signal voltage maximums and warning indicators

[terminalgs]

I have a tube pre-amp/overdrive pedal with 2 gain stages in a 12ax7.  this is not a starved plate circuit.  I've 180V at the plate on each triode. for simplicity,  lets say this:

input -->  Stage.1 -> Pot.1 -> Stage.2 -> Pot.2 -> output.

If Pot.1 is turned "down" and Pot.2 is turned "up",  its functions as a clean signal preamp,  If Pot1. is turned "up" and Pot.2 is turned "down",  Stage.2 gets overdrive and thus it functions as an overdrive effect.  (I'm not tell anyone anything new with this., but just to be clear with the circuit description).

the problem is that if Pot1. and Pot.2 are both "up",  a guitar's 300mV single can become 30v !!   

I was considering using double gained volume pots and using the extra set of pots as variable series resistors to supply a "warning light"  6v LED.  (if both volume pots are turned "up" the VR's will go to zero ohms and a full 6v will hit the 6v LED.

does anyone have a better idea?  Is there a simple circuit that can monitor the output voltage?  I would like to avoid sending the signal through a complex circuit that might alter the signal just for the sake of monitoring..  (sort of how I got to the double gained pot idea...).

[JKowalski]

It sounds like the best idea would be to redesign your circuit....

What I would do is adjust the second gain stage input to reach JUST ABOUT clipping levels in the second stage with the overdrive pot off. That way, when you turn the overdrive pot on, the wave begins to clip immediately, and you do not get any significant boost in amplitude, just distortion. Then, you have your standard attenuation volume control at the end of the circuit. That way, the basic amplitude of the whole circuit remains relatively constant no matter the overdrive level.

Then, you want to put a pretty large resistor before you attenuation potentiometer so as to limit the maximum value to guitar signal level! Or slightly higher, if you want it to work as a boost pedal as well.

[terminalgs]

I usually use a trim pot to attenuate the max output,  turn both gain and level to max,  then adjust the trim pot down to about 1.5X the input source...  of course, the trim pot is accessible to the user, so they might adjust it up.   

I'd would have liked some way to have an overload LED that actually monitored the output signal voltage regardless of pot/trim pot settings  (rather than what I'm going to settle for... which is using double spots on GAIN and LEVEL to deliver 6v+ and GND to an LED when both pots are between 9 and 10).

the circuit is similar to many high voltage plate tube preamps,  so its simple and there isn't much to redesign.

[JKowalski]

I'm not sure if I understand. You say you can get up to a 30V signal - which is obviously a very bad thing to put into your amp- but then you say you attenuate it down to signal strength. Why do you need the LED indicator?


My suggestion would be tapping into a point on your output line to get say a 2VP-P waveform, and putting that through a simple sample and hold diode+capacitor and then using a comparator to trigger the LED when that sample and hold reaches a certain voltage.



Sorry, I just can't seem to get the whole idea here. Tried my best!

[terminalgs]

yeah, my explanation wasn't clear:

I'd like to let the pedal have two functions:  1) clean boost and 2) overdrive.  the first is handled by turning down the  gain knob and turning up the level.  and the second, overdrive,  is the opposite.  turn up the gain knob and turn down the level.    the problem is, a user of the pedal can turn both all the way, which is bad.


I do have an attenuator trim pot,  but I like to set the max output level with the trim pot with the level knob at 100% and the gain knob about 30%-50%.  This seems to allow for a lot more versatility in the controls.  I could set the attenuator trim pot when both gain and level are at 100%.  this is ultimately the safest thing to do,  but it seems to limit the versatility of the device.  especially when some pickups are louder than others.

I've got some pedals (like a BK.Butler Tube Driver for example) that if the level and gain are both at 100%, the output level is tremendously huge. (maybe dangerously huge). 

is this just a known hazard? or do people here on diystompboxes always trim back overall gain? 

