Signal Path in Tube Amplifier Circuit

Discussion in 'The Projects Forum' started by drillert450, Jan 12, 2010.

1. drillert450 Thread Starter New Member

Jan 11, 2010
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Hi all.... I need a little help understanding the path the signal to be amplified takes in the schematic I've attached to this post. From what I can see the input will travel to the grid of the 7N7 and the amplified signal will appear on the plates. I'm confused from here because as far as I understand in order to be amplified further at the 829 the signal will have to again enter the grid at the 829... however the only path I can see is directly through the output transformer through resistor 6 to pin 3 where it would then be amplified and then sent strait back through the same path it entered... Is this correct??? If not could someone please help to clarify what the actual path taken by the signal is.. Thanks for any help.

Jason

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2. eblc1388 Senior Member

Nov 28, 2008
1,542
102
No. Not quite.

Input signal changes the 7N7 tube current and this change of current produces a correspondent voltage change across the cathode resistor R9(25K).

A capacitor(value not marked in your schematic) couples this signal into 829B grids for further amplification.

3. drillert450 Thread Starter New Member

Jan 11, 2010
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OK that makes more sence... So then The Capactior in the upper left (unlabeled C1) and C2 are used to filter out the signal and allow only supply voltage to pass. Would that be correct??

4. Ron H AAC Fanatic!

Apr 14, 2005
7,050
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What's the point in having a cathode follower (actually, two CF's in parallel) driving the grid of a common cathode amplifier stage? I guess it would make sense if the source is really high impedance.

5. eblc1388 Senior Member

Nov 28, 2008
1,542
102
That's correct. There are ripples in the 320V supply, left by rectification from AC into DC and also voltage changes when 829B anode current changes. These capacitors bypass them.

Double check the polarity of capacitor C4 between 829B cathode and +320V line. It is shown wired in reverse.

6. Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
I found the complete schematic that drillert450's schematic was excerpted from.
That, combined with the 829B datasheet, explains the need for the cathode follower.

7. drillert450 Thread Starter New Member

Jan 11, 2010
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I redrew the schematic to combine the 7n7 halves and to try to get more familiar with the layout. I'm just about ready to assemble this amp. It's kind of a training excersice. I have no problems understanding and assembling circuits based on schematics, however I'm limited in my understanding of the actual workings of the circuit. I just would really like to have a handle on whats going on with the signal being amplified so that as I put this thing together I get the most out of it. If I get the Ideas and principles in my head now before I start they will really set in as I go through assembly.

8. Ron H AAC Fanatic!

Apr 14, 2005
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Are you building the entire stereo amp, including the 6J7's in the front end?

9. drillert450 Thread Starter New Member

Jan 11, 2010
4
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Yes.. The whole thing. I started looking at this amp 2 years ago. I was going to start by building the "Mighty Midget" From Pete Milletts page but was lured into this design. It seemed like a fairly strait forward and simple layout to put together and I like the turrets used under the chassis for making connections.