AMT SS-11 Clone

[Davebassman]

Hi,

I want try to build a clean two channel's rack preamp based on AMT SS-11, but using ecc82 as a output stage buffer, instead a jfet like the original. Anybody can help me if I do something wrong? or avoid something?

Thanks!

[Helmholtz]

No idea what an AMT SS-11 is, but you can't have the CF grid at 0V with a cathode resistance of 50k. Would bias the tube close to cutoff.

Either use a grid voltage divider to bias the grid close to the desired cathode voltage or add a biasing resistor of ~1k on top of the cathode resistor and connect the grid via a 1M reference resistor to the junction.

In both cases you will need a coupling cap before the grid.

Also the cathode output will sit on a relatively high DC voltage of maybe 50V, so you need an output coupling cap followed by a resistor to ground.

[Davebassman]

The AMT SS-11 is a guitar preamp. I attach the original schematic and my schematic with your advice.

[Helmholtz]

my schematic with your advice.

The CF should work as shown.
CF input impedance is about 390k, cathode voltage will be around 60V, meaning a plate current of 6mA and plate dissipation of about 1.4W.

Cathode bias would allow for higher input impedance (e.g. 1M) but I don't think that's necessary.

Maybe because my eyes are tired (2:30 am here) but I can't spot a Jfet in the AMT schematic.

[Davebassman]

The CF should work as shown.
CF input impedance is about 390k, cathode voltage will be around 60V, meaning a plate current of 6mA and plate dissipation of about 1.4W.

Cathode bias would allow for higher input impedance (e.g. 1M) but I don't think that's necessary.

Maybe because my eyes are tired (2:30 am here) but I can't spot a Jfet in the AMT schematic.

Sorry, I avoid the 2n page of the original ss-11 schematic.

And, If I use ECC83 directly coupled , like many fender amps?

Thanks.

[Helmholtz]

And, If I use ECC83 directly coupled , like many fender amps?

Same as with your original circuit.

Input direct coupling keeps the grid at 0VDC. Consequently the full cathode voltage is used as bias.
While this works with a common cathode gain stage having a low value cathode resistor, the result with a CF will be too low plate current.

As said you can use cathode bias, but you need to split the cathode resistor to make a DC voltage divider, examples here: http://valvewizard.co.uk/accf.html

The output of a standalone unit always needs a coupling cap to block DC.

Directly coupling the CF only works between 2 tubes but not at the output of an amp.

[Davebassman]

I think that I need to learn more, because I can't understand at all... I'm so sorry.

Nevertheless thanks for your help!

[Helmholtz]

As said your circuit from post #3 will work.
What don't you like with it?

[Davebassman]

Yes!! I like. But I can't understand all the things, I'm a bit confusing... I read more or less the Cathode Follower chapter from Merlin Blencowe but I need to depth more into concepts.

Thanks!!