One other old italian guitar amp - Very strange bias circuit ???

[kagliostro]

I've draw one other impossible to find schematic of an old italian brand

the amp is a Mac 6C, a friend has reverse engineered the amp and draw a hand version of the schematic

what is very odd is how the bias circuit is connected to the final tubes

there are 6 x EL84 final tubes and the 2.2k grid resistors of 4 of the 6 tubes are connected in a way you don't expect

my friend say he has revised the situation and the amp is build exactly as draw

Dose someone has an idea of what is happening there ??

May be the 2.2k resistor value is so low that is within the tolerance of the 220k resistor value and there is a near zero difference in practice ???

Thanks

K

[sluckey]

Looks like a mistake to me. Either the drawing is wrong or the actual circuit is wrong. It's nothing that would prevent the amp from working normally though. The 220K bias resistors should be connected to the junctions of the 47nf caps and those 2.2K grid stopper resistors.

[kagliostro]

Ciao Steve

here is the power section of the amp as draw by my friend

as you can see the note say that the bias circuit is really as drawn in the schematic

Franco

[HotBluePlates]

The hand-drawn schematic is different than the schematic you first posted.

Ignore the section where he has drawn V-, V3, V2, V1 connections at the bottom of the schematic. Carefully follow the connections from the raw bias, through the 220kΩ to the 2.2k.

If you look at everything other than that little diagram on the bottom of the hand-drawn schematic, it matches how every other fixed-bias amplifier is wired.

I agree with Sluckey and think he just got confused during drawing, since the bottom doesn't match the top of the hand-drawn schematic.

[kagliostro]

Just a moment ago my friend told to me he will give me some detailed images of the PCB of the Power Section with the values of components as to examine better the matter

---

Talking about the hand draw schematic I posted, the bottom and the upper version , to me, are identical, only the draw in the bottom can confuse

if you look to the bottom (that refers to a branch of the PP) you see -V intake, then the 220K resistor, followed by the 2.2k resistor, after the 2.2K resistors is connected the grid of a tube, and are connected the other two 2.2k resistors each of it, on the other side, is connected to the grid of a tube

The same situation is drawn on the upper side of the schematic

what is odd is that the 220k isn't connected as usual, usually all the 2.2k resistors are connected between the 220k resistor and grids, here the 220k is connected directly to the grid, then a 2.2k resistor is before the two 2.2k resistors going to the grids

On the schematic I've draw, in red, the necessary correction that I think must be done as to have the traditional connection of the 220k resistors

May be I'm wrong, but I think this will be the right way

K

[HotBluePlates]

Below is your same hand-drawn schematic.

The bottom section has been deleted, which I think is drawn incorrectly. Everything else is exactly as it was in the original drawing, except I moved the location of 220kΩ resistors to show clearly how they both lead to the same bias voltage.

Except for being drawn at the output tubes furthest to the right, this is exactly how every other amp is drawn.

That last point is why I think your friend simply made a mistake when drawing the detail of how the resistors connect, because if you don't have that detail, everything fits together in the typical way.

[kagliostro]

HotBluePlates Thanks for your attention and support

I agree with you that your draw is correct, but isn't the same of my friend

here are the images of the PCB look to image #3

seems that the 220k resistors aren't connected as they must be

what I see is the 220k resistor connected directly to the grid of V3 and V6
(the socket of V3 and V6 are 180° rotated each other)

if I'm not wrong, to me seems that the 220k side who is connected to the grid instead must be connected on the other side of the 2.2k resistor, after the 2.2k resistor not before with respect to the grid

K

[kagliostro]

And here image #01

K

[sluckey]

Your friend drew it correctly. His drawing and the actual pcb circuit agree. It's odd, but it works just fine. Remember, there is no current flowing thru the bias resistors and grid stoppers so it makes no difference to circuit operation.

[kagliostro]

Grazie Steve

Franco

[PRR]

It's a 1% difference (2K:200K) of audio signal on two of six tubes.

Since the tubes vary +/-20%, it's insignificant.

Odd, but life is full of odd things.

Maybe it simplified the PCB layout?

[kagliostro]

It's a 1% difference (2K:200K) of audio signal on two of six tubes.

Since the tubes vary +/-20%, it's insignificant.

Odd, but life is full of odd things.

Maybe it simplified the PCB layout?

At the end that was exactly what I was thinking, the % of tollerance covers all differences

and probably/may be this way the PCB was a bit less complicated

Thanks for your opinion PRR

Franco

[DummyLoad]

why the spark gap? 1st i've seen on a guitar amp.

--pete

[kagliostro]

Ciao Pete

why the spark gap? 1st i've seen on a guitar amp.

I too, and didn't know why 

at the most I've seen (like you for sure) some diodes (usually 3 for each branch) connected in revers

Franco