L-Pad Attenuator => Poor Tone?

[Nick C.]

I made an attenuator from this L-pad from Parts Express
http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=260-262.

I got the idea from someone going by "Major Tom" found at this guitar forum.http://www.christianguitar.org/forums/showthread.php?t=71602 I have never played my Fender Hot Rod Deluxe with the volume more than 4 and I thought it would be a good way to experience real power tube distortion.

I'm not happy with the tone I get with it. Certainly getting lots of overdrive, but the tone is dull and lifeless. Not exactly the magic I hoped for. Is this a common experience? What could be going on? The tubes are original, 9 years old, could that effect the sound? The amp still sounds great at the low levels I normally play at.

Any ideas?

Thanks

Hope this is not OT.

[Papa_lazerous]

it will sound a little dull and lifeless with an attentuator (I wont say every attentuator as they are not all created equal) but you will get some mud.  Its the nature of the beast. 

[Mick Bailey]

It does affect the tone as you described when used on its own. You need to include a bypass capacitor to recover some of the lost tone. I ended up with quite a large value to recover the sparkle - it still doesn't sound 100%, but it's pretty good. Can't remember the configuration or exact size, but I Googled the info quite quickly when I originally installed it.

If you get stuck I can take the amp apart and take a look.

[BrianJ]

This is one of those situations when, as soon as you fix one thing, another pops up.  Depending on how much attenuation you apply (in an attempt to get power tube distortion) you may be running the speaker so gently that is is not contributing the same way it has in the past.  When a speaker is pushed it changes the tone dramatically. 

[Nick C.]

Thanks for the response guys.

Mick, I found that Regis Coyne's Ampwellhouse http://regiscoyne.com/ampwell/ampwellschem.jpg  is similar to mine and uses a brightness cap. He shows where to put it and what values. Any idea what volt rating is needed or should the negitive side go to the speaker or amp?

Nick

[Mick Bailey]

I ended up with a 2.2U non-polarised metallised polyester filmcap rated at 100v.  You could try adding a couple in series if necessary, to get the right value for your particular amp. I don't know how a polarised cap would behave in this configuration - my experience with crossovers and speaker-side stuff has always been to use non-polarised capacitors. Seems that most commercial products are built this way. 

[Nick C.]

Thanks Mick. Not an electrolytic, I bet that's a big boy.

[5thumbs]

Thanks Mick. Not an electrolytic, I bet that's a big boy.

They're big, but not THAT big.  The CDE 50V 2.2μF metal film caps I use are about 1cm on each side and 0.5cm thick.  They obviously won't fit everywhere, but they are quite useful and sound great in many circuits.  I just wish they were a tad bit cheaper.

[Mick Bailey]

Mine are 30mm wide, 15mm high and about 8mm thick. Lead spacing is 22.5mm.

[5thumbs]

Mine are 30mm wide, 15mm high and about 8mm thick. Lead spacing is 22.5mm.

The 2.2μF 50V CDE flim caps I use are 10.2mm wide, 12.0mm high and 7.9mm thick.  Lead spacing is 7.5mm.  They're $1.81 each at Mouser (which is about the only drawback to them), but I was amazed at how small they were the first time I took them out of the package.

Data sheet: http://www.cde.com/catalogs/DSF.pdf.

[zombiwoof]

Mine are 30mm wide, 15mm high and about 8mm thick. Lead spacing is 22.5mm.

The 2.2μF 50V CDE flim caps I use are 10.2mm wide, 12.0mm high and 7.9mm thick.  Lead spacing is 7.5mm.  They're $1.81 each at Mouser (which is about the only drawback to them), but I was amazed at how small they were the first time I took them out of the package.

Data sheet: http://www.cde.com/catalogs/DSF.pdf.

Can you please post a link to those caps at Mouser?  I've tried every way of searching for them there, and can't find them.  I hate their search system, It's often difficult to find things.

Thanks,
Al

[zombiwoof]

Never mind, I finally found them.  They had them mistakenly described as "metallized polypropylene", when they are actually stacked metallized polyester.  Jeez, no wonder I had trouble.

Al

[QSQCaito]

Are you looking for an attenuator?

Have you heard about the air brake?

[zombiwoof]

Mine are 30mm wide, 15mm high and about 8mm thick. Lead spacing is 22.5mm.

