Vox AC4 clone ... Adding Depth to Tremolo

[old tube]

I'm just about to embark on my first Amp build. I've chosen to build an AC4 clone mainly using parts harvested from a 1963 Phillips reel to reel tape recorder. I'm building it point to point as per the originals.

Circuit I'm following is attached below. I've heard that the depth of tremolo on these can be a bit weak and I'm wondering if I should add a variable depth control to the build. I found a few articles about adding depth to tremolo but none specifically for this circuit.

Is it as simple as replacing the 220K resistor that goes across the two halves of the 12AX7 with a 250k log pot?

Thanks

[LeftyStrat]

Look at the Fender Tremolux schematic. You want a 250k pot in parallel with the 100k going to the cathode of the EF86. The 100k attaches at one leg of the pot and to the wiper.

http://www.freeinfosociety.com/electron ... er5e9a.gif

[martin manning]

Your link won't open for me, Lefty, but I think that the 5E9 depth control will work here.

[LeftyStrat]

Thanks Martin, that was the circuit I was thinking off. That link won't work for me now either. I see you also caught the missing line from the plate to the grid of the oscillator circuit. This oscillator circuit looks very close to the 5e9, except for the missing depth, and the 1M speed control instead of 2M.

Also, I remember reading a comment from a builder of this circuit that replacing the 5.5M screen resistor with 1M resulted in more gain. Might be worth experimenting with this value.

I've attached the 5e9-a circuit.

[old tube]

As always, thank you very much guys ... I've got a lot to learn!

Martin, I take it the red line on the 12AX7 indicates wire linking pins 2 and 6?

If I might ask another question ....... I've read that the 5M5 resistor at the EF86 can make the amp sound quite "swampy" and have heard tale of lower values being used to raise the "gain" of the EF86. Again showing my ignorance, but does this equate to you experts? I've read of values as low as 1M being used ... in your view could this cause problems other than the reported slowing down the tremolo a little?

thanks again.

[old tube]

Lol! ... you must have posted while I was writing, Lefty.

So you don't think going down to 1M would cause any problems?

Thanks

[LeftyStrat]

Lol! ... you must have posted while I was writing, Lefty.

So you don't think going down to 1M would cause any problems?

Thanks

Shouldn't be a problem, it's commonly used in other designs using the EF86. I'd probably try both values to see which I liked.

This is a great choice for a first amp build. It's a simple but magical circuit. Please give us updates on your results.

I'd probably also experiment with the difference between 1M and 2M on the speed control. If you can't find a 2M pot, you could wire a 'range' switch in series with a 1M pot to switch in another 1M resistor in series. Should give you a greater choice of speeds. You can find pots with a push-pull switch to save panel space.

[old tube]

Thanks again for the help.

I'll be sure to post some photos of the build, hopefully some here will find it interesting as I'm doing it point to point and using recycled transformers, carbon comps and mustard caps etc.

I'll also no doubt need a consultation or two as I put it together!

[tubeswell]

R8 (100k) is the load resistor for the CF stage that buffers that LFO, and R7 is the Rk for V1, but R8 and R7 also make a voltage divider for the trem depth. If you want the trem to be stronger try experimenting with this voltage divider. My first thought is to either increase R7 (say to 1k8 or 2k2) or try lowering R8 (to say 82k) or both.

Another idea could be to increase R8 to 220k and have it going to ground, and to put a 250k depth pot in parallel with it and have the wiper of the pot going to the Cathode of v1 (or replace R8 with a 250k pot to ground, and have the pot wiper going to the cathode of V1). Edit: If you adopt these suggestions, you'll want a blocking cap either side of the pot (i.e.; on the pot input side and on the pot wiper side).