Turret board (falsely labled Point-to-Point) -VS- PCB

Outlaws

New member
Turret board -VS- PCB for amp design.

This is a fantastic article. Written by Andy Marshall of THD Electronics.

http://www.bogneramplification.com/Pcboard.htm


Andy Marshall said:
Not all manufacturers choose to use PC boards just to save money. We use them for consistency more than for price, but making a somewhat affordable amplifier is a nice benefit. I don't think that someone should have to be a lawyer or Microsoft Millionaire to be able to afford a new amplifier that is hand-built, reliable and sounds and feels good to play.

If a PC board is designed correctly and the correct components are used, the amplifier production should be absolutely consistent from one unit to the next. No re-routing of traces should ever be necessary to make an amp function or sound right. If you find it necessary to change and re-rout wires in your amps, then you are not in production, but are just making a series of unstable prototypes. Treble reduction to the point where it reduces the clarity of the amplifier is not an acceptable stabilizing technique for either a PTP or PCB amplifier.

Recently, we got a call from a tech complimenting us on our old Plexi model amplifier (that we built between 1990 and 1995), but he said that it was just a little bit "stiff in the high-end" compared to a real Marshall Plexi. To back up his point, he told us that he had a real Marshall Plexi on the bench next to ours and was comparing the two side by side. What he did not seem to realize was that no two Marshall Plexis sound the same. They were terribly inconsistent with their component sources and values, not to mention the inconsistencies in wire routing.

Taking a point to point or a turret-board amplifier, if one moves the wires around, the entire sound and character of the amplifier can change, often dramatically. This is a well-recognized phenomenon.

If you understand these interactions well, you can design a PC board to sound and feel any way you want it to. Furthermore, every one will sound the same. How many times have you plugged into an old Marshall-50 watt head, only to be terribly disappointed by the sound and feel of the amplifier? While this may be caused by poor tubes, at least in part, inconsistencies in the internal layout of the amplifier often play a significant role.

If you understand how one component affects the component next to it and how one trace affects the trace next to it, then you should be able lay out a circuit board correctly the first time, not by building 10 and picking the best one. Mind you, it takes many years of experience to develop the sort of understanding of the capacitive and inductive interrelations involved. In the old days, I did this for a living for other companies, designing circuit boards for the audio sections of amplifiers, mixing consoles, signal processing equipment, etc... While I am under confidentiality agreements with almost all of my former clients, I can tell you that there is hardly a professional recording studio in the US or Europe that does not have some audio circuit board with my layout in some piece of equipment. After a few hundred such projects, one develops an intricate understanding of how traces and components interact.

A number of years ago, Guitar Player magazine did a review of one of our amplifiers. They stated that they, as a general rule, do not care for circuit board amplifiers, but also said that I had addressed every one of their concerns, and that they had nothing bad to say regarding our use of circuit boards. It felt good to see someone start to understand what it is that we do and why.

Certain components throw a rather large field. Others do not. Some components are very susceptible to the fields from other components, while some are not. Components can affect the signal passing through traces, and traces can affect the signal passing through components. It ends up being an enormous network of positive and negative feedback between components within each other's sway. This is why the distance between specific components on the board and the physical orientation of the components relative to one another (rotational orientation, as well as lateral placement) cannot be ignored. Furthermore, which traces are parallel to one another and at what distance, which traces are perpendicular to one another and that what distance, and the amount of ground plane in-between them can seriously affect the overall sound and feel of the finished amplifier.

Most people design circuit boards either haphazardly or for the greatest parts density/easiest and least expensive manufacture. Neither of these methods belongs in a high-end amplifier, and such approaches give PC Board designs a bad name.

If you know what you are doing, a thicker board is better than a thinner board (ours are .093" or 3/32", most are .062” or 1/16”) and that thick copper is a good idea (ours is 4 oz, most use 1/2 oz or 1 oz). One of the greatest problems facing most circuit board amplifiers is board flex. Board flex creates metal fatigue in the copper. As the copper cannot really "break", it just crystallizes and makes tons of noise. This is much worse in combo amps, of course. We go to the trouble to support our boards ever few inches. Our design standard is that 100 pounds of force on a 1/4" diameter probe should not be able to flex the board more than 20 thousandths of an inch at any point on the board. All of our amps designs must pass this test. For comparison, most Marshall and Fender circuit boards would break under such force, and would flex more than 3/8 of an inch just before breaking.

