Small pre

[assasukasse]

Hello everyone
i am looking for a schematics for a easy and nice sounding pre which would drive an amp solid state in class A.
i want to get a nice warm tube sound, but i want to stay low on budget and easy on construction
Thanks!

 

[ErikdeBest]

Hi

So simple I draw it in paint in about 2 min. Inspired by J. Broskie (ECC88 at 100V plate and 10mA), the output resistor is for stability. The 10k resistor dissipates 1W, so take at least a 3W one. If this resistor fails, so will the tube.

Erik

 

[ErikdeBest]

I recommend adding a relay with timer at the output...in desernegised position the relay should connect the output to ground. When the amp is turned on, after some 30sec the relay should be energised (through the timer) disconnecting output from gnd, and letting the music flow.

Or turn you power amp on AFTER the pre amp!

Erik

 

[assasukasse]

hi
thanks for your answer i have two question about the circuit:
first the trimmer is a 100K ohm one, but what is the 109 next to it?
second the resistor just before the 4.7nF capacitor, is written 100R, is it a typo for 100k or is just a 0.1k resistor?
Thanks

 

[SY]

Don't forget a diode connected between grid and cathode to prevent arcover on power-up and power-down.

 

[ErikdeBest]

Hi

It is not 109, but log...I mean a 100k log potentiometer, for volume control. 100R is right, it helps to stabilize the preamp. It isn't a 4,7nF, but a 4,7muF (1000x higher). Actually you must tell what the impedance from your amp is, maybe you should even use a 10muF capacitor. This capacitor should be rated 250VDC.

Here you see the diode and zener diode in place, to protect the tube at turn on and turn off.
http://www.glass-ware.com/tubecircuits/6dB.gif

Actually this is what I draw, I justed added a pot at the input and a cap at the output. More about the diode and zener
http://www.tubecad.com/articles_2001/Tube-Based_Crossovers/

Good luck

Erik