• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Looking for 7N7 preamp schematic

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GoatGuy

GoatGuy

Member
Joined 2012
PRR said:
> the cathode resistor (5600 Ω)

I *think* you meant to grab 333 Ohms (from step 7). You seem to have grabbed the plate load.

> cathode bypass capacitor ...low frequency cutoff. I like 5 Hz.
> Using "Z = 1/(2π FC)" set Z equal to the cathode resistor (5600 Ω), F = 5 Hz
> 5600 = ½ π 5 C

Strictly, you want to bypass Re||(1/Gm), the tube's internal cathode impedance.

merlin has picked a current right off the datasheet. Gm also depends on plate voltage, so we have a choice of 3,000uMho or 2,600uMho. 333 Ohms or 385 Ohms.

Take the lower value (bigger cap).

So far we have 333||333 or like (sliderule) 160 Ohms. At 5Hz this is 193uFd.

Caps vary 20%. Tubes vary 20%. If we MUST hit -3dB @ 5Hz, we better allow another 40%. 270uFd.

Strictly we should factor-in the plate load 5,600 divided by Mu. 5600/20 is 280 more Ohms which appears in series with internal cathode impedance 1/Gm.

333||(333+280)= 215 Ohms. Compare to 160 Ohms neglecting Rl/Mu.

Rules-O-Thumb:

1/Gm will be very similar to Rk in most practical designs. (Small triode at 9mA is pushing the limits and should be checked.) So it is expedient to compute on HALF of Rk to pick Ck.

Rl/Mu is generally a small correction, and is "conservative" (results will be better than computed) so this is often neglected.

I actually plotted the picture. Agree that ~~5K Rl makes more sense than my extreme bad-examples. 1.5K has low gain. 17K has no swing. ~~7K is a split-difference. I have used larger swings than 50V up, but that works fine also.
Click to expand...


NICE WORK.
It is slide-rule easy.
GoatGuy
 
Stixx

Stixx

Member
Joined 2003
original-ton.net
like that...?
 

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Stixx

Stixx

Member
Joined 2003
original-ton.net
Do you know how easy it is to try something yourself in PSUD?
Just type in a different value and see for yourself how you arrive at the 300V or whatever you need.
Or change the topology. Replace the choke with a resistor... and what do you get?
Please sit down and try for yourself instead of constantly asking people for the easiest things.
 
PRR

PRR

Member
Joined 2003
Paid Member
> problems with my tx to reach B+ 300VDC 18mA

Tubes are NOT fussy. It will work about the same with 280V.

If you do not have a choke, try 100uFd 1K 100uFd. The 1K drops 18V but gives a LOT more buzz-filtering than lower values. Still for a line-amp I suspect you want a 3-stage filter: 100u-500r-100u-500r-100u.
 
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