latest version of the Sopht amp with a bit of theory

[slajeune]

Hi All,

I am learning stuff everyday on tubes and it's a blast!  I am still learning how to use the oscilloscope, so, I can't say for sure if my design is as clean as I want, but, it sounds very good to my ears.

Here is the link to the lastest design (Sopht 12K5 - slight return part 2):

http://www.sopht.ca/index.php?s=26

And here is my first "theory" document.  All comments / corrections are greatly appreciated.

http://www.sopht.ca/index.php?s=35

Thanks,
Stephane.

[Alpha579]

Wow, very cool! the theory bit is great! i was just wondering, what kind of sound you get from the updated 12k5 design if you stick a treble booster before it?

BTW, have you built a finished sopht amp? Coz if so, can we see some pics  

[Alpha579]

Are you going to do a theory page for the 12k5?  

[slajeune]

Hi Alpha,

The sound right now is very clean and very articulate!  I dontt think that at a gain of 20 (max), I will be able to overdrive the power tube, so, I am going for a very clean sound.  With a clean sound, it's easier to put a nice distortion pedal infront of the amp.  Actually, when I tried the sopht amp with a bk butler tube overdrive, it was awesome!  I don't have a trebble booster and therefore, I can't try that setup.  I am planning on adding a simple tone stack (BMP style tone stack).

Unfortunately, I am still at the design stage and therefore, I don't have a complete build yet.  I can post pictures of my setup though .

I will do a theory section about the 12k5 as soon as I sort it all out.

Cheers,
Stephane.

[Ben N]

Very cool--a tube miniamp is now definitely on my to do list.  

Some questions: How is current draw--do you think it would be possible to actually run this on batteries, say either a small lantern battery or a rechargeable power tool power pack with a step-up, for a reasonable time per battery/charge?

Also, have you given any thought to either a push-pull output or a pentode input stage?

Anyway, time to start keeping an eye out for a suitable enclosure...

Ben

[william]

How do you use the tube chart to figure gain?  I've got myself 2 12U7's and a 12SK5 as well.  I've been thinking about  an a/b push pull design for use with two 12SK5's but want to start small first.  I prefer a little bit of distortion in my amps as opposed to clean (actually, alot of distortion) and want to know how to tweak the 12U7's for my desired results.

[tazwolf]

I have prototyped slajuenes latest version of the 12U7 circuit but only the dual triode preamp section.

I haven't actually listened to it yet because I got hold of a frequency generator progam and an oscilloscope program for the PC today so I should I'd try it out first.

When I view a sine wave directly (at any frequency) from the generator and compare it with the output from the preamp on the dual scope setting.
I can see that the preamp is nicely rounding off the peaks of the sine wave but only on the top of the waveform the bottom still looks the same
- Am I correct in that both tops and bottom of the wave forms should be smoothed.

BTW the smoothed top peaks become progressivly more squarish as I increase the output level of the sig-gen.

Any idea what is wrong here? I will continue debugging tomorrow evening and give the pre a listen as well but if any body can give me a clue I'd be grateful as I am new to this.

It's great being able to see stuff with the scope - even if it is only software and works through the PC soundcard.

/Taz[/url]

[tazwolf]

BTW : the oscillocope software and signal generator is freeware u can get it at

www.dazyweblabs.com

loads of other freeware stuff there - sorry if this link has been posted before.

/Taz

[slajeune]

Hi All,

Ben, the major current draw is from the heaters (.15 for the 12U7 and .45 for the 12K5).  That amounts to .6A just for the heaters.  The rest is pretty negligeable.  A 1A wallwart is more than enough.  Actually, we could probably get by by using a 800ma.  One could use a lead acid battery.  Some are rated pretty high (i.e. close to 3A!!!).

I actually have thought about a push pull using 12K5.  I got a beefy power supply for that (12V 2.4A).  Because of it being a push pull, the minimum current draw is 2X.15 + 2x.45 = 1.2A.  But, I haven't finnished my single ended explorations yet.  Might actually go back to the 12CX6 before moving on to a push pull design.  I have thought about using either a 12K5 or a 12CX6 as a preamp.  Although, I would probably try to put it after a 12U7 to try and push the 12K5 harder.

