I have been reading about lead dressing and look at quite a few gut shots and assemble drawing of amps. They are all pretty much following the same concept. The volume, tone control on the front panel, then the wired/turret board in the middle and the preamp tubes on the other side. Below is just one typical picture I copy from a post in this site. The amp or circuit is not important, just the first gut shot picture I found to show what I described, that the turret board always in between the control knobs and the tubes.
From what my understand, all the talks about lead dressing are to minimize crosstalk ( coupling) between stages or from the later stages back to the previous stages. My question is why don't people do away with the board in between and have the tube much closer to the volume and tone controls, then connect the components directly from the tube to the potentiometers. That will shorten the leads which make the circuit much less susceptible to cross talk and unwanted coupling between stages.
Of cause it is messier and harder to do, but you sure reduce the lead lengths. Is there reason people don't do that?
There is a good mod to do to Fender amps made with the board in the middle which is to move all the tone stack components onto the pots as this cleans out near two feet of grid wire which is the main issue 95% of the time when you have a oscillation or coupling problem like you posted?
This mod of course makes swapping out parts and or to play around and tweak you tone a bit more of a project!
In short in terms of build layout you want to keep the grid wires as short as you can, this is very important for the first gain stage and in the PI stage if the amp is a class A/B type.
In the PI section you really want to keep the wires to the output tube grids under 4 inches if you can.
If need be in the preamp section look at the process like this.
The output from lets say the first gain stage to the coupling cap is the plate circuit, only once you get to the out bound side of that coupling cap does that length of wire become the next gain stages grid wire!
So if need be layout wise you are way better to have long plate leads if it helps the layout to have short grid wires!
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There is some wisdom in following the established practice simply because it works. Actually, you can reasonably assume it became established practice because it works!
A while back (not sure where or when) someone did a layout for a board with holes cut through to allow tube placement along the midline of the board and turrets set in a radial fashion. This looked very innovative to me. I recall the results were very good.
What's really motivating the question? Just curious or have you run into some problem, or what? I'm asking because many times the context of the question helps to produce a better answer.
This subject requires quite a bit of consideration. Kinda impossible to cover it in a sentence or two. There are quite a few articles and websites that discuss it in detail. I recommend googling Lead Dress in Guitar Amps and reading up on the many results. RG Keen wrote something pretty comprehensive and so did Gil Ayan. There is also Merlin's stuff as well as Aiken's site. Then you can search here and read up on what has been discussed MANY times. This is all required reading before opening up a new thread, again. You should make an attempt to educate yourself before plowing in and starting from scratch. I know this doesn't fit well with the 'I want it all now' mentality.
Phil_S wrote:There is some wisdom in following the established practice simply because it works. Actually, you can reasonably assume it became established practice because it works!
A while back (not sure where or when) someone did a layout for a board with holes cut through to allow tube placement along the midline of the board and turrets set in a radial fashion. This looked very innovative to me. I recall the results were very good.
What's really motivating the question? Just curious or have you run into some problem, or what? I'm asking because many times the context of the question helps to produce a better answer.
I don't have a lot of experience in guitar tube amp, only building the third one. But I am a long time high speed/RF engineer that design system with signal integrity and electromagnetic consideration. I specialize in low crosstalk pcb layout and passing system CE test including the emission test.
When I look at the layout of tube amps, I see so many long leads. So my first thought is there must be some special reason like intentional coupling from some later stages back to the previous stages. I read the thread regarding to the primary leads concern of the OT. So I asked whether people do lead dressing and component placement to get special effect or to minimize crosstalk ( causing oscillation). Seems like consensus are to minimize crosstalk to avoid oscillation, not creating special sound. That's the motivation of this post to confirm one way or the other. Because with good layout and component placements, you really can shorten the leads specially at the grid.
