If I might offer a critique

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Posted by Ralph (M) on February 3, 2025 at 12:28:10

In Reply to: Question about Marantz 7 circuit posted by Tube747 on February 3, 2025 at 03:37:48:

I think this circuit can be improved in a number of ways.

First there really needs to be a grid stopping resistor at the input of the first tube. Probably about 4.75K. It would not be a bad idea to have a second stopping resistor at the grid of the second tube as well.

The coupling cap arrangement between the second and third tubes is a bit clumsy.

The second tube could be direct-coupled to the third tube, eliminating two 1M resistors and a 0.01uf coupling cap.

The coupling cap at the output of the second tube (0.22uf) could then feed just the feedback and nothing else.

I would increase the value of the output coupling cap to about 2uf. The 100K output load is a bit low if a 0.47uf cap is to be used since many amps have an input impedance about the same. That would put the cutoff at slightly less than 7Hz, which is a bit high if you want to avoid phase shift in the bass region.

The larger you make that cap the more the power supply must be stable otherwise with certain amps you'll get a low frequency instability. You may have to increase the value of the cathode resistor of the 3rd tube a bit to deal with the increased Voltage drop across it. 33K might do just fine.

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Follow Ups Full Thread

Topic - Question about Marantz 7 circuit - Tube747 03:37:48 02/03/25 (13)

RE: Question about Marantz 7 circuit - Tube747 00:03:32 02/06/25 (0)

CCS loaded?? - Tube747 01:59:41 02/05/25 (0)

Phono Section Circuit for Marantz 7 - Tube747 03:11:28 02/04/25 (2)

So many capacitors! - Triode_Kingdom 06:47:34 02/04/25 (1)

RE: So many capacitors! - Tube747 10:02:20 02/04/25 (0)

All all interested in building or updating the Marantz 7!! Here are the info!! - Tube747 00:55:09 02/04/25 (0)

If I might offer a critique - Ralph 12:28:10 02/03/25 (1)

RE: If I might offer a critique - Tube747 04:18:53 02/04/25 (0)

RE: Question about Marantz 7 circuit - Tre' 08:21:21 02/03/25 (4)

RE: Question about Marantz 7 circuit - Paul Joppa 09:37:53 02/03/25 (3)

RE: Question about Marantz 7 circuit - Tube747 01:10:34 02/04/25 (0)

RE: Question about Marantz 7 circuit - Tre' 19:58:55 02/03/25 (0)

Very good question! nt - Ralph 12:15:38 02/03/25 (0)

Follow Ups

If I might offer a critique - Ralph 12:28:10 02/03/25 (1)

RE: If I might offer a critique - Tube747 04:18:53 02/04/25 (0)

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