Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Madison on November 30, 2012, 08:03:02 pm
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How do those Marshall style boost pedals with a LED work?
How is the power supplied?
Battery?
Anybody got schematic or a gut pic.
Peace
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There is nothing to it, the LEDs are used as diodes for clipping, different colors have different forward drop voltages thus clipping characteristics, you can replace the 1N914's in the schematic with say a mixture of red and green LEDs (with the same orientation). http://www.geofex.com/PCB_layouts/Layouts/omnidrv.pdf (http://www.geofex.com/PCB_layouts/Layouts/omnidrv.pdf)
Jaz
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Pardon.
I meant a "Channel Select" pedal like on Marshalls.
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Pardon.
I meant a "Channel Select" pedal like on Marshalls.
You mean something like this? http://www.amplifiedparts.com/products/P-H471-LED (http://www.amplifiedparts.com/products/P-H471-LED)
If so, then the power is likely to come from the amp.
Jaz
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Do you mean a stand-alone pedal? Battery or One-Spot power for the internal workings can also light an LED.
Or do you mean a remote-control for an amplifier?
It is perfectly possible to feed power down and detect how much is eaten, use that fact to control switching.
Simplest case: Find 14V DC inside the amp. Feed this through a 12V 20mA relay coil, to jack to pedal. Inside the pedal a switch opens or closes the path through (~~2V) LED and back to amp power common.
Switch on: both LED and relay are "on". Switch off: no current flows, both LED and relay are inactive.
Well, if you do that much, the buyers want more. For two switches and LEDs, one way is to feed AC to the pedal. Diodes split the path + to A and - to B switch/LED. Inside the amp something clever splits + to A relay and - to B relay. However half-wave AC to relays is ugly stuff. Also it isn't always convenient to get voltages and currents in LEDs and relays in the same ballpark. Usually some chips are used to buffer and steady the drive to the relays.
OTOH, with a TRS plug you can have two discrete paths, and use the simple scheme twice. The product Jaz pointed to shows a typical pedal schematic, except the logic is reversed. If fed 14V through 680r resistor, the Tip contact is 14V when LED is "off", 2V when LED is "on". You could reverse the contacts of the relay to get the effect state back in sync with the LED state. They probably put more (and more) stuff behind the amp jack to work the relay indirectly.
2-switch pedals are an old standard. How do you get buyers to say "WOW!"? I was just looking at (and head-scratching over) a Mesa plan with 5 or 6 switches and according LEDs. It did use a many-pin DIN plug instead of simple 1/4" plug. They seem to have come to it round-about. Power to pedal was +2V and -3V DC derived from heater winding. There were logic chips inside the pedal, and a MML DAC. The amp had a bar-graph chip, plus some different MML logic in a variety of discrete devices. (All apparently to avoid using DPDT Latching stomp-switches. SPST switches are much cheaper, even with the extra chips to latch their states.)
These days it has become cheap to put a $2 mini-brain at both ends of the wire. The pedal brain can scan many-many switches (more than can fit), trigger LEDs or even a text/graphic display, send hi-speed code up the wire to a similar mini-brain in the amp, which works its own dashboard light-show and the various switching schemes in the amplifier. The actual power needed may be low enough to run power down and data back on the same wire.
Fender Mustang II has a USB jack. You load software on your laptop (or I think iPhone?) and select several of 99+ different possible patches at one time. (You can do this on the amp but without a groovy interface.) A super-simple one-button no-LED pedal selects patch A B or C, clean solo bridge or whatever; then the iPhone can dump different choices for the next song. And it was not high-price: you could buy it as a basic amp and discover the wizz-bang later.
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>>remote-control for an amplifier?
Yes.
I did this on another amp recently.
It was a boost via the NFB loop on a Fenderish circuit.
I had a panel switch and a pedal wired TRS on a cliff stereo switching jack.
It work and worked well (quietly) but not the sort of boost I am looking for on this current build.
Now, I am trying to have the b/p cap on the first triode be the boost.
Noisy as heck;buzz.
I have the front panel pilot and the boost LEDs wired in series with simple diode DC rectification off a spare heater tap.
The pre amp ground is reference for the switching.
It seems as soon as I hook up the LEDs the buzz comes in.
Rectifier cap helps not.
So the confusion sets in and the wheels start to turn.
I am wondering if it would be better to use AC for the LEDs AND, while I am at it, IF one leg of the AC is going to run out to the pedal and back......I might as well have a indicator diode on the pedal too.
IF...........there is even a way to even do this with a TRS situation.
I don't want to get into relays at the moment.
Thanks.
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Show us a schematic.
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This is the previous build with the boost.
Don't know how to draw a TRS switching jack or boost pedal so bare with me.
I assume the previous build worked because I was never really lifting a ground.
Haven't made a schematic for the current build but it is really similar with EL34s.
I want to use the b/p cap on V1 for the boost.
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Looks like I maybe off to relay land................
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This is the previous build with the boost.
Don't know how to draw a TRS switching jack or boost pedal so bare with me.
I assume the previous build worked because I was never really lifting a ground.
Haven't made a schematic for the current build but it is really similar with EL34s.
I want to use the b/p cap on V1 for the boost.
You only need a SPDT switch. Also, it looks like the LED is on when the boost is not active.
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>>You only need a SPDT switch.
I remember trying that and it injected a lot of additional noise.
I can't remember how or why at the moment.
>>Also, it looks like the LED is on when the boost is not active.
Thanks.
I drew it upside down as looking into the chassis.
Corrections made......I think.
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Same idea but wired differently (may be)...
(https://dl.dropbox.com/u/1326040/Boost%20Switch%20w%20LED.jpg)
Jaz