What is Zinky Thinking?

[Dave H]

Yes, it's a waste of components. I could understand it if the first CR was there to keep high voltage (DC) off the second C but it's not that as the first C is 250V and the second 630V.

[bob p]

The two switchable vol pots after V4b in the Dual Professional are weird as well. I can't really see why this makes any difference in a guitar amp.

I'm wondering if they're expecting you to choose the fat option on one channel but not the other, use the volumes to adjust levels, and then go channel hopping for lead and rhythm.

[bob p]

The second RC filter has a 93 Hz crossover and can be seen as a subsonic/mud killing network.
The first one has a 1.5Hz cutoff and is an absolute waste of components.
Why is it there?
I have seen many no-clue "designers" switch parts at random, until they hear something they like, then they superstitiously freeze the design at that point, fearing to lose what they achieved, so they freeze it errors and all.

Maybe it's not that the designers are idiots who superstitiously freeze the design. Maybe they just walk away and expect a subordinate to do a better job when they tell them to wrap it up.

Chances are that the "designer" is involved in the amp's design as an overseer, with a lackey subordinate doing the scut work for him. They proceed to do parts swaps and when they're satisfied with the circuit's performance, the "designer" tells the subordinate to finish it up as he moves onto something else. The subordinate guy doesn't bother with much attention to detail. He declares the job finished and moves along. In the end, nobody really pays attention to detail.

I've seen a lot of circuits where it looks as if the designers limited their engineering efforts to getting something far enough to meet their design criteria, and then shoved the project off onto a subordinate for finish work. All to often the subordinates either don't take it seriously, or they don't know enough to recognize what needs to come out, so things are stupidly left in the circuit when the circuit goes to production. I think this is typical for small production houses that don't have design engineering departments that are separated from production engineering departments.

In the big electronics companies when the design team finishes the project it goes to the production team for revision. The designers typically work in a cost is no option mindset, they just try to design an optimized circuit. The production team's job is to analyze that circuit and determine how to manufacture it more cheaply, sometimes trading away marginal performance improvements in favor of significant cost savings. I'm willing to bet that after Zinky and his lackey finished the design, nobody else went over it with a fine toothed comb with an eye to shave away cost -- they just put the circuit into production and didn't worry about parts cost. Fender charges so much for the Custom Shop amps that the cost of passive parts isn't significant to them. And the production volume is so small that it probably costs more to review the circuit than it costs to throw away a few unnecessary passive components.

[Gnobuddy]

Maybe it's not that the designers are idiots who superstitiously freeze the design.

Guitar amp design is fascinating to me, partly because successful designs have come (a) from superstitious and ignorant designers, (b)from well-schooled and logical designers, and (c)from people at various points along the spectrum in between. Maybe even from idiot designers, who happened to have good hearing and a talent for tweaking an amp to sound good...valve guitar amps are at the intersection of right-brain creativity/craziness/superstition and left-brain logic/deduction/engineering/math, so strange things can happen.

In this case, there is certainly a useless RC filter in the circuit. But if the amp sounds good, it is a success as far as guitar amp design goes. We all know there are other very famous guitar amps that include useless components (small value ceramic capacitors in parallel with larger value film caps, Teflon-insulated wire, etc), and which have also been good amps anyway.

Maybe that's how you create "mojo", by adding useless components that other people can ascribe supernatural qualities to. In my next build, I plan to glue a pigeon feather, a plastic unicorn, and a sea-shell to the inside of the housing.

-Gnobuddy

[J M Fahey]

Maybe that's how you create "mojo", by adding useless components that other people can ascribe supernatural qualities to

Don´t think designers create Mojo, or even know or care about it, they just got some sound, some Guitar God was (legitimally) fascinated by it, used it on some famous song, and *after* that Musicians who love the song or envy the player try to get some of that for themselves, it is *them* who assign Mojo.

Removed many degrees from original designer.

As of useless values or combinations,I see them all the time.
Say a designer is trying to get a particular sound, he´s almost there but amp is still somewhat muddy or farty .... he may replace a .047uF cap by a .022uF one and play a few chords.

If none available or nearby or he simply wants to go back to original sound, he might just lift a cap end and insert another same value in series.
He knows like anybody else that effective cap value will halve, and low cutoff will rise an octave.

So far so good, suppose he likes it.
Should he rush to the parts bin , grab a .022uF one and check that it ***really really*** sounds the same as 2 x .047uF caps in series?
Not really, what for? .... it´sjust a prototype, and mods and tweaks will go on.
BUT, when such amp reaches production, he should use the .022uF one and not 2 x .047uF ones in series, in principle because it´s the same, and too boot simpler and cheaper to achieve.

And in fact I guess most designers (even if they reached a particular sound by shotgunning and not involving any Math) will also use the proper value.

