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Old 22nd April 2009, 01:36 AM   #1
Minion is offline Minion  Canada
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Default Low Voltage Tube Solidstate Guitar preamp design ...

Hi, I am trying to come up with a pretty simple Tube guitar preamp design ... I"m not good with tubes (not even good with solid state) but I came up with a design borrowing parts from other designs with some other ideas thrown in ....

I was wondering if you guys would take a quick look and tell me if if would work ....

Click the image to open in full size.

The first stage is a basic Non-inverting opamp which is intended to give a signal boost to help overdrive the Tube stage ....

The next stage is a 12ax7 Dual triode , it runs off of a 65v Plate voltage and 12 for the heaters ... it has a Pot between triodes to controll the signal going to the second triode ....

The next stage is just a simple Non-Inverting buffer with a bit of gain , it is to give a low inpedance signal to the next Inverting stage ...

The next stage is a Inverting Active 3 band tone controll , the purpose is well, to controll the tone ....

I know it is probably a pretty crappy design but will is work ??


Thanx a Lot guys/gals ....

Chris
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Old 22nd April 2009, 01:54 AM   #2
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Try to experiment: move the 1'st opamp on place of the 2'nd one, with additional diodes in deedback (2 in series in one direction, and the 3'rd one in another direction in parallel with them). Such a way you may get more "tubey" overdrive.

As the result, you will have a tube preamp and a diode overdrive that gives nice logarythmic asymmetrical curves.
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Old 22nd April 2009, 02:59 AM   #3
Minion is offline Minion  Canada
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Hi Wavebourn ... Luv yer work... Thanx for the Tips...

Most of the Guitar amps I have built have been solid state and I usually use Diodes as clippers (symetrical and asymetrical) so I was thinging about adding some clipping doides (Red LED"s are My Favorite) ...

But overall does the design look like it will work ?? at least put out sound ??

I was going to put it in front of a Bridged 150W TDA7293 Power amp , I built an amp recently which this design is based which sounds awesome , accept It was a PAIA Kit and a 15 band EQ and a single TDA7293 (they sound good in guitar amps as they actually sound good when clipped and bit) ...


Thanx a Lot...
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Old 22nd April 2009, 03:19 AM   #4
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You are welcome.
Try both ways and listen. I did not check tubes' working points, though... But I believe that your buffer will start saturating before tubes even approach their voltage limit.
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Old 22nd April 2009, 03:17 PM   #5
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Minion
I would think that the AC signal on the plate of the second tube section would be 20+ volts P t P and that is too much for the op-amp to handle. Perhaps you don't need the second op-amp at all. By the way, can you point me to the values on the tone controls you have used? I cant quite view the drawing well enough.
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Old 23rd April 2009, 12:31 AM   #6
Minion is offline Minion  Canada
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I don"t know what the output signal voltage from the second tube stage will be but the design I borrowed the Tube stage from has an opamp buffer after the tube stage so I think it should work ...

As for the tone controll section here is a bigger picture ...

Click the image to open in full size.

Cheers

PS: here is the schematic I coppied the tube stage from with some minor modifications....

http://www.paia.com/prodimages/siabsch.pdf
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Old 23rd April 2009, 12:59 AM   #7
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Quote:
Originally posted by Minion
I don"t know what the output signal voltage from the second tube stage will be but the design I borrowed the Tube stage from has an opamp buffer after the tube stage so I think it should work
You should be OK. The cathode resistors on the tube are unbypassed so the gain won't be so high.
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Old 23rd April 2009, 02:13 AM   #8
Minion is offline Minion  Canada
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Hi, Thanx , Would I get more gain if I put a capacitor in paralell with the cathode resistor ?? What Value for how much gain ?? Just out of curiosity ...

Cheers
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Old 23rd April 2009, 02:54 AM   #9
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Quote:
Originally posted by Minion
Hi, Thanx , Would I get more gain if I put a capacitor in paralell with the cathode resistor ?? What Value for how much gain ?? Just out of curiosity
Well, if you put one there you'll probably overload one of the opamp stages as firechief suggested. You can put a 20-25uF there if you want to try. If you do your gain will be close to 100 for that triode.
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Old 23rd April 2009, 04:38 AM   #10
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While it is not completely low voltage the Hughes and Kettner Tubeman schematic might be worth looking at for some ideas.
It was discussed on this thread (with links to schematics).
H&K Tubeman Circuit Questions (added schematics!)
Cheers,
Ian
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