Lowering gain on Frank's 6SN7 preamp

[glenz75]

Hi, I built this 6SN7 preamp a couple of years ago and would now like to lower its gain. From what I've been reading on here this preamp appears to have gain of around 20db. What would be the best way to lower its gain overall?

Circuit is as below...

Thanks in advance...

Glen.

[aardvarkash10]

Morgan Jones on the subject, from pg 499 of Valve Amps 3rd Ed

[mach1]

You could try a feedback loop from the output capacitor to the input tube grid. Just experiment with various value resistors.

[artosalo]

You can try this topology:



http://kuva.termiitti.com/image/17219.gif

This makes possible to adjust the gain from -1 dB to some 20 dB.
With gain below 10 dB the output impedance is below 1 kohms and distortion below 0,1 % when RL is some 10 kohms and output level up to 5 Vrms.
The good feature of this topology is that the input impedance is constant unlike when the feedback loop is brought to grid and different kind of source impedances used.

[DavesNotHere]

adjust the 820 ohm resistor on the second triode to 1k - 2k for hard set gain.

if you keep the control, try a 250k for the input also.

[artosalo]

adjust the 820 ohm resistor on the second triode to 1k - 2k for hard set gain.

No. It is bias resistor for the second triode and has no effect to total gain.
Since the last triode is a cathode follower, its gain is already near unity.
The gain comes entirely from 1st triode.

What would be the best way to lower its gain overall?

With existing topology, just use attenuator at the input.
Now somebody can say that this ruins the S/N-ratio, but it is not the case since the line level is so high that it can be attenuated remarakably before the S/N becomes an issue.

[DF96]

Attenuate, then amplify - does this suggest that perhaps a line stage is not needed at all?

[artosalo]

does this suggest that perhaps a line stage is not needed at all?

I don't know.
If the gain is now 20 dB and only 10 dB is required, then...

[Original Burnedfingers]

Your thread doesn't make any sense to me at all. I built Franks 6sn7 line stage and do not see your point of needing less gain. When built the same as the schematic shows it is very adjustable. I even added a cap to parallel the cathode resistor in the input stage thus increasing the gain I believe. When used with a good quality pot on the input one can adjust this line stage and get ANY amount of signal out as needed.

If your not happy with the line stage then scrap it and purchase one stereo pot or two mono pots and do without a line stage.



Quote:
You can try this topology:

http://kuva.termiitti.com/image/17219.gif

http://kuva.termiitti.com/image/17219.gif

This makes possible to adjust the gain from -1 dB to some 20 dB.
With gain below 10 dB the output impedance is below 1 kohms and distortion below 0,1 % when RL is some 10 kohms and output level up to 5 Vrms.
The good feature of this topology is that the input impedance is constant unlike when the feedback loop is brought to grid and different kind of source impedances used.

Sorry but not a believer in this type of topology. A control on the output generally sucks in my opinion. Never heard one that sounded worth turning on.

[artosalo]

Never heard one that sounded worth turning on.

The reason is maybe that this does not generate distortion like Frank's topology.
Some people hear somewhat distorted signal sounding great, especially those who evaluate amplifiers by listening only.