Schematic with ECC88

[Shoog]

So we shall not know ..... but I was far too absent here too often, so never mind!

I read something about grid leak bias, and am going to risk giving unrequested and perhaps well-known advice. But for those who do not know:

Grid auto-bias (e.g. an 8,2 - 15 megohm grid resistor and cathode grounded) is conditionally in order. It must be fed from a low impedance source otherwise distortion is markedly higher than with cathode bias. (Tube data sheets confirm this.) That is because the grid actually draws current to bias it, though minute. That 'generates' a non-linear grid input impedance which needs to be swamped by a feeding impedance of no more than preferably < 1k.ohm. Okay, that is a very general rule, but it is typically used with tubes requiring a small grid bias voltage (say 12AX7 and sharp cut-off pentodes like EF86 etc.) For ECC88? Mmmmm - on the border!

End of tutorial .... but I have seen it used with tubes and under conditions not optimal for it. With modern low-voltage electrolytic capacitors being of small physical size, cathode resistors can be bypassed with high value caps, eliminating fear of hum etc. Those privileged to own a spectrum analyser can test.

Grid leak bias sounds good. Thorsten uses it in his simple phono stage and that is a proven design.
I have also used it in an input stage of a power amp, and its a great sounding amp (though if driven hard the distortion rises significantly more than cathode bias would).

In the right application it is a tried and tested technique.


PS _ Aikido preamp boards are readily available from John Broskie at Tubcad at very reasonable prices. Alternatively it is sufficiently simple to build P2P.
Shoog

 

[DF96]

Grid leak bias should only be used for low signal levels and relatively low impedance sources. This means pickup cartridge (perhaps) or microphone. Definitely not the input stage of a power amp.

The original purpose of grid leak bias was to allow the cathode to be grounded so that tiny amounts of heater-cathode leakage did not create hum. Nowadays, although modern valves can be worse in this respect, we have big cheap electrolytics and DC heaters so there is no need for grid-leak bias except for faradaphobes.

 

[suntechnik]

Hi suntechnik!
Please if you are able to recommend a proven preamp with tube 6n1p-EV.
thank you

6N1P, 6N23P, ECC88, 6922 they are all clones of 6DJ8. 6N1P likes higher B+ and demands higher filament current however. 6N30P that is the most exotic one. Conrad Johnson preamp is implementing 6N30P for example.

I would stick to 6922 (or 6DJ8 Amperex PQ if you can find them) mu-follower implementation that is what makes the difference.

Akido is a good ball park. Or you may try to find Graaf 13.5II schematics. I am owning Graaf that is most phenomenal sounding preamp I've heard so far (it implements 6922 & Black Gates).

SRPP+ All-in-One & Mu Followers

http://www.diyaudio.com/forums/tubes-valves/100290-6h30p-preamp-schematics.html