First Try on Tube Preamp

[s2kov]

Ok this is my first try for tube preamp, your opinion on the circuit will be highly appreciated.

Below is my operating point for 12AU7 twin triode connected in parallel:

Plate Voltage = 185V
Plate Current = 7.75mA
Grid Voltage = - 6V


Thanks in advance....

 

[s2kov]

BTW, i need your expert opinion with the circuit if i'm getting right with the values indicated and getting the right track.

Am i really on the right track?

 

[fscarpa58]

hi

How load resistance is expected for that
stage?

Federico

 

[Frank Berry]

You might get better performance by using a 180 ohm resistor and a 47ufd capacitor on each cathode rather than connecting both cathodes together.
That will eliminate the "current hogging" that can occur if the triode sections aren't matched.

 

[JojoD818]

Kabayan s2kov!

I see you are using 12AU7, the foreplay preamp of bottlehead uses 2 of those. Unfortunately, I think it is copyrighted so no DIY!

Regards,
JojoD

 

[Frank Berry]

Bottlehead is a simple common cathode stage followed by a cathode follower. Direct coupled between stages.

 

[s2kov]

Federico,

I'm using Dynaco ST70 and if i'm not mistaken input impedance is 470K.

Regards.


Andy

hi

How load resistance is expected for that
stage?

Federico

 

[fscarpa58]

Hi s2kov,

Yes, your working point is o.k.

if you want less distortion try 15k at anode
and 200 at cathode. You still have 5V peak to peak before
saturation. Gain will increase of 1.8 db. But output res will rise from 2.5k to 3.2k OHm.
Speaking about output res it seems quite high.
I will consider the use of an output buffer or of an SRPP stage.

...by using a 180 ohm resistor and a 47ufd capacitor on each cathode...

if res and caps as to be put at each cathode (advisable thing) the value of res has to be doubled and the value of caps halved to retain the original characteristic.

Bye
Federico

 

[s2kov]

Hi Federico,

Thanks for the help! Actually, operating point for each section of triode tube, Rp is 15K and Rk is 744 ohm. Since the design is connected both halves in parallel, Plate current will become 15.5mA (7.75mA + 7.75mA) thus, Rp = 7.5K and Rk = 372 ohm

Is it right?

How did you get 200 ohm for cathode resistor?

Regards,

Andy

 

[JojoD818]

Bottlehead is a simple common cathode stage followed by a cathode follower. Direct coupled between stages.

How I wish I have a schematic

 

[fdegrove]

Hi,

How I wish I have a schematic

Nothing special about it....
I don't know why it's said to sound so good, I suppose it all depends on what you're used to.

Technically speaking it's about the only topology I'd avoid using, the whole thing works like a badly balanced see-saw.

Sorry, if you want a really good preamp it'll have to be a little more than just that one.

Cheers,

 

[s2kov]

Jojo,

I just choose 12AU7 for my try on tube preamp design. That's why i'm collecting opinions from tube gurus here to let me know if indeed i'm doing my homework right.

Salamat kabayan!

Andy

Kabayan s2kov!

I see you are using 12AU7, the foreplay preamp of bottlehead uses 2 of those. Unfortunately, I think it is copyrighted so no DIY!

Regards,
JojoD

 

[JojoD818]

Hi,



Nothing special about it....
I don't know why it's said to sound so good, I suppose it all depends on what you're used to.

Technically speaking it's about the only topology I'd avoid using, the whole thing works like a badly balanced see-saw.

Sorry, if you want a really good preamp it'll have to be a little more than just that one.

Cheers,

Nice way of putting that see-saw thing Frank. I have seen writeups about that preamp so many times and keep on wondering when can I try it.

Thanks Frank!

s2kov,

You're welcome kabayan!

 

[Bryan]

I've been working on a 12B4A preamp, currently breadboarded. I've had LOTS of help from Frank and others. It is a work in progress, but this being said, it is by far better than anything i have owned to date....

I am very happy with this tube.

Bryan

 

[s2kov]

Hi s2kov,

if res and caps as to be put at each cathode (advisable thing) the value of res has to be doubled and the value of caps halved to retain the original characteristic.

Bye
Federico

Is this what you mean? Will i still get the an operating point right?....

 

[fscarpa58]

Hi s2kov

The resistor values I have suggested (ra about 15k and rk about 200) come from simulation
(I use Micro Cap 7 demo). These values allows for lower distortion.


Regarding the duplication of the resistance to assure more similar static current in case of badly matched tubes, the values are reported in the following pic.

Ciao
Federico

 

[s2kov]

Federico,

Thanks for the help! Sorry, i overlook the Rk connected in parallel and it should be 2*Rk....

Can you point me how to run the simulation in Microcap?

Many thanks....


Andy

 

[fscarpa58]

Hi s2kov

Can you point me how to run the simulation in Microcap?

have you downloaded it?
if not try at


http://www.spectrum-soft.com/demodown.shtm

bye

 

[billr]

Kabayan s2kov!

I see you are using 12AU7, the foreplay preamp of bottlehead uses 2 of those. Unfortunately, I think it is copyrighted so no DIY!

Regards,
JojoD

nonsense, this is a standard design, you cannot copyright something that has been in text books for at least 70 years. go ahead and build what you want.

 

[tubetvr]

quote:
Originally posted by JojoD818
Kabayan s2kov!

I see you are using 12AU7, the foreplay preamp of bottlehead uses 2 of those. Unfortunately, I think it is copyrighted so no DIY!

Regards,
JojoD



nonsense, this is a standard design, you cannot copyright something that has been in text books for at least 70 years. go ahead and build what you want.

Also I dont think anyone can stop you for copying a patented or copyrighted circuit for DIY purposes, what you are not allowed to do with a patented circuit is to build it and sell it . For copyrighted material you are not allowed to publish it without owners consent.

For DIY, use whatever patented or copyrighted circuit you want if you can find it!

Regards Hans