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Old 2nd January 2013, 10:43 PM   #21
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I take it you're driving it with something more than just a guitar, like the output of a high gain pedal, and that's why you need the massive headroom.
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Old 2nd January 2013, 11:22 PM   #22
m145mcc is offline m145mcc  United States
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Yeah, I was going to build the real mctube guitar pedal. Then I thought well what if I integrated a tube screamer clone into it. then I found the schematics for an orange tube head so I took the 3rd gain stage and part of the "phase inverter"(I think that's what they call it) then dropped a high voltage op-amp in-front to makeup for the first two gain stages. Not exactly design work but its not a perfect clone either. To answer your question.... yes I plan on putting a high gain metal petal in-front of my mutant preamp. I'm note sure how everything will pan out but it looks promising.
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Old 3rd January 2013, 09:40 AM   #23
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Quote:
Originally Posted by m145mcc View Post
Yeah, I was going to build the real mctube guitar pedal. Then I thought well what if I integrated a tube screamer clone into it. then I found the schematics for an orange tube head so I took the 3rd gain stage and part of the "phase inverter"(I think that's what they call it) then dropped a high voltage op-amp in-front to makeup for the first two gain stages. Not exactly design work but its not a perfect clone either. To answer your question.... yes I plan on putting a high gain metal petal in-front of my mutant preamp. I'm note sure how everything will pan out but it looks promising.
The op amp, as is, is less amplification than a single 12AX7 or similar gain stage.

You could probably increase it more without it clipping, depending on which pedal you put into it, (you don't want to clip the op amp). A Tube Screamer gives out about 1 volt peak-to-peak, and the op-amp can probably handle 70 volts peak-to-peak before clipping. Not sure how much gain the OPA453 can take and stay stable, but you could try dropping R5 by degrees down to about 1K5 for maximum gain.

Of course, for a higher gain pedal it will probably be fine as configured. In fact if your pedal is giving out really high signal, like 7V peak-peak (pretty much the most possible from a 9V battery) then you might want to keep R5 as it is and lower R6.
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Old 3rd January 2013, 06:55 PM   #24
m145mcc is offline m145mcc  United States
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Your absolutely right, I swapped the feed back resistor on the opamp for a 100k pot changed R5 to 1.2k, removed all other pots except for R21. and I'm thinking about changing the anode resistors on the tubes, but I'm not exactly sure how to go about doing that. I was skimming through that book recommendation ya sent me. From what I read your suppose to choose a val for the anode resistor then consult the chart for the current then use that information to calc the cathode resistor.... right? If this is the case then at 150 volts with an anode resistance of 47k biased at 1 volt my cathode resistance should be 15k. but for some reason I still think I'm missing something here. I'm looking forward to your response.
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Old 3rd January 2013, 07:28 PM   #25
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Click the image to open in full size.

This is the load line for 47K as 150/47K equals 3mA near as damn. Notice how you will get less swing of plate voltage as the line is steep. So 47K means less gain than 100k.

At 1V grid you get 0.7mA. 1V/0.7mA gives about 1K5, so I think you missed a decimal point somewhere.

You can play around with biasing by biasing hot (more mA, less grid voltage, so smaller cathode resistor) or cold (the opposite) to change the clipping characteristics. Basically, as the signal swings the grid voltage around it will do different things when it moves up and to the left on the load line as when it moves down and to the right. I've heard said the former is a harder clip, and the latter a softer compression, but I'm not sure this is right, as when you move toward 0V bias grid leak starts affecting biasing, which, to my ears, softens the clipping too.

A 10K trimmer on the cathodes is a good plan. Or a log pot (log is a better taper here). Just remember to turn off and bleed the caps before you poke around with a screwdriver. The second triode has a lot of volts at the cathode, and even though the first one doesn't, you could slip.

The bias at the second triode will make a bigger difference to the sound, by the way.

(Also, this load line is not so great an approximation AC effects like gain, and clipping, as you have to factor in the AC load, which includes all that comes after the anode. You can pretty easily draw an AC load line though)

Last edited by Robert Kesh; 3rd January 2013 at 07:41 PM.
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Old 3rd January 2013, 10:54 PM   #26
m145mcc is offline m145mcc  United States
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wow, your right I don't know how I did that. This is the first time my TI-89 has ever let me down. thanks a bunch.

Last edited by m145mcc; 3rd January 2013 at 11:01 PM.
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Old 3rd January 2013, 11:20 PM   #27
m145mcc is offline m145mcc  United States
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(head explodes) I had to read your response a few time before every thing sank in. its absolute genius. This poor engineering student thanks you.
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Old 4th January 2013, 04:19 AM   #28
m145mcc is offline m145mcc  United States
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Update... I'm note sure if this is a step forward or back, I may simple eq.
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Old 4th January 2013, 05:19 AM   #29
m145mcc is offline m145mcc  United States
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I added a simple tone control, but I think I will leave it out.
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Old 4th January 2013, 09:31 AM   #30
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Quote:
Originally Posted by m145mcc View Post
I added a simple tone control, but I think I will leave it out.
You've changed second triode to an AC coupled cathode follower. You'll get no voltage gain out of it. You've also missed out the grid leak resistor (which also biases the grid) and the cathode bias resistor.

Also, the high cathode voltage means you'll probably have to elevate the heater.

Last edited by Robert Kesh; 4th January 2013 at 09:33 AM.
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