[slacker]

Easiest way to fix this is just don't turn up both knobs

What I'd do is string some LEDS across the output, like you would with clipping diodes in a distortion pedal. If you used 3 or 4 superbright LEDs in series they would clip at about 6-7 volts. That would limit the output volume and you could use them as indicators as well.

[solderman]

I have a tube pre-amp/overdrive pedal with 2 gain stages in a 12ax7.  this is not a starved plate circuit.  I've 180V at the plate on each triode. for simplicity,  lets say this:

input -->  Stage.1 -> Pot.1 -> Stage.2 -> Pot.2 -> output.

If Pot.1 is turned "down" and Pot.2 is turned "up",  its functions as a clean signal preamp,  If Pot1. is turned "up" and Pot.2 is turned "down",  Stage.2 gets overdrive and thus it functions as an overdrive effect.  (I'm not tell anyone anything new with this., but just to be clear with the circuit description).

the problem is that if Pot1. and Pot.2 are both "up",  a guitar's 300mV single can become 30v !!   

I was considering using double gained volume pots and using the extra set of pots as variable series resistors to supply a "warning light"  6v LED.  (if both volume pots are turned "up" the VR's will go to zero ohms and a full 6v will hit the 6v LED.

does anyone have a better idea?  Is there a simple circuit that can monitor the output voltage?  I would like to avoid sending the signal through a complex circuit that might alter the signal just for the sake of monitoring..  (sort of how I got to the double gained pot idea...).

This one includes an clipping indicator that fires of a LED if exceeding a certain voltage level You cold probably tweak it to your purpose.

http://www.solorb.com/elect/musiccirc/reverb2/

//Solderman

[Gus]

What is the max peak to peak you want to allow?  Maybe a diode clamp/limiter on the output

Once you decide make a diode fullwave bridge.  Signal and ground into the AC pins now the cool part you can just add diodes in series (can be different types) anodes to the + and cathodes to the- you can even add series resistance.  This cuts down on the amount of diodes needed.

  As slacker posted about using leds as indicators/clamps you would just need the LED in the middle of the bridge clamp/limiter.  It would be two diode drops of the bridge plus what is between + and -

[terminalgs]

slacker wrote: Easiest way to fix this is just don't turn up both knobs

YES!  that's true.  my friend was recording with a Marshall 100W head and a 4x12.  he had the Marshall at full tilt,  used my pedal,,  I think with both knobs at full tilt (against my warnings).  -- at mid -volume settings on an amp, you can tell when both gain & level knobs are at full tilt..., --> volume goes way up...  BUT with the amp at "11".,,  increasing the input signal doesn't do much to the volume coming out of the speakers...    at some point not long into that recording session,  the head lost a couple power tubes.   sooo you can warn them,, but it won't help.  with an 'overload' LED,  next time I can say "well, was the 'overload' lit?

Gus wrote:  What is the max peak to peak you want to allow? 

good question?  what is a good safe max level that amps can deal  with in a healthy manner?   hot pickups will push .5V - 1V at peak,  so  with some boost,,  maybe 2V - 3V ?  what is safe to push into an amplifier?

here is a passive warning light idea I was kicking around: I'm using 1M audio pots for the two vol controls, and I have some double stack types, so an extra 1M pot on each if I want to do this:

http://s270.photobucket.com/albums/jj119/GSstudio/?action=view&current=Picture12.png

since 1M is a bunch of resistance and the 6V+ doesn't get through until the very end,  I added R1 and R2 to "turn on the LED sooner"....
again, this is passive and doesn't save an amplifier.

[JKowalski]

I have an idea. How about you put in a switch that alters the voltage divider for the gain pot one way, and the voltage divider for the clean the other way?

Basically, since you already have to attenuate before the pots, why don't you have it so the switch adds in a little more resistance before the pot you want to limit, so you can't go as high on one as you can the other?

So you have either the GAIN POT (WIDE RANGE) + CLEAN POT (SMALL RANGE) or switchable to the GAIN POT (SMALL RANGE) + CLEAN POT (LARGE RANGE)? This way you won't be able to turn both up to max at the same time.