Happy Jack Posted December 29, 2011 Share Posted December 29, 2011 I have an early 70's WEM Dominator 25 combo, all valve of course, and great tone - just not enough volume for modern gigs. I'm looking into introducing a switchable socket that would allow me to choose whether to run the amp through the on-board Rola Celestion 15" or into an external cab, most probably a Barefaced Compact. The on-board Celestion is rated at 15 Ohms, the Compact is 8 Ohms. Do I have a problem? If so, is there an easy solution? Quote Link to comment Share on other sites More sharing options...
thepurpleblob Posted December 29, 2011 Share Posted December 29, 2011 Unless you know for a fact that the WEM will work at 8 Ohms then, yes, you have a problem. I'm not a big expert on valve amps but I seem to recall that they are much fussier about the output impedance than transistor amps (another reason valve amps are a bad idea, but lets not go there ). The only thing you can do is to get another 8 ohm cab and run them in series. This gives you 16ohms which will do. However, you will need a weird cable and the end result will probably be rubbish. Quote Link to comment Share on other sites More sharing options...
Mr. Foxen Posted December 29, 2011 Share Posted December 29, 2011 It will probably work into 8 ohms, but it won't be ideal, the most destructive mismatch is much further mismatched, and mismatched the other way, high impedance cab, or no cab at all. Too low impedance is just tone trouble, and you may like the tone just fine. Easiest mode is put a socket on the amp and a lead on the speaker, or another socket, so you can choose internal speaker of external with your plugging. Just make sure you don't choose neither. Quote Link to comment Share on other sites More sharing options...
icastle Posted December 29, 2011 Share Posted December 29, 2011 Why not just wire an 8Ω 25W resistor in series with your new output socket to fool the amp into seeing 16Ω? A 1Ω difference is nothing in the big scheme of things. Use one of the switchable sockets so you disconnect the internal speaker as you push the plug in just to be on the safe side. Quote Link to comment Share on other sites More sharing options...
Mr. Foxen Posted December 29, 2011 Share Posted December 29, 2011 For a start you lose half the power of the amp into the resistor. Then that power turns to heat and the resistor explodes. Quote Link to comment Share on other sites More sharing options...
thepurpleblob Posted December 29, 2011 Share Posted December 29, 2011 (edited) [quote name='icastle' timestamp='1325192378' post='1480860'] Why not just wire an 8Ω 25W resistor in series with your new output socket to fool the amp into seeing 16Ω? A 1Ω difference is nothing in the big scheme of things. Use one of the switchable sockets so you disconnect the internal speaker as you push the plug in just to be on the safe side. [/quote] Because half of the power will go to warming up the resistor which means that the cab is only getting 12W max (i.e. not much) EDIT: >>snap<< the resistor will work because you get one rated at 25W. It's just a bit pointless though. Edited December 29, 2011 by thepurpleblob Quote Link to comment Share on other sites More sharing options...
icastle Posted December 29, 2011 Share Posted December 29, 2011 [quote name='thepurpleblob' timestamp='1325192710' post='1480869'] Because half of the power will go to warming up the resistor which means that the cab is only getting 12W max (i.e. not much) EDIT: >>snap<< the resistor will work because you get one rated at 25W. It's just a bit pointless though. [/quote] Well I never claimed it was a perfect solution. Quote Link to comment Share on other sites More sharing options...
icastle Posted December 29, 2011 Share Posted December 29, 2011 [quote name='Mr. Foxen' timestamp='1325192668' post='1480868'] For a start you lose half the power of the amp into the resistor. Then that power turns to heat and the resistor explodes. [/quote] The first part is true, but the second part isn't if you use a big enough resistor. Quote Link to comment Share on other sites More sharing options...