Through-plated holes are an absolute must, with solder pads on both sides. This makes it much harder for a repairman to inadvertently lift a pad or a trace by overheating or from poor technique. The way that we have addressed this is to start with boards that are clad with 2 oz copper, and in the through-hole plating process we add another 2 ounces. This leaves us with traces and ground planes of 4 ounces, and through plated holes with 2 oz copper in the holes themselves. I have seen some other people start with 3 oz copper, plating on an additional 1 oz, and I have not like the results I have seen. The through-holes pull out too easily.

Contrary to popular belief, “Orange Drop” film capacitors are far from great. They are OK for certain position in certain circuits, but their consistency from one to the next is atrocious. Maybe this is part of why so many people who use them in PTP amps find the need to make wire adjustments. This is a big part of what I mean by using the right components.

As for PCB solder joints becoming problematic with time, this is no more a problem than on PTP. A good solder joint with absolute minimum stress on it (using the right component with the right lead length and the right mounting technique) will yield the longest and most consistent life. Assuming that the flow-solder machine is correctly set up, the right solder, right flux, right solder temperature, right flux temperature, right pre-heat, right cooling, etc… are done, a flow-soldered board will last longer and have higher quality solder joints than a hand-soldered board. If you doubt this, ask yourself the following questions: How do you decide what solder to use? Do you choose SN60, SN63, SN96, Savebit or some other? How do you decide what flux to use in your solder and how much? How do you decide what temperature to set your iron at? It all makes a HUGE difference in the quality and consistency of your solder joints. If you cannot answer all of these questions, then you cannot even have a clue about the long-term consistency and life expectancy of your products. This, along with countless other points, is part of what separates the hobbyist from the professional.

In a PTP amp, the entire surface of the solder joint is exposed to air, and thus, to corrosion. In a through-plated PCB amp, only the top and bottom surfaces of the solder joint are exposed to corrosion, not the majority of the joint, which is within the through-hole, which is where most of the contact is made.

We use only FAA-approved aircraft assemblers in every stage of our manufacturing. They have to understand all of these points completely. The FAA is even more stringent than the military. Also, the aircraft industry is just about the only industry left that uses PCBs for the electronic components wired to chassis-mounted electro-mechanical components like the controls and connectors. They do this because countless FAA tests have shown that devices built this way last longer and are more reliable and consistent than any other method, even taking cost out of the picture entirely. This is, of course, why we use the exact same methods.

In closing, I absolutely believe that circuit boards, when they are well-designed and laid out, are in all ways superior to other manufacturing techniques when one is building amplifiers in quantities. If I did not believe this firmly, I would not be doing it. This said, I think it is a terribly expensive and cumbersome method for hobbyists to attempt. If you don't have a great deal of experience under your belt designing circuit boards, you won't like the results. Point to point and turret-board techniques offer the hobbyist and the small-scale amp shop the opportunity to easily tweak their designs, as is so often necessary. So, unless you're going to be building 50 amps a month or more, it is probably best to stay away from circuit boards.

Andy Marshall President, CEO THD Electronics, Ltd. www.thdelectronics.com
 
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"Point to point and turret-board techniques offer the hobbyist and the small-scale amp shop the opportunity to easily tweak their designs, as is so often necessary. So, unless you're going to be building 50 amps a month or more, it is probably best to stay away from circuit boards."


Exactly.
 
capnkid said:
"Point to point and turret-board techniques offer the hobbyist and the small-scale amp shop the opportunity to easily tweak their designs, as is so often necessary. So, unless you're going to be building 50 amps a month or more, it is probably best to stay away from circuit boards."


Exactly.

Yes, but the point is that a lot of people state as fact "PTP is better" without know why.....just that somehow it has to be better for technical reasons, when in fact it is not better for technical reasons.

Then there is the fact that PTP isn't even what they are talking about anyways, they are referring to turret boards. PTP is a mess next to a turret board, and no two PTP amps will ever sound the same.
 
Outlaws said:
Yes, but the point is that a lot of people state as fact "PTP is better" without know why.....just that somehow it has to be better for technical reasons, when in fact it is not better for technical reasons.

Then there is the fact that PTP isn't even what they are talking about anyways, they are referring to turret boards. PTP is a mess next to a turret board, and no two PTP amps will ever sound the same.

I like the fact that he pointed out orange drop capacitors aren't "all that" in certain areas.

But now I have a problem. My 18 watt has junky point to point wiring, and the controls are in back.

