Marantz 7 clone tape head preamp

[Foxbat]

Of course one could do that, but is there a solid reason to do it? My personal impression was that while the gain was on the low side, it was still more than sufficient with medium inductance head. I generally believe in the "less is more" philosophy when talking about audio circuits. Plus there is something to be said for the tube purity of that circuit.

 

[tapepath]

I agree completely, if possible just use the M7 by itself and raise the volume sufficiently. However, not everyone will find that a satisfactory solution. For use with an outboard Dolby unit the lower output might not permit calibrating properly. Or, just not wanting such a wide disparity with other higher output sources. Or, not being able to calibrate a deck to factory specifications can be frustrating. Since finding matching step up transformers is difficult the use of a really good quality MC "head amp" is a viable alternative.

 

[Kcin]

I have found the volume to be manageable with both a technics RS1500 and an Otari MX5050BII--- and @Foxbat is absolutely right about the tube purity of this circuit it is fabulous.


... the only thing that I have found is that the replacement of the 47K resistor with a 10K pot in the feedback cct... --- that pot if rotated to the extreme position ( and I have to check this -- whether its 0 or 10k - the preamp is out on loan now) causes instability - like motor boating- I may have to stick a resistor in there to stabilize it ...

Anyone encounter this?

 

[Foxbat]

I agree completely, if possible just use the M7 by itself and raise the volume sufficiently. However, not everyone will find that a satisfactory solution. For use with an outboard Dolby unit the lower output might not permit calibrating properly. Or, just not wanting such a wide disparity with other higher output sources. Or, not being able to calibrate a deck to factory specifications can be frustrating. Since finding matching step up transformers is difficult the use of a really good quality MC "head amp" is a viable alternative.

Yes, in a calibrated system such as Dolby you will have an issue with its gain and you would have to make a decision. In which case a pre-scaler might be a convenient answer. However, there could be other options too, like adding a tube output stage.

I would even recommend a wonderful device for that application - the 6C45 tube... I love that one. Tons of gain and low plate resistance, just what you need.

 

[hannemanpl]

A few days ago, I received the potentiometers I had ordered. Yesterday, I found some time to install them in my version of the preamplifier and play around with the settings a bit.

Imagine my surprise when I connected the preamp to an oscilloscope and started turning the knobs.


Everything came to life, and I was able to perfectly adjust the gain with the potentiometer installed in place of the 510 ohm resistor. The current setting is about 220 ohms, which gives me about 0.75V at the output. In my case, I leave the head matching resistor at the 47k setting. It is optimal. Turning the knob causes audible changes in the sound and adds or removes low tones. The third potentiometer is used to set the tape speed and for 15 ips it is 7.5k ohms.


I recommend using single potentiometers for each channel because not every tube is identical, and in my case there is a 7% difference between channels. I can correct this with the gain potentiometer.


I am now satisfied with the result, but as you can see in the photos, I will need a new enclosure, which makes me happy because it means that my work on the preamp is not over yet.


I am still waiting for a shipment of step-up transformers. But I treat it more as an experiment and I will see what changes it will bring and what the potentiometer settings will be then.



Best regards colleagues!

 

[tapepath]

Thank you for your suggestion, Foxbat. A prescaler is completely unknown to me. It appears to be used to change high frequency signals to lower in pitch, useful in clock driven devices.
A tube output stage is what I was thinking about. I even experimented with a line stage I built for another project, but the result was drastic overloading of the add-on stage's input. Useful for guitar amplifiers, not so much for music listeners. Could you, perhaps, reference a possible design using that tube?

 

[Robnec]

If I could help a little. 6c45 is my favorite tube also. I have built two phono pres based on this tube. They are now converted to head pres as I do not use phono pres any more. The Ongen build is more costly due to interstage transformers.
The Conara build is great too. It is in use now. As always the bigest problem is power supply. It is three times bigger.

 

[Foxbat]

You are absolutely correct, Robnec - such a gain stage would need its own power supply, I would try to have 5-10mA through the tube, with a cathode resistor, and then it becomes very linear and has great sound, so I use them in all my SE phono stages.

With its huge transconductance the tube naturally become a consideration for the input stage, implying very low noise, but that high Gm also makes the grid sensitive to vibrations, resulting in mictophonics, so I never moved beyond experimentation in that application.

 

[tapepath]

Would this work:

 

[Foxbat]

Yes, that will work. Personally, I would reduce the supply to 150V, increase the plate resistor to 5K and put a 220K resistor to ground after the output cap. I would put the grid stopper to the other wide of the 470K, but it might work as it is. With 100V on the plate it will have plenty of output swing. For the output capacitor I would use the 3.3uF 400V V-Cap ODAM, they are currently my favorite signal caps.