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screen capacitor function

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Posted on September 28, 2016 at 08:26:32
banpuku
Audiophile

Posts: 1008
Joined: January 19, 2006



Newbie question: What purpose does the 2.8uF capacitor do in this circuit? I have read where this capacitor can be removed. So with a minimalist point of view, is this capacitor required or can it be eliminated?

Thanks,
Pat

 

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RE: screen capacitor function, posted on September 28, 2016 at 08:39:15
coffee-phil
Audiophile

Posts: 1444
Location: Shingle Springs CA
Joined: January 7, 2010
 Hi Pat,

In your circuit example the screen grid is supplied via a resistor. The cathode, control grid and the screen grid essentially form a triode vacuum tube. If the capacitor were not there a signal would appear across the screen dropping resistor which is out of phase with that on the control grid. This out of phase signal will partly cancel the signal current flowing to the plate, greatly reducing the gain.

Phil

 

RE: screen capacitor function, posted on September 28, 2016 at 08:43:44
Triode_Kingdom
Audiophile

Posts: 10048
Location: Central Texas
Joined: September 24, 2006
 For pentode operation, the screen should have a steady DC voltage relative to the cathode. That requires bypassing the screen to eliminate the signal AC that would otherwise appear there.


 

RE: screen capacitor function, posted on September 28, 2016 at 10:12:31
banpuku
Audiophile

Posts: 1008
Joined: January 19, 2006
 Thanks you!

 

RE: screen capacitor function, posted on September 28, 2016 at 15:56:33
FenderLover
 Should this cap be connected to ground or the cathode? I've read both.

Thanks!

 

RE: screen capacitor function, posted on September 28, 2016 at 17:38:08
Triode_Kingdom
Audiophile

Posts: 10048
Location: Central Texas
Joined: September 24, 2006
 The primary requirement is to hold the screen steady relative to the cathode. In many designs, the cathode is bypassed to ground, so the screen can be bypassed either to ground or directly to the cathode.



 

RE: screen capacitor function, posted on September 28, 2016 at 18:12:17
FenderLover
 In this case, the cathode is tied to ground via a resistor and bypass cap network (cathode biased). Would that make a difference?

Thanks, again!

8^)

 

RE: screen capacitor function, posted on September 29, 2016 at 08:16:19
Triode_Kingdom
Audiophile

Posts: 10048
Location: Central Texas
Joined: September 24, 2006
 Well, when you ask whether it will make a difference, the answer depends on the context. Technically - for instance, if you SPICE it - there's no difference. Caps to a common ground from both the screen and cathode essentially connect the two points together for AC. However, I'm sure there are audiophiles who will say sonics are better if the screen is bypassed directly to the cathode. :)


 

Thanks! (n/t), posted on September 29, 2016 at 10:31:23
FenderLover
 mt

 

RE: screen capacitor function, posted on September 30, 2016 at 07:29:28
dave slagle
Manufacturer

Posts: 5430
Location: NYC
Joined: April 27, 2001
 As an aside... when I was simming dt667's RH84 amp, I noticed that in both V1 and V2 the designer opted to not use a screen bypass cap. In V1 he uses a 2.2K resistor from B+ and in V2 he uses a Zener.

Since I had it built in spice, I decided to see what happens if I bypass the screen and interestingly enough distortion went up slightly. This got me thinking about all the people who say that a "triode is just a pentode with internal feedback" and how this type of connection starts to slide down that slippery slope of "partial" triode wiring.

Not sure what my point is... or if I even have one but the reduced distortion from the lack of bypass is interesting.

dave

 

RE: screen capacitor function, posted on September 30, 2016 at 11:35:39
banpuku
Audiophile

Posts: 1008
Joined: January 19, 2006
 Dave,

Have you ever tried not using the bypass cap in real life (not just in the computer simulation)? If so, what were your findings?

Pat

 

RE: screen capacitor function, posted on September 30, 2016 at 12:53:31
banpuku
Audiophile

Posts: 1008
Joined: January 19, 2006
 I tried removing the cap. While there appeared to be less distortion and a lower noise floor (completely subjective), the output was significantly lowered to where it was unusable. FYI.

Pat

 

RE: screen capacitor function, posted on September 30, 2016 at 13:57:09
dave slagle
Manufacturer

Posts: 5430
Location: NYC
Joined: April 27, 2001



which cap are you talking about? In the RH84 there is no screen bypass cap.

 

RE: screen capacitor function, posted on September 30, 2016 at 13:58:32
banpuku
Audiophile

Posts: 1008
Joined: January 19, 2006
 In my schematic in the original post. It is shown as a 2.8uF cap.

 

RE: screen capacitor function, posted on September 30, 2016 at 17:26:47
Steve O
Audiophile

Posts: 12383
Location: SE MI
Joined: September 6, 2001
 FWIW, your observation is entirely predictable. Removing the bypass cap causes the pentode to approach triode behavior...much lower gain and possibly lower distortion.

 

RE: screen capacitor function, posted on September 30, 2016 at 17:29:44
banpuku
Audiophile

Posts: 1008
Joined: January 19, 2006
 Steve, I was thinking of trying a 10Y (triode) tube as the driver. Would the 10Y driver give less output, assume using the same output tube (2A3)?

Thanks, Pat

 

RE: screen capacitor function, posted on September 30, 2016 at 19:06:08
Steve O
Audiophile

Posts: 12383
Location: SE MI
Joined: September 6, 2001
 If you plan to replace the 6C6 with a 10Y you're going to have serious gain structure problems. Can't calculate right now but a 6C6/6J7 as a proper pentode will have a gain ~100 and peak output ~ 100V. 10Y will have vastly lower gain so if you don't like gain with partial triode 6C6, you'll probably be very disappointed with a 10Y.

 

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