William, you don't use the charts to calculate gain.  You use several math equations + specs from the spec sheet.  Here is a nice link to calculate the gain: http://www.rru.com/~meo/Guitar/Tubes/t2.pl .  If you are using a bypassed cathode capacitor, the cathode resistance is considered null.  For example, a 12U7 with a cathode bypass cap with a plate load resistor of 100k, a plate resistance of 12500 and the transconductance of roughly 1600 would give a gain of 17.6.  Altough, I'm not sure that I am actually getting a gain of 17.6.....

The only distortion I can see you generating with one 12U7 and one 12K5 is preamp distortion.  You can bias the 12U7 to be closer to saturation (i.e. grid closer to 0v, say -0.25v).  To do this, simply decrease the cathode resistor until the grid voltage is close to -0.25v (or higher, the higher you go, the more distortion you'll get).  Let me know how it works out!

Cheers,
Steph.

[william]

Yea, I'm actually planning on using two 12U7's.  That should give me 4 gain stages, and if I still don't get enough, I'll resort to diodes.  But I'd prefer using just the tubes.  Thanks for the link, I'll check it out right now.

[slajeune]

Hi Taz,

cool, your one step ahead of me (I'm planning on putting my scope to work this week!!!).  Basically, what you are seeing is the tube clipping the signal.  What you are seeing is the tube reach saturation (top part of the sine wave).  Therefore, this setup would generate preamp distortion.  BTW, what is your output level (i.e.  100mv or bigger?!?!).  A typical guitar generates a 100mv signal.

Thanks!
Stephane.

[slajeune]

Hi All,

I had a bit of time to play around with the amp again tonight.  My setup stayed pretty much the same as what I have posted as my slight return part 2 but I added a second bypass cap on the cathode of the second triode.  This produced a nice smooth distortion.  Here is a link to the clip:

http://www.sopht.ca/sounds/sopht_12k5_v3.mp3

Ben, I know that you are looking for WAY much distortion than this, but, it's a start.  If you look at really big gain monsters (soldanos, boogies, bogners, etc), they tend to generate a lot of preamp distortion, so, with 2 x 12U7, you could make a serious attempt a that!  I might try something similar, but for now, this sounds nice to me!

Enjoy,
Stephane.

[tazwolf]

Slajeune: I can vary the  output as my signal generator is PC software so I can vary thefreq from the program and the output level from the PC soundcard volume control.

So by starting at very low outout vol (from the PC) and increasing it I can see the tube rounding off the sinewave and then going into clipping as I continue to increase the output level but I still don't understand why only the top part of the waveform (the positive going part) is clipped and not the tops of the negative half of the waveform - have I biased the tube incorrectly?

/Taz

[slajeune]

Hi Taz,

Well, in typical class A setup, both parts of the sine wave should be cutoff (i.e. top and bottom should be cutoff).  So, in that regards, I think that yes, the stage seems to be mis-biased.  If anybody has a better explanation (or if I am mistaken), then, please jump in!!!

BTW, what are the readings for the grid voltage for that triode stage?  Calculate the grid voltage by putting the negative probe on the cathode and the positive probe on the grid (for stage 1, cathode = pin 3 and grid = pin 2).

Cheers,
Stephane.

[puretube]

triodes clip for 2 reasons:
1.: when grid is very negative, tube cuts off = no conduction = plate voltage stays at max. value, can`t get higher;
2.: when grid is zero, tube is at max. conduction = plate voltage is at min. value, can`t get lower;

if biased right in the middle of these grid voltages, "clipping" occurs at both halves - however a little different on both sides.

(roughly said...)

:idea:

[Alpha579]

that sound clip sounds great!

[tazwolf]

stephane and puretube - thanks - I'll try out your suggestions when I get home after work.
Alpha - i don't agree with you - i don't think the sound clip sounds great - .........it sounds absolutely fantastic.

[Peter Snowberg]

.........it sounds absolutely fantastic.

I'll second that!

Take care,
-Peter

[Ben N]

That sounds great!

Steph, I actually wasn't looking so much for a lot of distortion (that may have been someone else), just a way to get power tube saturation--that is why I suggested a pentode preamp.  I guess I'm thinking along the lines of Dr. Z amps that make that gorgeous sound with next to no preamp distortion at all, like the Rt. 66 and Z-28.  It's all about getting big clean gain at the input, so maybe a cascode would also do it.  But what you have there does sound terrific.

Ben

[slajeune]

Hi Ben,

you're right, William was looking for max gain.  I am also interested in power tube saturation as oposed to preamp distortion.  I will have some time this week to hookup my oscilloscope to it to do a bit more debugging.

Cheers,
Stephane.