Light bulb across output transformer secondary - noise reduction!

[Bluemeany]

I have performed this trick in the past, basically I placed a low voltage (12v 4watt) incandescent bulb across the secondary leaves of the output transformer in a highly modified valve junior.

While the amp has no signal running through it the bulb is off (low resistance) and contains the idle hum level. As you play the light bulb comes on, the resistance increases and you get a subtle expansion effect.

3 things are cool about this - 1) noise reduction, 2) a neat expansion effect and 3) it lights up as you play!

I haven't seen any discussion of this little trick for a while so I thought I'd bring it up again

[Willabe]

Expansion or compression?


               Brad     

[Bluemeany]

Expansion. When the bulb is off (not playing) it dumps signal, as it turns on it dumps less signal. I.e the harder you play the less signal is dumped. A compressor does the opposite - the harder you play the more signal they dump the less you play the less signal they dump, hence the increase in the idle noise floor when you use a standard compressor.

If you connected the light bulb to a ldr and strapped it across the wiper/ground lugs of the volume control in the amp you could create a rugged compressor.

[Edit] I went back and played with the amp a bit with the light bulb in the circuit. It's a very strange mix. Brad, I think in practice (from what I'm hearing) it's a bit of both - compression and expansion. The compression doesn't come in for a long time (when you compare it to the attack time of standard pedal compressors), as you play with the values of the bulb the ratio changes from expansion to compression. I'm not really sure I can truly describe it as solely either but as an interesting (and musical) combination of both - you get the initial quiet-loud expansion then the soft compression of the top end as the bulb eats some watts. Pretty cool and very usable!

[Willabe]

That's interesting, thank you for posting that.


           Brad    

[Bluemeany]

If you want pure compression you can place the light bulb in series with the speaker. Do beware with this method - if the bulb burns out it will leave your output transformer without a load! Bad news. It'll behave more like a pure attenuator/compressor than placing the light bulb in parallel where you get the cool expansion thing going on.

[jeff]

so when you play soft you load is less than 8 ohms and as you play louder and louder the bulb lights more and more and your load is getting closer and closer to being 8 ohms?

That somehow doesn't sound good for the amp?

In other words your speaker load is 8 ohms(speaker) in parallel with a resistance(bulb) that changes as you play harder, and only when you're fully cranked do you have close to an 8 ohm load?

Seems like series is even worse. Wouldn't you have greater than 8 ohm load? Can you explain more what's really going on here, because at first glance this seems like a bad idea to me.
Thanks.
  Jeff

[sluckey]

Tungsten is the most common filament material in an incandescent light bulb. The resistance of tungsten is lower when it's cold than when it's hot. Grab a bulb and make the cold/hot resistance checks. Then it will make more sense.

[Bluemeany]

Yes the impedance of the speaker is in parallel to the light bulb. I measured the idle resistance as 6 ohms (ish) so in parallel with an 8 ohm load that's about 3.5 ohms - plug into the 4 ohm output and you're good. The resistance can't rise far above 6-7 ohms. impedance changes with frequency anyhow. Some transformer/speaker combos can see 30ish ohms at high frequencies and people use zobel networks to counter that without problems. I've had it in my valve junior for years with no probs too. THD uses the same idea in the univalve and I believe some Matchless? (if my memory is right) also had these in.

The series light bulb with no protection, like a large wattage resistor in parallel, is a bad idea and you're just waiting to damage your amp.

[kagliostro]

Some time ago I've seen something like that and asked in the forum about

http://www.el34world.com/Forum/index.php?topic=14404.0

Read what PRR told me


K

[jeff]

So basically it's like putting a resistor in parallel with your speaker and the resistors value changes the louder you play/hotter it gets.
So when you play quiet your amp sees a 4 ohm load,
 a little louder it sees a 6 ohm load,
a little louder it sees a 8 ohm load,
But not only that, because the bulb is a resistance and the speaker is an impedance the power is split differently between the two depending on the frequency AND the volume. So when the bulb is hot enough to be 8 ohms, at frequencies where the speaker is 8 ohms the speaker gets half the power but at frequncies where the speakers impedance rises the bulb gets most the power.

Seems like a wierd idea. Kinda a variable attenuator?

[Bluemeany]

Yeah, its a cool effect. If you add a switch to make it selectable you'll be able to find situations where you enjoy the tone and others where you want to take it out.

