Resistor Types vs Placement in Circuit

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tmod
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Re: Resistor Types vs Placement in Circuit

Post by tmod »

There is, no denying it. And I don't see any conflict either in this case. Just saying Allen and RG are reliable even without having $30k amps to their name...

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FourT6and2
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Re: Resistor Types vs Placement in Circuit

Post by FourT6and2 »

And to be fair, none of those guys are making high-gain amps. And by high-gain, I mean at the very LEAST, something like this: https://youtu.be/1WY2lICcJvo?t=49s I think noise—whether pink or white or purple or blue—is probably a bad thing in this case.
JD0x0
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Re: Resistor Types vs Placement in Circuit

Post by JD0x0 »

Interesting. After seeing that pic, I'm very inclined to believe that composition was carefully chosen. It looks like high quality parts all around. Many MF's except the one on the plate and what looks like the NFB resistor?.
It's true i've lost my marbles and i cant remember where i put them
JD0x0
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Re: Resistor Types vs Placement in Circuit

Post by JD0x0 »

FourT6and2 wrote: Sun Jul 09, 2017 3:31 pm And to be fair, none of those guys are making high-gain amps. And by high-gain, I mean at the very LEAST, something like this: https://youtu.be/1WY2lICcJvo?t=49s I think noise—whether pink or white or purple or blue—is probably a bad thing in this case.
I've heard (and own) high plate ODS clones that can get into this gain territory FWIW. It is interesting that my ODS has a CC on the input grid of the very first stage, and the rest of the grid stoppers in the amp are MF. I wonder why the most sensitive stage used a CC?
It's true i've lost my marbles and i cant remember where i put them
tmod
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Re: Resistor Types vs Placement in Circuit

Post by tmod »

Doesn't seem to make much sense, signal and dc level isn't high enough to get resistor distortion (sweetness), and the other benefits of cc (overload characteristics and rf stability don't seem relevant in this stage either... The only effect of cc resistor resistors at the input grid would a tiny amount of extra hiss during low signal situations...

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MakerDP
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Re: Resistor Types vs Placement in Circuit

Post by MakerDP »

tmod wrote: Sun Jul 09, 2017 8:41 am Otherwise I'd stick with the advice of guys with proven expertise when it comes to amp knowledge. Guys like Allen, RG, Merlin Blencowe, Zoe and Iain Hartney, Celeste Hall, Alex Hall.
Gurus like Ken Fischer and Howard Dumble has some amazing amps to their names, but the science behind their work is sometimes a little, er, lacking...
Don't forget Kevin O'Connor... love him or hate his approach he is also a metal-film-only guy. Look at his kits and builds... only MF and he also uses nothing but film-type box capacitors and Nichion/Panasonic EL's throughout. Doesn't believe in "mojo" parts at all.
FourT6and2
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Re: Resistor Types vs Placement in Circuit

Post by FourT6and2 »

JD0x0 wrote: Sun Jul 09, 2017 4:45 pm Interesting. After seeing that pic, I'm very inclined to believe that composition was carefully chosen. It looks like high quality parts all around. Many MF's except the one on the plate and what looks like the NFB resistor?.
What pic? The one I posted? I don't see any MF there. They are all carbon film.
tmod
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Re: Resistor Types vs Placement in Circuit

Post by tmod »

There is a definite reason why he wasn't added; he's much more controversial. I have a couple of his books, and they're good reads, but he's been associated with more "unproven mojo". He "invented" Power scaling, which was really just regular voltage scaling in a guitar amp. Clever idea, and he's done some great implementations over the years, but hardly the moon landing breakthrough it's been touted as. Also, his ideas tend to cost a lot of money, the others give away their knowledge for free...

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tmod
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Re: Resistor Types vs Placement in Circuit

Post by tmod »

Could find the edit:
But KOC has indeed shared some nuggets for free, I have enjoyed some good suggestions over at music electrics forum, but he's not always correct in his technical advice, and don't seem willing to admit mistakes... (Also a lot of the advice seems to be "just buy these three books I wrote at $50 a piece and you'll figure it out yourself.").

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ChopSauce
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Re: Resistor Types vs Placement in Circuit

Post by ChopSauce »

tmod wrote: Sun Jul 09, 2017 5:05 pm Doesn't seem to make much sense, signal and dc level isn't high enough to get resistor distortion (sweetness), and the other benefits of cc (overload characteristics and rf stability don't seem relevant in this stage either... The only effect of cc resistor resistors at the input grid would a tiny amount of extra hiss during low signal situations...

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I am sorry I am discussing amp design without being capable of doing some math for a first approximation. Also I rely on Aiken and Keen's discussion about resistors but I am even less competent in such matter and I do not even know how Aiken's contact noise relates to "yours" (maybe Keen's ? ) distorsion one :?:

In fact, I am just an apprentice amp builder managing to summarize all my readings, in order to analyse some practical ways of doing things.

