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newbie question regarding gain reduction

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Say I'm looking at a standard cathode follower gain stage with too much gain: what is the best way to reduce the gain? Between throwing a voltage divider on the output side and changing the value of the grid/cathode resistors, it would seem that the voltage follower route would decrease the S/N ratio but altering the grid/cathode resistor values would be more work to calculate.

Is there an easy way to calculate the change in gain from varying the grid/cathode resistors or does this throw the whole thing off too much to make it easy? Thanks in advance!!!
 
Cathode followers have variable current gain Unless you clip or exceed plate current, cathode followers have a voltage gain of nearly one, but never more than one (that's why they are called followers).

The only practical ways to reduce their voltage gain is to divide the input signal or divide the cathode resistor. You can increase the plate resistor above zero Ohms, but I think that by the time this reduces the gain substantially the plate resistor value is proportionately large and clipping or saturation is a real danger.

Or did you mean a common cathode stage?
 
If you have way too much gain....even with the cathode un-bypassed....then someone choose the wrong tube for that stage..
Need to go back to the drawing board and first figure what Gain you need then work backwards at that point in determining what tube to properly choose for the job..... Then you need to look at the problem from two perspectives....bypassed or un-bypassed cathode resistor.....
As the other poster mentioned, you don't want to reduce the plate load resistor too much or else you sacrafice the linearity of the stage as well as the headroom running out into early clipping.....
You also need to make sure you take into account the amplitude of the input signal to that stage, and that you can properly bias the stage to avoid overloading the input...... Lower mu tubes will generaly have larger input voltage swing to play with....

Chris
 

cbutterworth

Member
2006-10-03 11:02 pm
The amount of gain required by a system seems a little slippery. Most power amps can take line-level input without the need for preamplification. In my case, my 55WPC power amp works very well with a passive preamp - in others words a source selector and an attenuator.

HOWEVER.....having an active preamp in front of my power-amp results in firmer, more solid bass, plus it simply sounds better to my ears. Depending upon tubes, my preamp has either 10dB or 30dB gain. In fact, the 30dB sounds better than the 10dB.

The upshot of this goes something like this: while a power amp may not require active preamplification, it may simply sound better with it than without.

All that said, you could add signal attenuation on the signal input and a simple L-pad should work very well for this. In my system, I found that placing the L-pad on the input to my power-amp worked the best.

I expect that if you have a well-designed tube amp, then you probably would be better off avoiding tinkering with the plate, grid resistors, etc. I would go for an L-pad on the input. You can calculate the resistor values for different levels of attenuation. The calculations are simple, but you can use an online calculator such as http://www.lalena.com/Audio/Calculator/LPad/

Good Luck,
Charlie
 
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