Happy Jack Posted December 29, 2011 Author Share Posted December 29, 2011 [quote name='Mr. Foxen' timestamp='1325191585' post='1480841'] It will probably work into 8 ohms, but it won't be ideal, the most destructive mismatch is much further mismatched, and mismatched the other way, high impedance cab, or no cab at all. Too low impedance is just tone trouble, and you may like the tone just fine. Easiest mode is put a socket on the amp and a lead on the speaker, or another socket, so you can choose internal speaker of external with your plugging. Just make sure you don't choose neither. [/quote] So I can try this without blowing anything up, but I may not like the resulting tone? Quote Link to comment Share on other sites More sharing options...
thepurpleblob Posted December 29, 2011 Share Posted December 29, 2011 [quote name='Happy Jack' timestamp='1325194185' post='1480895'] So I can try this without blowing anything up, but I may not like the resulting tone? [/quote] No you can't. That's not good advice. You would be running an amplifier "expecting" 16ohms into half that resistance. A partial short-circuit to rather over-state it. Quote Link to comment Share on other sites More sharing options...
Mr. Foxen Posted December 29, 2011 Share Posted December 29, 2011 [quote name='icastle' timestamp='1325193924' post='1480889'] The first part is true, but the second part isn't if you use a big enough resistor. [/quote] And a heat sink. If you look at 25w resistors, they are made in such a way you screw them to a heatsink. [quote name='Happy Jack' timestamp='1325194185' post='1480895'] So I can try this without blowing anything up, but I may not like the resulting tone? [/quote] Stuff can always blow up in a vintage amp not serviced. But they can cope with a short, which is why usually the output jacks are shorting ones, so you are semi safe if nothing is plugged in. A mismatch a step either way isn't generally harmful unless you get silly with it. Quoting relevant nerd, was trying to find BFMs because he is less wordy, but the wordyness makes it easier to search: [quote name='alexclaber' timestamp='1306166247' post='1241870'] The AD200 is a valve amp so the rules are totally different to the solidstate amp ones. It looks like it has both an 8 ohm tap and a 4 ohm tap on the output transformer and you can use either one but not both. You could safely run three 8 ohm cabs on the 4 ohm tap, that won't bother it at all as valve amps are sensitive to overly HIGH not low impedances. Likewise I wouldn't envisage a problem running all those cabs you describe off the 4 ohm tap. [/quote] [quote name='alexclaber' timestamp='1306237656' post='1242844'] It's a valve amp with an output transformer, not a solid-state amp! All the posts talking about increased current flow at low impedances are wrong - valves and output transformers do not behave like that. This is quite informative: [url="http://www.metaltronix.net/metaltronix-amp-basics.htm"]http://www.metaltron...-amp-basics.htm[/url] 4 ohm cab on an 8 ohm tap and you'll get about half the 8 ohm power. 16 ohm cab on an 8 ohm tap and you'll get about half the rated power. Go lower than 4 ohms and the power decreases further but the amp doesn't care. Go higher than 16 ohms and the power decreases further AND you risk killing your amp. (The reasons are complicated but explained in that link). I'll say it again - valve amps do not behave like transistor amps! [/quote] Quote Link to comment Share on other sites More sharing options...
icastle Posted December 29, 2011 Share Posted December 29, 2011 (edited) [quote name='thepurpleblob' timestamp='1325194540' post='1480901'] No you can't. That's not good advice. You would be running an amplifier "expecting" 16ohms into half that resistance. A partial short-circuit to rather over-state it. [/quote] +1 I certainly wouldn't want to risk it. With valve amps an ohm or two here or there isn't too bad but as soon as you start halving or doubling the load it expects to see then you're going to get a repair bill somewhere down the line. Jack - to go back to your original requirement, the point that's being missed in this thread is that, even if you added a different cab to your combo, it's limitation is that it's only ever going to be a 25W into 16Ω amp. I very much doubt you'll ever be able to successfully gig with it unless you mic it up and stick it through the PA (at which point you'll probably lose half of the tone you liked so much in the first place of course). Edited December 29, 2011 by icastle Quote Link to comment Share on other sites More sharing options...
thepurpleblob Posted December 29, 2011 Share Posted December 29, 2011 Yesss.... the stuff about the output stages of valve amps is all quite true (the output stages work a transformer so it's a whole different ball game). I actually have never studied the theory of it, BUT... if it was my amp if the output stage said 15 ohms then I would be thinking of connecting something in the region of 15/16ohms to it. Just to be on the safe side. I'm going to have to read more about valve stuff.. sigh! Quote Link to comment Share on other sites More sharing options...
