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a new project: building a modular tube vco
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zipzap



Joined: Nov 22, 2005
Posts: 559
Location: germany
Audio files: 24

PostPosted: Fri Mar 03, 2006 9:15 am    Post subject: a new project: building a modular tube vco Reply with quote  Mark this post and the followings unread

Hello electromusicfriends
I post u a shematic i discovered in a book. Its a parametric eq working with half a ecc83 (12ax7) tube. That means you can build 2 of those out of one tube and keep things modular as we like it. (paralel/seriell/feedback2-1/1-2/ stuff like that)
The parts (in the book) are
R1 10m
R2,3 100k
P1,2 100k
C1,4 22nf
C2,3 470p-5n
C5 10uf/350v
First of all: for r2 wich is the feedback resistor we want to use a pot. The book gives a warning that this thing can oscillate at some settings. Exellent!
Next we want cv controll. P1 (i think it determines the freq?) is a variable resistor that can be changed by a ldr or fet. here we go.
This calls for some experimentation and tweaking. Great thing is that this is a low part project so its gonna be fun!
Well, ready to go exept for one thing: a power suply is needed. The one in the posing will work for 2 ecc83. thats 4 eq stages or 2 stages and a booster. Each tube needs 12v*0,15A for the heating and here comes my problem- about 230-270v dc for operation. the good thing: only 4ma (or even less) are needed for two tubes.
As you can see in the shematic 2 transformers are used. the first one provides 15v and it must be strong enough to be transfered to 12vdc/300ma and to power the second transformer thats connected in opposite direction giving 230v (in europe). If you want to try this do not try to transform to more than what you put into the first transformer- the book says so!!!!!!!!!!!
Here come my questions:
230v can be dangerous. at 4ma it should not be dangerous.
How can i protect this ps that it can give no more than say 5ma, even if its shortcut or touched by my unlucky hand? A fuse will do i guess, but it takes some time to blow. can i just limit the current with a resistor?
I still need fuses. where should i place them?
What are good values for r and c in the shematic?
Hope someone knows, as soon as this runns i´ll let you know how it sounds.
cheers zip


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toppobrillo



Joined: Dec 10, 2005
Posts: 766
Location: oakland, ca
G2 patch files: 1

PostPosted: Fri Mar 03, 2006 9:11 pm    Post subject: Reply with quote  Mark this post and the followings unread

hey i like tubes theyre cool, but honestly i think some of this new tube stuff is just hype- and i think that by doing things the 'wrong' or i guess the 'not careful' way with solid state, you can get really similar results as heard in sound samples of certain modular tube products that just really aren't that impressive to me.

[this is just my humble personal opinion, of course, this is coming from 10+ years of making sound almost exclusively on SS gear, i may very well hear something someday that will change my mind about that- hopefully coming off of my workbench Wink ]

anyways i digress sorry umm have you thought of running the tube at 'low voltage' maybe, like 48v or so? the PAIA preamps do this, and they sound perty good. this voltage is much less intimidating to me, and i have components to tame it!

the way that paia gets it 48v+ from 12v+ is via a voltage multiplier circuit using diodes and caps. same type of thing found in some condendser mic power supplies.

as far as voltage ripple [your R and C values] there are convenient formulas to find that stuff. i'll see if i can dig one up for ya. only 4mA current draw? hmm. that wouldn't require too much, but, if you are going to build a power supply, build a capable one i say!

josh
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zipzap



Joined: Nov 22, 2005
Posts: 559
Location: germany
Audio files: 24

PostPosted: Sat Mar 04, 2006 2:57 am    Post subject: Reply with quote  Mark this post and the followings unread

I agree! Thats true what you say about that hype thing! There are many products now that use the tube for promotion only, and suddenly people are willing to pay 400 bucks for a noisebox... On the other hand, if i compare a tube guitar amp with something solidstate, well its like talking about beethoven or the beatles (and i love them both).
I don´t know about the paia preamp, but many of those designs run a tube at low voltage to get more distortion, while the main circuit and amplification is solid state or ic. With a supply of 250v the 12ax7 can work in its ideal range. Of course it can still be overdriven, and of course you could include a voltage divider to get that tubeclipping sound at lower levels (jimy did this back then - rip).
All you need to power 1 12ax7 (thats two triodes) is 1,2ma. A small transformer for it costs about nothing, so why not go for it.
If the ps is limited to say 5 or 6 ma its enough for 2-4 tubes and should not be dangerous. What i don´t know is how to technically limit the current coming from the ps to make it safe. I touched a broken power cable once and i don´t want to repeat that experience...
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toppobrillo



Joined: Dec 10, 2005
Posts: 766
Location: oakland, ca
G2 patch files: 1

PostPosted: Sat Mar 04, 2006 4:09 am    Post subject: Reply with quote  Mark this post and the followings unread

hey speaking of tubeamps,, not to mention a ton of synth, fx and other schematics, dig this if you haven't already

http://www.freeinfosociety.com


this is what john simonton had to say about tubesound and 'starving' tubes

http://www.paia.com/tubesnd.htm#tubework


yes a resistor limits the current [and dissipates it as heat] check this for an 'active' current limiting with MOSFET, acts faster than a fuse!

http://www.geocities.com/tjacodesign/300vsup/300vsup.html

josh
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