Attenuating a Laney LC15R

JamieWakeham

Backstory - I've had this LC15R amp since I bought it new, in about 2002.  I've always absolutely loved its tone - the way it can do lovely Vox-y cleans (admittedly not much headroom there) and just-breaking-up crunch, but also go to a pretty good impersonation of a Marshall.  BUT it's always been too loud for driven sounds at home volume - if I want an AC/DC-esque tone, about six on the gain, I have to have the master volume down at just approaching one.  So, I bought an AC30VR a year ago.  This certainly can do the trick of medium-gain tone at sensible volume, because it's mostly solid state.  All good.

But I've just got the Laney out to test everthing is working before I put it up for sale, and I made the mistake of A/B-ing it with the Vox, and... I can't sell it.  It's so much more articulate than the Vox. The cleans shimmer more, the drive is more organic.  It's lovely.

I don't need to sell it- I don't need the money or the space that badly, so for the time being I'll hang on to both and see what I'm thinking in a few months.  I suspect I will, in the fullness of time, think about selling the Vox.

So here's the point of my post - what's the best way to mod the Laney to get lower volume whilst driving it a bit harder?  I'm considering an L-pad attenuator, as discussed extensively on here - I'm planning to use the 'Elevenator' circuit from guitar dot com, but without bothering with the 4/8 ohm switch and taking note of ICBM's comments about the bypass.  But I do see that whenever attenuators are mentioned on here, the advice seems to be to ask if an attenuator is the best way to go for your particular amp, so I guess question one is - is this attenuator a good call with this amp (three ECC83 into two EL84)?

Or is there another way I should be considering?  I know that some Laneys have a very useful 1w settting - is there any sensible way to mod my LC15 to obtain this?  I'm reasonably capable with a soldering iron but I am by no means an electical engineer!

Would switching to a less responsive speaker (maybe a Greenback) make much difference?  I suspect not.

Or - an idea I've wondered about in the past - what would happen if I got three knackered 8ohm speakers, removed their cones, and wired them with the existing speaker as two parallel pairs of two in series (to retain 8ohms overall)?  Would this result in three quarters of the power going to more or less silent speakers, and a corresponding volume reduction?  Or is it just too much faff to contemplate?

Thanks for any advice!

ICBM

Yes, the L-Pad is the way to go. The 1W setting on the other Laneys isn't actually a proper attenuator, it's a 'virtual power reduction' which uses effectively a fixed post-phase-inverter master volume to limit the input to the power amp, rather than reducing the output. You could fit one, but it's actually a bit of a faff since the LC15 is a single-PCB amp without separately-mounted valve sockets, so you'd need to modify the board. Even to use an attenuator you'll need to modify the speaker connections, but that's quite easy.

A less sensitive speaker - probably one of the Jensens will be the lowest - won't make much difference at all, it will knock off at most a few dB, which is like turning the master volume from 6 down to 5 or something similar. Even using the three-dead-speaker idea would only reduce it by 6dB, and it would be a faff to knock up too. (But with only about 4W going to each voice coil, probably wouldn't burn them out - normally you can't do this at higher power levels because the coil relies on the air movement to cool itself.)

For what it's worth, if you don't really want to sell the Vox either, try it with a better speaker too - the stock one is crap.

JamieWakeham

Thanks ICBM - I was rather hoping you'd pop up when I asked my question!

Looks like step one is to build an attenuator then.  I'll also look into a Jensen - Even if it only saves a dB or two, that still reduces the amount of attenuation needed.  

I've always been tempted to switch the speaker on this; although to my ears it sounds pretty good with the HH, so many people say that the speaker is the weakest link that I need to try it with something better.  Looks like I can find a C10Q or C10R very easily. 

As for the Vox - once I have the Jensen, I might try running it into that speaker as well, to see how it reacts.  The stock speakers (it's a 2x12) are labelled Vox VX12 but have Celestion labels on the side.  I rather think, though, that if I can get the Laney doing what I want, the Vox will be tonally quite similar, and it won't be worth keeping both...

In fact (I'm getting ahead of myself here) I wonder about trying to find an 8ohm 1x12" greenback extension cab.  Being able to run the Laney either into a 10" Jensen or a 12" Greenback, depending on what tone I'm after, seems quite flexible.  The aux output on the laney supports 8-16ohms, so I could even run them both in series. Not sure yet what the attenuator will think of seeing a 16ohm load, though.