The 2.2μF 50V CDE flim caps I use are 10.2mm wide, 12.0mm high and 7.9mm thick.  Lead spacing is 7.5mm.  They're $1.81 each at Mouser (which is about the only drawback to them), but I was amazed at how small they were the first time I took them out of the package.

Data sheet: http://www.cde.com/catalogs/DSF.pdf.

Are you sure you got those 2.2µf caps at Mouser?  Even though I finally found that type of cap there, they only list as high as .22µf, and there is no catalog page given for them.

Al

[drewl]

Another trick is to plug a dummy load or LPad into the extension speaker jack so it soaks up half the power and it cuts the output to the speaker without altering it's tone.

[DougH]

An L-pad is just basically a "power resistor voltage divider". So the load on your amp is purely resistive when you use it. A real speaker has an inductive component to the reactance that varies with frequency. My experience with resistive loads is they work okay for low-wattage stuff. The speaker jack on my 1/2W Firefly amp has a power resistor that switches in when I unplug the speaker and use the line-out. It's fine for its intended purpose.

But I experimented with attenuating 10-12W amps I have and they didn't sound good with just a resistive load. The highs were rolled off and the amp took on a muddy, midrangey character I didn't like. I ended up building an attenuator using a MASS speaker motor for the load which provided the inductance and the frequency response drastically improved.

[Nick C.]

Certainly the more power I attenuate the worse the the tone gets. With my 40 watt amp this thing, which is rated at 100 watts, gets pretty hot. I've used it conservatively so far and I wouldn't trust it for a gig and wonder what damage it might do.

Another trick is to plug a dummy load or LPad into the extension speaker jack so it soaks up half the power and it cuts the output to the speaker without altering it's tone.

That's interesting! It would be great if it could absorb more than 1/2 the power. How would the L-Pad be hooked up in this case? Would you put some thing across the speaker connection? A resistor? How about a speaker without a cone?

[drewl]

Yeah, cutting power in half is only a 3dB reduction, more is usually called for.
I also found the LPad's getting very hot when used as the main load and didn't want to trust it so I built a box with a couple power resistors in it and the Lpad so the Lpad gets maybe half or a 1/4 of the amps power and then only a fraction of that is sent to the speaker.
For example, with an 8ohm pad get 3 8ohm power resistors wired series/parallel so now the Lpad is only getting a 1/4 of the amps power and that's the most that can be sent to the speaker or it could be dialed down lower.
Each resistor is only seeing 10W (w/40w amp) and everybody's happy.
YMMV.
That's why I have a Hotplate, or you could try a Weber Mass attenuator, they're fairly cheap.

[Ardric]

Part of what makes a tube amp sound good is the poor damping factor.  The output impedence isn't near zero like a solid state amp but instead has a few ohms of impedence.  That "light grip" on the speaker tends to boost the bass notes where the speaker resonates, and higher frequencies where the speaker impedence is climbing up.  It's a well-known effect.

An L-pad ensures that the amp always sees the same load impedence.  It doesn't do the same for the speaker!  The speaker sees a very small resistance (say .22 ohms) in parallel with it's terminals.  This has reduced the effective output impedence of the amp as seen by the speaker.  This causes the bass/treble peaks to recede and the midrange to be more apparent.  In effect, the amp power section sounds less tubey and more solid-state.

The solution is T-pad instead of an L-pad.  It keeps a constant impedence on BOTH sides.  But since they're hard to find, a quick&dirty fix is to put a smallish value resistor in SERIES with the speaker, after the L-pad, and only when the L-pad is engaged.  Try something like 2-4 ohms on an 8-ohm speaker.  It should restore most or all of that loose sound/brightness without resorting to a bright cap.

[ubersam]

Mine are 30mm wide, 15mm high and about 8mm thick. Lead spacing is 22.5mm.

The 2.2μF 50V CDE flim caps I use are 10.2mm wide, 12.0mm high and 7.9mm thick.  Lead spacing is 7.5mm.  They're $1.81 each at Mouser (which is about the only drawback to them), but I was amazed at how small they were the first time I took them out of the package.

Data sheet: http://www.cde.com/catalogs/DSF.pdf.

Are you sure you got those 2.2µf caps at Mouser?  Even though I finally found that type of cap there, they only list as high as .22µf, and there is no catalog page given for them.

Al

click here for 2.2uF CDE film caps @ mouser.com