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Trem pulse LED or Why Won't This Work?

Started by galaxiex, September 14, 2014, 10:11:42 PM

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Quote from: Roly on September 18, 2014, 08:00:53 AM
Something to paste in your hat; frequency-selective networks also produce phase changes, lag for low pass and lead for high pass.
Sooo, that seems to imply that all the frequency shaping networks throughout an amp might be responsible for the "shimmer" I often hear when playing clean esp. thru something like my Deluxe Reverb?  :)
Love that sound, never knew why/what made it happen.

Quote from: Roly
If you look at the runoffgroove "Umble" preamp you will see that the Source 1k5 resistors are bypassed by 4.7uF caps.  If you change the Drain resistor (e.g. with the trimmer) you are changing both the DC conditions and the AC load, so the signal gain will also change.  If you vary the bypassed Source resistor however you are only changing the DC conditions because the AC signal is bypassed around the resistor, and that is what is actually required.  This is the right way to DC re-bias the FET without changing the AC gain (significantly).

I did think it odd that the trimmer was on the Drain side.
"Kinda" knew this already from reading Kevin O'Conner's stuff, although his material is more focused towards tube amps, the biasing and R's surrounding a triode tube (eg. 1/2 - 12AX7) are very similar to what is done with transistor circuits, and Vice-Versa. That much is obvious to me.
Thanks for the clarification.  :)

Quote from: Roly
Before Electronics Australia was killed off by Hi-Fi marketing types it was a very credible electronics mag.  This is not to say they didn't sometimes have some real bloopers (a certain RIAA equalised phono preamp comes to mind) and they may have been a bit less than forthright about their mistakes, but they did issue errata to correct those mistakes (at least until the dying days when things seriously turned to $@#t).  The only mag I know of today that 'fesses up to its mistakes is Elektor, and they generally maintain a blistering standard of electronic engineering that others struggle to keep up with.

Elektor Nice!  :tu: I think I've seen the mag here on the occasional blue moon.
Not many electronics mags around, depending on what store you visit.

Quote from: Roly
Errors are one thing, but outright misrepresentation is another.  I remember a ham telling me about a "100 watt" RF amplifier he built from an article in an American ham radio magazine and whatever he did he couldn't get more than a few watts out of it.  Later he traveled to the US and made a point of catching up with the author.  When he commented that he couldn't get it to make watts he was astonished to be told "Yeah, I couldn't get more than that out of it either".  So how did it get to be published with the claim of "100 watts"?  Who knows, but it certainly undermines your faith in the magazine editorial committee.  Mistakes I forgive, deception I don't.

That's just down-right borderline criminal! Talk about being only concerned about selling magazines.
The bean counters rule! No integrity!  :grr
If it ain't broke I'll fix it until it is.


Quote from: Roly on September 19, 2014, 06:57:26 AM
Well one thing you don't do is draft circuits like the one above without values or transistor types.   :grr

NEVER! I hate that!  :grr
If it ain't broke I'll fix it until it is.


Quote from: J M Fahey on September 19, 2014, 07:35:40 AM
All of them are drawn that way, except the odd one which is cut and pasted from somewhere else.
All of them with the same primitive software .... Paint?
Or something as crude.

Attracts beginners because they post incredibly simple projects, like this 30W power amp .... irresistible:

Just 3 transistors !!!!!!

He even publishes specs, such as 10 to 100KHz frequency response, 0.4% distortion, etc.
All either pulled out of thin air or, maybe, simulated.

Problem is, simulators imagine perfect heatsinks, parts always at 25 DegC , no ripple, stable supplies, etc.

In the real world, this uncompensated amp will overheat, thermal runaway and die.

And the poor beginner gives up in frustration and instead gets into some easier hobby, such as surfing without a board, parachuting with an umbrella or learning the Ramayana, in Sanskrit, by heart.

Yeah, I can see how a beginner would be attracted to that.... (ugly schematic not withstanding...)

So simple! why do I need all that other complicated stuff to have an amp?
I'll just build this super simple easy circuit and have have a great amp!  ::)
If it ain't broke I'll fix it until it is.


So I found this > http://www.generalguitargadgets.com/pdf/ggg_eat_sc_improved.pdf
updated version of the EA Tremolo.

These guys trustworthy?
If it ain't broke I'll fix it until it is.


Well ... no. (unless you are "shimmering" a tone control at the time).

