one of the leads running back to the amp are disconnected before you try measuring the speaker.
I'd not come across the trick of multiplying the read value by 1.3 before, but it works really well

[bumnote]

[quote name='icastle' post='1051732' date='Dec 8 2010, 02:41 PM']Yep.

Modern 'off the shelf' replacement speakers of the size you are looking for only come in three impedances:
4Ω - well below the minimum impedance of the amp and carries a high risk of damaging it.
8Ω - nearest value to the 6Ω minimum impedance of the amp.
16Ω - would work fine but output would be reduced.[/quote]

Unlike transistor amps, providing the output from the amplifier is set to the impedance value of the speaker, you will get the rated output from the amp, it doesnt matter if its 4, 8, or 16

However, a valve amp must be set to the impedance value of the speaker, otherwise when a signal passes through it there is a real danger of damaging the amplifier.

Therefore 'IF' it is a 6 ohm output and you connect a 15 ohm speaker, you will not get reduced output, you could very well fry the amp.

[Christophano]

[quote name='bumnote' post='1052036' date='Dec 8 2010, 07:49 PM']Unlike transistor amps, providing the output from the amplifier is set to the impedance value of the speaker, you will get the rated output from the amp, it doesnt matter if its 4, 8, or 16

However, a valve amp must be set to the impedance value of the speaker, otherwise when a signal passes through it there is a real danger of damaging the amplifier.

Therefore 'IF' it is a 6 ohm output and you connect a 15 ohm speaker, you will not get reduced output, you could very well fry the amp.[/quote]

Crikey.... There isnt an impedance changer switch or anything... If such a thing became a problem, how would one change the impedance the amp was chucking out? Knock three times?

[bumnote]

[quote name='Christophano' post='1052208' date='Dec 8 2010, 10:41 PM']Crikey.... There isnt an impedance changer switch or anything... If such a thing became a problem, how would one change the impedance the amp was chucking out? Knock three times?[/quote]


I dont know your amp well but if you had for example a marshall from that period it would have a plug on the back, which you could change from 15 ohms to 7.5 ohms, depending if you were using 1 cab or two.

With your amp, it was designed as a package, ie the speaker was chosen to match the characteristics of the amp. Actually in those days, it was probably what was available.

There may be different tappings on theoutput transformer to allow you to change, I have no idea.

Much easier is to find accurately the impedance of the original speaker, and get a replacement to that spec, or have the existing one rebuilt which will retain the vintage characteristics.

Phil Star says measuring with a multimeter and multiplying x 1.3 will give you a good idea and he knows far more than i do. Alternatively contact WEM and ask them

This part of an article from sound on sound

Whereas most modern solid-state amplifiers are virtually bombproof in terms of whether their outputs see proper loudspeakers (of any nominal [size=4]impedance) or a short or open circuit, most valve amplifiers are far less tolerant. In fact, the majority of vintage valve amps will self-destruct if driven without the correct speaker load attached! The reasons are complex and depend to some extent on the design of the output circuit, but can be boiled down to what are called 'reflected' impedances.
[/size]
Most, if not all, practical valve amplifiers employ an output transformer. The use of the transformer is principally to translate the effective load impedance between that required by the valves, and that of a practical loudspeaker — a typical valve output circuit requires a load of between 5k(omega) and 10k(omega), whereas a practical loudspeaker presents a nominal impedance of between 4(omega) and 15(omega). The transformer does this by 'reflecting' the loudspeaker's impedance through the transformer (as a function of the square of its turns ratio) to create a different (in this application, higher) load impedance for the valve output stage. Thus a 15(omega) speaker will appear to the output valves as a 9k(omega) load, say. It is important to note that it is the physical loudspeaker's impedance that defines the operating load for the output stage, and that valve amplifiers are very fussy about their load impedance. If a loudspeaker with a different speaker is connected, the output valves will see a different load and their performance and operating characteristics will change as a result.

Consequently, to make the system more flexible in accommodating different loudspeakers, many valve amplifiers have different output terminals (or some way of selecting nominal output impedances) for different loudspeaker loads. This is achieved by using different tappings on the output transformer so that an 8(omega) speaker connected to the correct terminals will produce the same reflected impedance to the output stage as a 15(omega) speaker connected to its appropriate terminals.

So what happens if the loudspeaker is disconnected? Well, instead of the 15(omega) load being reflected into a 9k(omega) load for the valves, we now have an infinite load, which will be reflected as an infinite load to the valves. For a given current, an infinite load requires an infinite voltage. Imagine a brief positive transient audio signal (a drum strike, perhaps) driving the output valves to the unloaded output transformer. When that transient stops, the magnetic field developed in the transformer collapses and generates a reverse polarity signal called the 'back EMF'. With an infinite load impedance, the back EMF will tend towards an infinite reversed voltage spike and this is applied directly to the valve anode plate. Depending on the valve in use, this huge back EMF is likely to far exceed its rated values and so may cause the valve to break down, damaging or destroying the grids or anode plate, and resulting in one very poorly amplifier.

However, this huge back EMF can only be generated if the amplifier is being driven in the first place. If there is no input signal to the amplifier, there will be no output signal, and so no back EMF. Under these rather exceptional circumstances there is unlikely to be any damage. The most sensible thing to do, however, is to always check that a suitable loudspeaker is connected to a valve amplifier (with the correct output terminals or transformer taps selected), before you connect or turn up the input.

Edited December 9, 2010 by bumnote