Bias modulating trem idea

[octal]

I have an idea for an improvement to bias modulating trem- as I understand it, the bias level in an amp which bias mod trem is critical, as the bias swings down on the "cutoff" portion of the cycle and and swings up (increases) on the other half of the trem oscillator's cycle. If you're biased too hot, it won't cut off enough and may redplate the tubes at deeper trem depths.

Let's say the bias is normally -35v and the trem osc makes it vary from -45 to -25. The part of the cycle from -35 to -25 isn't useful- it doesn't change the volume of the output signal- all it does is push up the average bias current. Here's a circuit idea to allow the trem osc to pull the bias more negative to cut off the output tubes but not allow it to disrupt the bias in the more positive direction. The added diode will clamp the trem voltage and prevent it from getting any more positive than the amp's normal bias setting.

Pardon the longwinded explanation and the poor drawing. I can't get to my scanner right now....

Will this work? Is it a good idea?


Nathan

[loudthud]

This is not a bad idea and I'd like to suggest some improvements. But first, a couple of points. From the "cut and try it" point of view, Gerald Weber suggested to me that bias tremolo required that the bias be set on the cold side to sound it's best. This validates Octal's point. However, I think it's a good idea to limit the top half of the LFO's (Low Frequency Oscillator) waveform but it is usefull at low amplitudes and doesn't need to be eliminated completely.

In one of his books, Kevin O'Connor presents a solid state tremolo pedal that uses a triangle LFO waveform. I've never tried it but it brings up the point that the modulating waveform doesn't need to be a sinewave. I have looked briefly at the modulation of a blackface neon-LDR type tremolo and it wasn't much like a sinewave and varied with amplitude. Simply apply a signal from a generator at say 1KHz and sync your scope on the LFO at low sweep speed to observe the modulation.

So one of things to try might be a zener diode ( maybe 5V) in series with the diode on Octal's schematic. Something to be aware of is that shunt type clipping will tend to 'push the LFO waveform down'. What happends is the .1uF cap that the LFO signal goes through will build up charge such that the centerline of the LFO wave will be more negative than the bias. This will affect the first few cycles of the LFO wave when the tremolo is first switched on and you may hear something as it is switched off.

There are possibilities to clip the LFO waveform at the cathode follower tube. This will alter the LFO wave at all amplitudes regardless of the setting of the Tremolo Depth control. First, insert a 1meg resistor between the plate of the oscillator tube and grid of the follower tube. This will preserve the gain of the oscillator so it won't stop oscillating. Now, decrease the 470K to ground at the cathode of the cathode follower until you start to get some clipping. You could go as low as 47K or so. You may also adjust the 4.7K cathode resistor of the oscillator and these will interact.

The bottom line is there are lots of things to tweek on that will alter the operation of the tremolo so heat up your soldering iron and have fun.

[octal]

Thanks for your input, Loudthud. You make some interesting points.

I thought of the "pushing the waveform down" issue of charge building up on the coupling cap.

I think your idea of using the cathode follower stage to limit the positive swing of the LFO's output (and hence the bias) is very interesting. I suppose all of these methods would change the duty cycle and shape of the LFO waveform.

If I ever end up trying these ideas, I'll let you know how it works out.


Nathan

[6267]

If you're already using a cathode follower, you could try running some of the tremolo into the screen grid with a big cap instead of the grid or cathode. That will have a fairly direct effect on gain without being as sensitive (in most cases) as the first grid. They also used to bias the suppressor (I guess this would only work for EL34 in common guitar amp tubes) positive in some tv circuits to sharpen the characteristic knee...might not be audible, or might be pretty cool sounding...
Tube regulated screen voltage with the trem signal coming into the regulator and "unregulating" it rhythmically would be pretty neat.

[NorCalTuna]

So one of things to try might be a zener diode ( maybe 5V) in series with the diode on Octal's schematic. Something to be aware of is that shunt type clipping will tend to 'push the LFO waveform down'. What happends is the .1uF cap that the LFO signal goes through will build up charge such that the centerline of the LFO wave will be more negative than the bias. This will affect the first few cycles of the LFO wave when the tremolo is first switched on and you may hear something as it is switched off.

Hey thud, this is a great idea. I have a few questions if you don't mind...

I don't see why this won't work on the non-cathode following princeton reverb variant. Do you?

Does the value of the zener determine value of the clamp? So, more voltage at the zener, more centerline "dip"?

The cathodes of the two diodes are lined up, right?


I can't wait to to try this... I have a "PR" build that loves to run hot but the player loves intense vib settings!

thanks!

[octal]

On something of a tangent- this is the weirdest bias modulating trem circuit I've ever seen (see attachment- gibson GA40) the 3rd 6v6 (V7) is triode connected and appears to work one of two ways- it should pull the screens down a bit as it conducts, but it's also going to pull the cathodes of the power tubes up by dumping current into the cathode resistor. Well, you have to commend them on their originality!

Who is going to build and try out this one?

[loudthud]

I don't see why this won't work on the non-cathode following princeton reverb variant. Do you?

Any load on an oscillator reduces it's gain and if it's too much, will stop it from oscillating. So try it.

Does the value of the zener determine value of the clamp? So, more voltage at the zener, more centerline "dip"?

The clamp will limit the positive excursion of the LFO signal in relation to the bias. I think there would be less centerline dip than just a simple diode.

The cathodes of the two diodes are lined up, right?

Yes.

I don't have an amp I can try this on at the moment. You guys have fun and report back how it sounds.

[oskar]

I am trying this thing on one amp , instead of a zenner I am using a blue led with a voltage drop around 3v , I noticed that when I put this led instead of a simple diode , the negative side of the modulating signal , also clips , and the modulating signal goes less negative , doing the tremolo effect less efective.
I am thinking about it but I dont see why with the blue led clips the negative side and with a 1n4007 it doesn't.

[octal]

I am trying this thing on one amp , instead of a zenner I am using a blue led with a voltage drop around 3v , I noticed that when I put this led instead of a simple diode , the negative side of the modulating signal , also clips , and the modulating signal goes less negative , doing the tremolo effect less efective.
I am thinking about it but I dont see why with the blue led clips the negative side and with a 1n4007 it doesn't.

You are exceeding the reverse biased breakdown voltage of the LED and it's behaving like a Zener. Why not try an LED with a 1n4007 in series with it? Better yet, you could just use a zener...

Nathan

[oskar]

Thank you , you are right , I was not thinking about the reverse voltage aplied at the led , .......
yes of course I can use a zenner , it was only for experimentation , and to try something diferent ... if you use a led , you can use it as a tremolo speed monitor led , I noticed that for this aplication you must use a ultrabrigt led , otherwise dont ligth too much due to the little current that pass thru it.