Schematic Help

[J Rindt]

http://www.el34world.com/charts/Schematics/files/marshall/marshall_18watt_schem.pdf
OK.....In the above schem, with nothing connected to the Lo input and a guitar connected to the Hi input, it looks like the Tip of the Hi is connected to the Tip of the Lo through the Switch of the Lo.
Is that right.? Does that put the two jacks in parallel.?  I am having a hard time trying to figure this out.....
Thank You

[John]

IF I understand it right, the hi input makes that a parallel triode by "feeding", or connecting both grids, 2&7. The low input jack feeds only pin 2, and since there's nothing in the hi jack, that grid (7) is then connected to ground. I believe some amps do the same sort of thing with a switch instead of 2 jacks.

[J Rindt]

That is how I am seeing it.....my first experience (I think) with this type of input.
Would that also put the two 1 Meg resistors (to ground) in parallel.?
If it does.....the Hi Tip to ground would read 500k.....is that right.?
Thank You

[HotBluePlates]

Looks right to me.

Since the lower input impedance might slightly load a guitar with 500k pots (such as most older humbucker-equipped guitars), perhaps it might be better to make R4 2.2M or so (~688k total resistance). Then again, it might not make a difference to the player.

[sluckey]

Sounds like you already got it. Take a look at page 6 of this scrapbook for a visual reinforcement...

[J Rindt]

That is a great page sluckey......
Thank You
Thanks to everybody......guess I had a hard time believing my own eyes. I kind if live in a BF Fender world.

[J Rindt]

So, another question.
I have very fast "motor boat" problems with this build......so I am checking everything.
What should the resistance to ground be from the Tip on the Hi and Lo jacks.?
With nothing in the Lo and a blank plug in the Hi, the Tip of my Lo jack reads 4 Ohms.
Should it not read one of the 1M resistors to ground.....or maybe both in // for 500k.?
Thank You

[HotBluePlates]

"Blank plug" means a plug with nothing connected to either solder lug? Is the case off it, so that we know it's not shorting hot to ground?

Tip of the Low, with an empty plug in High, should read 500k.

[J Rindt]

I just measured to make sure....the plug reads wide open O.L.
If I have it in the Lo and nothing in the Hi I get 1M. Put the blank in the Hi and measure the Tip (hot) of the Lo Jack I get 4 Ohms. Seems like I have a problem somewhere.
As you say....it should be 500k......right.?
Thanks

[sluckey]

It ain't wired right. 

[PRR]

Motoboating probably is not the input jacks.

[J Rindt]

No, I doubt it is.
But, because of the MB I was checking everything on this new build. When I checked the jack resistance it seemed there was a problem......so here we are.

[sluckey]

Does the motorboat speed change as the trem speed changes?

Maybe these pics will help with the input jack wiring...

[J Rindt]

Hey Sluckey.....Thanks Again.
I am not sure pictures are the problem....but I will definitely take a look at yours.
One problem is that I used (because of space limitations) Switchcraft style jacks. So you have to do some thinking when comparing Cliff to Switch.....which is one reason I drew it out on paper.
Anyway......I MUST have something wired wrong somewhere.......
The MB even changes with the volume and tone controls of the Non-Trem channel......with no instrument plugged in. And it is FAST......not a Putt-Putt sound....but a motor boat at full throttle.
Maybe motor boating is the wrong term.?

[sluckey]

The MB even changes with the volume and tone controls of the Non-Trem channel......with no instrument plugged in. And it is FAST......not a Putt-Putt sound....but a motor boat at full throttle.
Maybe motor boating is the wrong term.?

Disconnect the wiper of the tone control from ground. Connect a 100 or 120 ohm resistor between the tone control wiper and ground. Does that fix it?

[sluckey]

Here are some switchcraft jacks wired for 18 watt normal channel. Maybe this will help...

[J Rindt]

That Switchcraft picture is Awesome.....I already found my problem. How can I look at something a dozen times, and not see a simple mistake. I had the Lo Jack switch wired correctly.....AND to ground. I swear, sometimes I must automatically BF Fender everything. I'm an idiot.
I will try the Wiper....resistor....ground application.
Thank You

[J Rindt]

The MB even changes with the volume and tone controls of the Non-Trem channel......with no instrument plugged in. And it is FAST......not a Putt-Putt sound....but a motor boat at full throttle.
Maybe motor boating is the wrong term.?

Disconnect the wiper of the tone control from ground. Connect a 100 or 120 ohm resistor between the tone control wiper and ground. Does that fix it?

It quells the MB problem as it relates to the Tone Knob. The Volume Knob (normal channel) still sets it off.
So what am I doing with that 100 Ohm to ground exactly.?
What is motor boating.? That is....is it typically a lead dress, component, ground, or something else problem.?
Thank You

[HotBluePlates]

What is motor boating.? That is....is it typically a lead dress, component, ground, or something else problem.?

Yes.

That is, motorboating is defined as a low-frequency oscillation. "Low-frequency" means it is often below the audio range, and therefore sounds like a putt-putt-putt several times per second. If it is fast "putting" but still slow enough to not turn into a smooth low hum (like 20-30Hz), that's still a slow, low frequency oscillation.

Contrast this description of "low frequency" with "high frequency" oscillation, which is often well above the audio range, and manifests as all sorts of strange phenomena; this kind is best seen with a scope.

Motorboating is often related to the power supply somehow. It could be in-phase coupling from one stage to another due to insufficient decoupling in the power supply (i.e., 1st & 3rd stage of an amp, all inverting the signal, and all connected to the same filter cap). This coupling through the power supply could be due to a failed filter cap (which now no longer decouples as it should) in an old amp. In a new amp or repaired amp, it might indicate a bad ground for a filter cap, or some other circuit element that is normally grounded and through which large current flows.

I would be looking hard at the whole power supply chain; PT, rectifier, CT's, filter caps, series decoupling (aka "dopping") resistors, and how they find ground.

If this is a new build, expect to find dumb errors. We all make them. The smarter/experienced guys find their dumb errors faster, and hope to remember not to make them on the next build.