ICBM

JamieWakeham said:

The stock speakers (it's a 2x12) are labelled Vox VX12 but have Celestion labels on the side.

If I remember correctly they’re another relabelled variant of the G12P-80... not a great speaker at all.

When I was working on one of those I thought it was a bit weak and flat-sounding for a 30W Vox, so I hooked it up to a pair of Mesa OEM V30s - the difference in tone and volume was amazing.

JamieWakeham said:

Not sure yet what the attenuator will think of seeing a 16ohm load, though.

It’s no problem at all, it will just give a slightly different volume taper.

If you do need to mismatch amp/attenuator/speaker for any reason, always match the amp to the attenuator since that’s what the amp sees as the load - the speaker very quickly becomes irrelevant once the attenuator is much below -3dB.

JamieWakeham

Thanks again.  I have a Jensen C10Q on its way, and I'll report back when I have that installed and the attenuator built.

I quite fancy the idea of adding that 1x12, and having a 10" Jensen / 12" Greenback / both switch...

JamieWakeham

I'm sitting here with my new Jensen, and need some help before I do something stupid!

I've got the front board out of the amp, and it's facing downwards - the speaker is cone-down and magnet-up.  There are four screw-threaded bolts coming out of the board, and the speaker is held on with a nut on each of these bolts.  But I can't unscrew the bolts because when I rotate them, the whole threaded bolt rotates as well.

I've tried to clamp the thread to hold it still while I unscrew the nut but it's not possible - if I clamp it any tighter I'll strip the thread.

Do I need to remove the grille cloth and get at the front to secure the bolt from rotating that way?  It seems very unlikely that I will be able to get the cloth back on again neatly...

ICBM

That's quite a common problem... what I do is get two more nuts and lock them together on the thread above the speaker nut, which should allow you to hold the bolt in place while you loosen the lower nut - but that requires two more of the right nuts, which you may not have.

Don't remove the cloth, you're right that you'll never get it back on straight. If you're careful you may be able to get the tip of a screwdriver through the weave of it without damaging the threads - actually a small flat-blade one will usually grip the bolt head enough, and will be easier to get through the cloth.

JamieWakeham

Thanks - got it.  What a terrible piece of design!  The threads were much too short to get the pair of nuts on as you suggest so I've had to go though the cloth.  And had to use a larger screwdriver than I'd have liked, as the nuts were bloody welded tight to the bolts and my finest screwdrivers didn't give enough purchase.  I'm now trying to carefully rearrange the weave of the cloth to hide the holes  

When it comes to attaching the new speaker, rather than re-use this terrible system, is there any reason I shouldn't just rotate it by a few degrees and fasten it on with very short screws straight into the board?

Edit: I'm an idiot.  There's no way to get the bolts out so I have to re-use them.  Back on they go, then.

ICBM

JamieWakeham said:

There's no way to get the bolts out so I have to re-use them.  Back on they go, then.

If you haven't done it yet, soak some superglue around the threads - that should stop them turning. Just make sure it's completely dry before you put the speaker on!

JamieWakeham

Saw this too late - 'twas a good idea though! 

I have to admit that having seen how flakey the construction is on the entire thing, I'm now wondering about cutting it in half to make a little 15W head (leaving enough depth for the attenuator to be installed) and building an whole new speaker cab. 

But first things first, I'm planning and ordering the kit for the attenuator.

JamieWakeham

I'm just putting together the order for the kit I need for the attenuator.  I am really quite taken with the idea of, later on, getting an 8ohm 12" Greenback, and setting it up so I can run the Jensen, or the Greenback, or both in series.  It seems hard to find out what the actual resistance values used in the L pad are, but having run a few likely values, it seems that switching the speaker load from a single 8ohm to a series pair at 16ohms actually makes very little difference either to the total impedance seen by the amp or the total power output to the speaker(s).

So, I'll order an extra switch for the dual speaker setup at the same time as buying everything else, so I don't have to put another order in later and pay an extra delivery charge.  I think what I need here is a dp3t on-on-on switch to enable me to build this circuit:

http://i68.tinypic.com/f3cl74.jpg

...and I think that this one ought to do the job: https://www.digikey.co.uk/product-detail/en/nkk-switches/M2044ES1W01/360-2864-ND/1047121

That's the right idea, isn't it?

