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Old 14th May 2008, 01:21 AM   #1
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Default Seperating Preamp from Amp in Integrated Amps

Just like the title suggests.......I am looking to seperate the pre amp section from the amp section of a Knight KN 3050(Integrated tube "pa"/mic amp). Though I have not been able to get the schematic for the kn3050, I do have the schematic for the kn3035, and they appear to be VERY similar after pulling the bottom of the chassis on the 3050 and doing some quick comparisons. Here is the insides for those who are interested.....

Click the image to open in full size.

My intuition is that I can simply cut the signal wire coming OUT from the "master" volume and route it to an "output" rca/phono jack. That then will go out to my active crossover(Peavey ECM), get "processed", and then come back to the Knight though a similar "input" jack(again rca/phono) that would be connected to the "other side" of the before mentioned "cut" wire from the master volume control returning the "processed" signal to the amp section. Here is a pic of the "master" volume control pot in question.

Click the image to open in full size.

I believe that I have determined which wire is input and which is output....the YELLOW one appears to come from the tone control, and the GREEN wire goes to a 12ax7(my amp input). The Green wire is the one I plan to "cut".

Now if this works as I think it should, I would also like to be able to add a secondary volume control for the the amp section(my other amps all have one, and it makes tuning much easier as my crossover has no level controls) I have another "master" volume control(500k trim pot from another old Knight Int amp) from parts unit that should work just fine, but I wanted to get some input before I start cutting wires.

I assume that I can add the "secondary volume control" in the same manner as the original "Master volume control"(same wiring technique), though there is one thing that I am unfamiliar with, which is the ceramic cap that bridges the ground and the output section of the trim pot. Do I need to repeat this for the secondary volume control for the amp?

Thanks for any and all help or suggestions.....FWIW....I already know Im crazy, but if you feel the need......let it fly!!!!

Nate
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Old 14th May 2008, 04:23 AM   #2
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If you cut the green wire, you will need to install a 'grid leak' resistor to replace the pot, unless there is one already there.

If you replace the pot with another pot for a secondary volume control (which begs the question: why not leave the existing pot in place and use that as a secondary volume control?), then you probably don't need to add a 'grid leak' resistor because the replacement pot will serve as a 'grid leak'.

However, I would add a 'grid leak' resistor anyway, because it helps guard against nasty noises if there is anything wrong with the pot's slider connection.
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Old 14th May 2008, 05:08 AM   #3
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Thanks for the reply. The "existing" pot will continue to be the "master" volume control for the whole system. I wanted another pot for just the amp section as that will be running just the midrange portion of a three way active crossover setup, and being able to attenuate just the amp section would be a great advantage for tuning.

I'm fairly new to "working" on tubes, so "grid leak" is still a bit foreign to me. Where exactly do I put that in? Is that what is already installed on the existing pot between the green wire and the black wire?

Will I need another one on the secondary pot for the amplifer section?

Thanks again for your help, sorry if my wording is confusing......
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Old 14th May 2008, 09:10 AM   #4
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Quote:
I'm fairly new to "working" on tubes, so "grid leak" is still a bit foreign to me. Where exactly do I put that in? Is that what is already installed on the existing pot between the green wire and the black wire?
A grid leak resistor is required in a normal (common-cathode) stage between the control grid and ground. Its value must be less than the maximum value recommended for the tube in question, usually around 1 Meg for voltage amplifiers and 100k or less for many power tubes. There is usually no current drawn by the grid but it needs to be grounded by a resistor high enough in value not to load the previous stage too much.

That thing installed between the green and black wires is a capacitor. I'm not sure why it's there, but its effect would be to roll off the high frequencies by an increasing amount as the volume control is wound up. Maybe that has to do with PA use, I don't know

Quote:
Will I need another one on the secondary pot for the amplifer section?
I don't understand what that means, sorry.

I suggest you read a bit of background information on tubes, it will help you get your mind around them. The following websites have good, straightforward explanatons: Aiken Amps and Max Robinson. If you're really interested, you could buy Morgan Jones's very useful (and readable) Valve Amplifiers, available on line from several booksellers including Amazon.com.
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Old 14th May 2008, 05:18 PM   #5
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Thank for the reading suggestions.......will definitely follow up.

What I was referring to in......

Quote:
Will I need another one on the secondary pot for the amplifer section?
Was simply if I would need ANOTHER grid leak resistor (GLR) for the second pot. From what I can tell I will not.

I was pretty sure that the "thing" on the existing pot was a capacitor, but once you got me thinking of resistors that I was unfamiliar with, then I though that it might have been a GLR.

Well I guess that I have some reading to do.....

Thanks for your help......

Nate
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Old 17th May 2008, 12:03 AM   #6
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Default IT WORKS!!!!

Well me being the hard headed stubborn SOB that I am.......went ahead and did the mod that I had planned out. And though I am very grateful to moth for the suggestions on the GLR, it does not seem to be needed, or it was already in the circuit somewhere else.

I wired it up as I described, and even added that secondary pot to attenuate the tube amp. For those who are interested I have linked a few pics here.......gotta admit.....I'm a little proud. And though it might not be everyone's cup-o-tea.....it sure makes me grin..... though maybe not my neighbors so much.......

anyway, enjoy!!!!!(I certainly do).......

circuit mod....yeah.....the soldering is a little rough.....dont have my good iron with me at the moment......

Click the image to open in full size.

back of unit mod.....simple input/output.....

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front of unit mod....anyone got a matching knob for me?????

Click the image to open in full size.

finally the beast in action with the rest of the system.....full details in my other post "looking for schematics".....pardon the cheap Ibanez......my custom "axe" is still in production.....

Click the image to open in full size.
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