Grid current and direct coupling
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 27th July 2003, 10:16 PM #1 jeff mai   diyAudio Member     Join Date: Aug 2002 Location: Melbourne Grid current and direct coupling I'm curious to learn about what happens when direct coupled circuits conduct grid current. I've got lots of questions. Where is the grid current supplied from in a direct coupled stage? Is it supplied from B+ through the anode load of the previous stage? What is different if the anode load of the preceding stage is a choke? And what if the choke also provides the bias for the following stage like a DRD or Monkey topology? Thanks in advance! Jeff
dhaen
diyAudio Moderator Emeritus

Join Date: Jun 2002
Location: U.K.
Jeff,

Quote:
 Is it supplied from B+ through the anode load of the previous stage?
Must be.

Quote:
 What is different if the anode load of the preceding stage is a choke?
If direct coupled: the same, but the output swing can be above +B due to the reactance, so it'll come from that stored energy.

Quote:
 And what if the choke also provides the bias for the following stage like a DRD or Monkey topology?
It should ve the same as if the grid was connected from a driver transformer or grid-choke.

...........I think............

So I think that A2 must be possible with monkey, as long as the source z is low enough - was that the question?

Cheers,

jeff mai
diyAudio Member

Join Date: Aug 2002
Location: Melbourne
Quote:
 So I think that A2 must be possible with monkey, as long as the source z is low enough - was that the question?
Well, it was the question...sort of.

Actually, my main question is this: cathode followers aside, is direct coupling a good idea for intentional A2 operation?

Going back to your statement, what forms the source z for the grid in a monkey configuration?

Also, because the choke in a monkey sets the bias of the output stage, wouldn't you get some sort of feedback if you drove the tube into grid current?

Thanks,

Jeff

fdegrove
diyAudio Senior Member

Join Date: Aug 2002
Location: Belgium
Hi,

Quote:
 is direct coupling a good idea for intentional A2 operation?
No.

Cheers,
__________________
Frank

 28th July 2003, 08:00 AM #5 dhaen   diyAudio Moderator Emeritus     Join Date: Jun 2002 Location: U.K. Hardly a trivial question...... Jeff, I'm in the area of speculation here, so please indulge me I think that the source Z of a monkey driver is set mainly by the Rp of the driver. The DC source resistance being the source z in series with the winding resistance. Aha, and the output stages cathode components will affect as well- now I'm beginning to see where you're coming from. It needs someone to try it, I think. Frank, you may well be right. Please convince us..... Cheers,
TG
diyAudio Member

Join Date: May 2002
Location: Kiev, Ukraine
Hello,

Quote:
 is direct coupling a good idea for intentional A2 operation?
I'd say, capacitor coupling is a bad idea for A2...

Good luck!

 28th July 2003, 08:57 AM #7 Sch3mat1c   diyAudio Member     Join Date: Jan 2003 Location: Milwaukee, WI The thing with a monkey is the grid current will cause the grid voltage to fall, while at the same time the cathode rises due to increased tube current. But the grid current is nullified because whatever goes into the grid comes out of the cathode, so bias depends upon cathode current, which should go up in this situation. Overall I think it would be a pretty stable situation to apply current to. Put some local NFB on the driver to drop its Zo and you should be good to go... Tim (shouldn't be typing this tired) __________________ Seven Transistor Labs, LLC Projects and Resources / Electronic Design and Consultation
dhaen
diyAudio Moderator Emeritus

Join Date: Jun 2002
Location: U.K.
Quote:
 Originally posted by TG Hello, I'd say, capacitor coupling is a bad idea for A2... Good luck!
Hhha

 28th July 2003, 09:25 AM #9 Pjotr diyAudio Member     Join Date: Nov 2002 Location: Netherlands Hi, Maybe this can shed some light on it: http://melhuish.org/audio/46amp.htm Some real schematic helps to discuss. Cheers
jeff mai
diyAudio Member

Join Date: Aug 2002
Location: Melbourne
Quote:
 Originally posted by Sch3mat1c The thing with a monkey is the grid current will cause the grid voltage to fall, while at the same time the cathode rises due to increased tube current.
If the grid voltage falls and the cathode voltage rises, the bias has shifted. Not good! I don't think you'd want any grid current to occur in a monkey or DRD amp because of this.

Let's simplify things by just choke loading and direct coupling the driver stage to the output, using a separate B+ supply. Does grid current do evil things to the driver stage's bias since that grid current must be supplied through the driver's plate choke?

Jeff

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