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ECC82 pre with cathode follower
ECC82 pre with cathode follower
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Old 15th November 2019, 02:18 PM   #11
Ketje is offline Ketje  Belgium
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Quote:
Originally Posted by artosalo View Post
Your operating point is quite extreme, i.e. -Ug is high (-4.3 V) and therefore Gm is low.
At the attached photo it is marked with red dot.
I would use something like the operating point marked with green dot.

Increase R8 to some 5k6 and fine tune (=search) R7 to get some 20 mA anode current.
Yes,linearity is better, but dynamic Gm (and Zout) is worse.
Mona
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Old 15th November 2019, 04:43 PM   #12
disco is offline disco  Netherlands
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They shoot over the moon these sputniks; got ten pieces and they all measure differently. After a lot of nitpicking I ended up with 43 ohm for cathode resistance and a beafy 4K 10W load. On the graphic is a sample. I measured Vak=137,5V and Ia 114,5/4035=28,4mA while the grid is at 112,7-114,5=-1,8V. That's half a volt off from the graphic. 7788 better choice?

Oh, and there's a hum (AC out 2 - 3mV) but faintly noticeable between songs. That could be because of the 1 meter B+ cable plus AVO7 in between No ground loop, no progress when changing AC to DC glow or grounding the glow. When I short the sputniks grids it's silent. Time to insert anode stoppers too?
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Last edited by disco; 15th November 2019 at 04:50 PM.
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Old 15th November 2019, 04:56 PM   #13
artosalo is offline artosalo  Finland
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Yes,linearity is better, but...
Yes, a lot better. See THD readings.
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Old 15th November 2019, 05:13 PM   #14
disco is offline disco  Netherlands
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Originally Posted by artosalo View Post
Yes, a lot better. See THD readings.
90% 2H probably?
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Old 15th November 2019, 05:17 PM   #15
disco is offline disco  Netherlands
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Originally Posted by Ketje View Post
Yes,linearity is better, but dynamic Gm (and Zout) is worse.
Mona
Could drive a LED probably...
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Old 15th November 2019, 05:31 PM   #16
Ketje is offline Ketje  Belgium
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Originally Posted by artosalo View Post
Yes, a lot better. See THD readings.
20Vpp is to much for a follower with 12V on the cathode, with 5Vpp the situation will be less catastrophic.
The last solution of disco is much better.But the big current draw is probably the cause of the hum.
Mona
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Old 15th November 2019, 07:58 PM   #17
artosalo is offline artosalo  Finland
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Quote:
20Vpp is to much for a follower with 12V on the cathode...
True. I was a bit hasty with my simulation.

At some 6 Vpp out the difference is still huge: 0.008% vs. 0.26%.
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Old 18th November 2019, 03:05 PM   #18
disco is offline disco  Netherlands
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Hum problem solved; sometimes the clue is in the datasheet The 6C45 heater (6V AC) is raised 43V by means of the Morgan Jones "Thingy" ladder with two BJTs. Loading this pre down to 10K is no problem. I'm gathering parts for the PSU and timber for a case; will post some pics and building instructions when done.

If you like solid bass this is the one. It does a good job preserving dynamics, ranging from fine details down to voices and contra bass strings plunging. To my (limited) experience some pre amplifiers have a tendency to loose some quality in the process of controlling the output level. Either low end or highs get attenuated when turning the volume control, although it would come down to a silly part of a deciBell, normally not observed.

The only thing worrying might be the decline of transconductance, note the 3000hrs remark in the datasheet.
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Last edited by disco; 18th November 2019 at 03:09 PM.
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Old 21st November 2019, 01:11 AM   #19
PRR is offline PRR  United States
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> The only thing worrying might be the decline of transconductance, note the 3000hrs remark in the datasheet.

The rated Gm amounts to 37 Ohms at the cathode. The projected change of Gm means it could be 26 or 52 Ohms in 2 or 3 years (or 4 months at 24/7).

The 1u cap and hi-fi expectations suggest it shouldn't be driving even as low as 10k load.

Note that 1uFd is still 160 Ohms reactance at 1kHz. It dominates output impedance over most of the audio band.

10,000/10,026 -0.023dB
10,000/10,053 -0.046dB
+/- 0.04dB or 0.01dB per month working steady

The voltage-gain stage and passive parts will drift at least this much. The output stage Gm variation vanishes in the haze.
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Old 21st November 2019, 06:02 AM   #20
disco is offline disco  Netherlands
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So, this plus token gives you a ride on Metro. Which screen is foretelling?
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