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Old 5th May 2010, 12:55 PM   #1
youngb4 is offline youngb4  United States
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Default Blending 12ax7 parallel gain stages

Hey all,

I have a quick question: I'm in the process of designing a preamp for my parallel 6v6 el84 SE guitar amp project and I was curious if it would be relatively easy to blend between two different voiced 12ax7 gain stages in parallel? I would only be using one 12ax7 and my initial thought was to tie the clockwise and counterclockwise pins of a pot. to the grids of the 12ax7 and attach the wiper to the input of the amp, thus i could vary the signal going to each of the triodes. would I still connect the plates in parallel though and use a single plate resistor? I am also planning on using different cathode resistors and bypass caps for each of the triodes (to voice them differently). Could anyone think of a better way to do this, or would this work out okay?

Thanks for your help in advance!
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Old 9th May 2010, 10:43 PM   #2
youngb4 is offline youngb4  United States
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Does anyone have any input? Please?? I've attached a schematic to hopefully spark some interest, the only thing I'm a little worried about is the fact that the grid resistor (68k) will be far away from the grids of the 12ax7, but if I plan on using shielded wire will this be a problem? Will a 100k lin pot. give enough attenuation so a difference can be heard between the two stages?

Thanks for the help in advance.
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Old 9th May 2010, 11:02 PM   #3
llwhtt is offline llwhtt  United States
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I thinking since there is no current flow in the grid circuits the pot won't change anything, you are basically adding resistance to the 68K. Need some current flow to do anything. Now if you use a dual pot and wired them reverse from each other and ground the ends it might work.

Craig
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Old 9th May 2010, 11:11 PM   #4
youngb4 is offline youngb4  United States
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Thats what I was just actually thinking! Thanks!
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Old 10th May 2010, 11:57 AM   #5
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You can place 1-2megohms on each triode grid.

This is actually a good idea. It will work, and I'd love to hear what you think.
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Old 10th May 2010, 12:01 PM   #6
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Damn, I type faster than I think!
With a 100k pot you need lower resistances. But why not use a 1meg pot?
As for the grid stoppers, the 68kohms, place one resistor on each grid, right on the tube socket. btw it's ok to experiment with those grid resistors, I prefer a value about half the norm, and you may find you like something entirely different.
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Old 10th May 2010, 02:57 PM   #7
Cassiel is offline Cassiel  Libya
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Two different voiced 12ax7 gain stages is a good idea, the rest is not. Two inputs, each one going to a different grid and forget about the pot. Try grid leak in one and a bypassed resistor with the other. Do a Google search for grid leak biased 12ax7, plenty schematics out there.
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Old 10th May 2010, 09:56 PM   #8
youngb4 is offline youngb4  United States
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SemperFi-while doodling around with my schematic I actually changed it to a dual ganged 1meg linear pot. I just didn't repost it! Glad you're interested in this though, I'll be sure to let everyone know once I get a prototype up and running!

Stalker, thanks for chiming in, your advice is always appreciated. Could you elaborate a little more on why you think the dual pot is not a good idea? (Just curious) Also, when I googled grid leak bias, all that seemed to come up was people wanting to convert that method to cathode bias but I will def. research it more. About the two inputs, I was originally going to do this, but I'm pretty set on having the two stages being blendable now and would really like to make it work!
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Old 11th May 2010, 02:03 PM   #9
Cassiel is offline Cassiel  Libya
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It's OK with me, you definitely want something different.
I still think it's not a great idea - a waste of time. I don't want to be a killjoy though. I also believe young people must learn from their own mistakes so go ahead and build it, hey I might be wrong after all.
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Old 11th May 2010, 02:17 PM   #10
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Stalker has a good point. You might end up with more work than pleasure. But it's nice to see I'm not the only one who likes to try weird stuff. The day this gets serious I'm buying a boat.
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