6sn7 why the soft clipping. - diyAudio
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Old 31st August 2015, 09:14 PM   #1
Celsius is offline Celsius  Spain
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Default 6sn7 why the soft clipping.

This weekend I have checked by the scope the amp to see that all is under control, well my surprise have been that:

When the first plate give more than 5V RMS begin to clipping in the upper wave.

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At 11V RMS very visible.

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Schematic :

B+ 140V AC double voltage give 350V DC.

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Old 31st August 2015, 09:18 PM   #2
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Is the second stage supposed to be DC coupled? If it's supposed to be AC coupled like the schematic shows there is no grid leak resistor for the the second stage.
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Old 31st August 2015, 09:33 PM   #3
Merlinb is offline Merlinb  United Kingdom
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You have no grid stoppers and the second triode is biased to 6V, so it will clip at about 6Vpeak or 4.2Vrms on the grid, so it is no surprise that you are seeing clipping at 5Vrms at the first anode. (Also you need a grid leak,as noted above)

Last edited by Merlinb; 31st August 2015 at 09:47 PM.
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Old 31st August 2015, 09:40 PM   #4
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Quote:
Originally Posted by Merlinb View Post
The second triode is biased to 6V, so it will clip at about 6Vpeak or 4.2Vrms, so it is no surprise that you are seeing clipping at 5Vrms.

Agreed but the grid of that stage is floating compared to cathode. There is no ground connection. Doesn't he need to add a resistor?
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Old 31st August 2015, 09:44 PM   #5
Celsius is offline Celsius  Spain
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Quote:
Originally Posted by famousmockingbird View Post
Is the second stage supposed to be DC coupled? If it's supposed to be AC coupled like the schematic shows there is no grid leak resistor for the the second stage.
Have the grid leak resistor 270k I forget to draw it
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Old 31st August 2015, 10:05 PM   #6
Celsius is offline Celsius  Spain
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Quote:
Originally Posted by Merlinb View Post
The second triode is biased to 6V, so it will clip at about 6Vpeak or 4.2Vrms, so it is no surprise that you are seeing clipping at 5Vrms.
With 5v RMS I measure 80Vrms 224Vpp enough to drive the 6c33 but a little close I move to 8v and 6mA the second stage to have more head room.

This is the problem when don't have more reserve of voltage the 6sn7 need 400v in the second stage to drive correctly.

Thanks
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Old 31st August 2015, 10:14 PM   #7
Merlinb is offline Merlinb  United Kingdom
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You could remove the second stage cathode bypass capacitor. Or do you need more gain?
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Old 31st August 2015, 10:32 PM   #8
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If you already have a coupling cap between the preamp stages, why not go fixed grid bias? You also already have a negative supply, for the power stage bias.

Then you'd have 8V more B+ headroom for the second stage, and you'd have gotten rid of that cathode bypass cap.

Also overall better fidelity - with a CCS load, 6SN7 should not be clipping that softly. The cathode bypass cap is probably not good for transient response.
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Old 1st September 2015, 12:29 AM   #9
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Is it safe for the 2.5V NiMH battery to be constantly trickle-charged with 8mA like that? I would move it to the grid.. and following the suggestion of others, use fixed bias for the second 6SN7, perhaps another battery grid bias.
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Old 1st September 2015, 03:54 PM   #10
Celsius is offline Celsius  Spain
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Ballpanciil, I use in many preamps a amp the NiMh with any problems for me give better quality than Led or Diodes and Resitor w/w cap.

Merlin and MrCurwen I did not know that cathode cap with CCS are not good, the sound now is very good tonight I will remove.

About how to increase the Vpp to drive the 6c33 now I have 112v peak to and the 6c33 bias is at -85v now if I reduce the CCS current to 7mA and increase the cathode bias of the 6sn7 to 7 to 8V will have 150v I think is more than acceptable.

The amp is very chaotic inside no have more free space, this amp Bravo 6c33 had a 6ac7 in the preamp stage in pentode mode with very high Ri, in this case the NFB is mandatory, my target have been to find the way of low THD without NFB, now with the 6sn7 with CCS the sound is very good and dont need the NFB. I want to try without the cathode cap and don t open the amp for a long time,

Last edited by Celsius; 1st September 2015 at 03:56 PM.
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