Dad3353 Posted December 29, 2011 Share Posted December 29, 2011 A great deal of the tone of your amp comes from the speaker/cab combination. It will do no harm at all to try coupling to a Barefaced instead, and, if you really like the result, modify permanently, but I seriously doubt that the result will be anything comparable to the present, standard, sound. No harm in trying, and you may get a very pleasant surprise. I'd be curious to hear how this turns out, if you decide to go for it. I did quite a bit of mixing/matching back then (anyone remember the 'Concord' valve amps from Tottenham Court Road..?), and every combination of amp/speaker/cab was radically different,mostly in unpredictable ways. Really, the answer's a lemon; suck it and see..! You won't blow the WEM. Quote Link to comment Share on other sites More sharing options...
Beer of the Bass Posted December 30, 2011 Share Posted December 30, 2011 One other thing worth a look; a lot of old valve amps have unused taps on the output transformer, so there might even be a 7.5ohm tap which could just be wired up to a socket. It might be worth either you or a tech checking it out to see if this is the case here. Quote Link to comment Share on other sites More sharing options...
pete.young Posted January 1, 2012 Share Posted January 1, 2012 [quote name='Happy Jack' timestamp='1325185364' post='1480756'] I'm looking into introducing a switchable socket that would allow me to choose whether to run the amp through the on-board Rola Celestion 15" or into an external cab, most probably a Barefaced Compact. [/quote] I thought that the Barefaced Compact was a cabinet that requires loads of power to get the best out of it - is a 25W Wem amp really going to have enough power to drive it? What about close-miking it and running the result through your PA? Quote Link to comment Share on other sites More sharing options...
Mr. Foxen Posted January 1, 2012 Share Posted January 1, 2012 [quote name='pete.young' timestamp='1325416456' post='1482848'] I thought that the Barefaced Compact was a cabinet that requires loads of power to get the best out of it - is a 25W Wem amp really going to have enough power to drive it? [/quote] The woofer/mid Barefaced cabs want the power, the Compact is pretty comparable or better to any other 15 in terms of output to power, it can just take much more power and thus go louder. Quote Link to comment Share on other sites More sharing options...
Happy Jack Posted January 1, 2012 Author Share Posted January 1, 2012 [quote name='Johnston' timestamp='1325421661' post='1482924'] [url="http://www.ampmaker.com/store/Box-of-Matches-30W.html"]http://www.ampmaker....atches-30W.html[/url] [/quote] This looks like an interesting idea, but it would worry me that they describe their product as a [b]matching tansformer.[/b] For some reason this leaves me wondering about their attention to detail. Quote Link to comment Share on other sites More sharing options...
Happy Jack Posted January 1, 2012 Author Share Posted January 1, 2012 [quote name='pete.young' timestamp='1325416456' post='1482848'] What about close-miking it and running the result through your PA? [/quote] Believe me, I've thought about it, especially with the current offer from Red Audio: http://www.red5audio.com/cgi-bin/sh000001.pl?REFPAGE=http%3a%2f%2fwww%2ered5audio%2ecom%2fcgi%2dbin%2fsh000001%2epl%3fREFPAGE%3dhttp%253a%252f%252fwww%252ered5audio%252ecom%252facatalog%252fMicrophones%252ehtml%26WD%3dbass%2520amp%26SHOP%3d%2520%26PN%3dDynamic_Microphones%252ehtml%2523a78&WD=rvd1&SHOP=%20&PN=Drum_Kit_Mics%2ehtml%23a13#a13 Trouble is, we'll still be a pub rock band and if all I want is to feed a bit of bass through the PA then running off the DI just makes so much more sense. No one in the audience at a typical London pub will be able to hear the wonderful tone I get from the WEM, and neither will I since I stand behind the FOH speakers. Quote Link to comment Share on other sites More sharing options...
Happy Jack Posted January 3, 2012 Author Share Posted January 3, 2012 I think I'll start with the cheapest option and try mic'ing up the WEM. I have a variety of mic's that I can experiment with, including a Red5 Audio RVD1, a Thomann SM55 copy, and a bunch of uber-cheap vocal mic's. If I experiment by trying out expensive vocal mic's (i.e. a genuine Shure SM55 and a Shure Beta SM57) is there much I can do to wreck 'em by mistake? Quote Link to comment Share on other sites More sharing options...
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