Marshall Bias Circuit Diode Direction

[Sowhat]

I was checking out the Marshall 4145 to see how it's Post Phase Inverter Master Volume control was hooked up when I noticed something a bit strange. The bias circuit diode is reversed from a Pre Phase Inverter Master Volume amp like on a 2203/4. I don't know if this has anything to do with the master volume at all but the circuit in this area are nearly identical except for this reversal.

What exactly does this diode do in the bias circuit and why is it's direction important (or not important)?

[Enzo]

What are you looking at, a schematic or the amp?

It matters a whole lot which direction it is in. Diodes are one-way devices. In the bias circuit the diode direction is what makes it a positive or a negative supply.

The 4140/4145 schematic I have shows the diode correctly. The 2203/4 drawings I have are also correct, I see no difference in the way the bias supply diodes is wired.

[Sowhat]

I'm looking at schematics.

That's kind of what I thought but I figured it as worth an ask anyway.

The JCM 800 Series Models 4140 4145 schematic has this diode with the anode to the 15K resistor and the JCM 800 Lead Series 2203/4 schematic has the cathode to the 15K resistor. Is this one of Marshalls infamous schematic errors?

[Sowhat]

Here's what I see...

[Enzo]

It is a negative supply, so the anode should be towards the DC side.

[g1]

As Enzo has suggested, the 2203/2204 schematic you attached shows the diode backwards.

[Jazz P Bass]

Oops!

[Enzo]

Oh I see typos all the time, it is not just Marshall. I have two version copies of the 2203, and both of mine are correct. I have no idea how many versions of that schematic were ever issued. So far that makes three. But i have to say, I could have looked at that wrong one a hundred times and not noticed it was drawn wrong. I already know bias will be negative, and I already know which way a diode must face to make that, so I wouldn;t have even thought of it. Yes, of course it would be confusing for someone not as well versed. But these are complicated drawings, and often done by a draftsman (or whatever they call that job today) rather than a tech or engineer, so mistakes happen. Other common errors are things like lines that cross get shown as connected when they really just cross over one another. Or the two sides of a 12AX7 get reversed, so the input might be drawn going to pin 2 rather than pin 7.

[Sowhat]

I've use that schematic for years and never noticed that error until now as well. Just thought it would be of interest to someone seeing that this schematic is all over the place. Now at least there's some definition on this for others out there who wire up wrong based on it. Thanks guys.

[Bruce / Mission Amps]

Hmmm... I don't see it... isn't the source of the AC on both drawings connected to the cathode?
It's a small snip of the actual schem so I can't tell for sure.
One is left to right and the other is right to left....

[g1]

Bruce, in the drawing on the left, the AC feed is shown going to the anode.

[Sowhat]

Hmmm... I don't see it... isn't the source of the AC on both drawings connected to the cathode?
It's a small snip of the actual schem so I can't tell for sure.
One is left to right and the other is right to left....

No they're not. The 2203/4 schematic (left) has the anode towards AC, the 4140/45 schematic (right) has the cathode towards AC. As I said, I never noticed this for years and years but there it is... as Jazz P Bass says, Oops!

[Bruce / Mission Amps]

Dang, I looked it two or three times and you guys are right!
I've never noticed that before either!!!