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

[Alpha579]

Steph, how do you go about rebiasing the output tube?

[Peter Snowberg]

That looks GREAT Steph!

I'm really looking forward to hearing the new clip, but even more to getting some of these tubes and building a Sopht!

Take care,
-Peter

[puretube]

I need some of these tubes!

Now there's also the four stage 12V tube overdrive pedal to be made here.

I wonder if Sovtek/NewSensor/EH will ever explore this area?

Take care,
-Peter

I heard that! (back from a little Turkey-vacation...)

If s.o. got a stock of tens of thousands of those tubes, it might be interesting to start thinking about such pedals...

If there is s.o. to have such stock, it is probably the company
you mentioned...

for me, (feeling addressed...) when I started to re-vive my old tube-passion in the eighties, my intention was to do anything possible FX-wise with only the tube-type which I was sure would gonna survive for the next 30 to 50  years, and that is the 12AX7.

Although I`m using some less common HQ-tubes (some of them still/again in current production) in a couple of 19" rack studio projects, I don`t wanna bother too much with "exotic" types, which do not offer the features of the better known types, and bear the danger of becoming obsolete or over-expensive within a short time period.

[slajeune]

Hi All,

I'm learning so much that it isn't funny .  Anyways, played a bit with the design today and found one thing that was anoying me.  Bascially, the amp had too much 'bite'.  It was very sensitive to pick attack and it would really be anoying how sensitive it was.  Someone on the AX84 did build a sopht amp and came to the same conclusion.  After a bit of searching, I saw that the grid voltage for the first section of the triode was a bit too high (i.e. very close to 0v).  From the loadlines in the spec sheets, I knew this had to be below 0v.  These 12v tubes seem a bit harder to bias (or maybe I don't know how to bias a tube, which is a very good possibility also).  Anyways, adding a cathode resistor didn't help in briging down the voltage on the grid.  So, I increased the grid resistor (R2).  This droped the voltage of the grid.  I settled on 470k and it seemed like a good choice (grid now reads -0.2v).  The bite is now gone and we are left with a better sounding amp.  Please note that increasing the grid resistor will roll off some highs.  I might try adding a 'brightness' cap in case someone needs to have a 'brighter' input.

Cheers,
Steph.

[Alpha579]

wouldnt that lower the gain somewhat?

[Boofhead]

I've created new sound files which a voice like a different amp.  I used the original sound files posted the other day.  I started this poll because I believe the rig can have a large effect


http://www.diystompboxes.com/sboxforum/viewtopic.php?t=22392

[slajeune]

Hi Alpha,

Steph, how do you go about rebiasing the output tube?

There are very good biasing information at:

http://www.aikenamps.com/Why70percent.html
http://www.aikenamps.com/Biasing.html
http://www.aikenamps.com/CrossoverNotchBiasing.html

Basically, everything in http://www.aikenamps.com/TechInfo_2.htm is a good read!

Cheers,
Steph.

[slajeune]

Hi Peter,

I think that may be a different one actually. I remember independant 22 ohm cathode resistors for some reason. The splitting transformer looks the same though.

Is this the link that you were looking for:

http://www.jogis-roehrenbude.de/Leserbriefe/ECC86-Audion-Diskussion/ECC86-Audion.htm

I have been playing with the 12AL8, but, I'm not sure I totally like it.  I will go back to a 12K5 design.  The 12AL8 has the luxury of having an extra triode, but, we don't have any loadline information making it hard to find the 'sweet spot'.

Cheers,
Steph.

[Peter Snowberg]

Thanks. That's got to be the site . I was mixing up two schematics in my head I think.

Take care,
-Peter

[Peter Snowberg]

I was just looking at the Hamond 125s.

http://www.hammondmfg.com/125.htm

It looks like using terminals 1&6 or 2&6 with a 4 ohm load would be a nice match for a pair of 12K5s or 12AL8s.

