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Best way to set very low tube gain
Best way to set very low tube gain
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Old 12th September 2019, 07:53 AM   #1
AKSA is offline AKSA  Australia
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Default Best way to set very low tube gain

I want to use a 6SN7 to deliver a gain of 3 from a B+ of 250. Output voltage should reach 120Vpp with good linearity.
My thoughts are: 22k plate load, 6.8k cathode resistor, and self biased resistor under the cathode to deliver about 4mA. Will this work well?

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Old 12th September 2019, 09:47 AM   #2
DF96 is offline DF96  England
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Plot a load line for 28.8k anode load and see.
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Old 12th September 2019, 09:54 AM   #3
artosalo is offline artosalo  Finland
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120 Vpp to what load impedance?
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Old 12th September 2019, 11:01 AM   #4
tikiroo is offline tikiroo  New Zealand
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Simulation shows that 120V PP could come close to clipping as the cathode voltage comes up and the anode voltage comes down, with distortion pretty ordinary. How about an "anode follower" with feedback to the grid like an op amp inverting amplifier? You could combine this with CCS loading for even better performance. The sim shows 0.36% distortion (nearly all 2nd harmonic) with a gain of 3 and 120V PP.
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Old 12th September 2019, 11:33 AM   #5
indra1 is online now indra1  Indonesia
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With 6SN7 Ayumi model. 125.18 Vpp from 40 Vpp.
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Old 12th September 2019, 01:43 PM   #6
AKSA is offline AKSA  Australia
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Thanks to All! A great brains trust.......
My idea seems to create more h3 than h2 not very good.
I do like the plate follower - it has far less distortion and offers about three times Zout which will de needed for a target impedance of 33k.
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Old 12th September 2019, 03:02 PM   #7
Lingwendil is offline Lingwendil  United States
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If you only need a gain of 3, are you opposed to using other tubes? A triode connected 6V6 with an unbypassed cathode should be around there. Might need to CCS or choke load it, though.

A triode connected 6P43P-E will be close to the same, but will be able to do it with less supply voltage (~200 or so) to give you more headroom for a traditional anode resistor for similar swing.
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Old 12th September 2019, 06:54 PM   #8
kodabmx is offline kodabmx  Canada
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Best way to set very low tube gain
Quote:
Originally Posted by Lingwendil View Post
A triode connected 6V6 with an unbypassed cathode should be around there.
So will a 6P45S Seems like a waste though.
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Old 13th September 2019, 12:47 AM   #9
PRR is offline PRR  United States
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What load impedance??! Critical bit of amplifier design.

> more h3 than h2 not very good.

Not going to happen with a simple single triode. The curve is one-way, except at extremes you do not want to reach.

The way to hit "gain of 3" is to use a low-Mu tube, un-bypass the cathode resistor, and then pad-down the input to make your gain. (Or NFB.) The values below (for a similar tube) give gain of 3.2, adjust to taste. THD @ 3.2V peak out is 0.2%, "all" 2nd (as expected from triode curvature).
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