Anyone want to trade? :D
 
I have a relatively new handwired jtm-45 clone and also a marshall 1987x MKII circuit-board amp, which I believe is basically a plexi 50 (correct me if I am wrong). I also own a 1964 blackface bassman (also basically the same amp). What I really want is to either upgrade the jtm-45 to a jtm-50 (30 watts isn't enough) or replace the circuit board in the MKII with a turret board. WHY? because the marshall amp is flaky (the only one to break down so far) and the tone isn't as good (but its still good). Also having pots and jacks on the circuit board sucks.

but obviously this guy knows more about it than I do. I am just making an observation.

Also I have a crate 50 watt tube amp that runs EL84's. I have wondered would it be possible to pull out the board and replace it with a vox AC30 turret board circuit.
 
This from the article..

Through-plated holes are an absolute must, with solder pads on both sides. This makes it much harder for a repairman to inadvertently lift a pad or a trace by overheating or from poor technique. The way that we have addressed this is to start with boards that are clad with 2 oz copper, and in the through-hole plating process we add another 2 ounces. This leaves us with traces and ground planes of 4 ounces, and through plated holes with 2 oz copper in the holes themselves. I have seen some other people start with 3 oz copper, plating on an additional 1 oz, and I have not like the results I have seen. The through-holes pull out too easily.

I agree that plated through holes are a must. But you can achieve a completely reliable PTH without going to heavier copper. The trick is how you specify the board to be built and to what standards. But the more reliability you want the higher the cost.

There is some shoddy fabrication of PCB's out there and those boards will tend to come up with reliability problems.

I design PCB's for aircraft flight control systems and they are highly reliable. These boards must endure the harshest of environments and survive for decades. Poor reliability of a PCB is due to faulty design and or crappy fabrication, not to mention poor assembly work.

Good article!
 
PTP or turret board amps are better than poorly or cheaply designed circuit board amps (most Fender or Marshal circuit board amps), but a well designed circuit board amp is AS good as a well designed PTP or Turret board amp, though no better.

There are, however, some advantages to PTP or turret boards, namely the ability to run your wires close to the chassis (which adds a very small amount of capacitance to the run, which cuts down on some types of noise and RF interference - or so my amp guy told me when he was berating me for some of the wire runs in the first amp I made myself). The other advantage to PTP or Turret boards is that it is MUCH easier to replace components, so if you want to tweak things, or when you need to repair things, it is much easier.

Still, none of these things are major advantages, so there is certainly no reason to poo poo a well designed circuit board amp, as you so often see on the internet.


Light

"Cowards can never be moral."
M.K. Gandhi
 
I couldn't have said it better.

Point to point was the only budget option back in the day, then as production requirements came about, so evolved the turret board. Now in the day of modern manufacturing, there's no reason not to have a circuit board. The word "vintage" carries too much weight IMHO but I will say that the modern approach does tend to focus on reduction of production costs which often means lower quaility.

It's totally possible to make circuit board amp as good as a PTP or turret design. The question is, will someone actually make it?
 
My preference for PTP over PC boards always stemmed from the fact that it's simpler to troubleshoot/fix PTP wiring.

Also, if a fat dude like me gets lazy and thoughtless and sits on an amp with a PC board in it, the PC board can crack.
 
PTP vs PCB in a digital world

I am having an interesting discussion with my musician son. He claims anything other than PTP construction, no matter the constructor, provides inferior sound - specifically in pro class studio mixing boards. He claims that the increased cost of a custom PTP board should not be a consideration when upgrading studio equipment even in a digital world. My experience has been the opposite in the energy sector I work. Most if not all the electronic equipment used is PCB stuff. The environment used can only be described as extreme. For example, down hole electronics are design to work in pressures up to 20,000 psi and 400 F. 300 F and 10,000 psi is common. My musician claims that audio is so different that any comparison to my experience is wrong minded. Anyone want to weigh in on the conversation? ..
 
As one that has to work on both PTP (turret) wiring and PCBs I agree with the comments that turret is easier to "mod" but if the boards are of decent quality, component swopping should be fairly easy IF you have the techniques and tools.

There is also no doubt in my mind that PCB based amps will be vastly more consistent in performance not to mention much cheaper.

From a servicing point of view there is little in it. Hand on heart I only had to change about 6 components on turret wired amps in 6 years and they were carbon comp' resistors fitted for "fashion" that had gone noisy!
A well designed valve amp will also not suffer "collateral" damaged even for an OP valve blow. The worst I had ever seen from a spectacular "Purple flashed" KT88 (which took out the HT and mains fuses) was a rather browned 10k grid stopper but being metal film it still read 9.89k!

Dave.
 
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