How much gain can 2 tubes and 12v get you? Answer inside!

[puretube]

tu-be or not to-be  

[slajeune]

Hi all,

Taz, thanks for pointing out the caps between tubes.  I will have to try it.  I think that an extra triode stage will definately increase output volume.  You are right, it can be pretty exciting to play with low voltage tubes!!!

JD, yes, a jfet would definately increase the output.  But, as puretube points it, I was looking for an all tube solution.  A jfet would be a good solution to drive the 12U7 into clipping, which could be an interesting sound also.

Thanks,
Stephane.

[puretube]

it`s good to know, that all is happening @ low voltages in this thread   ;

A certain Mr. Barkhausen had interesting findings about in-phase/outa-phase relationships many decades ago   :?  ; .....

It makes me smile/grin/happy, to see that there are still people,
who don`t believe in the sayings of >40 years ago, like:
"we don`t need that old hot glass-crap anymore, because that new germanium stuff does everything better"  

[Peter Snowberg]

"we don`t need that old hot glass-crap anymore, because that new germanium stuff does everything better"  

:shock:  :shock:  :shock:

Huh??? How the hell is it supposed to work if it doesn't glow?

Take care,
-Peter

PS: When I worked for the U.S. Gov't, my e-mail account name was "tubetech".

[puretube]

it did glow (for a moment), when you gave it 5% above specs..  :lol:

(grin)

[tazwolf]

stefane - the thanks is all to you for sharing this 12v tube thing with the forum.

One thing I did was to put a 470k potentiometer in place of the (330k data sheet schematic or 10k your schematic ) resistor going to the anode of the tetrode - ths allows me to vary the gain in the output stage I don't know if this is the correct way to do it but it seems to work.

Another idea I had was to take a small crummy old 8 ohm speaker and tear off the cone and connect it on to the secondary so that the transformer would still have a load (but without sound) and then to take a serial feed off the speaker as a line out kind of thing.

[tazwolf]

Sorry - correction my previous message should read potentiometer connected to the anode of the triode!

riode dode bonode bonometer - time to get some sleep.

[Travis]

I tried the RS 1k-8 transformer today vs. a generic c. 10W fenderish transformer.  The RS transformer cuts out way too much of the sound for my taste, killing pretty much all bass below 300hz (if I recall the specs correctly).  The bass is important to the perception of round PT distortion, at least for me.  With non-guitar specific speakers, the tranny matters much less.  

The 12AL8 (driven by a 12U7) responds very well to effects.  Boosters, etc, increase volume dramatically.  I drove it with a McTube and a Big Muff last night, and the results were impressive.  I need to get some of the ROG stuff perfed to see how it reacts.

I mic'd the speaker just off center, as close to the cone as possible.  It gets very good tubes/speaker about to blow sounds.  For recording, I've been running the mic into a nearby amp, giving nice, controllable feedback.  The feedback further improves the perception of volume.

I'm finishing a .04 Watt with an EA trem in front of it (nice sounding boost, always on), kind of a precursor to 6GW8 (13W) or 6BM8 (9W) 18 Watt adaptations.

:?: A couple of questions:
Is there any reason to have a bypass in this thing?
How large a signal did the designers of these 12V tubes expect to feed them?

[puretube]

How large a signal did the designers of these 12V tubes expect to feed them?

maximum pk/pk = grid bias voltage...

[slajeune]

Hi Travis,

I am really interested in your 12U7/12AL8 design.  I had some issues with my design and am currious in seeing this work!  I just got an Oscilloscope this week and I will debug the circuit when I have some free time.

Thanks,
Stephane.

[Travis]

Right now it isn't much of a design, just pots connecting everything together.  It sounds good enough for me to show to other people, however.  I have three other projects (probably about a week and a half) that I need to finish before I can pull the 12U7/12AL8 back apart and take all of my measurements for a final copy.  It was initially built to specs, but was unspectacular as such. I still need to play with som eof the cap values, hopefully opening it up a bit.

I've had a few pros play it over the last couple of weeks, and they were really impressed.  Anyone who has spent time in a recording studio can appreciate the ridiculous low volumes of this thing.  They enjoyed it more than the Filter S/H that I left out for them.

The 12V tubes are an insane amount of fun.  I like being able to touch the circuit I'm working on without fear of immediate death.  A paralyzing sleep deficit keeps me too scared to work on high voltage tube stuff very often.

I'm hoping that a couple of the 55mu triodes will give me enough gain for a 12V tube wah.  Can you roll your own Darlington tube (without anything exploding)?  Also, is there any real use for the included tube diodes?  If there is, I may work them in.

Anyway, I should have a variety shipment of the 12V's coming in sometime next week.

[puretube]

problems with Tube-Wah (TM) is not so much depending on gain, but on impedance issues...  

[Travis]

Nice, thanks.  Guess I need to buy one of your envelope filters now.  Fortunately, I've been looking for an excuse to do just that.

I was thinking of building along the lines of the GEO tube wah (or valve wah, if it makes you more comfortable ), so that I have a simple established design (inductor/Vox) and rely on the tubes for distortion behind the sweep.  I really don't have the time to do anything fancier.  

How low-gain can you go?  Would an amplification of 20 be too low?  How about 10?  I suppose that if I can get a sound I like out of it, I can always tack on a gain recovery stage.

http://www.geofex.com/Article_Folders/tube-wah/tube-wah.htm

Stephane, I'll see if I can get the 12AL8 sorted soon.  What problems did you have with yours?

[slajeune]

Hi Travis,

the issue with the 12AL8 is that the distortion just seems to harsh.  It happens when I use the triode of the 12AL8.  Using the circuit in the spec sheet, I get ugly distortion.  If I raise the grid voltage just a bit (from -.9 to say -.75), some of that harsh distortion goes away.  If I try to bring it higher, I get some sort of oscillation...

I still need to wrap my head around all of that.  Maybe I can start learning how to use an oscilloscope to see some of this, I am fairly visual at these things.

If you have any ideas, please let us know!

Thanks,
Stephane.

[tazwolf]

Slajeune: I connected up a 12U7 (according to your schematic ) on the fron of the 12AL8 and I am getting the same results as you. A harsh and "blatty" distortion as well as oscillation - although the volume is quite a bit louder like you said. Could it be that the 12U7 triodes are providing too high a signal to the 12AL8 triode? I'll keep on experimenting.

Travis: When you write down your schematic would it be possible for me too have a look at it?

Taz

[puretube]

Slajeune: great thing you got a scope now !  

From now on, you`ll be able to take bigger steps, faster and better...

[slajeune]

Hi Puretube,

yes, yeah are right, I will be able to advance faster, once I learn how to use the oscilloscope!!!

Taz, thanks for confirming my problem.  With old tubes, the possibility of having a bad tube is always there.  I need a few components to start using the oscilloscope.  I am getting these today and hopefully, I will be able to debug a lot of these things next week.

Right now, I have a very clean low volume amp with one 12u7 and one 12k5.  I did play with a distortion pedal infront (a tube distortion pedal) that goes from somewhat clean boost to a good heavy distortion and it was very sweet!  With the clean boost, the volume can get pretty loud whitout breaking up!

Cheers,
Stephane.