Modified Marantz 7 line amp
HI,
You may change the last buffer tube with 12AU7 for better vocal with the following changes in cathode resistors:-
680 ohm => 2K ohm
27K ohm => 15K ohm
What type of coupling capacitors are used in yours?? I am using Jensen copper-in-oil. They sound very nice and sweet indeed.
Johnny
HI,
You may change the last buffer tube with 12AU7 for better vocal with the following changes in cathode resistors:-
680 ohm => 2K ohm
27K ohm => 15K ohm
What type of coupling capacitors are used in yours?? I am using Jensen copper-in-oil. They sound very nice and sweet indeed.
Johnny
Hi Johnny
Thanks for your suggestions. I have already repalced the buffer tube with 12AU7 (Phillips JAN) but did not change any resistors
Coupling caps are all Sprague.
What should be the value of the output cap?
Some DIYers use as high as 5uf although the original schematic is only about 0.22uf
Thanks
Audiohifi
Thanks for your suggestions. I have already repalced the buffer tube with 12AU7 (Phillips JAN) but did not change any resistors
Coupling caps are all Sprague.
What should be the value of the output cap?
Some DIYers use as high as 5uf although the original schematic is only about 0.22uf
Thanks
Audiohifi
shyeung said:
No, unless what you really want is an M7. There are many, many far better preamp circuits out there- there has been some progress since mass market designs of 50 years ago. For a start, read the preamp book from Allen Wright and, of course, Morgan Jones's "Valve Amplifiers" for both insights as to how to make a first-rate preamp and the reasons that designs like the M7 fall short.
The control is a hum balance adjustment. The easiest way to set it is by listening to the speaker with no signal and volume turned up. Adjust for least hum. The setting is normally near the middle of it's range. Depending on your system, it may not make much difference or be difficult to hear a difference.
Ideally, observe the output waveform on an oscilloscope. Short the input chosen (probably the high gain (phono or microphone)- whichever input you choose then make sure a shorted phono jack is inserted in that input. Set the scope to look at 60hz hum. It will manifest itself at the noise floor as a thick base line. You would adjust the pot to make the base line as thin as possible. You would need to magnify or use 10X. Then repeat for the other channel. You may look at both channels simultaneously on a dual trace scope. Ray
You are looking in the wrong place although it might now be out of print since his death several years ago. You'll have to ask them.
The SuperCables CookBook,The TubePreamp CookBook
Actually the latest edition is available.
Not sure you would find my books on Amazon either, not sure, haven't checked.
The SuperCables CookBook,The TubePreamp CookBook
Actually the latest edition is available.
Not sure you would find my books on Amazon either, not sure, haven't checked.
Much water over the dam since 1958!
In it's day, the Marantz 7 was fine as long as you could get Telefunken 12ax7s. We've learned much since then. Morgan Jones VALVE AMPLIFIERS is the best book. Jones was an engineer for the BBC. About the only place you need a 12ax7 is in the first stage. There are better circuits now with regulated power supplies and active constant current sources and sinks rather than passives. Ray
In it's day, the Marantz 7 was fine as long as you could get Telefunken 12ax7s. We've learned much since then. Morgan Jones VALVE AMPLIFIERS is the best book. Jones was an engineer for the BBC. About the only place you need a 12ax7 is in the first stage. There are better circuits now with regulated power supplies and active constant current sources and sinks rather than passives. Ray
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