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Old 17th June 2003, 12:11 AM   #1
SHiFTY is offline SHiFTY  New Zealand
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Question Schematic help!

In the image below, what is the purpose of the circled bit? It is the preamp section of an 807 PP amp, and uses a direct coupled EF37 to 6SN7.

It is a bit hard to see, but there is a 220uF in series with a 12.5K resistor which goes from the grid of the 6SN7 to ground.

This is taken from

which is a great site and has a lot of interesting stuff on it.

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Old 17th June 2003, 12:47 AM   #2
dhaen is offline dhaen  Europe
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It sets the HF limit of the earlier stages. In practice it probably reduces ringing. The cap will most likely be 220pF BTW.

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Old 17th June 2003, 02:46 AM   #3
Colt45 is offline Colt45  Serbia
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it shunts the HF to ground.

wiht a value like 220pf, its quite high freq.. like radio, not audio...
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Old 17th June 2003, 04:23 AM   #4
DSummer is offline DSummer  United States
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100% sure it's 220pF, you can find this kind of design in most vintage amps....
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Old 17th June 2003, 06:23 AM   #5
SHiFTY is offline SHiFTY  New Zealand
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Thanks, I was thinking it was some kind of attenuator to reduce the frequency response, and 220uF made no sense at all...

Oh well, on to the build stage I guess!
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Old 17th June 2003, 07:11 AM   #6
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Does this amplifier have global feedback? The reason I ask is that these kind of RC networks is sometimes used as phase compensation in feedback amplifiers in order to increase phase margin and reduce risk of oscillation, the RC network will reduce the phase shift at higher frequencies and therefore make the amplifier more stable, I use this kind of compensation in some of my amplifier designs.

If it is indeed used for this purpose I advice against removing it as it will increase risk of oscillation.

Regards Hans
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