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Old 11th February 2011, 11:44 AM   #1
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Default Value of coupling caps

I have a schematic for an amp that uses 6sn7 drivers for 4-El34 outputs. The coupling caps between these stages are 1uf Can I reduce this to .22 or perhaps .47 without affecting performance. Thanks for the help. John Dozier
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Old 11th February 2011, 11:48 AM   #2
Loren42 is offline Loren42  United States
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Originally Posted by john dozier View Post
I have a schematic for an amp that uses 6sn7 drivers for 4-El34 outputs. The coupling caps between these stages are 1uf Can I reduce this to .22 or perhaps .47 without affecting performance. Thanks for the help. John Dozier
Or parallel two .47 F caps...
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Old 11th February 2011, 11:59 AM   #3
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Quote:
Originally Posted by john dozier View Post
I have a schematic for an amp that uses 6sn7 drivers for 4-El34 outputs. The coupling caps between these stages are 1uf Can I reduce this to .22 or perhaps .47 without affecting performance. Thanks for the help. John Dozier
That can't really be answered without looking at the schematic. Can you link to it?
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Old 11th February 2011, 12:59 PM   #4
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A lot could depend on whether there is global NFB, and where the OPT LF rolloff starts. It is easy to create or solve LF stability problems by changing coupling cap values.

Why do you want to reduce them?
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Old 11th February 2011, 01:45 PM   #5
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High quality caps in 1uf are hard to find and expensive. I cannot link to the schematic as I do not have it on computer and do not have a scanner. I can calculate the cut off of the RC network though if that would be helpful. Thanks
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Old 11th February 2011, 02:13 PM   #6
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There is a 6.8k resistor in series with each coupling cap. This seems to give a cutoff around 20hz. Check my math please! thanks
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Old 11th February 2011, 02:22 PM   #7
DF96 is offline DF96  England
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The 6.8k resistors are grid stoppers. They don't affect LF. The relevant resistance for LF is the grid leak (to ground, or the bias supply). Probably in the region 200-500K.

If you reduce the capacitors you might simply get a bit less LF. Or, because of the increased phase shift, you might get an LF peak around 10-20Hz. With digital sources this does little harm. If you listen to LP then you may get cone flapping. Mains voltage variations can also do this. We can't tell you the answer, because it depends on all the other LF poles in the circuit too.
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Old 11th February 2011, 03:50 PM   #8
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I appreciate all the help. My knowledge of tube theory is limited. Since high quality 1uf are usually limited to 200v, I think I will use 2-,47uf at 600v, bypassed with .1 and .01 600 v. I hope I can fit them in the chassis I have in mind. thanks again
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Old 11th February 2011, 04:04 PM   #9
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0.22uF will generally be ok for a decent LF response, assuming something from 100k upwards has been used for grid stoppers of the power tubes. Bypassing the coupling caps with yet smaller values makes very little sense IMHO, but whatever floats your boat; it's a free world. Expect very little improvement with values higher than .47uF.
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Old 11th February 2011, 04:09 PM   #10
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The grid stopper is 100k. I think perhaps a .47 600v might be a good value. This way I can use a high quality cap, have it fit, and do away with the bypasses. What say You? Thanks again
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