DHT as cathode follower?

[vetmed]

Can this be done? Or too complicated to pursue any further?



Regards
     Robert Lees

[Paul Birkeland]

Hello Robert,

Yes, you can run a DHT as a cathode follower.  What do you have in mind?

-PB

[vetmed]

Hello Paul
   I am very impressed with the Crack design, I've modded mine with Speedball update to run Russian 6n30 with 6bx7 on cathode follower duty.
   Next experiment was modding an original SEX amp to run VT25/10 using suitable output iron and a separate outboard driver stage running 01As. While the output stage is noisier than I would like this combo has a very special sound.
   Nearly all of my listening these days is with headphones, and given the above the idea of an OTL headphone design based on Crack chassis using DHT as both driver and cathode follower is very appealing. I would be willing to purchase another Crack and Speedball just to modify if it seems feasible. Perhaps something like 01As as driver and 71A as cathode follower?
   Certainly from a technical stand point I'd like to know how to set up a DHT as cathode follower even if only as an intellectual exercise.
        Regards
           Robert Lees

[Paul Birkeland]

Hello again Robert,

While you could certainly style an amp after the Crack topology, you would want to go a little further in terms of giving yourself chassis space and a transformer that is more in line with this idea.

As you have noticed, DHT's can be noisy if you do not pay careful attention to the filaments.  For something like the Crack, this would mean four separate isolated and ideally regulated DC supplies! 

As far as headphones go, you would want the tube used as the cathode follower to have very high gm, and this will be the big challenge with such a design (high gm will lead to low tube output impedance, resulting in the coupling cap/headphone combo to follow to be easily driven).  I would say 5000+ umohs would be a nice number to shoot for (A 71A is less than half of that).  A triode strapped 46 would probably get you decent drive into 600 ohm cans.

An easier approach might be to build a crack with a pair of DHT drivers, then keep the 6BX7 for the cathode follower.  A pair of 1LE3's should do the job nicely!

If you wanted to build an all DHT preamp with a DHT cathode follower, these restrictions can be loosened greatly. 

-PB

[Paul Joppa]

PB pretty much covered the basics. I think you'd want a 300B or possibly a 2A3 for the output - that gets you to 5000 uMhos.

As you may have seen we are working diligently on a 300B preamp, which will necessarily have two independent DC heater supplies. That might be the better starting point for what you have in mind.

Meanwhile, just to add confusion  :^)  consider the idea of a shunt-regulated Crack. Shunt regulation works some real magic on cathode followers, that's why the Foreplay now has such regulation even in stock form. Unfortunately, cathode follower shunt regulation requires the regulator current to equal or exceed the follower current - parafeed has a much more moderate requirement. So a pair of monoblock Cracks could be a reasonable approach.

Well, "reasonable" in the context of this community's shared lunacy! Geez - a PAIR of huge amps, just so you can have your headphones in stereo?

[Paul Birkeland]

I build a differential Crack about four years ago, I've been seriously considering harvesting it to build a shunt regulated crack instead... 

[vetmed]

My sincere thanks for taking the time to answer. I think I may wait for the new preamp. I also thought that using the indirectly heated tube as follower for directly heated driver is a great idea, allows one to get the "flavor" of DHTs without too much bother. All relative of course! OTOH I have a big stash of 46s...



Robert Lees