Yet, *sometimes* I find such unnecessary kludges (instead of the proper value) which are even more inconvenient to use rather than the sensible way, that´s what puts in my mind the idea that Designer is afraid of changing anything and losing that magic sound.
As in: "yes, I ***know*** .022uF is the same as 2 x .047uF in series and should sound the same .... but .... what IF not? ..... I am taking NO risks and ordering production as-is"

Also remember seeing , say, 2 x 47k in series instead of 100K or even worse, say 15k in series with 1M
FWIW Mesa stuff often shows such things .... take your own conclusions

Sometimes it is needed, but such cases are way more unusual.

Here´s a tiny sample, the Mesa Boogie V Twin.

In my eyes designer was tweaking values; so far so good, then he added a smaller resistor in series and a smaller capacitor in parallel.
So far so good again ... on a protoboard or test setup, but by a funny coincidence, both tweaked values resulted in standard next, easy to find, most common value progression ... why not just use the preferred value (at which designer did arrive, by the way) , instead of keeping the experiment approximation?
All I can think is "I will NOT touch those values, period" ... even if same is available in a more convenient way.

Of course, pedal cloners copy everything they find in a schematic, including the ketchup stain, or inside a pedal, including the quite alive roach and her 1200 egg nest.

[Enzo]

We have guys log on here all the time who want to do something over the top, like auxiliary cathode resistors that snap into the circuit if the main one burns out, or ten turn pots on four individual bias settings. I can imagine a designer with a thing for subsonics, an obsession with things like stage noise, and so puts a real low filter on a circuit no matter what. In other words, some litle circuit serves no real purpose but solves a non-existent problem in the designer's mind. (I hope I articulated that well enough.)

[nickb]

Bear in mind that the loop gain at speaker resonance is likely to be somewhat higher than at nominal, as the impedance may be >10x nominal; so even a bit of NFB may have more of a damping effect than might otherwise be apparent.

From memory, at high signal levels, OL output impedance of regular tube guitar amps tends to be ~3x nominal load impedance; at low signal levels is a fair bit higher, maybe 10x.

LtSpicing an open loop 6L6 amp for Zo I get this at low signal levels



and this at high signal levels ( done by disabling one bottle and cranking the bias current up)



The transformer has a big effect due to the high plate-plate impedance. I did this by strapping a current source across the load. I think that's a valid way to do it.

[teemuk]

Two triode stages seems to be de rigeur for a roach trem -- first triode does the oscillating, second one drives the lamp. Looking at this amp, I get the impression that he just cut and pasted an AB763 trem circuit.

And I wouldn't be surprised if the input stages were actually "xeroxed" from a design where low frequency modulation is introduced in input stages, with bias voltage modulation, after which the LF modulation artefacts are filtered away with a bunch of sub-sonic filters in series. The same filter circuit, of course, serves no purpose in a design where LF modulation is introduced -post- that filter.

We may never know.

[darkfenriz]

Also remember seeing , say, 2 x 47k in series instead of 100K or even worse, say 15k in series with 1M
FWIW Mesa stuff often shows such things .... take your own conclusions

Every experienced designer knows by their experience and countless evidence that the values including 4 followed by 7 (470, 4k7, 47k, 4u7 etc.) are magical and cannot be swapped by any other value or any series or parallel combination.
39k and 9k1 bring different qualities, that may be desirable somewhere but not elsewhere. Avoid 5k6 by all measures.

All digital pull-ups must be 10k, preferably 0.01%, 0.1% tops. Don't even think of 1%.
All power emitter resistors must be 0.22R, no other value is permitter.
All decoupling must be 100nF. No, that 68nF or 150nF lying around won't work and even if it will, bad luck guaranteed.

Practice, yo.

Edit: would've nearly forgotten: the elcap must have a 'c' in its brand name.

[Leo_Gnardo]

All power emitter resistors must be 0.22R, no other value is permitter.

Yes, something I noticed a long time ago, "AB" brand solid state power amps used 0.43 ohm resistors, a hard to find value. No matter though, every AB amp I ever saw was thoroughly trashed: "it looks like a bomb went off in there." Now whenever I'm invited to try & fix one, I avoid it with all my might. Heck, there's already half a dozen lying around here, anybody need a couple boat anchors?

[Gnobuddy]

...Avoid 5k6 by all measures.
<snip>
All digital pull-ups must be 10k, preferably 0.01%

LOL. You hit the nail on the head there! Sometimes it feels as though we're still stuck with the ancient E6 resistor decade values, and I almost never see anything that's not in the E12 series. Even when we're using 2% resistors.

-Gnobuddy

[rjb]

Every experienced designer knows by their experience and countless evidence that the values including 4 followed by 7 (470, 4k7, 47k, 4u7 etc.) are magical and cannot be swapped by any other value or any series or parallel combination.

470K is my very favorite resistor value.
I like the Easter colors (yellow/violet/yellow).

[Justin Thomas]

470K is my very favorite resistor value.
I like the Easter colors (yellow/violet/yellow).

I would laugh my ass off if you made an amp out of nothing but different series/parallel combinations of 470K resistors! Might be a bitch for the 1-ohm cathode resistors, though...

Justin

[loudthud]

When I play the lotto, I use resistor values.

[Justin Thomas]

Who needs to worry about Zinky-Thinky with <US> around? Sheesh!

Justin