Question: Is a 30-watt amp head going into a 2x12 cab good enough for medium-sized gigs?

Old Music Guy said:
I really don't want to start an argument, I really don't.
Firstly, what do you mean by "physical description" ? Fit, form, and function?
Secondly, how would someone go about measuring the "inductance" of a transformer i.e. inductor?
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Strictly speaking a transformer, when correctly terminated is not an inductor. This is how audio transformers 'can' give a flat, extended frequency response but beware! They are rarely perfectly driven or loaded and so are not totally' flat'.

Transformers all have losses of course and one test is to measure the impedance of say the primary with all the other windings shorted. In theory the primary should be just the DC winding resistance but in practice there will be a small inductance. This is termed "leakage inductance".

Dave.
 
ecc83 said:
Strictly speaking a transformer, when correctly terminated is not an inductor.
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I'm sorry, but that is totally incorrect. Properly terminated or not, a voltage across a primary winding of a transformer, wrapped around a common core, induces a voltage on the secondary, that is directly proportional to the ratio of the primary winding to the secondary winding.
How can it possibly NOT be an inductor?
 
Old Music Guy said:
Firstly, what do you mean by "physical description" ? Fit, form, and function?
Secondly, how would someone go about measuring the "inductance" of a transformer i.e. inductor?
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Diameter and height.
It will narrow down who I bug for a transformer. Because certain people only deal with certain core sizes.
 
Old Music Guy said:
I'm sorry, but that is totally incorrect. Properly terminated or not, a voltage across a primary winding of a transformer, wrapped around a common core, induces a voltage on the secondary, that is directly proportional to the ratio of the primary winding to the secondary winding.
How can it possibly NOT be an inductor?
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OK, LAST thing I wan ti is a cow with YOU friend but consider a "perfect" 1:1 ratio transformer in the OP stage of a mixer say? We expect and find, to a first approximation, the the frequency response is decently flat from say 40Hz to well beyond human hearing. Were the transformer behaving as an inductor that could not be! Of course, if you put a winding on an LCR bridge you can measure its inductance...WITH the coils O/C.

If there are any Power engineers here they would tell you that line transformers have to be "balanced" so as to present a reasonably resistive load otherwise the power Co gets very miffed and you can have terrifying voltage spikes. The same applies with large capacitive loads but in that case you get huge out of phase currents.

Transformers are not perfect of course, they have resistance, leakage inductance and Iron losses. These failings can be minimized of course but Sod's Law states that "better" is always more "expensive"! Traffs are also made in a range of ratios and it is a fact that the further you go from one to one the harder it is to keep high performance

All sorts of electronic "devices" LOOK like one thing but behave differently in circuit. EQs often use "Gyrators" these are a collection of just Cs and Rs round op amps but BEHAVE like inductors.

A single loudspeaker in a cabinet is a small inductor but over its operating range the complex impedance caused by the cone movement and the air load can make it "look" like a capacitor at some frequencies.

IF this stuff was easy anyone could do it and I would be a freaking sight better off!


Dave.
 
ecc83 said:
That is a solid state amplifier and will therefore equate to about 15, maybe only 10 watts from a good valve amp. Vox AC30s are extremely loud! Maybe if the 2X12 contains some really sensitive speakers such as the Celestion V30s it might cope but I doubt it. The main problem is that sstate amps do not sound nice if you push them into distortion.

As others have said, go through the local PA. You could still use the amp as a personal monitor and then have the cab micc'ed up or run a line to the desk from the FX send, probably need a passive DI box for that but the sound "person" should have a few of those. Mention was also made about the impedance of the cab? Check that the amp is happy with whatever it is. Very few 2X12s are as low as 4 Ohms but some sstate amps are only safe down to eight. Check it.

Dave.
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It sounds like snake oil and 1 watt on a factory spec scale should equal another and maybe they do but I’ll say this in support of above…the loudness (maybe just perceived) is different in both SS vs Valve, but I’ll even suggest vintage vs new. Buddy has a newer Yamaha power plant stereo with tuner for his modular home sound system. Nothing fancy but decent Bose speakers. I have a ‘76 Marantz 2275 75 watt power plant (also with tuner though the glide wheel not digital). We’ve never put them side by side but I would wager my Marantz, at a minimum, produces the same ‘loudness’ as his Yamaha. I may be wrong but that’s my perception.

As for guitar amps, I’ll add this:
First ‘real’ amp after a Sears thing with a 6” speaker) was a Peavey Bandit 50 watt with 12” speaker. Second amp was a Peavey Musician head with a Oeavey 4x 12 oversized (and unnecessarily HUGE cab. That was a cool amp, SS. Third was a 1982 Marshall 2204 50 watt head and 4X12” cab bought used 1985. Then a JC-120 Jazz chorus 120w (60 x 2 in stereo) SS beast, Fender tube Suoer Reverb 40 watt and Orange TT combo 15 watt tube. Marshall Fender and Orange I still own.

I’ll argue all day long my Marshall at 50 watts is WAY louder by loudness than the Bandit 50. The Jazz Chorus? Never really opened it up enough to say for sure, but (like the Marshall), it was loud on 2. The Fender also loud on 2. The Orange 15w is sweet but on 2 you’ve got plenty of room to increase volume before you start laboring your ears.
Just wanted to respond to a fellow tuber in here
 
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And regarding mic’ing the amp,!yes that is an absolute reality and a standard for even medium-sized bars/venues. Most will have a house PA system and smaller ones come with a sound man to run it. Any sound man will tell you first thing at soundcheck to turn your amp down. And it’s annoying. But they want to be able to give you that practice place live sound through on stage monitors and be able to mix front of house without physically walking up on stage and adjusting you aml’s volume if too loud and fucking up the mix.

