Noob Amp Question

[Freddy V-C]

See? Even the thread title is an apology.

So I've finally sold my Vox Valvetronix abomination, after realising that everything sounded horribly digital through it. But of course, this means I have to buy another decent-sized amp for gigging with. I'm pretty sure I need to get a tube amp of some sort, but due to my lack of a job currently I can't spend many hundreds of pounds all at once, so the plan is to buy a 2x12 cabinet now (I'm thinking this one because it's cheap and, as far as I can gather, the cab doesn't make an enourmous difference to the sound) and then, when a kind soul finally decides to employ me, buying a fairly decent tube head at a later date. So the question is, until then can I use my 15w tube combo amp as a head? I have no idea how any of it works, really, so I'm just checking beforehand so I don't do this and then make something explode...

Oh, and also, what's the difference between guitar cables and speaker cables? I've only ever used combo amps until now, so I've never connected a head to a cabinet, ever.

[Bacchus]

See? Even the thread title is an apology.

So I've finally sold my Vox Valvetronix abomination, after realising that everything sounded horribly digital through it. But of course, this means I have to buy another decent-sized amp for gigging with. I'm pretty sure I need to get a tube amp of some sort, but due to my lack of a job currently I can't spend many hundreds of pounds all at once, so the plan is to buy a 2x12 cabinet now (I'm thinking this one because it's cheap and, as far as I can gather, the cab doesn't make an enourmous difference to the sound) and then, when a kind soul finally decides to employ me, buying a fairly decent tube head at a later date. So the question is, until then can I use my 15w tube combo amp as a head? I have no idea how any of it works, really, so I'm just checking beforehand so I don't do this and then make something explode...

Oh, and also, what's the difference between guitar cables and speaker cables? I've only ever used combo amps until now, so I've never connected a head to a cabinet, ever.

It will be doable, the using the combo as a head thing. Is there a cable coming out of the back of the amp bit of the combo going into the speaker part? If not, then you might have a bit of drilling and such to do. Someone here can talk you through that, but it might be easier getting someone else to do it. I'll be doing something similar soon to a bass amp, but for the opposite reason (I want to know what the 15" speaker in the amp sounds like being driven by a guitar amp).

The cabinet will make a difference to the sound, but I've not heard people say bad things about the Harley Benton ones.

Guitar cables and speaker cables are very different. They just look the same. As far as I know, one ought to have as high a resistance as possible, and one as low a resistance.

[Freddy V-C]

See? Even the thread title is an apology.

So I've finally sold my Vox Valvetronix abomination, after realising that everything sounded horribly digital through it. But of course, this means I have to buy another decent-sized amp for gigging with. I'm pretty sure I need to get a tube amp of some sort, but due to my lack of a job currently I can't spend many hundreds of pounds all at once, so the plan is to buy a 2x12 cabinet now (I'm thinking this one because it's cheap and, as far as I can gather, the cab doesn't make an enourmous difference to the sound) and then, when a kind soul finally decides to employ me, buying a fairly decent tube head at a later date. So the question is, until then can I use my 15w tube combo amp as a head? I have no idea how any of it works, really, so I'm just checking beforehand so I don't do this and then make something explode...

Oh, and also, what's the difference between guitar cables and speaker cables? I've only ever used combo amps until now, so I've never connected a head to a cabinet, ever.

It will be doable, the using the combo as a head thing. Is there a cable coming out of the back of the amp bit of the combo going into the speaker part? If not, then you might have a bit of drilling and such to do. Someone here can talk you through that, but it might be easier getting someone else to do it. I'll be doing something similar soon to a bass amp, but for the opposite reason (I want to know what the 15" speaker in the amp sounds like being driven by a guitar amp).

The cabinet will make a difference to the sound, but I've not heard people say bad things about the Harley Benton ones.

Guitar cables and speaker cables are very different. They just look the same. As far as I know, one ought to have as high a resistance as possible, and one as low a resistance.

Sounds a little bit more complicated than I was expecting, in that case. I was just thinking I could connect the cab to the combo amp via the Line Out/External Speaker jack?

[Mityushikha]

1) I didn't think the valvetronix series was bad at all, since I got a tube amp my lead guitarist uses my ad30vt at practice and live!! Of course it depends entirely on the sound that you want to get out of it.

2) The cabinet is going to make a difference. Also, speakers are heavy...

3) There is a good chance you can use the combo amp as a head, what type is it though?

[paul_]

Because low-impedance/passive or line-level instruments (guitar/bass/keyboard) send far less current out than an amp/mixer, instrument cables are not built to handle large power loads, and the resulting strain in using one to plug your amp into your cab can fry your amp's output transformer.