To the other posters:
I think whether you can separate the anode circuit of the previous stage and the grid circuit of the following stage really depends on the coupling cap. If the value is 0.022uF. at 1KHz ( where oscillation usually are above), the reactance is 1/(2\pi f C)=7.2K ohm. Compare to the plate resistor of typical 100K and grid impedance of over 100K, the coupling cap is like a short circuit. BUT if you have low value coupling cap, then I agree that you separate the anode side and the grid side by the coupling cap. JMHO.
vibratoking wrote:This subject requires quite a bit of consideration. Kinda impossible to cover it in a sentence or two. There are quite a few articles and websites that discuss it in detail. I recommend googling Lead Dress in Guitar Amps and reading up on the many results. RG Keen wrote something pretty comprehensive and so did Gil Ayan. There is also Merlin's stuff as well as Aiken's site. Then you can search here and read up on what has been discussed MANY times. This is all required reading before opening up a new thread, again. You should make an attempt to educate yourself before plowing in and starting from scratch. I know this doesn't fit well with the 'I want it all now' mentality.
As I replied above, I might be new in guitar amp, I am a long time engineer specialize in high speed and RF, many years in signal integrity and EM design and in charge of passing CE test and design system grounding. So layout, wiring is nothing new to me. I worked with mixed signals in nV and pA level in SMPS and high speed digital environment.
I even published a paper in American Institution of Physics on what you called lead dressing for electron multiplier in mass spectrometer system to reducing coupling and reduce ringing of the signal. Which result in unpreceding efficiency of pulse counting. http://rsi.aip.org/resource/1/rsinak/v6 ... horized=no
VacuumVoodoo had posted years ago a diagram of individually decoupled gain stages, I can't seem to find it??? Anyhow, being that you are versed in RF/analog design, you understand the importance of localized decoupling. So many of today's amps are built on "historical" designs, many of which were built long before any thought of high gain structures. Older Fenders quickly come to mind with their doghouse caps - placed a fair distance away from the actual circuit with random grounds placed not particular to the circuit.
One of the best clean to moderate distortion sounding "fender" style amps that I built used individually decoupled stages with poly caps. There is a difference in sound, try it and be the judge whether it is better or not. IMHO, the largest misuse of "historical" information is the very poor decoupling of the power supply / gain stages along with random ground points. As we know, the power supply is the other half of the equation.
Alan0354 wrote:I am a long time engineer specialize in high speed and RF, many years in signal integrity and EM design and in charge of passing CE test and design system grounding.
I even published a paper in American Institution of Physics on what you called lead dressing for electron multiplier in mass spectrometer system to reducing coupling and reduce ringing of the signal. Which result in unpreceding efficiency of pulse counting.
Then you should be the one who could prove to us why it is a non-issue in this realm of relatively low gain, low resolution, and low frequencies.
vibratoking wrote:This subject requires quite a bit of consideration. Kinda impossible to cover it in a sentence or two. There are quite a few articles and websites that discuss it in detail. I recommend googling Lead Dress in Guitar Amps and reading up on the many results. RG Keen wrote something pretty comprehensive and so did Gil Ayan. There is also Merlin's stuff as well as Aiken's site. Then you can search here and read up on what has been discussed MANY times. This is all required reading before opening up a new thread, again. You should make an attempt to educate yourself before plowing in and starting from scratch. I know this doesn't fit well with the 'I want it all now' mentality.
As I replied above, I might be new in guitar amp, I am a long time engineer specialize in high speed and RF, many years in signal integrity and EM design and in charge of passing CE test and design system grounding. So layout, wiring is nothing new to me. I worked with mixed signals in nV and pA level in SMPS and high speed digital environment.
I even published a paper in American Institution of Physics on what you called lead dressing for electron multiplier in mass spectrometer system to reducing coupling and reduce ringing of the signal. Which result in unpreceding efficiency of pulse counting. http://rsi.aip.org/resource/1/rsinak/v6 ... horized=no
"It is very important to attach all components directly to the electron multiplier
inside the vacuum with minimal lead Zength. The same components outside the vacuum provide no improvement."
