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Edwind 17th January 2012 02:52 PM

New Schematic Linestage Tube Preamp using EF86 & 12AX7
This a new linestage preamp using EF86 & 12AX7.
What do you think?

kevinkr 17th January 2012 02:58 PM

Hi Edwind, Welcome to the forum!

In terms of your schematic I am wondering what the intended purpose would be? The gain for a line stage application will be very high. I've not done an analysis of this design, but generally gains of well under 20dB are the norm for modern designs, and in most cases less than 10dB is a reasonable choice.. (I would guess the gain here could be 60dB - in phono stage territory.)

The output impedance will be extremely high as well which will make it a poor line driver..

The input stage has limited signal handling capability, I suspect a couple of hundred mV would cause this design to clip as there is no way of dealing with the very high signal levels of digital sources (typically 2Vrms @ FS) and even most analog sources..

Filament circuit should not allow current to flow through chassis/circuit ground as this is an excellent recipe for hum.

DF96 17th January 2012 03:15 PM

A guitar preamp? Certainly not a line stage. It seems designed to add noise and distortion.

Edwind 17th January 2012 04:13 PM

Hi, kevinkr annd DF96 (is it tube type?).
This are the gain for every tube that use: +22dB (12AT7), +16dB (12AU7), +21dB (12AX7).
This is low quality circuit, fun for beginners (low cost) and a little homeworks for experts :D
I already build this. Input directly from CD player.
There is a very little of hum, noise, and distortion (for my ear, may be no for others).
This circuit not like Quad II phono stage.
The output impedance still to high since it's use common cathode and may add a voltage follower to boost the output signal.
Many-many thanks for advices, it will give a positive feedback for next circuit.
By the way, will somebody upload the new scheme for upgraded version of this circuit?

SY 17th January 2012 04:31 PM

It looks like there's a 20dB attenuator in the front of the circuit. Followed by amplification. The logic here is not something I'm getting.

DF96 17th January 2012 05:03 PM

Only in a guitar amp would someone think that ECC81,82 and 83 are directly interchangeable. Beginners can have much more fun with sensible circuits; cheaper too as you would not have to throw it away once you know better.

As a line stage this circuit is simply wrong. I will leave it to you to do the homework. It can't be upgraded, only replaced. Sorry to be blunt, but you asked what we think.

Edwind 18th January 2012 03:25 AM

It's ok, no need to sorry DF96.
You told the truth.
Thank you very much.

revintage 18th January 2012 08:20 AM

Hi Edwind,

As DF96 says the circuit is useless. But there is actually a solution for making it work:

1 Move the volumecontrol to the input
2 Triode-strap the EF86 Ra=47k Rk=820
3 Make the second stage a CF, DC connected to the first, and use ECC82.

The triodestrapped EF86 sounds great as lineamp.

Edwind 18th January 2012 03:19 PM

Hi, revintage/Lars,
Thanks for the solutions that yo give to me.
I'll try it.

kevinkr 18th January 2012 04:58 PM


Lars' solution is a good one.. :D

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