$29 for a 125A at Angela (3 watt transformer)
$20 for a 125C at Angela (8 watt transformer)

Take care,
-Peter

[aron]

I have been playing with the 12AL8, but, I'm not sure I totally like it. I will go back to a 12K5 design. The 12AL8 has the luxury of having an extra triode, but, we don't have any loadline information making it hard to find the 'sweet spot'.

Rats. I was ready to order. This project has got me interested.

First of all, what transformer should I get? Secondly if 1K is possibly a better match, would that tiny Radio shack transformer work? It's 1K:8 ohms I think.

I'd like to build this amp. How loud is it exactly?

[slajeune]

Hi Aron,

well, 40mw isn't a lot but, when looking straight into the speaker, it's loud enough (sorry, I can't really compare to anything...).  No feedback, though.  I will go back to the 12K5 design and see if I can make it a bit better sounding distortion wise.  I am learning, so, I am not always sure about what I do...  Having said this, buy a 12K5 and an 12AL8 and try both!!!  Both are rated for 800ohm load, so, the radio shack 1k:8 ohm is allright.

Cheers,
Steph.

[aron]

Steph,

How big is the Hammond transformer??? The Radio Shack one is so tiny. I would imagine bigger iron would be better? Not sure here.

[slajeune]

Hi Aron,

I just recently got a few 1.2k ct : 8 ohm transformers and they are really small (made for transistors).  The hammond is about 5 times bigger.

Cheers,
Stephane.

[aron]

Do you think the size makes any difference? Or would make a difference?

[slajeune]

Hi Aron,

hard to say.  This is my first shot at making a tube amp!  If I do remember somewhere, they did mention that the size of the output transformer does have an impact on sound.  But, more importantly, it's the frequency range of the OT.  Even if it's HUGE, if it doesn't have a good frequency range, it might suck.

Cheers,
Stephane.

[tazwolf]

I've breadboarded my first attempt at an amp on breadboard using just the 12Al8S and a hammond output transformer. I used the schematic from the datasheet. i must say it sounds very good i drive it with a Vox valvetone or a supreaux.

It is a bit low in volume - I wonder if it would be an idea to to remove the transformer and iuse a solid state amplifier stage to bring the output to 1-2w maybe using an LM386 or one of those TDA chips.

would this be viable?

It's amazing seieng those little tubes glow.

/Taz

[slajeune]

Hi Tazwolf,

glad you tried the 12AL8!  I didn't have much success with this tube, but, I am probably doing something wrong.  Do you have another triode infront of the 12AL8 or are you just using the 12AL8?  If you are only using the 12AL8, this could be one of the reasons why it is so quiet.  Typical low volume single ended tube amps have 2 triode stages followed by a pentode or tetrode stage.

You might want to try adding a single triode stage prior to the 12AL8 triode stage.

As for using the output of the 12AL8 to drive some solid state, this is possible as this was the initial entent behind these tubes.  They were meant to drive transistors.  Having said this, I am unsure how to go about setting it up that way.  Some potention issues:

1- Impedance mismatch between the tetrode section and the LM386 section
2- Voltage coming out of the tetrode might be to high for the LM386 (could be easily fixed with a voltage divider)

Issue one could be a show stopper.

Cheers,
Stephane.

[tazwolf]

I'm using the triode thats already in the 12AL8.
I have also got some 12U7 so I'll try putting one in front as per your schematic.
One thing I noticed though your capacitors between the tubes are smaller than the ones in the example schematic from the 12AL8 datasheet (0.1uF).

I suspected that the impedance mismatch would be an issue.
Oh well I'll keep experimenting - the smell of warm 12v tubes is rather addictive.

[Lonestarjohnny]

Stephane, why don't you take like the Fetzer, Jfet's my friend, exactly like wireing in a half of a preamp tube, just remember it'll take 2 Jfet's to equal  1 dual stage preamp tube, and this would give you a nice place to park a good tone stack and it should drive your tube the same as the jfet preamp on a PV drive's the tubed power section, all you have to remember with tube's is keeping the signal in phase with the output or if not sometimes all you'll get is oscillation. :idea:  :idea:  :idea:
JD