[Audiobliss2]

My speakers are very sensitive (105dB per 1 watt per 1 meter). Furthermore, my amp has 75Kohm input impedance compared to the 2K input impedance of the MP-701. Even at the gain of 4, and the volume level at the 9 o'clock position the level is already at earsplitting.

Maybe you should go for some lower gain 12au7 or 6sn7.

 

[pythagore]

I read on an another forum that you shouldn't bypass coupling caps, supposedly causes phase shift or something like that. Only the power caps.
Any truth to that.

I disagree for this particular scenario. These coupling caps in combination with the input impedance of the amplifier being fed by the MP-701 form a high pass filter. By high-pass, to only pass in the frequency range you desire. In my previous post I commented about if you want to improve the bass output of the MP, you simply have to increase these capacitors withing Garry's provided range. (See my attached picture below, on how the coupling capacitor and the input resistance of that amp contribute to allow pushing the cut-off frequency of that high pass to get more bass or less of it. )
Here is where I disagree more. A capacitor has what we called an Equivalent Series Resistance (ESR). That ESR, the lower it is, the better. Why, because these account for some of the signal lost across that capacitor. Therefore, putting two capacitors in parallel causes the ESR of the resulting equivalent capacitor to decrease. In fact, you may notice in some design the designer chooses to put multiple small capacitors in parallel instead of using one big one for the same value, because he/she wants a lower ESR.

 

[pythagore]

Maybe you should go for some lower gain 12au7 or 6sn7.

I have 4x 6SN7 and lot of 12au7, but I don't have the adapter yet. I ordered the adapters but I haven't received them yet. 12au7 sounds too warm for my taste.

 

[Bkido]

I have 4x 6SN7 and lot of 12au7, but I don't have the adapter yet. I ordered the adapters but I haven't received them yet. 12au7 sounds too warm for my taste.

For gain I'm also at the lowest setting of the pre. Feeding it already with a slightly hot signal form my Holo Cyan 2 and then going to my Soncoz SGP1 power amp. My Cabasse Murano are not the most sensitive but 12 o'clock is the maximum I'd want to listen at probably. Still very much in a usable volume range for me. Super sensitive speakers combined with high power/gain power amp might introduce some limitations concerning volume range though.

 

[Audiobliss2]

I disagree for this particular scenario. These coupling caps in combination with the input impedance of the amplifier being fed by the MP-701 form a high pass filter. By high-pass, to only pass in the frequency range you desire. In my previous post I commented about if you want to improve the bass output of the MP, you simply have to increase these capacitors withing Garry's provided range. (See my attached picture below, on how the coupling capacitor and the input resistance of that amp contribute to allow pushing the cut-off frequency of that high pass to get more bass or less of it. )
Here is where I disagree more. A capacitor has what we called an Equivalent Series Resistance (ESR). That ESR, the lower it is, the better. Why, because these account for some of the signal lost across that capacitor. Therefore, putting two capacitors in parallel causes the ESR of the resulting equivalent capacitor to decrease. In fact, you may notice in some design the designer chooses to put multiple small capacitors in parallel instead of using one big one for the same value, because he/she wants a lower ESR.

I will leave the formulas to you, but good to know now I have more options to try, as I do have a variety of small value caps from previous projects over the years comprising of,

1 pair mundrof Supreme SO .22uf
1 pair mundrof supreme SO .1uf
1 pair auricap .22 uf
1 pair Obbligato Premium + 1.0uf
1 pair Cardas golden ratio .22uf
1 pair Cardas golden ratio .10uf
1 pair Audio Note copper foil paper in oil .01mf
1 pair Musiccap 5.ouf
A single Audio Note unleaded pure tin foil .47uf
A single Jensen paper .27uf

So the experimenting seems endless .

 

[pythagore]

Nice collection of bypass caps for you to try. The .01mF (a typo?), you meant .01uF.

 

[Audiobliss2]

Well, I went back and checked them again, and actually I found two typos.
The Auricap is .22mf, and the Audio Note is 0.1mf.
The Audio Note came from a M3 preamp kit I built many years ago, and the Auricap was in a pair of UL KT88 mono bloc amps many years ago also. No idea why they used those values.
I guess they will not be good candidates for the current project, right?

 

[pythagore]

Well, I went back and checked them again, and actually I found two typos.
The Auricap is .22mf, and the Audio Note is 0.1mf.
The Audio Note came from a M3 preamp kit I built many years ago, and the Auricap was in a pair of UL KT88 mono bloc amps many years ago also. No idea why they used those values.
I guess they will not be good candidates for the current project, right?

They are correct. Based on the size of them they are actually (Auricap 0.22uF, Audio Note 0.1uF in today's marking). The marking "mf" indicates that these are pretty old capacitors. I'll say 20-30 years old. Nowadays microfarad is indicated by "uF" and milifarad by "mF", in the 90's some brands use to indicate microfarad by "mf, or mF", due my surprise when I saw the .01mF.

 

[Audiobliss2]

You are correct, they are from the late nineties, they had very little use so I guess i will try at least the Audio Note and the Mundrofs.

 

[Audiobliss2]

Got my adapters yesterday, got a few more 6sn7 also to make up some 3 sets.
So now have to test them make sure they are at least as good as what I already have.

This preamp is surely a tube refresher course.

 

[Audiobliss2]

Have my 6SN7s tested and ready to install, but the Tesla 6922s sounding so good now, I hesitate to take them out. Can't imagine it getting better.

 

[pythagore]

Have my 6SN7s tested and ready to install, but the Tesla 6922s sounding so good now, I hesitate to take them out. Can't imagine it getting better.

I love the 6922's as well. Garry recommended the 6922's to me as the best option for this preamp.

 

[Audiobliss2]

Help!, my adapters doesn't work for some unknown reason. I plugged the 6sn7s in them, then plugged into the preamp sockets, I did it that way because the circuit board doesn't seem like it can take much stress.
Anyway, plugged up everything and powered up but only the rectifier lit up.
It was like the 6sn7s were not getting any power. So I disconnected everything again and put the 6922s back in to test to make sure nothing broke, like a tube pin solder joint.
The preamp is fine again with the 6922s, so it has to be a issue with the adapters.
Is there a difference between 6sn7 to 12au7 adapter, compared to a 6sn7 to 6922 adapter?
I didn't think so that's why I got the 6sn7 to 12au7, they are both 6sn7 to 9 pin.
Am I wrong? That's the only thing I can think of.

 

[Audiobliss2]

One other thought, the adapters says 6.3v on them, so are they converting the voltage? Because a 12au7 uses 12v, so are these supposedly to reduce 12v to 6.3v?
If thats the case, that's probably the problem. It's not just a straight wire from 9 pin to 8 pin.
Maybe I will just stick with the 6922s.

 

[pythagore]

The pin-outs of the 12Au7 and the 6922 are identical if you ignore pin 9. Pin 9 is not a used pin for both 6922 an d 6SN7, but it is used as the plate heater center tap for the 12Au7. So converting from 12Au7 or 6922 to 6SN7 should be the exact same wiring as long as the pin 9 is ignored. Assuming that your 6SN7 tubes are good, then that adapter may be using the pin 9 while it shouldn't. See attached pin mapping for the three tube types below. Plate=Anode