The article doesn't say why this is true. It seems to be more show and tell, rather than "This is why it matters". Could you comment on why the observation regarding vacuum occurs? Is it just a matter of shielding from the vacuum chamber?
Had I been the reviewer of your manuscript, I would have asked for these details before publication.
John_P_WI wrote: being that you are versed in RF/analog design, you understand the importance of localized decoupling. So many of today's amps are built on "historical" designs, many of which were built long before any thought of high gain structures. Older Fenders quickly come to mind with their doghouse caps - placed a fair distance away from the actual circuit with random grounds placed not particular to the circuit.
One of the best clean to moderate distortion sounding "fender" style amps that I built used individually decoupled stages with poly caps. There is a difference in sound, try it and be the judge whether it is better or not. IMHO, the largest misuse of "historical" information is the very poor decoupling of the power supply / gain stages along with random ground points. As we know, the power supply is the other half of the equation.
No reason stage by stage PS filtering couldn't be employed in amps. You could keep the big filter caps off in their corner of the chassis or in the doghouse/bathtub below and add "helper" filters close to the circuitry as done in mixing consoles and plenty of digital equipment. Manufacturers don't do it (yet) because it increases the cost, and isn't part of the way things were done. Stage by stage supply filters, sure why not for the individual builder. Perhaps some commercial amp companies will pick up on the idea eventually. I just serviced a bass amp I made in 1997 and bingo, I built stage by stage filters in that amp without even thinking about how "revolutionary" it was.
Alan0354 wrote:I am a long time engineer specialize in high speed and RF, many years in signal integrity and EM design and in charge of passing CE test and design system grounding.
I even published a paper in American Institution of Physics on what you called lead dressing for electron multiplier in mass spectrometer system to reducing coupling and reduce ringing of the signal. Which result in unpreceding efficiency of pulse counting.
Then you should be the one who could prove to us why it is a non-issue in this realm of relatively low gain, low resolution, and low frequencies.
No, I am not trying to prove anything. Human ears are so much more sensitive than test equipments can measure, I never take for granted that this is a non issue.
FYI, EM radiation is not limited to RF. Case in point, 60Hz power gives so much interference. Even with higher harmonics, it's still in audio range. It is a false assumption that just because this is low audio frequency that it does not radiate. As I talked about the magnetic loop, the loop does not have to be 1/4 wave length to radiate. even at audio frequency, it radiates. As I said I have seem more case of magnetic loop coupling than capacitance coupling. Look in the table of this link and you can calculate the coupling capacitance between two wires of certain distance, length of the parallel run and diameter of the wire.
Calculate a 6" run with one inch separation. You will find the capacitance is very low. But a magnetic loop can emit field quite far, like your radio totally get interrupted within 50' of the power line. Those are EM loop couple, not capacitor coupling.
Alan0354 wrote:Calculate a 6" run with one inch separation. You will find the capacitance is very low. But a magnetic loop can emit field quite far, like your radio totally get interrupted within 50' of the power line. Those are EM loop couple, not capacitor coupling.
Do you really work in this field? The EM coupling is dependent on current, frequency, angle between conductors, and distance. You're blowing smoke and waving your arms at the same time.
Zippy wrote:"It is very important to attach all components directly to the electron multiplier
inside the vacuum with minimal lead Zength. The same components outside the vacuum provide no improvement."
The article doesn't say why this is true. It seems to be more show and tell, rather than "This is why it matters". Could you comment on why the observation regarding vacuum occurs? Is it just a matter of shielding from the vacuum chamber?
Had I been the reviewer of your manuscript, I would have asked for these details before publication.
In this case, it is due to coupling of long running wires through the flange, inductance of the lead that make it become a mismatch tx line and reflection causing part of the problem.
There are plenty of PHDs in AIP reviewing this, going through a lot of Q&A. This is American Institute of Physics.
This is not the only paper I published, I own two patents under my name.
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