Preamps

[vendettav]

Ok so, I was thinking, how dya define a preamp. if it's like a distortion pedal and you feed it into the PA it's gonna sound Sandy ... if you make a tube unit like valve caster.. it's a booster. so erm if you have a preamp what are you gonna use it with? that's a little nitro to my real question actually

now here's the question: is it possible do design a 12v tube preamp to have a clean channel and a crunch channel? not totally tubey but i want that tube overdrive/distortion, guess it could be fine by pushing the tubes with a jfet or some other type of boost. so once again how shall one go obtaining tube distortion and not having the poweramp section? im really thinking of making a preamp like that. and im doign a lot of reading but thought i'll also drop a line here. dont need exact schematics or something but like ideas (detailed may be) as on how shall one go doing it?

[newfish]

V,

I think a good plasce to start would be the 'Tube Boost / OD on 9v' thread.

Cascade multiple stages for more gain if you need.

No matter how many projects I start off *thinking* I've designed, it always comes back to what someone else has done before.

May as well short-cut to playing with an existing (proven) design - then see what takes your fancy as regards filtering / tone controls etc.

[vendettav]

Hey Ian, yeahi already have a little idea of how what to try but it's like a boost tube thingy like valve caster. what im here wondering is where or how do you get the distortion. the last stage of the tube? so like overboosting the tube gives you the overdrive? also given pedals in front of PAs sound sandy guess you'd need some sorta acb emulation for a preamp like this im talking about? (to be plugged into PA)

[defaced]

so once again how shall one go obtaining tube distortion and not having the power amp section?

Loosely speaking, there are two kinds of tube distortion people talk about with regard to guitar amps.  One is power amp distortion, the other is preamp distortion.  With a preamp, the only thing you can deliberately control is preamp distortion.  If the signal coming out of the preamp is hot enough, and you've got a power amp that you can over drive, you can distortion that power amp.  This is the case if say you had a rack preamp, rack power amp, and a guitar cab.  It'd sound just like a regular guitar amp but the big parts are put into two different boxes insdead into a single box.  Power amp distortion is usually associated with the classic rock sound, preamp distoriton is usually associated with modern high gain amps used for metal (Soldano, Mesa, Diezel, Fryette, Bogner, Framus, Engl, etc).

Now, if you want to plug your preamp into a PA and it sound like a guitar amp, you have to re-create somehow the tone shaping and distortion that take place in the power amp, speaker cab, and mic that you would have otherwise used.  This is not easy and attempts usually result in varying levels of success. 

As for the channel switching and 12v idea, yea, it's totally doable.  Running your tubes at low voltages and channel switching are completely unrelated parts of the circuit. 

[iccaros]

Ok so, I was thinking, how dya define a preamp. if it's like a distortion pedal and you feed it into the PA it's gonna sound Sandy ... if you make a tube unit like valve caster.. it's a booster. so erm if you have a preamp what are you gonna use it with? that's a little nitro to my real question actually

now here's the question: is it possible do design a 12v tube preamp to have a clean channel and a crunch channel? not totally tubey but i want that tube overdrive/distortion, guess it could be fine by pushing the tubes with a jfet or some other type of boost. so once again how shall one go obtaining tube distortion and not having the poweramp section? im really thinking of making a preamp like that. and im doign a lot of reading but thought i'll also drop a line here. dont need exact schematics or something but like ideas (detailed may be) as on how shall one go doing it?

you can use my project http://www.diystompboxes.com/smfforum/index.php?topic=91745.0

If you insert a bypass switch for the pentode side you would have a clean front end and kick in the pentode for distortion. This sound really good infront of the amp with nothing between. I have put a rebote delay before and it sounds good, just not between the amp and this pedal.

This gives the best of both worlds in my opinion, A pre amp tube like a 12ax7 overdrives in a way that can be really grainy. but a power tube like a pentode, well they are smooth when overdriven.
This with the two volume controls cleans up completely  unlike a valvecaster.  It has a lot of clean boots and when it gets dirty.. well its nice.. with the delay in front with out a true bypass it really tightens up and has a good solid metal lead tone.

I'll finish the the design, I was thinking of putting an switchable opamp for that final boost to metal land.

That tube you were talking about before would work well in this design, you could add a mosfet to make up for the missing triode in it. I'll draw it out if you need it.. 

[vendettav]

thanx iccaros! yeah i saw your design and i was thinking of trying to do that once i can try the other thing you suggested with my current tube

so yeah this makes quite the sense now. I was actually thinking of some opamp or JFET boost cause i can't really get any mosfets here... tho if everything goes well i'll order the 12AX7 (or may be AU) for your project.

so finally it's possible to make a tube distortion to appear in the preamp section and then feed it into an amp (be it a tube or SS)

ok here's another question, where would a tone stack go? I'm thinking of a dedicated eq for each "channel" like a stomp switch would take the output from the last triode to a tone stack>volume>out or bring the pentode (with the gain knob (p1?) before it) into the chain and then take it to another tone stack>volume>out... you'd get a separate EQ and volume for each channel...

