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Old 12th November 2004, 02:22 PM   #1
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Question advice for 6L6GC P-P amp

Since Ill rebuild my amp because I have some hum problems (if youd like to see what happened go over here ), Id like to start a new thread to ask some questions to develop the schem.
The bias of 6L6GC is 320V at about 70mA.

1) If you believe that I must change any value of the components for better results, please tell me which one to change.
2) Instead of using 6SN7 as an input tube, do you believe its better to use a single triode driver? I read somewhere that this tube doesnt have internal screening and this may compromise channel separation.
3) Is it better to use bypass cap at the first stage or to leave it as it is?

Thanks
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Old 12th November 2004, 02:24 PM   #2
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And here is the PSU
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Old 12th November 2004, 02:58 PM   #3
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Too much C in the PSU, use a choke instead. But that's not going to cause hum unless the current spikes are drawn in via ground loop.


I don't like star grounding. Too damn messy. Just ground the first stage (which should be a 6SL7, you'll have way too little gain as-is) to the input jack, then connect those to the rest of the grounds however you please.

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Old 13th November 2004, 02:09 PM   #4
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Hmmmm....
A 6SL7 or to parallel the two halfs of the 6SN7??
I'll have more gain and not the problem with channel separation.If I'll use 6SL7,one half at L channel and the other at R, I'll have the same problem.
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Old 13th November 2004, 02:12 PM   #5
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Geez, it's going into the same damn room, you WON'T notice 2pF of crosstalk.

You need more gain because you need to remove as much with NFB to get it to sound good.

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Old 13th November 2004, 02:13 PM   #6
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Quote:
Too much C in the PSU, use a choke instead.
Where is the problem with that?
I prefer not to use chokes!More ringing problems!
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Old 13th November 2004, 02:23 PM   #7
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Geez, it's going into the same damn room, you WON'T notice 2pF of crosstalk.
So,there's no any problem to use each half at each channel?

Quote:
You need more gain because you need to remove as much with NFB to get it to sound good.
I'm planning to use about 8dB of FB.I think I'm fine with only one half of 6SN7.A Mu of 70 (6SL7) is too much for my purpose.
I think so.That's why I'm asking.Please give me a more clearly explanation.
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Old 13th November 2004, 09:20 PM   #8
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You want more than 8 dB of feedback. At that low level, the THD does decrease, but third and higher actually increase over the open loop condition. 15-20 dB is probably more appropriate.

I don't get the ringing comment. I think the cap situation you show is OK, but a choke at the first pi filter would help efficiency and knock down hum quite nicely.
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Old 15th November 2004, 06:36 AM   #9
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I don't get the ringing comment. I think the cap situation you show is OK, but a choke at the first pi filter would help efficiency and knock down hum quite nicely.
Since I use PSUDII for my PSU designs I don't like to use chokes.I can't design easilly a stable and not ringing PSU.With caps and Shindo style,things are better.Chokes are more expensive,too.

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15-20 dB is probably more appropriate.
So, I must change the first tube?

Use 6SL7 as sch3emat1c said or to parallel the two halves of 6SN7?I prefer the last one cause I allready have the tubes and I also can make the two channels identical.
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Old 15th November 2004, 11:41 AM   #10
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With a C-input filter, I don't see any ringing in any of the amps I have on hand. That's not the same as a simulation, granted, but has its virtues. L-input filters will respond nicely to snubbing, but that's not an issue here.

Paralleled 6SN7 will not give you enough gain- you don't increase mu, you just drop rp. Use the 6SL7.
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