train wreck grounding

[jestaudio]

just starting a big bottle rocket with master volume, i,m a little suprised by the grounding scheme which appears to be all ober the place rather than from one common point, how does this affect the amp

[rooster]

You need to think vintage Fender. Go look at a layout page for a Super Reverb for example. Basically the chassis is the ground plane but the various parts of the circuit tap in along the way. Many say it works much better than what you refer to, BTW. And try to remember that a lot of what you see today in pc board amp construction is based on the manufacturer's goal of spending as little time as possible in the chassis. If they can present a reasonably quiet amp with just one ground point - well now, that's easier for them. Cool. Not.

[Cliff Schecht]

There are a lot of schools of thought on grounding. Star grounding, buss grounds, using the chassis as a current return path or simply a shield for the circuit, the list goes on really. But in the end your goal with layout out the grounds is to keep the high current return paths away from the sensitive nodes of the circuit. The Trainwreck scheme does a fine job of this albeit with some goofy practices that I follow to a T only when building a Trainwreck clone.

[RJ Guitars]

One interesting technical tidbit - At high frequencies - well above the audio range, you can get a frequency dependence on which ground a signal will choose to use - that is the path of least resistance is frequency dependent. However in guitar amps this probably isn't going to happen and if it did you're probably not going to notice. For the biggest part of guitar amp history various "technically incorrect" grounding schemes have been used and the amps sound great. If it sounds good and doesn't hum, then the grounding scheme is as good as it needs to be... however if you design something of your own from scratch, I suggest that you do it right. The best guitar amp tutorial on this topic that I have seen is on Randall Aiken's web site... worth the read. YMMV rj

[Colossal]

For what it's worth, I used a distributed star grounding method with a single ground point on the chassis in a recent Trainwreck build. I return the grounds (cathodes, pot grounds, grid leaks, etc) for each B+ node back to the negative terminal on the respective filter cap serving those components and each negative terminal is linked via a common bus. Finally, a single ground connection to chassis is made near the input jack. The IEC Mains has its own dedicated chassis ground near the point of input. Those are the only two chassis ground connections. The amp is dead quiet and can idle at full volume with the shielding plate off and not oscillate or howl. Turn up the guitar and it will start to feed back of course.

[RJ Guitars]

What Colossal said +2...

[M Fowler]

I'm rebuilding my Liverpool and Express right now and both will be converted to Larry grounding scheme which I am very confident will make a substantial difference in both of my amps.

Mark

[Colossal]

I'm rebuilding my Liverpool and Express right now and both will be converted to Larry grounding scheme which I am very confident will make a substantial difference in both of my amps.

Mark

I've also had very good results with Larry Grounding in a recent modded Marshall build.

[sliberty]

What recommendations can you guys make with respect to grounding when a single cap can is used? My example is a Fender circuit with a 40/20/20/20 cap can. The problem is that here is only one single ground for all caps, and so keeping preamp and power amp grounds isolated is not possible.

[joCCe]

What about the input and speaker jacks when using the Larry scheme?

Do you isolate the switchcraft jacks or replace them with Cliff's or do you leave them as they are?

[M Fowler]

What about the input and speaker jacks when using the Larry scheme?

Do you isolate the switchcraft jacks or replace them with Cliff's or do you leave them as they are?

I just use Cliff jacks now before I was using isolating washers with switchcraft jacks.

a 40/20/20/20 cap can. The problem is that here is only one single ground for all caps, and so keeping preamp and power amp grounds isolated is not possible.

Correct the can cap ground is single so it would go with the HT center tap ground.



Mark

[sliberty]

a 40/20/20/20 cap can. The problem is that here is only one single ground for all caps, and so keeping preamp and power amp grounds isolated is not possible.

Correct the can cap ground is single so it would go with the HT center tap ground.

And then would you pull all of the preamp gounds together to their own spot near the input jack? If i am reading you correctly, this would result in essentially 3 gournd points.

1. Preamp grounds (not preamp filters though)
2. Power stage grounds (including all filters, all center taps, power tube cathodes, etc.)
3. Mains ground

Is that what you are proposing?

[kwijabo]

too long/don't read version: you're at the mercy of the cap-can. if your OT ground is not grounded to the chassis at the output jack and is isolated from it there, there shouldn't be much of a difference if you ground the pre-amp to the chassis or directly to the cap-can. if you're using a fender style output jack, ground the preamp on the cap-can or as close as possible to it.

long version:
i think when talking about grounding, you're trying to minimize two/three different things, all of which have been hinted on in this post.

1. "The end your goal with layout out the grounds is to keep the high current return paths away from the sensitive nodes of the circuit." This is done by the physical layout/orientation of lead dress and component placement. For this, you're looking to eliminate the high current signal coupling over into sensitive parts.

2. Because we don't live in an ideal world, all ground is not equal. so you don't want the return path of your output section to have any chance of sharing a return path for the preamp. this is what you're trying to minimize the effect of when you follow a merlin grounding or larry style.

2.1. Similarly to 2. but not exactly the same, RJ brings up a great point that those of us in the microelectronics field know all to well. "At high frequencies - well above the audio range, you can get a frequency dependence on which ground a signal will choose to use - that is the path of least resistance is frequency dependent." again, you're concerned here with one return path being interfered with by another return path.

so sliberty, as to 1. above you still have some control on that. with 2. you're at the mercy of the cap-can. if you're OT GND is isolated from the chassis at the jack, when you ground your preamp to the chassis it takes a path to the cap-can. when you ground it to the cap-can, it does the same thing only this time through a wire you can manipulate.

and then when you follow the theoretical advice, it won't work and you'll end up doing all sorts of crazy things and it will work. but one of my favorite expressions is, "it is doing what it is supposed to, we just don't know why." good luck!

[M Fowler]

And then would you pull all of the preamp gounds together to their own spot near the input jack? If i am reading you correctly, this would result in essentially 3 gournd points.

1. Preamp grounds (not preamp filters though)
2. Power stage grounds (including all filters, all center taps, power tube cathodes, etc.)
3. Mains ground

Is that what you are proposing?

I have build Trainwrecks ken's way but I plan to do it different with my current rebuilds.

1. Input jack to V1a cathode resistor/cap ground, heater center tap, and then to chassis ground lug right in front of the board.

2. Volume pot ground, V1b cathode ground, and preamp filter cap ground.

3.Mid and presence pot grounds, phase inverter filter cap ground, and output jack common ground.

4. bias circuit ground

5.screen filter cap ground, main filter cap ground, HT center tap, and the EL34's cathodes.

6. the IEC or power cord ground (green) has it's own separate ground.

The Komet 60 has similar grounding to the Larry grounding. The attachment is a JTM 45 but this grounding works in any amp.

Mark

[M Fowler]

Actually I think if I had a 40-20-20-20 can cap I would ground that by itself. The main filter can cap 30/30 or 40/40 or 50/50 not sure what is being used I would ground with HT CT.

Then if your going to use TW buss solder to the pots I would run the remaining ground near the input jack rather over by the cap stack as TW does it.

Mark