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 Post subject: Amp Schematic Questions
PostPosted: Mar Wed 20, 2019 8:15 pm 
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Things were a bit slow at work today so I did what any caged animal does, I tried to entertain myself. I took out a schematic of a recently deceased Hammond M2 organ amplifier and tried to improve my understanding of how it works. Two questions came up that I cannot answer and hope someone here can:

the two halves of V4 (6SC7) share a cathode, so why on the schematic do they show different components attached?

the cathode of V2 (6SJ7) shows +1.5 (from V4) does this mean it is attached to the point on V4's cathode that is labeled +1.5V?

I do not currently have the amp where I can see it.


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 Post subject: Re: Amp Schematic Questions
PostPosted: Mar Wed 20, 2019 8:38 pm 
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I just came in from outside, beautiful Spring sunshine. Don't worry, you'll get your share soon enough. 8)

The schematic shows the components on the first half of V4, then just shows the voltage on the cathode of the other half of V4.
To draw the same components twice would cause even more confusion. There ought to be an X or other symbol showing that pin 6 is indeed the same pin on both tube diagrams. The teardrop might mean that.

Yes there is a jumper from pin 6 of V4 to pin 5 of V2.

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 Post subject: Re: Amp Schematic Questions
PostPosted: Mar Wed 20, 2019 9:11 pm 
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All three cathodes are connected together as John said. Sometimes they just label the connected points instead of drawing the wires.

This is a place where bypass capacitor C14 is important. If it isn't in place, there will be a signal path among the three stages through the cathode connections. That path is likely to result in an oscillation or strange frequency response.

Jay


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 Post subject: Re: Amp Schematic Questions
PostPosted: Mar Wed 20, 2019 9:24 pm 
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Thank you both.

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 Post subject: Re: Amp Schematic Questions
PostPosted: Mar Fri 22, 2019 2:58 am 
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Yep they all got the same DC bias on the cathodes thus grids and the bypass cap makes all three cathodes think they are merely connected to ground chassis or near so, at audio frequencies. But what is the advantage other than saving maybe 4 cheapish parts?

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 Post subject: Re: Amp Schematic Questions
PostPosted: Mar Sun 24, 2019 3:35 am 
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No particular reason that I can see, most likely it was just that, lower production cost.

But there is one thing that might alter that opinion. Since the the first stage is capacitor coupled the DC parameters are not a major factor. But the plate of V4A has 120 volts on it and is connected to the grid of V4B through the Expression Control. I am unsure what that control is or whether it can pass DC. (I would assume not, otherwise there would be 120 V on the control grid of V4B! If it is actually a capacitive or open coupling, no problem. But if DC can pass through it, the connection of V4 cathodes to a common point would allow bias to offset based on changes to cathode current on either stage. The key question becomes, What is the Expression Control and how does it work? Can it pass DC voltages?


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 Post subject: Re: Amp Schematic Questions
PostPosted: Mar Sun 24, 2019 3:54 am 
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The drawing clearly suggests that the expression control is a split stator variable capacitor.
Hammond experts please confirm.
-Chuck


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 Post subject: Re: Amp Schematic Questions
PostPosted: Mar Sun 24, 2019 10:59 am 
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K7MCG wrote:
The drawing clearly suggests that the expression control is a split stator variable capacitor.
Hammond experts please confirm.
-Chuck


Certainly no Hammond Expert here but you are correct. the shaft has the moveable plates at the end of the arms, the stators are mounted at the bottom of the box


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