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Old 25th February 2008, 06:45 PM   #1
sgerus is offline sgerus  United States
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Default Using a Grid Choke On the 2A3 when the input/driver is a Pentode?

Iíve 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 thatís 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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Old 25th February 2008, 08:09 PM   #2
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Default Re: Using a Grid Choke On the 2A3 when the input/driver is a Pentode?

Quote:
Originally posted by sgerus
Iíve 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 thatís 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.
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Old 25th February 2008, 08:19 PM   #3
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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...
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Old 25th February 2008, 11:44 PM   #4
sgerus is offline sgerus  United States
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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
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Old 18th March 2013, 05:40 PM   #5
painted is online now painted  Luxembourg
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This question was asked a long time ago but I had the experience a grid choke can work excellent if it's cathode driven (from low impedance). The penthode is direct coupled to the 2A3 here.
I was curious about the origin of the shifting operating point of the penthode, as the DC-level is slowly shifting 0,15V max on bass notes. Anyone?
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