[punkykatt]

Would you say the expansion effect is the same as bloom effect?  Does it change the character and feel (touch sensitivity) and tone of the amp?

[phsyconoodler]

I like the idea of the noise reduction when not playing.I have a single ended high-gain amp that's a bit noisy at idle when the gain is cranked.
  But I have VVR on this amp and the bulb may not light up when using the VVR cause the output voltage will be really low.

[Bluemeany]

Would you say the expansion effect is the same as bloom effect?  Does it change the character and feel (touch sensitivity) and tone of the amp?

Yes, bloom is how id put it. It does alter the feel, the attack isn't so immediate on single notes and they do expand. It certainly doesn't destroy your tone - it alters it but not in a "bad" way, its just different - it softens the high end and gives a nice expanding feel to chords. It'll work well with jazzy stuff and I love it for higher gain rock n roll chords.

With vvr you will need lower rated bulbs to get the effect. The bulb's rating needs to be tuned to the amp to get the best response.

It's nice to get this sort of bloomy/expansion/compression effect in a single ended amp as you miss out on all that lovely push pull tube rectifier sag.

[Willabe]

I just remembered seeing this.

The Night Light was designed specifically for the ST-6V6se, STR, AST, and AST Pro.   But even though optimized for the Swart brigade, the Night Light will work with ANY amplifier 22w or less with the light compression, 30w with it off.

Attenuation settings
Filament BULB Compression Mode
This mode runs the attenuated signal through a Swart jewel lamp filament, bringing a subtle tube compression/brown sound goodness.  Ever wonder what your signal sounded like through a light bulb?   You'll be addicted in seconds. NOTE: This switch ONLY works with LEVEL chickenhead at 12oclock

Bypass compression mode
Removes the light compressor out of the circuit for more natural dynamics. With level at 12oclock, this is least attenuation/highest volume setting.

Heavy duty 3 position Switch
This main chicken head switch allows you to switch between FOUR different attenuation settings.  At first setting, or 12oclock, the NL can cycle between the COMP and DECOMP switch which brings two levels.  DECOMP is least attenuated or loudest setting with COMP position bringing more attenuation via the bulb filament. Turn the switch to 2oclock will bring more attenuation, bypassing the DECOMP/COMP switch. The lowest setting is about the same loudness as your guitar strings.



http://ep.yimg.com/ca/I/humbucker_2249_343155457


           Brad    

[Bluemeany]

That's a cool looking little box. This certainly has the light bulb in series with the speaker, it looks like there is a big old 8 ohm resistor in there which is always in the circuit that will offer protection if the bulb pops. I think I'll build one up and see how it sounds on my 18 watter..

[punkykatt]

Bluemeany, If I were to try the parallel bulb trick with a 15 watt amp, would i need a 24v 15watt bulb? Is there any rule of thumb to go by?

[Bluemeany]

In my amp there is a single el34 pushing around 10 watts iirc. The 12v 4 watt lightbulb is fine there. In parallel you don't have to worry about causing any damage, if the bulb burns it burns open so you have the normal speaker load. The thd univalve is rated at 15w and that uses a 13v 4.4w bulb. So somewhere around that value. In any case any power not needed by the bulb will be taken by the speaker (within reason). Get a range of different bulbs and try them, each one will give you a different response. Lower rated ones will kick in sooner and not give you such a long bloom.

Don't worry about bulbs not being rated super High, the voltage/wattage you want them to sink isn't huge.

[PRR]

> 12v 4 watt lightbulb ---- 13v 4.4w bulb

1895R Mini Indicator Lamp - 14 Volt - 0.27 Amp - G4.5 Bulb - Miniature Bayonet Base
Voltage : 14
Amperage : 0.27 Amps
(3.78 Watts)
ANSI Code : 1895R
Base Type : Miniature Bayonet (BA9s)
Length : 1.07 in.
Diameter : 0.59 in.
Life Hours : 2,000
typically used as an instrument panel light in domestic cars.

The "R" is red, "B" is blue, no suffix is clear.

NAPA or other car-part shop can sell you one for a buck or two. Going rate in bulk is about $3 for a 10-pack.

[Bluemeany]

That is certainly a good candidate.

I drew up a schematic for the night light, I'll build one up after I've finished with my airbrake and compare the two. I'll be rigging up some vvr in the 18 watt to so I can directly compare all three - see which is "the best"...