low voltage tube amp -> new name, sound clip posted

[slajeune]

Hi All,

just a quick update.  I have a name for the low voltage tube amp, the Sopht amp.  Furthermore, I have posted a small sound clip of the amp at the following site:

http://www.geocities.com/slajeunesse/low-volume.html

Sorry for the less than ideal recording and the dreadfull playing....

Cheers,
Steph.

[slajeune]

Seems like I already went over my bandwidth limit.  Try this link:

http://sopht.250free.com/low-volume.html

Cheers,
Steph.

[Kleber AG]

Looks cool! And sounds nice too!
Only 12V that's really interesting....... for plates and heaters...
Thanks for sharing it!  

Kleber AG

[Peter Snowberg]

Very cool!

I looked around a bit for some data on the 12CX6 and 12K5 and found that both tubes are able to output in the 35 to 40mA range when properly matched to their output load. In the case of the 12CX6, the typical plate current is 3mA which makes for about 37mW at 12.6V. I found one datasheet (can't find it now of course) that showed output power vs. plate load for one of these 12V tubes. A change of 50 ohms in the load can make an enormous change in output capability. 50 ohms too high or too low when the load is 800 ohms is a big fraction.

http://www.mif.pg.gda.pl/homepages/frank/sheets/137/1/12K5.pdf
http://www.mif.pg.gda.pl/homepages/frank/sheets/049/1/12CX6.pdf

If you have not done so, you might want to think about trying a big cap (100uF) for C5 in your 12K5 design.

Again, Great work!

Take care,
-Peter

[slajeune]

Hi Peter,

thanks for the hint (C5), I will try it tonight!  BTW, the 12K5 is biased so it has 8ma on the plate.  Therefore, this is roughly 100mw!!!

Cheers,
Steph.

[Doug H]

Hey that sounds real nice! Nice open tone. I like it!

Doug

[Peter Snowberg]

I looked at the 12K5 data again and it says that with 8mA being drawn from the plate and 85mA being drawn from the space-charge grid, the output power (Pout) is 35mW. It appears that there is always some plate current so you can't just multiply the dissipation by the voltage to get power.

I've never played with a space-charge grid tube before so this territory is a little new to me.

It looks like you have done something that is fairly rare in DIY these days which is to present a *NEW* tube distortion character.

I'm also curious if you have experimented with larger grid resistors (R7) for the 12K5? The datasheet says you can go up to 2.2M.

I'm really looking forward to building one of these 12V amps! Too Cool!

Take care,
-Peter

[slajeune]

Hi Peter,

currently, I'm using cathode biasing.  I have to try 'grid leak biasing' as these little tubes apparently shine with grid leak biasing.  It was just simpler for me to bias them using cathode resistors.

Building these little tube amps is fun!  Even if you don't like their sound, they make for a great conversation piece!!!!

Cheers,
Steph.

[Gary]

Very nice work, Stephane!  Nice sound!

[Doug H]

Hi Peter,

currently, I'm using cathode biasing.  I have to try 'grid leak biasing' as these little tubes apparently shine with grid leak biasing.  It was just simpler for me to bias them using cathode resistors.

You will probably find the distortion sounds pretty ugly with grid-leak biasing. Grid-leak biasing is very sensitive to input level. I tried it on the pentode-pre of my amp and it sounded pretty horrible when I drove it with a pedal.

Here's a good tutorial on grid-leak biasing:

http://www.tpub.com/content/neets/14178/css/14178_45.htm


Your amp sounds really good, BTW!

Doug

[slajeune]

Thanks for all the kind words!!!  What is fun with this amp is you can easily breadboard and tweak it to your hearts content SAFELY!!!

Thanks for the info on the grid leak biasing.  I would try it only on the tetrode section and not on the preamp side of it.

BTW, the problem with sound clips is it's very hard to capture all of the sound from the amp.  I am not the best one at creating sound clips.  The amp sounds better live than on the clip.

Thanks,
Steph.

[Alpha579]

Do you think it would make a difference if you had a valve rectified supply? Give it a bit of squash?

[slajeune]

Hi Alex,

that could be interesting but keep in mind the following:

- For a tube rectifier to work, you still need to have the heaters on, therefore, you can't rectify the current that feeds the heaters.  You would need a dual supply for this to work, 12V (or 6V depending on the rectifier used) for the heaters and whatever power supply you need to rectify.
- Most tube rectifiers that I know are for high voltages (i.e. > 120V).  Not sure how this would work with smaller voltages (my guess is that it would work, but, I am not sure).

If I were to try this, I would get a beefy 12V wallwart (i.e. 2 Amps) and a 24V AC wallwart.  The 12V would supply voltage to your tube heaters and you could then rectify the 24V to get that 'sag'.  could be interesting to investigate!

Cheers,
Steph.

[Peter Snowberg]

The 5AR4 or GZ34 have the lowest forward voltage drop of the common HV regulators and it's about 17 volts. I haven't seen any 12V rectifier tubes.

You can always emulate sag with a couple resistors and a capacitor.

Take care,
-Peter

[slajeune]

Hi Peter,

the 12DL8 has a power tetrode and 2 "diodes" in 1 tube.  This might be an interesting start.  Although, I am not sure about the max voltage for the "diode" section.

Cheers,
Steph.