Return to breadboarding, Ruby Tuby questions

[Luke51411]

I breadboarded the ruby tuby from Sopht amps http://www.sophtamps.ca/schematics/ruby_tuby_1.jpeg
I only have 12au7 tube. It sounds pretty good. I'm trying to wrap my mind around how it works. The 100k pot controls volume to the 386 from the second stage correct? This is different than the typical output pot to ground. Is there a certain point where if turned too high the 386 will clip instead of just clipping from the tube stage? I guess it's a bit hazy how the volume control setup works with the 386. I do occasionally get some ticking, I take it this is from the 386? I used a jrc as suggested. The 500k pot controls the gain of the tl072 correct? If I change the 220k resistors from the tube stages to a lower value the tube will have less gain correct? Can I have different values for each stage such as a high gain first stage and a lower gain second stage? Will it change the character of the sound or just affect available gain? And lastly (I think) could I put in a charge pump multiplier and run the tube at say 33-40 volts without harming anything? I know you have to run the heaters at 12v.

[Luke51411]

I'm hoping to get a 12AX7 to try out in this setup in the next few weeks though the 12AU doesn't sound bad. Does any one have any experience with this build?

[GibsonGM]

To my ear, a 12AU7 sounds better at lower plate voltages than an AX7.  Actually, to most ppl, LOL.  AX are known to be harsh when starved.

100K controls vol. to 386, yup.      The vol. is actually quite the same as any other vol. control, if you look closely.  One end to signal, other end grounded, wiper is the output.  It's just 'on the front end' instead of at the output.   It's really acting like a gain control....you'll still get hiss in the speaker since the 386 doesn't get turned down.

Which leads to:
I ASSUME if you input a high enough level to the 386, it will clip (just not sure what the output of the tube is).    I would bet, tho, that those 2 stages can clip a 386, LOL.   Some people like the sound of it.   One way to know if the 386 is distorting your signal is to connect the output of the vol. pot to your amp instead of the 386, so you hear just the tube!   Use the vol. pot tho, OK?   You'd want to start off low in volume to be sure your amp does ok with it.  Don't want to blow your speaker, etc!!

The 500K is the input section's gain, yes.   
"Tube stage" resistors:  The 220K plate resistors, you mean?    They set the operating point (bias) of the tube...GENERALLY speaking going lower will be 'cooler' and going up more may make things "hotter", but we're really not at the proper voltages to hear the difference, IMO....you might need to settle for "sounds ok at this value" and "sounds like JUNK when I change it!"           Go ahead and play with the plate resistor values...say down to 47K, up to 470K, just for grins (no real reason I'm giving those values, voltages aren't high enough here to get you in trouble...later on, they may be, ha ha).

Yes, you can run this on a higher voltage IF everything is rated for said voltage.  Be careful.

Go read up on triode gain stages!  HERE:  http://www.valvewizard.co.uk/

Merlin is on here, and is very knowledgeable...buy his book if you still can get it!   It will open a lot of doors for you. You have great questions, and some reading in and amongst his site will answer most of them for you! 

[GibsonGM]

PS - haven't built this particular setup, but I bet it will sound nice.  Not sure how the AX will sound, but Sopht don't play around, so I'm sure it's nice.  Have a go, well worth it just to tweak it!! 

[Luke51411]

Thanks, yeah it sounds pretty good! I'm trying to apply what I learned in building a valvecaster to this experience. Speaking to the voltage ratings of the parts other than the tube, can I just supply the ICs and heaters with 12 and the plates at higher? Would any values need to be changed? Would giving just the tube 30-40v affect the ICs? As long as the caps and resistors are rated high enough.

[PRR]

> Is there a certain point where if turned too high the 386 will clip instead of just clipping from the tube stage?

Yes.

The output from the tube is at-best 5V p-p (and probably less).

Try pot full-up. 5V p-p into the '386.

'386 as shown has gain of 20. So 5V in makes 100V out.

This '386 only has 12V supply, can make at-most 10V p-p out.

Working backward: 10V p-p out of '386 is 0.5V into '386 and 0.5V out of a tube which might make 5V p-p.

So pot on full, the '386 clips while the tube is at 1/10th of its clipping.

To be fully tube-clipped, the pot has to be turned down to 10%. "1"(!) on a linear pot, or around "4" on an Audio taper pot.

This is over-simplified. The tube runs a 220K plate load but has a 100K audio load, so it's straining. The '386 has 50K input, further strain. You can probably turn up above 10% and still be tube-clipped.

Since the tube's own ~~5V p-p is more than most guitar amps need to puke their guts, and the ~~100K impedance will drive most g-amp inputs, we might wonder if the '386 is really needed.

But why ask why? Take the wire off the pot wiper, and tack an output jack in place of the '386 input, go right to your amp. Does that work? (It will be not-as-strong, so turn-up the amp.)

> run the tube at say 33-40 volts

Distortion is reduced and clipping happens later (relative to input level). That is NOT what you want.

If you really object to the sound of low-voltage tube, don't piddle around, go to 300V. And cut the heck out of the signal before you go into a guitar amp!

[GibsonGM]

If you really object to the sound of low-voltage tube, don't piddle around, go to 300V. And cut the heck out of the signal before you go into a guitar amp!

Absolutely.  The purpose for these circuits being run at 12V ("starved plate") is to DECREASE the amount of clean headroom the tube has to work with...so you can swing a lot, lot less before you clip.      You may have a GREAT, fat and creamy sound running at 300V - NOT overdriven, distorted, etc.   It takes many stages cascading into one another before you achieve Marshall greatness.

Look into load lines, and you will see exactly how this works, and also (and more importantly!) what the plate resistors do.   

As for a higher voltage affecting other parts of circuits: well, as paul pointed out, there is going to be a 'max input level' for an IC, where past that the thing just craps out and sounds totally disgusting.    Plus, some voltage level will really be absolute MAX, where beyond that fries the chip (see data sheet).       Amplification devices may use high(er) voltages to do their thing, but at the output that blocking cap...blocks the DC.  So the output is limited (again, Paul showed that as ~5 p-p here).     
-- IOW, The audio rides on the DC level, which is then blocked, so your IC should never see it.   
What changes is the level of the audio (AC) signal.  And that is why he talked about 'cut the heck out of the signal before going into an amp'!   It might be 60VAC at the output if you run at 300V!  

[Luke51411]

All great info! I understand the limitations of running starved plate and this was probably never going to be a project that goes beyond the breadboard but it is a great learning experience. As for the clipping, I'm getting plenty running at 12v and curious how a few more V will effect the sound. I'm working my way up to a full on tube amp build but it will probably be a while before I'm comfortable with that. Thanks for helping me along in the learning process.

[GibsonGM]

Good deal, Luke.    Feel free to experiment with this one, change resistances and note the effects, change coupling caps....a bit higher voltage and it should clean up (but might sound 'fatter' or 'better', too, you never know).   Well worth playing around.   It seems like rocket science when it's new, but it will all come together.

Learn about load lines   Most important tube thing you can do!