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Old 12th February 2012, 12:42 AM   #1
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Default grid stopper and grid choke on pentode input

I've been using a pair of amps for years which use a plate loaded pentode input tube driving it's single ended output stage DC coupled. They have sounded great and I built it with quality parts but it eventually came time to try to make it sound better. One of the latter things that I did was to put grid chokes on the grids of my input pentodes. I did that because I remember a day about 30 years ago when I had connected up some gear to and amp that had an input transformer to ground in parallel to the input I was driving. The manual said to remove the input transformer (in an octal socket) if using the unbalanced input but it always sounded better with the transformer still in the socket. I never knew why and no one could tell me why it sounded better. Years latter I started to read about grid chokes on output triodes and It got me thinking again. So I bought a pair of grid chokes and set up an amp upside down on a keyboard stand at a point where I could stand and listen in the sweet spot. I soldered up the grid choke to ground and got out a clip lead so I could hot switch between the 100K input shunt and the grid choke. I could hear a slight improvement with the choke but it was so small that I would not have noticed it had I just substituted for the resistor and turned it back on. It was nothing near what I experienced 30 years back but it was still an improvement so it became permanent. Certainly not anything I would recommend on a cost basis. Now on to part two...
After that I decided for the next upgrade to change my input pentodes to a high transconductance type. That was a big improvement in sound (and more gain) but it became an RF oscillation problem where I could get it to make noises by putting my hand near the tube and sometimes it would just be a random noise maker. I had gotten away without using a grid stopper on the initial build but now I had to add one. I put in a grid stopper and a couple of ferrite beads in places and it sounds fine now. I've been puzzled though about which side of the pentode's grid stopper should the grid choke attach.
I've poured through Radio Designer's Handbook but not found a clue. Can somebody with a better understanding of theory tell me which way is correct? I have a ferrite bead on the grid choke lead too.
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Old 12th February 2012, 07:28 AM   #2
dgta is offline dgta  United States
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Grid stopper is always attached as close as possible to the tube pin, so directly to the socket with very short lead resistor.
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Old 12th February 2012, 02:03 PM   #3
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hey-Hey!!!,
Occaisionally you'll find you want to break open the glass bottle and solder that stopper right to the grid support rods I bet...

You will probably wish to put stoppers on their g2 connection as well. It doesn't usually take much. I rigged up a pentode linestage with a 0A3 shunt reg'd supply for their g2 and had it work w/o a g2 stopper. When I went to a lower impedance shunt element( ~2 Ohms v. the 0A3's ~90 ) it would oscillate. A 56 R stopper fight on the g2 pin put a stop to that misbehaviour.
cheers,
Douglas
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Old 12th February 2012, 09:32 PM   #4
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Quote:
Originally Posted by Bandersnatch View Post
hey-Hey!!!,
Occaisionally you'll find you want to break open the glass bottle and solder that stopper right to the grid support rods I bet...

You will probably wish to put stoppers on their g2 connection as well. It doesn't usually take much. I rigged up a pentode linestage with a 0A3 shunt reg'd supply for their g2 and had it work w/o a g2 stopper. When I went to a lower impedance shunt element( ~2 Ohms v. the 0A3's ~90 ) it would oscillate. A 56 R stopper fight on the g2 pin put a stop to that misbehaviour.
cheers,
Douglas
I already had a 1K series on the screen so that was never a problem. I was concerned that I might need big fat ferrite beads on the filament but it looks good so far. The screen is regulated with half of a dual triode and using a tiny wire lead VR tube (about the size of a firecracker) for reference which lights at 100V. It sounds better than the zener diode string that it replaced but it produces a pop in the speakers when it reaches it's strike voltage.
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