I need to clarify something.  A fixed frequency selective network produces a fixed phase change (and for "phase change" here you can read "a tiny delay in time") which is generally inaudible.  If you vary a tone control the phase will change while the control is being changed, pushing the frequency up or down a tiny amount, but when you stop moving it the phase change stops moving and the frequency returns to normal.

Phase vibrato and the like depend on the phase constantly changing (due to the LFO) which causes a constant change of phase back and forth and therefore a constant up and down shift of the signal passing through, and this change is audible.

To a large extent FETs and triodes are very similar, while BJT circuits are fairly similar.  The very different voltages aside, how you bias a FET is pretty much the same as how you bias a valve - you alter the Source/Cathode resistance.

The idea of a circuit is to communicate a story, and if there are no component values the story may as well be written in a foreign language.

The power amps in my Twin-50 aren't particularly complicated and have given great service over the decades since they were designed and built, so complexity is not essential in a good circuit.

The main difference I see here is that a FET buffer has been added in front.  Jack Orman - credible, Robert Strand and JD Sleep - unknown, runoffgrove - well we've covered that.

Now Jack may have done a mod, but we are looking at an interpretation of that mod, and straight away I notice something that rings an alarm bell; C9 10uF is simply in series with C1 0.22uF.  Why?  The effective capacitance between the FET Source and the following transistor Base is 0.22uF, so why the 10uF at all?  This suggests to me that whoever drafted this circuit doesn't understand capacitors in series - which is pretty damn basic.  The 0.22uF will also prevent any DC voltage getting to the 10uF electro so it won't have any polarising voltage across it, and for an electro that's not good.  {a single resistor to ground between them would fix that, but it doesn't explain the ten lane super highway coming in and a goat track going out}.

I also notice at the input of the FET there is R1 1Meg to ground, then C8, then R15 1Meg to +ve and another R16 1Meg to ground.  For AC signals these are all effectively in parallel and therefor the input impedance presented to the guitar will be 1Meg/3 or 330k, which IMO isn't enough.  All of these resistors could be considerably increased in value, say to 3.3Meg, then giving a more respectable 1.1Meg input impedance.

So like the Curate's egg, this modded circuit is "good in parts".  I think this is a situation where what was a reasonable circuit to begin with has been "contaminated" by a little knowledge being a dangerous thing.

"trustworthy?"  Even the best designers and magazines can have off days, drop the ball, so no matter how highly thought of they may be, we always have to keep our critical wits about us and be alert to designer brain farts and drafting bloopers - and to the blind leading the blind.  One stupid mod can turn an otherwise reasonable circuit into a pile of steaming dung.
If you say theory and practice don't agree you haven't applied enough theory.


Thanks for all that,  :) and for pointing out the (now) obvious flaws in that "interpretation" of what may have once been a decent circuit.

I'm still working on this Trem, waiting for some parts to arrive.... meanwhile... there's a couple other things I've been tinkering with.
Will post new threads about those in due time (if anyone's interested  ;)).

Other projects...

Yet another Teisco... tiny little practice amp.
Checkmate 10 combo with a 6" speaker in a flimsy (real wood  :o) cabinet.
It came to me with not much more volume than a whisper.
The UN-plugged electric guitar was louder than what came from the speaker.  :lmao:

Heath TA-16 with a few problems... really bad harsh distorted sound and not much volume.
Lousy (for guitar) Baxandall tone stack.
The good. It came with 2 new Fender 12" speakers.
The original trem circuit and reverb work. (new reverb tank)
There are a few existing threads already here on these amps...

Trying to not have too many irons in the fire all at once.  ::)
If it ain't broke I'll fix it until it is.


If the basic design objectives of that mod are to provide a reasonable input impedance, and maybe a moderate amount of gain, I start to think of a TL072 FET input (or other dual) that would do the job with no fuss.

The padding of the depth control really depends on the individual FET used, they are just an idea to get a good spread on the depth control really.
If you say theory and practice don't agree you haven't applied enough theory.


Hi Roly,
I thought about a TL071 for the input buffer, may yet try that. (Life's are a bit chaotic these days...)

Meanwhile, here's a couple of R.G.Keen's papers on the EA Tremolo.

I thought the "Vibra-matic" quite interesting.  :)

On the basic tremolo circuit, I notice he merely adds the 1M input resistor to the existing original EA circuit.
If it ain't broke I'll fix it until it is.