Using a Grid Choke On the 2A3 when the input/driver is a Pentode?

sgerus · 25th February 2008, 06:45 PM

I’ve been reading about the benefits of using a grid choke in place of the grid leak resistor on the power tube (2A3 or 300B). Some place I read that’s not a good idea if the input/driver is a pentode?

Has anyone used a grid choke on the power tube when the input driver is a pentode?
Is this a bad idea with a pentode (wired as a pentode).

Here is what I have:
The pentode is an EF37A (6j7)
RL = 60K
Ra = 2.5M
Coupling cap = .47uf
Grid leak resistor (2A3) = 470K

Miles Prower · 25th February 2008, 08:09 PM

Quote:

Originally posted by sgerus
I’ve been reading about the benefits of using a grid choke in place of the grid leak resistor on the power tube (2A3 or 300B).

Yep, takes care of the grid current problem, as these DHTs tend to pull grid current even before the Vgk actually goes positive. It also takes care of the overdrive blocking problem by providing a low resistance path to ground.

Quote:

Some place I read that’s not a good idea if the input/driver is a pentode?

Has anyone used a grid choke on the power tube when the input driver is a pentode?
Is this a bad idea with a pentode (wired as a pentode).

Here is what I have:
The pentode is an EF37A (6j7)
RL = 60K
Ra = 2.5M
Coupling cap = .47uf
Grid leak resistor (2A3) = 470K

It's not a good idea at all. Take a look at the specs for the 6J7: rp= 1.0M; gm= 1.9mA/V. You're gonna need a gigundous choke to avoid loading that pentode down to the point where it won't have any gain at all. We're talking kilohenries (hundreds) here. That thing would be as big as a substation phase correction choke, and would have an absolutely horrible high frequency behaviour. You'd be better off running it into something else, like a White cathode follower or a MOSFET.

Tom Bavis · 25th February 2008, 08:19 PM

Your 60K load on the pentode is the source impedance. A 150H choke is 18K at 20 Hz... so you're down 12 dB, and -3 dB at 60 Hz... what you need is a lower impedance driver. If your driver was a 12BY7 you MIGHT stand a chance...

sgerus · 25th February 2008, 11:44 PM

Tom / Niles,

Thanks for the replys

The grid choke I had in mind was a
MQ 16NI 4800H
or a
MQ 16M6 1000H

XL=6.28*20K(hz)*4800=602K
or
XL=6.28*20K(hz)*1000=125K

Do either on of these work with my EF37A?
Then the real question: is it worth $100-$150

25th February 2008, 06:45 PM	#1
sgerus diyAudio Member Join Date: Nov 2005 Location: Milwaukee, WI	Using a Grid Choke On the 2A3 when the input/driver is a Pentode? I’ve been reading about the benefits of using a grid choke in place of the grid leak resistor on the power tube (2A3 or 300B). Some place I read that’s not a good idea if the input/driver is a pentode? Has anyone used a grid choke on the power tube when the input driver is a pentode? Is this a bad idea with a pentode (wired as a pentode). Here is what I have: The pentode is an EF37A (6j7) RL = 60K Ra = 2.5M Coupling cap = .47uf Grid leak resistor (2A3) = 470K

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25th February 2008, 08:19 PM	#3
Tom Bavis diyAudio Member Join Date: Jun 2002 Location: Macedon NY	Your 60K load on the pentode is the source impedance. A 150H choke is 18K at 20 Hz... so you're down 12 dB, and -3 dB at 60 Hz... what you need is a lower impedance driver. If your driver was a 12BY7 you MIGHT stand a chance...

25th February 2008, 11:44 PM	#4
sgerus diyAudio Member Join Date: Nov 2005 Location: Milwaukee, WI	Tom / Niles, Thanks for the replys The grid choke I had in mind was a MQ 16NI 4800H or a MQ 16M6 1000H XL=6.2820K(hz)4800=602K or XL=6.2820K(hz)1000=125K Do either on of these work with my EF37A? Then the real question: is it worth $100-$150

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