Furthermore, using a speaker cable to plug an instrument into an amp/mixer will produce horrible noisy interference, because an instrument cable is a hot core wire surrounded by a stranded/braided shielding wire; the conductors in a speaker cable are the same as each other, simply braided with each other and unshielded.

You should be able to output your combo to a speaker cabinet, though you should always make sure the cab has an impedance (ohm rating) higher or equivalent to your amp's to avoid a stinky, smoking fracas.

So if your amp is 8ohm, get an 8 or 16 ohm speaker cab... not a 4ohm one.
Also more ohms = more resistance so you want to try and match it or not be too far ahead with the cab, for efficiency... basically if the ohms are too high [on the cabinet -edit], you'll be running at less power.

[Freddy V-C]

1) I didn't think the valvetronix series was bad at all, since I got a tube amp my lead guitarist uses my ad30vt at practice and live!! Of course it depends entirely on the sound that you want to get out of it.

2) The cabinet is going to make a difference. Also, speakers are heavy...

3) There is a good chance you can use the combo amp as a head, what type is it though?

1) Yeah, my main issue was that I've started using pedals a lot now, and it's really difficult to get a completely clean sound.

2) I meant like, one 2x12 cabinet isn't going to sound that much different than another 2x12 cabinet, rather than absolutely all cabinets sounding the same regardless of size.

3) It's a Rocket 15T. Good luck finding any info on it though, I've never seen one other than at the shop I bought it from, and even they seem to have stopped stocking it.

[Bacchus]

Does it say anything beside the external speaker jack? Like does it tell you a number of ohms?

[Freddy V-C]

So if your amp is 8ohm, get an 8 or 16 ohm speaker cab... not a 4ohm one.
Also more ohms = more resistance so you want to try and match it or not be too far ahead with the cab, for efficiency... basically if the ohms are too high, you'll be running at less power.

Didn't think about ohms... :\
Doesn't seem to say how many ohms the combo amp is anywhere, and like I said, it's pretty difficult to find any information about it on the internet.

[paul_]

How about the internal speaker? Anything about ohms on there?

[Mityushikha]

I've not tried putting pedals through my VT but the other guy using it does, it works for him I guess. I know I definitely prefer the sound of a tube amp though.

Like Paul said, if you can get a look at the speaker it might have a label or stamp telling you how many ohms it is.

[Freddy V-C]

How about the internal speaker? Anything about ohms on there?

Nothing that I can see...

This cab I'm planning on getting is 16ohms, though, when it's in stereo. My little combo amp can't be outputting at more than 16, surely?

I think I'll just get the cab and use this combo as a head until I can afford a half-decent tube head. It's possible I might even like the sound of the combo going through the cab, and then I won't have to spend any more money. Hooray, go me!

[Bacchus]

This cab I'm planning on getting is 16ohms, though, when it's in stereo. My little combo amp can't be outputting at more than 16, surely?

I'd be careful. It doesn't work that way. It's most likely to be putting out 4 or 8, but it's not like wattage where a much higher rated cab will work with a lower rated amp.

[Reece]

is a 4 ohm head into a 16 ohm cab going to cause any problems with the amp or would it just be less power?

i never ran my champ through my PPC112 'cause i was never sure if it'd be alright but if i ever get around the getting the thing sorted i'd quite like to try it out cranked through a cab.

[Bacchus]

is a 4 ohm head into a 16 ohm cab going to cause any problems with the amp or would it just be less power?

i never ran my champ through my PPC112 'cause i was never sure if it'd be alright but if i ever get around the getting the thing sorted i'd quite like to try it out cranked through a cab.

It won't sound as good as it would with a matched cab. I've read that it's okay to have the ohmage on the cab greater than on the amp, but my experience with my Champ isn't consistent with that. It blew up when I used it with my 8 ohm 412, but only when I used the Saltbooster on it.

(the output on the Champ is rated at 4 ohm)

[Freddy V-C]

is a 4 ohm head into a 16 ohm cab going to cause any problems with the amp or would it just be less power?

I went and did some reading up on the interwebs, so brace yourselves for some intellectual speak. Unless I've horribly misinterpreted things, the cab can be greater than or equal to the head, but it affects the power. A 50 watt head putting out at 16ohms will be putting 50 watts of power into a 16ohm cabinet, but then only 25 watts into an 8ohm cabinet and so on and so forth.

I would post a link to the site where I read this stuff, but it was a few hours ago and now I can't find it... feel free to point out various flaws in this poorly informed explanation.

[Bacchus]

is a 4 ohm head into a 16 ohm cab going to cause any problems with the amp or would it just be less power?

I went and did some reading up on the interwebs, so brace yourselves for some intellectual speak. Unless I've horribly misinterpreted things, the cab can be greater than or equal to the head, but it affects the power. A 50 watt head putting out at 16ohms will be putting 50 watts of power into a 16ohm cabinet, but then only 25 watts into an 8ohm cabinet and so on and so forth.

