Is Tremolo Plate Voltage Critical?

[dwinstonwood]

Hey everyone, a new mini-project in the works...

The first tube guitar amp I built was a Vox AC4 V-1-1 "sans" tremolo. It still sounds great and I use it as much as my other amps.

But, while I'm waiting for the cabinet to arrive for my latest build (a Vox AC15 with a cathodyne PI) I thought I'd revisit the AC4 and add a tremolo circuit.

I've decided to use the Vibro Champ tremolo circuit - because it has both speed and intensity, and it sounds good - and connect it to the EF86 cathode, just like the original Vox tremolo circuit in the V-1-1 schematic.

Now, the three B+ nodes on my AC4 provide 305VDC, 252VDC, and 234VDC, respectively. But, the Vibro Champ schematic shows 340VDC on the pin 6 plate of the circuit.

So, my questions are: How critical is plate voltage to the operation of tremolo circuits? Is higher better? Is 305VDC adequate for the Vibro Champ circuit?

[schematics attached]

Many thanks in advance!
David

[PRR]

> because it has both speed and intensity

The Fender intensity network will go right into the VOX at the 68k resistor location.

[dwinstonwood]

Thanks PRR, but I don't see a 68K resistor in the Vox schematic. There's a 6.8K grid resistor on the EL84... are you referring to that location? 

Do you mean I can simply add the intensity pot in place of the R9 100K resistor in the Vox schematic? Or, should that resistor be changed to  68K and left in place?
Thanks.

[PRR]

I mistyped.

[dwinstonwood]

Great! Thanks for your help PRR. I'm going with that circuit.

[sluckey]

Not a big deal but... The footswitch is labeled wrong. The tremolo will actually be ON when the switch contacts are open.

[dwinstonwood]

Thanks sluckey. I'll see if I can figure that out.

But, this has me confused as well. Why is there a second B+ connection (highlighted in green) running through the 10M resistor to the junction of the two .01uF caps? The Vibro Champ doesn't have this. If I left that connection out wouldn't the circuit operate just like the Fender one?

[sluckey]

That's a "kickstart". It is used to make the oscillator start quickly. The oscillator will still work without it but it may take a few seconds to start oscillating. Musicians can't wait for that. They want tremolo immediately when they stomp the switch.

How it works... When the footswitch contacts are closed, there is a ground at the junction of two of the feedback caps. This disables the oscillator. As soon as the switch contacts are opened, a voltage is immediately applied to the junction of those two caps. The quick transition from zero to some positive voltage is coupled through the caps to the grid and causes the oscillator to quickly start up. After the oscillator turns on the voltage at the junction of the two caps is inconsequential and tremolo works until you stomp the switch back off. Why did Vox decide to pick up the voltage from a different power supply node? Maybe because the 260V node is a little cleaner than the 270V node.

The Fender also has a kickstart. Notice how the 1M resistor connects to the cathode of the tube? It could have been connected to ground but then the oscillator would be slow to start. The cathode connection provides 1.6V to be used to quick start just as the Vox voltage divider. That quick transition from zero volts to 1.6V causes the oscillator to quickly start. You can think of it as slapping a sleeping person to immediately get their attenion, versus gently nudging them to slowly wake up.   

So, you gotta ask, "Do I want a snappy ON/OFF or can I live with a sluggish ON/OFF?"

[dwinstonwood]

Wow Steve, that makes total sense! Thank you for that clear and complete explanation. I've included the Vox kickstart in my layout.

But, I see now that the 1M resistor I have going to ground could also connect straight to pin 8, and I could do away with the Vox method. (I don't have any 10M resistors, just 5.6M's and 2.2M's, so the Fender kickstart would save me having to order more parts)

Here are two layout details showing how I arranged the components for both methods.

[shooter]

just 5.6M's and 2.2M's

5.6+2.2+2.2 = 10, temp solution while parts arrive   

[dwinstonwood]

Thanks shooter. That's a math equation I can actually solve.

I'm in no hurry with this project. I need to buy another chassis anyway, but this one's cheap at around $15.

[dwinstonwood]

I'm finally making progress with this re-build of my AC4 (adding vibrato)
I spent most of the time planning and arranging/re-arranging everything in the small 11x7x2 chassis.

[Bieworm]

I'm finally making progress with this re-build of my AC4 (adding vibrato)
I spent most of the time planning and arranging/re-arranging everything in the small 11x7x2 chassis.

Oh man... that's almost...cute 🥰😊

[dwinstonwood]

It could fit in the dashboard of a car. It's the last miniature amp I'm building.

[dwinstonwood]

Ok... The good news is that the Vibrato sounds great. Both the speed and intensity controls provide plenty of range.

The bad news is that turning up the Intensity control causes the volume to go down! Not completely, but enough to be an issue. I'm guessing it's some weird interaction with the EF86?

I've attached my schematic and layout. Thanks in advance for help with this. Otherwise, it's a nice sounding amp with really cool Vibrato.

[sluckey]

The bad news is that turning up the Intensity control causes the volume to go down! Not completely, but enough to be an issue. I'm guessing it's some weird interaction with the EF86?

Maybe that's why Vox had no INT pot.   

[dwinstonwood]

Thanks Steve, I'm sure you're right.

But, a search revealed that others have encountered the same problem. One solution was to use the 5E9-A depth control. It can't do any harm to try it.

Another solution was to determine the point at which the volume gets attenuated by the intensity pot and add a resistor to the pot to "stop" it from going past that point. Sort of like the safety resistors we put on bias pots.

[dwinstonwood]

This morning I wired in the 5E9-A depth circuit and it cured the volume attenuation issue I'd say by about 80-90% or so. There's just a slight increase in volume with the Intensity knob at zero. This might simply be a "perceived" difference since the signal is then constant and smooth (without the voltage or current fluctuations). And, naturally, the amp sounds better in the cabinet than playing through the old, cheap test speaker sitting upright on the table.

I did notice a slight decrease in the intensity, or depth, of the Tremolo, but I think it's still plenty strong. But, assuming the red LED mod that sluckey has covered would work in this circuit, I could always grab one the next time I place an order with Doug.

One other thing I did was to try different tubes. I ended up using Tung-Sol Ef806, EL84, and 12AX7. It's hard for me to say how much difference that made. At any rate, it's an amp I can crank in the bedroom without rattling picture frames.

Thanks for the help and advice with this mod PRR and sluckey!

If anyone's interested in the final schematic and layout I'll update them and post the PDF's.

[sluckey]

The LED should not be needed in that circuit. You already have more than enough LFO signal to wiggle that EF86 cathode.

[dwinstonwood]

Thanks Steve. After playing the amp for a while I think increasing the Tremolo any further would - for me - just push it into the "unusable" range. One last thing I might do is to step down to a 33K grid stopper on the EF86 to make it more like a traditional Hi input (it's a 47K "Mid" input now).

I need to state that I stole this fix off the internet (another forum). I take no credit for the 5E9-A mod, of course.

I'll go ahead and attach the final files because it's a cool little amp that's relatively simple and inexpensive to build, especially if you have an old Fender Frontman Reverb cabinet to build it in.

 

[dwinstonwood]

I took the basic voltage readings and added them to this schematic. They all look good to me, given my slightly lower b+ voltages.
I changed the 100K resistor in the depth control circuit to 82K. This made a small improvement and raised the EF86 cathode voltage to 2.66V which is near enough to the 2.7V on the Vox schematic.
With the 150 Ohm cathode resistor the EL84 is biased a bit cool at 10.55W - 87.9% dissipation.