CCCP SUBTUBE: my last project

[orson56]

This is my last project: CCCP SUBTUBE, an all soviet subminiature tube OTL headphone amplifier. Finished today and works great at the first switch on.

It sounds really good at the first listen: powerfull with my AKG 50ohm and my Sennheiser 300ohm, only a really small problem of background motor boating due to the onboard switching PSU, maybe a ground/shield problem, touching the little ferrite core with a finger the problem disappear.

The circuit is composed by an SRPP with 6N28B-V double triode at the input and with two 6S31B-R triode in White Cathode Follower mode. The supply is a micro switching power supply from Taylor Electronics 1363 HVPS (originally used for nixie tubes) that is perfect for this amp (12V input 150/250V at 50/60ma output) so... no iron!!

I tested this circuit with B2Spice, using Eagle to draw the PCB (this is a test PCB the last PCB design comes in the next time after two little corrections) with a great help from Tube Cad (great soft! when a pentode version?).

I really like the russian tubes, when used in the right mode they sound great and are very cheap. I love them! In all my project I use them and there are so many types that you can experiment for all your life...

my next projects: an OTL headphone amp with 6S19P and an SE headphone amp with 6P30B-V power subminiature pentode.

ciao

[tomchr]

Your last project? As in no more projects ever? Hopefully you meant your latest project.

Anyway. Really nice work. I love those little tubes. Neat trick with the switchmode supply. I would appreciate it if you'd post a schematic.

Motor-boating, eh? That's a low-frequency instability. Could also be caused by a cold solder joint in the supply. What frequency is it? I bet it's a bad connection in the supply or its connections to the board. I doubt it's a ground/shielding problem.

~Tom

[tricomp]

Hi,

I was thinking about using the miniature-tubes myself in a pre-amp project but I was thrown off the idea because someone on a forum claimed that they are extremely microphonic.
Whay is your experience in this field?
ARE they more microphonic than ordinary tubes? Of course you may also find 9-pin types that are not all good.
Out of 4 NOS 6AN8A I used in a stereo-amp I built recently, only 2 were useable. The other 2 were highly microphonic, one even developing some ultrasonic, but still noticable 'whine' after running for an hour or so. Sound emitting from the bulb, NOT the speakers!

Oh, and...

CCCP is NOT CCCP when you choose to write in Latin characters. Then it will be SSSR, the acronym for 'Sojus Sowjetskich Sozialistitscheskich Respublik'

rgds,

/tri-comp

[orson56]

Originally Posted by tomchr

Your last project? As in no more projects ever? Hopefully you meant your latest project.

Anyway. Really nice work. I love those little tubes. Neat trick with the switchmode supply. I would appreciate it if you'd post a schematic.

Motor-boating, eh? That's a low-frequency instability. Could also be caused by a cold solder joint in the supply. What frequency is it? I bet it's a bad connection in the supply or its connections to the board. I doubt it's a ground/shielding problem.

~Tom

sorry about my english... I mean "this is one of my latest project"

attached the schematic.

"motor boating" problem solved! with a little copper foil over the top of the ferrite core and ground connected the boating disappear. Now it's perfectly silent. And dissipate some hot. Nice.

The sound is great, and after some running will be better.

Now it's running at 150V but I'will try to run it at about 200V. The HVPS is voltage regolable with only one resistance. This HVPS is a nice and cheap circuit and with a little trick it can give from 70V to 200V in output.

in next post I'll put some measure.

any advices are welcome!

Carlo

[orson56]

Originally Posted by tricomp

Hi,

I was thinking about using the miniature-tubes myself in a pre-amp project but I was thrown off the idea because someone on a forum claimed that they are extremely microphonic.
Whay is your experience in this field?
ARE they more microphonic than ordinary tubes? Of course you may also find 9-pin types that are not all good.
Out of 4 NOS 6AN8A I used in a stereo-amp I built recently, only 2 were useable. The other 2 were highly microphonic, one even developing some ultrasonic, but still noticable 'whine' after running for an hour or so. Sound emitting from the bulb, NOT the speakers!

Oh, and...

CCCP is NOT CCCP when you choose to write in Latin characters. Then it will be SSSR, the acronym for 'Sojus Sowjetskich Sozialistitscheskich Respublik'

rgds,

/tri-comp

CCCP -> USSR i know, my wife speak perfectly russian language, here in italy was URSS, but also CCCP. Some years ago in italy there was a rock group called CCCP ecc. ecc.

the tube I have mounted don't have any microphonicity, maybe due to the low gain of the input stage (total amp gain 12 dB). But if you read the tube characteristics you can see that the mechanical noise is very low. I knock with my nail over the tube and there isn't any noise.

Carlo

[MaXwellHouse]

nice work! and 12v to 200v converter is very interesting, can you explain something about it (my english is not perfect too)

[orson56]

Originally Posted by MaXwellHouse

nice work! and 12v to 200v converter is very interesting, can you explain something about it (my english is not perfect too)

I've found it at Tayloredge - Welcome! , it's a real tiny High Voltage Switching Supply, max input 16V max output 200V at 55mA. You can find on Taylor site the pdf datasheet and other stuff. Buyable on ebay.
Maybe in the future I'll project a powerfull switching power supply for tubes by myself. Many other projects on net.

ciao

[piano3]

I am curious about the appearance of these 6S31B-R tubes; I do see any getter of any kind in the photograph.