ICBM

The resistors in the L-pads are specially designed to maintain a constant impedance as seen by the amp.

Your wiring is unnecessarily complex and potentially unreliable - all you need is an SPDT centre-off switch. Connect the speakers in series (as you have) and use the switch to bypass one or the other. That way the speakers are always connected to the amp and the worst that can happen if the switch fails is that both are then on.

Wire the switch with the centre terminal to the junction between the two speakers and the end terminals to the other sides of each speaker.

JamieWakeham

Oh crikey - that's embarrassingly simple compared to my method!  Thank you.  I'd got caught up in the idea that I needed to connect to each side of each speaker, whereas all that's needed is to bypass one or the other.

JamieWakeham

Finally about to start putting this together.  I've read several times that 5A or 6A cable is fine for carrying post power amp signal, but I've also seen one or two mentions that you shouldn't use too heavy a cable.  Is there any reason I shouldn't use regular 13A twin & earth - both pulled apart to provide wire for individual connections, and as a cable (ignoring the earth) for the final speaker runs?  I happen to have some left over, whereas I'd need to go and actually buy 5A 'garden' cable... I can't see any reason it shouldn't be fine, and it just fits into the NP2Xs.

ICBM

Any mains power cable that will physically fit into the plugs is fine. There’s no disadvantage to using the biggest you can. If it’s three-core cable you can either leave the third unconnected, or parallel it with one of the others depending on which you think is ‘neatest’ - it makes no other difference.

(Well, to be completely accurate if you parallel it the total resistance of the cable is reduced by a quarter, but with the gauges and lengths you’re talking about here it makes no practical difference.)

JamieWakeham

Thanks for confirming.  I didn't think there could be any issue - I guess the references I've read to not using too heavy a cable are simply about getting it to fit into the jacks!

Nerine

 I wasn't sure what an L pad attenuator was. I googled it, and do these small knobs really attenuate between 50 and 100 W amplifiers? 
Where does all the heat go? Do they literally just work on their own, or do they need to be connected to something else for them to work?
i.e. Of course they need to be connected to other things in the amplifier, but they look tiny. I can't imagine that one knob with whatever network of resistors it may have attached to it happily taking the full output of a 100 plexi... ?

JamieWakeham

If you look at the article in guitar.com (they call their circuit the 'elevenator') it's explained quite well.  The L pad is a pair of variable resistors, cleverly set up with one in parallel across the speaker and a second in series, such that whatever setting you have it at the total resistance remains at 8 ohms.

The one they use (and which I'm using) is quite a chunky bit of kit - it's larger than you think from the photo (about 7cm across) and feels substantial.  They're purely resistive and as such are simply turning amp output power into heat.

But they were originally intended for hifi applications, and their power ratings are optimistic when applied to guitar amps.  So you're right, they won't bring a 100W plexi down to bedroom levels! 

ICBM

Nerine said:

 I wasn't sure what an L pad attenuator was. I googled it, and do these small knobs really attenuate between 50 and 100 W amplifiers? 
Where does all the heat go? Do they literally just work on their own, or do they need to be connected to something else for them to work?
i.e. Of course they need to be connected to other things in the amplifier, but they look tiny. I can't imagine that one knob with whatever network of resistors it may have attached to it happily taking the full output of a 100 plexi... ?

No, you’re right - they can’t. This is a big problem with the way they’re described, which is based on their original purpose.

They were originally designed as treble and midrange controls for ‘hi-fi’ speaker systems - not really ‘hi-fi’ by modern standards! You can see the L-pad knobs on these vintage Wharfedales. (These ones are very sophisticated with bass controls as well and separate tweeter boxes!)



The ‘100W’ is the rating for the whole system. Given that in a full-range music programme mix each of the frequency bands only use about a third of the total power, that means that the L-pad is really only capable of handling about 30W.

And since an overdriven amp - also not what they’re designed for! - will put out up to double the clean rating, the real “guitar amp rating” on them is more like 15W, possibly 20 at the outside.

prowla

The folks selling the Strandberg basses at the guitar show had some reactive load attenuators at rather good prices.