[vendettav]

oh and another question, is it worth ordering few 12au7s? I am quite low on budget so i gotta be careful with what i order. so if i order 12au7 and don't like say that schematic's sound can i try and get some other sound?? (stupid question right? I jsut need someone to say I'm safe ordering those haha)

[iccaros]

thanx iccaros! yeah i saw your design and i was thinking of trying to do that once i can try the other thing you suggested with my current tube

so yeah this makes quite the sense now. I was actually thinking of some opamp or JFET boost cause i can't really get any mosfets here... tho if everything goes well i'll order the 12AX7 (or may be AU) for your project.

so finally it's possible to make a tube distortion to appear in the preamp section and then feed it into an amp (be it a tube or SS)

ok here's another question, where would a tone stack go? I'm thinking of a dedicated eq for each "channel" like a stomp switch would take the output from the last triode to a tone stack>volume>out or bring the pentode (with the gain knob (p1?) before it) into the chain and then take it to another tone stack>volume>out... you'd get a separate EQ and volume for each channel...

I was thinking about the tone stack on the airplane.  I have to play with it. The primary location is between the two triode sections but that does not help in with the pentode section.  I was thinking of three switching options, one where you have just the triodes (clean boost) with a tone stack between the triodes, then hit the switch and in places the pentode in line. Another switch takes the clean boost out of line and puts  a opamp driving the pentode with  a tone stack between the opamp and the pentode.


But that is a lot of pots.. Would need some 9mm to make it fit a case well
as for 12au7's in the valvecaster they can sound good, in mine also you just have to adjust the pull up resistor until you get about 6 volts on the anode (from a 12volt source) I used  a 1meg pot and a 220k and adjusted until I liked it and measured what the pot says added to the 220K.

for your Russian tube, the 12au7 could make a good preamp section, where you put one 12au7 and two of the russian tubes to get a push pull amp of around 1/2 watt. May be a good practice amp that you can overdrive

[vendettav]

nice info, mm so you haven't tried the tone stack between the trioe dand pentode or after the pentode? i wonder how it'd sound and if it'd need a recovering stage (could probably be a tranny stage, say i am not too keen on that if you get the overdrive from the tubes then a little recovering stage won't harm it or somehting... idk)

so AU7 better than AX7 for my situation (like you know im not into amp building) ?  

oh and about the sockets, thanx man, i'll keep that in mind, im now thinking if i shall order some stuff (enclosures and switches) from abroad, paypal is quite a hassle in armenia so i gotta think twice.. hmm also thinking if i can find time to go to this flea market they'll suerly have bunch of old tubes and sockets as well i bet

[iccaros]

12au7 will work better if you decided to just build a valvecaster.

12ax7 is a good tube and easy to find, well they both are. My design (i say that like I invented something, really my frankenstein of a research I found with Ricks pentaboost)  works with either, you get more gain with the 12au7, before someone screams at me.. read .... this is strange with the 12ax7 having more gain as a tube, but in low voltage the 12au7 is close to the 12u7 which was made for 12volts (my theory). Its more linear on the load line at low voltage than the 12ax7.  So the 12au7 is harder to keep clean in the clean boost part.

So which is better? both, but for building pedals the 12au7 is more flexible. If you can find a 5751 @ a swap meet I think that is the best tube for the valvecaster @ 30 volts..

[vendettav]

so 12u7 will be even better for 12 volt pedals??? i checked the datasheet and thought it didnt make much sense to me but the heater voltage was 12.6v which will also waive the voltage drop resistor need. but these things are dual-triodes. what shall i get for a pentode? also how about submini tubes?

[iccaros]

There are many pentodes, the 6au6 is a good one, for submini the 5840. There are a crap load of Russian pentodes

I have my mini with one 6111 and one 5840. These would require that you place a resister in parallel with the 6111 for heaters

(6.3 volts/ .3amps = 21 ohms) and it would need a  rating of (6.3v * .3ma =  1.89 watts).. or say 2 watts (4 watts to be safe)

some to look at:
EF732
6ak5
EF94
6136
6111
5840
6au6

if you run the 6au6 you can run the heaters from both off the 12volts. but you have to do it weird..

you tie pin 4 and pin 5 of the 12au7 (or 12ax7) to positive then tie pin 9 of the 12au7 to pin 3 of the 6au6 and then pin 4 to ground.

they both pull the same 300ma this way and will split the voltage more or less.

Nice compactron tube to try, 6AF11  which has two triodes and a pentode in the same bottle.

[vendettav]

nice thanx again for the info man!
so probably 6AF11 is what i'll look for. will be more convenient for a pedal use

[iccaros]

I can tell you this weekend, when I get home from travel, I connect one up and make sure.

[vendettav]

that's cool, definitely waiting for your update  (if it's not a trouble for you of course )