English Channel / RDV

[Ge_Whiz]

Ricky - I've just finished the English Channel and had a query for Brian which he suggested I refer to you. Comments from other EC builders welcome, of course.

The query is about the 'Top Cut' control which, on my build is - ahem - subtle to say the least. Of course, it may be my tin ear, but I can barely hear its effect. I've checked the wiring and it's all there, but, well, it ain't doing much at the moment. Is this typical?

Answers on a postcard, please.

[RDV]

What are you testing it on amp-wise?
When I use mine in the house on the Ruby amp I can barely tell, but with the Marshall, it can really make it shrill. I keep mine only about 1/4 open. I think I have it wired backwards actually, cause mine gets more trebly as I turn it up, which may be good considering I used an old Guitar pot that is an audio type, which I suppose gives me a reverse taper? I will say the the range of the control is right at the outer range of human hearing. :wink:

HTH

RDV

[Ge_Whiz]

Thanks for the prompt reply, Ricky.

I've been testing it on my HH solid-state amp on clean, and at fairly low levels. I've noticed that it has more effect as you turn the wick up on the EC, as the main effect is on the harmonics generated (which I guess is the purpose).

Maybe I'll tweak it a bit.

[Ge_Whiz]

It's amazing, isn't it, just how easily you can believe in something that's not there.

Last night I connected the EC up to a signal generator and 'scope. What it demonstrated quite surprised me. The effect of the gain and tone controls on a sine wave is quite astonishing - there is a huge range of harmonic structures available as the circuit distorts, hence the range of sounds available. The bass and treble controls are very interactive. But the top cut control, with the component values shown does absolutely nothing to the waveform across its entire range. I used both sine and square waves to generate as many harmonics as possible and looked for very subtle effects, but nothing. When I substituted a 100nF capacitor for the 4.7nF cap, the effect was seen but only in the last 5% of the pot's range.

Clearly, these two components need some tweaking. I guess the idea of the circuit is to implement a guitar-like tone control - the 47nF cap and 220k resistor fix the input impedance to the top cut circuit to something akin to a guitar's 250k volume control, and the output impedance is very high. For a guitar, 4.7nF here would have a significant effect on the tone - but not here. I wonder why not?

Anyhoo, experiments to do.

[Ge_Whiz]

Well, looking further on the internet I found this:

http://www.blueguitar.org/new/articles/other/topboost.pdf

It's also available as an html file elsewhere on the web. Doesn't answer my query about the top cut control, but has some interesting things to say about the wierd tone controls, and a circuit error which has been carried through to the EC...

[B Tremblay]

Hmm, very interesting!

I've given your question some more thought and I seem to remember that while testing the EC, it seemed that the Cut control simply set the upper limit on the Treble pot's range.

I think that RG is well versed in the intricacies of Vox amps, so maybe he would have some insight regarding the Cut control function or the Bass pot wiring.

Have you tried lifting the Bass pot from ground as they suggest in that article?

[Ge_Whiz]

No, I haven't tried removing that ground connection, since I found the article during my coffee break (honest guv) at work. I'll add it to the list of tonight's experiments. Actually, I kinda like the tone controls the quirky way they are, as they provide some extreme sounds, quite apart from the 'authenticity' factor.  

[RDV]

I use mine to get some treble bite from my Les Paul, works wonders on "Nowhere Man".

RDV

[Ge_Whiz]

Okay, some experiments later...

Lifting the ground from the bass control does very little - the quirky features in the waveform remain, though ever-so-slightly less prominent. After some experimentation, I think that 15nF is a better value of capacitor for a decent effect. To achieve full control over the top cut with this capacitance, with no dead space, a 47k pot is sufficient. I prefer linear track.

Having said all that, it doesn't seem to achieve anything that couldn't be done with the treble control alone. Next time I build this circuit, I probably won't bother with it. Still, this is not to disparage ROG - I'm building this for a friend, who will love it, and I may have trouble parting with it...

[Jim Jones]

On the old Vox amps, the Cut control was a cap and pot (wired as  variable resistor) across the power tube grids.  I don't know if the ROG application would provide the same effect...  The effect  is quite noticeable on a Vox amp...

Jim