Personally, I love some good volume on my amps live and hate to rely on monitor mixes…I like everything to sound like rehearsal in a (usually) small basement room. Only way to get that in a bigger space is to mic and mix it.

Is 30w 2x12 cab enough for a 350 seat venue. I could do it with my 15watt Orange.
 
There was a time decades ago where PA’s weren’t that advanced and audience volume was needed from each instrument. Those years are long past. Big rock/punk/whatever style bands you see with stacks of speakers behind them? Most are void of heavy speakers because they’re unnecessary.

UNLESS (and I’m assuming you are not) putting a mic through your guitar amp with the guitar and maybe you are) then you’re good with 30w 2x12 and will only use a fraction of that when mic’d up.
 
I’ve always had a simple formula.

If at rehearsals your amp isn’t drowned out by the drummer you’re golden. Usually at your rehearsal space the only PA gear is for vocals.

Small bar gigs, club gigs are pretty much the same.

You get to a bigger venue, usually you have bigger PA, and things besides vocals get miced up.

So the key is if you can keep up with your drummer, you’re good.
 
I'd like to examine the oft quoted idea that "valve are louder than transistors". Basically it is true and for very sound engineering reasons.
The first is the way speaker Voice Coils behave. Made of Copper they have a DC resistance of around 10 Ohms for a nominally 16R speaker. As the coil heats up its DC resistance increases and thus, for a given input voltage, current drops and so does power.This is known as "Thermal Compression". However, if you can somehow increase the drive voltage, current is maintained and so is power. Valves can do this because they have a much higher output resistance than transistors and of course run at far higher voltages. The OPR of a typical 30W cathode biased amp will be 30-50 Ohms, a 50W or bigger fixed bias amp with a whiff of Negative FeeBack might still be 10-20 Ohms. The HT supply will be anything from 300V to 500V+ In contrast a 50W transistor amplifier will have an OPZ* of less than 1 Ohm and the internal supplies are unlikely to exceed +&-50V.

Valves are also very rugged (but not AS rugged as 50yrs ago!) and can stand serious over voltages and momentary 'shorts'. They will keep chucking out the watts even when driven into serious distortion and operating at peak anode voltages of 2kV or more. Transistors need some form of protection against gross overload. Electronic, "VI" protection just sounds awful and 'clamps down' the signal. Fuse protection just shuts you down and often doesn't save the transistors anyway! (though it has to be said that is not the proper reason for fuses in the first place)

Distortion is very important to our perception of loudness. The case cited of two transistor amps can be explained to a degree by saying the amplifier which distorts earliest is likely to sound the loudest. Then of course there is no connection between the numbers on a VC dial and subjective loudness! Pots vary in their "resistance v angular position" due to the track law and even nominally the same pot law can be different and have different 'break points' maker to maker. The surrounding circuit can often distort the law as well.

You CAN make a transistor amplifier which seems as loud as the equivalent valve jobby...Start with a sstate amp capable of perhaps 3 times the advertised amp power, so 150-200W for a "50W" amp. Then use some clever frequency and level sensitive "tricks" to not only give the subjective impression of "50 watts of valves" but also protect the amplifier very well to boot. At least one guitar amp company has done this very successfully.

*Yes, there are some sstate amps that use "current feedback" to simulate to some degree the characteristics of valves but this is done more for "tonal" reasons than loudness.

Dave.
 
Old Music Guy said:
I did not know that. Thank you
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Yes, good speaker makes put the level drop in their specification, it can be 3dB power loss at 50% of the power rating.

This is the reason multiple identical speakers can go louder than you might expect. A single V30 say, rated at 60W might start to compress at 30-40W but two will compress less and four hardly at all. Anyone who has put even a few watts through a 4X12 knows how 'punchy' they are?

Dave.
 
Seeker of Rock said:
I’ll argue all day long my Marshall at 50 watts is WAY louder by loudness than the Bandit 50. The Jazz Chorus? Never really opened it up enough to say for sure, but (like the Marshall), it was loud on 2. The Fender also loud on 2.
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The differences is internal gain structure of the amps. The Roland that was mentioned was one of the few solid state amps with the same gain structure and internal signal levels as a tube amp. The output stage of a tube amp has no real advantages or disadvantages compared to changing it out with a solid state equivalent.
 
drtechno said:
The differences is internal gain structure of the amps. The Roland that was mentioned was one of the few solid state amps with the same gain structure and internal signal levels as a tube amp. The output stage of a tube amp has no real advantages or disadvantages compared to changing it out with a solid state equivalent.
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I do not agree.

Dave.
 
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It’s always been my understanding that the ‘magic’ of a tube amp is coming from the power tubes saturating. Preamp tubes on their own just won’t do it. Over the years there’s been tube preamps marketed and sold to give you that sound. It never happens.
 
RFR said:
It’s always been my understanding that the ‘magic’ of a tube amp is coming from the power tubes saturating. Preamp tubes on their own just won’t do it. Over the years there’s been tube preamps marketed and sold to give you that sound. It never happens.
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Well,...the "tone" of a valve amp, the whole thing, is a mix of pre amp and power amp distortion and that is a complex subject I am happy to discuss but my main point was about the loudness of valves compared to a similarly 'labeled' transistor model.

But, circuit configuration matters. Take two EL34s and stick 450V on their anodes and cathode bias them to about 20W apiece, nice chunk of NFB and you will get 30-35W at decently low distortion. Very Hi Fi, drive it harder and it will clip at ~50W and do no more. Now run the valves at 15W each, fixed biased and you will get 'fairly' clean 50W (about 5%thd) and 80W or more of deafening filth.

Dave.
 
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