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Old 16th February 2009, 04:26 AM   #1
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Lightbulb Just an Idea?

Ok, Well I decided to throw together a small transistor circuit, and upon finishing it, I figured maybe I can make a guitar pedal with it... then I decided to throw in some tube warmth with none other then..... A TUBE! a 12AX7 actually. After doing that I threw a few transformers on and wallah!

Now ofcourse I only did this on a schematic, havn't build anything yet. And why you ask? Well simply put - I have no idea what i'm doing

I figured there's bound to be someone on this forum who knows a thing or two about tubes and circuit's n' such. so here it is, fully open to ALL critism and idea's. Infact, I'm VERY cetain this schematic wont even function. so I'm open to any idea's that may help getting this to work, what resistance's and capacitance's and all that!

Also: Let me know what ya think of it anyway? i'm sure this has already been done 10,000 times before and in way better versions - but this one I made from scratch so i'm proud of its inability to function

Thanks,
Weston

P.S. I know! I know! 12Vdc B+, I'm still uncertain about that, I heard somewhere that the 12AX7 can run on a 12V plate. ofcourse set me straight if this is false info. (the plate is really only just to add warmth not amp (I guess?)).
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Old 16th February 2009, 05:24 AM   #2
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OK....yep it won't work..First off No a 12AX7 will not function with 12 VDC..tubes are high voltage devices(Read DANGEROUSLY HIGH voltages).
Second, your power supply is not even a DC supply...it is currently configured as a 'pulsing' DC supply. Your signal off the full-wave-bridge has no Capacitor to 'store' a charge when the signal waveform swings from zero to the top of the peak (12v).
Then again, transformers are rated as an RMS value. RMS means Root Mean Squared (Math term!).
If your math does not extend deep into Algebra you are going to have a tough time dechiphering this stuff.
Perhaps you heard wrong of the 12AX7 tube using 12VDC on the plate, rather the filiment voltage can be configured in either 6.3 V or 12.6 V.
Read up on this stuff first.....your schematic is way out in left field...WE can coach you on alot of stuff but you need to learn some basics first.
I would suggest some basic electronics to start you off, some power supply study, transistor operation, OP amps,Algebra for electronics & then you can turn toward towards tubes.
_____________________________________________Rick. ......
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Old 16th February 2009, 05:51 AM   #3
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Hi Weston,

The things that occur immediately to me are:

* The 12AX7 is not a particularly good choice if you just want warmth and not especially gain. A better choice might be a 6CG7 or 6FQ7.

* The B+ is about 1/10th of what it should be for a tube to operate properly.

* The B+ supply needs smoothing, not just rectification. That means at least a C-R-C ("pi") network.

* The tube won't do anything without a plate load, such as a resistor, between the plate (and signal take-off) and B+.

* The tube needs a grid leak resistor to ground - say 100k.

* Q1 won't operate like that because the tube grid needs to be at ground potential, so the NPN transistor won't have a voltage across it, nor would it get any current even if the grid were at a positive voltage because the grid cannot source current for the transistor. (I'm not sure what Q1 is supposed to do anyway?)

* C2 capacitance depends on the input impedance of whatever the tube is supposed to be driving. 2.2uF is a high value and suggests that the impedance of the next stage is low - probably too low to be driven from the tube's plate.

* C3 and R4 serve no purpose, as far as I can see, and they shouldn't be in there.

Just a few observations.

I suggest that you acquaint yourself with the principles of tube amplification. A good book would be the best approach, IMHO, such as Morgan Jones' excellent Valve Amplifiers. If you don't want to go to the trouble of getting a book straight off, you might get some useful information from Max Robinson's "Fun with Tubes" website.
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Old 16th February 2009, 06:04 AM   #4
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I don't really want to point out the level of fail in this design

Maybe look at / build something similar to Fred's "real mctube". He had a good way of explaining things

http://www.dogstar.dantimax.dk/tubestuf/mctube.htm
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