I would post a link to the site where I read this stuff, but it was a few hours ago and now I can't find it... feel free to point out various flaws in this poorly informed explanation.

Forget about wattage for a second, and concentrate on the ohmage (or load).

This is the common theory that I found when I read about this a year or so ago:

By mismatching the ohms you are seriously risking damage to the amp. An amp with no cab on the end (therefore no load on the amp) will blow up. An amp with a load lesser than what the amp is putting out is very likely to damage the amp. An amp with a load greater than the output is less likely to damage the amp, but won't sound as good as if it was a matched load.

I have blown up an amp that was putting out 4 ohms by using an 8 ohm cab, so I would advise against any sort of mismatch, despite what it says above.

What we really need is someone with actual amp know how in here to explain this. The best thing to do is to get a cab that matches your amp. There must be some way of finding out what the output on the amp is, even if it means measuring the impedance of the speaker or something.

[Doog]

I have blown up an amp that was putting out 4 ohms by using an 8 ohm cab, so I would advise against any sort of mismatch, despite what it says above.

I did this with a solid-state head at gig volumes for years with zero problems. Most SS heads seem to put out a non-switchable signal of 4 ohms, just because it'll work fine with the vast majority of cabs (4, 8 and 16 ohm) and is cheaper to manufacture.

But yeah, definitely don't do it with a tube head.

[NickS]

Echo Doog. Most solid state amps will drive 4 ohms or more and even not care if there's nothing connected. Most SS amps are basically voltage amplifiers; the more volts on the supply rail the more volts an amp can put on the speaker. Lower ohms = more current for a given voltage = more power.

Valve amp output stages want to see a pretty-damn-close-to-specified output impedance of several thousand ohms. To make that compatible with speakers, they need a transformer. The ratio of turns of wire on the valve side to turns of wire on the speaker side determines the ratio of impedance. Say the output stage wants to see 4800 ohms: using a turns ratio of 600:1 gives you a connection for 8 ohm speakers, and a turns ratio of 1200:1 gives you a connection for 4 ohm speakers; a lot of amps have their output side "tapped" at different points in the winding to provide outputs at 16, 8 or 4 ohms.

If you connect the "wrong" impedance to the output the effects vary. Plug your 4 ohm cab (or two 8 ohm cabs) into the 8 ohm output and you're presenting a lower impedance than you should; the valves will pass more current and get hot; the transformer will get hot, and eventually something will die. Like my acquaintance Kevin's Mesa Boogie, which he ran into two 8 ohm cabs from the 8 ohm output for 2 years, then had to pay over £400 for repairs including blown output transformer and cooked PCB. If you connect it to too high an impedance (8 ohm cab into 4 ohm output) you may get away with it; some amp manufacturers even say it will be OK with a maximum mismatch of 2:1. It does mean that any nasties are less likely to be absorbed, and this is where good amp design and layout is really important. My son's Marshall JCM 2000 TSL has an early issue main PCB with bad power stage layout and the end-most EL34 will die at the drop of a hat. He's currently running it with only the two centre valves, as a 50-watter - to do that safely he plugs his 8 ohm cab into the 4 ohm output.

That brings us to fiddling with impedance matching for odd cabs Remember, that output transformer may have multiple turns ratios. You can use that to connect cabs of different impedances. For example, you can stick an 16 ohm cab into an 8 ohm output and a 8 ohm cab into a 4 ohm output (as long as they're both active at the same time; some manufacturers switch the outputs to save on sockets.) That will give equal power to both cabs, whereas simply paralleling the two and connecting them to the 4 ohm output would (a) reduce the overall power as you're putting 5.33 ohms on the 4 ohm output (b) mean the 16 ohm cab is getting half the power of the 8 ohm cab. If you don't have that option, use the output that's lower than the combined impedance of the two cabs

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(by Ohm's Law: i.e. combined impedance of A and B in parallel is (AxB)/A+B)); 8 ohms in parallel with 16 ohms = 128/24 ohms = 16/3 ohms = 5.33 ohms

i.e. the 4 ohm output. Don't ever do anything dumb like plug a 4-ohm cab into the 4-ohm output and an 8-ohm cab into the 8-ohm output at the same time.

[Mike]

The general rule is this:

- Always match with Tube amps

- Solid States generally state a minimum and you are fine above this.

Of course some people do say that you can mismatch Valve stages up and down by a factor of 2, but this is putting extra stress onto an Output Transformer. Old Fender amps were super-engineered and could take that kind of abuse, do you think something off the shelf today would be so solidly made? Exactly.

- Match.