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Is there a no/gain preamplifier schematic using high-distortion EC90?

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Layberinthius

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Joined 2004
#1
Driving the EC90 (same as half a 12AU7) where it can add colouration to material which benefit with plenty of distortion..Rock circa 1970's\vinyl.

But I need a schematic that will provide all the EC90 can give me in terms of gain for microphone sources and little-to no-gain for line level sources while maintaining that tube distortion quality.

Using my rather extensive supply of Mullard EC90 tubes.

So I can sample it with an ADC.

From my search it's as if no-gain tube preamps are hard to find?!
 
#2
For the "unity" gain block, a cathode follower with only a bias resistor will be non-linear.

CCS loading a common cathode wired 6C4/EC90 will allow you extract the "full" mu of the tube in a very linear fashion.

If you use a linear gain block with "mikes" only and feed both amplified "mikes" and line level stuff through the non-linear CF, you will achieve your purpose. A CCS loaded CF to drive the load would finish things off nicely. Use the level control as the linear CF's grid resistor.
 
#3
What an interesting question! I don't know of any, and am no expert anyway, but looking into guitar effex pedal circuits may help. IIRC, that's what the early ones did -- intentionally clip, noon-linearly amplify, etc. Probably a lot of sites out there with details.

BTW -- is the pin/scoket type the same as 12au7? Just curious.
 
#6
For the "unity" gain block, a cathode follower with only a bias resistor will be non-linear. CCS loading a common cathode wired 6C4/EC90 will allow you extract the "full" mu of the tube in a very linear fashion. If you use a linear gain block with "mikes" only and feed both amplified "mikes" and line level stuff through the non-linear CF, you will achieve your purpose. A CCS loaded CF to drive the load would finish things off nicely. Use the level control as the linear CF's grid resistor.
Click to expand...

Yo frank you catch the number of that bus!? nay, train!

Yeah it went in one ear and out the other...

CCS loading = active loads instead of resistance.
CCS

Common cathode

If you use a linear gain block with "mikes" only and feed both amplified "mikes" and line level stuff through the non-linear CF, you will achieve your purpose.
Click to expand...

So "CF" or cathode follower is another way of saying you have the plate of one 'preamp' tube in my case an ec90 going into the cathode of a second more linear preamplifier tube which is then loaded by an active load on it's plate to drive whatever on the end essentially giving a finished signal?
 
#7
For the linear "mike" gain block, a mu stage with either a MOSFET or a 6AU6 "upstairs" will get the job done. Cap. couple the mu stage to the distortion generating block.

The distortion generating block is a simple cathode follower with only a bias resistor under the cathode. A 470 KOhm grid leak resistor is fine here. Cap. couple the distortion generator to the O/P buffer.

The O/P buffer is a cathode follower with an active load "downstairs". Again, either a 6AU6 or a MOSFET will do nicely. If you can source 500 KOhm volume controls, use them as the grid leak resistors for the buffer triodes. You could use stacks of 3X red LEDs to bias the buffer grids 6 V. below the cathodes' potential.

The unit consists of 3 stages: a linear "mike" only preamp, a near unity gain distortion generator, and a linear O/P buffer. There are 3X 6C4s plus 2 other active devices per channel.

BTW, if the distortion generator is a bit much, placing a smallish load resistor under the bias resistor will reduce the distortion level.
 
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