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takashi 17th October 2003 03:20 PM

tube buffer USING 12au7
2 Attachment(s)
dear all,
the circuit is a tube buffer using 6dj8 and low power supply.
can i use 12au7 to replace it?
if yes, need to change any resistor?

any any reference circuit that using a 12au7 to construct a tube buffer?


ashok 17th October 2003 04:17 PM

Hi Takashi,
I have used a ECC82 in place of a 6DJ8 in a headphone amp. The amp worked off a 36 volt single supply. The tube had about 18 volts across Plate and cathode.
It worked . It also sounded OK. The measured distortion is higher than the 6DJ8. The 6DJ8 tube sounded better - more open and clearer. Quite noticeable but not an enormous difference. You could live with the 12au7 till you get a 6DJ8.

ashok 17th October 2003 05:04 PM

12au7 seems OK.
I checked the circuit. The 12au7 will work as is but it will have far more distortion. Running it at a higher plate current will be better say around 1 mA at least.

Make R3 = 820 ohms and R4 = 4.7K if you plan to use the 12au7. It should sound good. You will have to try out the 6DJ8 or any other tube to see how good (or bad) the 12au7 is. IMHO the sound depends on more than just the tube and so you can't trash the 12au7 because it sounds bad elsewhere. Maybe that was the wrong application ! Here I think it should be OK .
Remember that gain is less than one ( about 0.85 or so ). Distortion at 0.707volts (rms) input is just over 0.1%. Looks like you can feed about 5 volts peak at the input without clipping into 1M load.

takashi 18th October 2003 04:13 PM

thanks a lot.

James D. 18th October 2003 06:04 PM

Stefano Perugini has written an intereesting article on using the 6C45pi at 24V instead of the ubiquitous 6DJ8. Battery 6C45pi is the link and pick Battery 6C45pi from the menu. It's worth reading even if you don't intend using the 6C45pi.



JojoD818 17th January 2004 08:31 PM


I am so very new to tubes and would like to start my journey with a tube buffer such as the one shown above (takashi's post). I would like to build it because it uses low supply voltages so I can start asap. However, I can only find the 6N1P tube locally, can I use it in place of the 6DJ8? If not, which resistors must I change?

Thanks in advance,

huangyong 21st December 2005 02:30 AM

is there any better design using Constant Current Source?

ashok 21st December 2005 04:40 AM

You can use a constant current source at the cathode and you will get a huge reduction in distortion . That is not because the ccs does any magical thing but just because the ac load on the tube is now much higher ( in ohms !) than if you used a resistor in place of the ccs.
You could try setting the current at 5mA for the ccs. The 6N1P also might work well here , without any changes. I don't have its data sheets .

Leave out the input capacitor . A capacitor at the output is essential though it needn't be very high voltage types. However if you pick a 'botique type ' cap it might just be a HV type.
Have fun.

ashok 21st December 2005 04:55 AM

2 Attachment(s)
Maybe something like this.
V1 could be 6DJ8/6922 or possibly 6N1P.
Q1 can be an IRF830 or even a bipolar if you want ( BC639 ?)
Q2 is a BC546b .

EC8010 21st December 2005 08:13 AM


Originally posted by huangyong
Is there any better design using Constant Current Source?
Do a search for the "Heretical" line stage on this forum...

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