As such I do not even know the bulk amplification factor of an ODS, for the full signal path, including the OD section, from the input to the speaker, but _IF_ it is even close to ten times, then a negligible (<1%) pink-alike contact noise of carbon comp resistors would gain about one order of magnitude, and might thus become significant. Broadly speaking, of course -
tmod
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Re: Resistor Types vs Placement in Circuit

Post by tmod »

Didn't mean to be snobbish, sorry if I came across that way! :-)

I'll give a short reply, just so I won't contribute to more confusion... There are two relevant phenomena involved with resistor types: noise and distortion. Noise is covered well in online tutorials, but the important bit here is that noise is unreleated to the guitar signal (for example hiss). Distortion is different, and related to the signal. CC resistors are unique in that they have a relatively high degree of non-linearity, they "change value" when there is a voltage across them, and the change is bigger for bigger voltage swings, ie large signals. The end result if I recall correctly is to add a little bit of 2nd harmonics, or a little bit of an octave effect. The effect is very small, and depends on a large static voltage across it and a large signal across it.

Hence my comment that it won't make sense as a early grid stopper; there's no static voltage across it, there's the smallest signal of the whole amp, and the noise gets amplified the most.

I don't have an ODS in front of me, but expect every gain stage to amplify the noise by a factor of 40-60, so noise early in the signal chain can quickly add up...

The noise is mainly unrelated to the signal level at the input, so during loud parts ut will probably be overshadowed by the guitar sound, but during quieter parts the noise will be as high and thus more noticeable...

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FourT6and2
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Re: Resistor Types vs Placement in Circuit

Post by FourT6and2 »

Seems like too much to worry about over something so small. I've also been told one should use carbon film on all the grids. Not so sure about that one either.
tmod
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Re: Resistor Types vs Placement in Circuit

Post by tmod »

Film resistors are made by putting a resistive film (metal or carbon based) onto a non-conductive tube. Then there is made a spiral cut, the number of turns deciding the length of the resistive film and thus resistance. Unless special care is taken with how the cuts are arranged there will always be some parasitic inductance, and earlier film resistors were much more inductive than older carbon comp resistors. Inductance on the grid can in some cases cause oscillations, and thus cc where recommended over film resistors. Today it's less if an issue, modern metal film being much less inductive (and you can easily get special non-inductive resistors for peanuts). Screen grid resistors is a special case, in that there can be significant current swings, and carbon resistors can handle momentary overvoltage much better. Hence you can get away with a lower wattage screen grid resistor if using carbon film.

One further issue with carbon comps is that they have significant drift over time due to soaking up moisture from the air, so avoid them where exact values are critical, and be prepared to change them out every few years or so. Also, they take heat really poorly, so they should be avoided where they are near heat sources, or they will need to be changed even sooner. They also become brittle with age.

In short, carbon comps are now harder to find, much more noisy (at least ten times), less reliable, and only influence the tone (a tiny bit) when late in the signal chain, outside of feedback, and where there is a high static voltage across it. Use sparingly if at all.

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ChopSauce
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Re: Resistor Types vs Placement in Circuit

Post by ChopSauce »

tmod wrote: Sun Jul 09, 2017 9:46 pm Didn't mean to be snobbish, sorry if I came across that way! :-)
No worries 8)

My main concern is about my ability to explain myself well enough - through the computer, in a foreign language. Sometimes I read replies that makes me feel like I felt. So, that was not meant. I was just feeling frustrated. That was before I could read your technical insights, thanks!
pdf64
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Re: Resistor Types vs Placement in Circuit

Post by pdf64 »

FourT6and2 wrote: Sat Jul 08, 2017 10:25 pm...I wouldn't use CCs on the input plate stage (V1a) or the second stage in a high-gain amp (V2a) for noise reasons. The stage just before the PI in the amp I'm building is a cathode follower and just has the single 100K resistor on the tube socket as seen in typical Marshalls. The article says the PI Plates would be a fine place to use CCs. While a global NFB affects how much a CC will distort there, it still does distort a little, no?..
As your 2nd stage plate only seems to have the gain of the power amp after it, I think that the ratio of likely signal level to potential CC noise is high, and so a CC would be fine there, as per RGs guidance.
As the gain of a CF stage is close on unity and pretty much independent of the cathode resistor value, the voltage coefficient of characteristic of a CC cathode resistor won't be able to have any effect on the stage output, ie there's no potential a CC cathode resistor on a CF stage to add harmonics.

The degree to which a global NFB loop is able to reduce distortion is dependant on the open loop gain and the negative feedback ratio.
Generally, the loop reduces gain by about 6dB, so (within the linear range) I think that the distortion generated within the loop will be reduced by a similar amount.
Be aware that although NFB reduces distortion overall, whilst reducing low order harmonics, new higher order harmonics are introduced, so the tonality may be less pleasant.
Also, with NFB, because the large signal performance within the linear range is more accurate, with slighter higher signal levels the onset of clipping will be more pronounced / extreme, with a potentially harsher tonality.
With amps that have a presence control in the NFB loop, at higher frequencies the degree of NFB will be dependant on the presence control setting.

I think that even order harmonics that are introduced equally in both sides of a balanced system (eg CC LTP plate resistors) will result in odd order harmonics when the balanced signals are combined back to single ended
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