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Constant Current Source for Dynaco ST-35

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Posted on December 12, 2014 at 11:40:16
altecboy
Audiophile

Posts: 404
Location: Northern NJ
Joined: May 29, 2004
 I keep moving along with my scratch built Dynaco St-35. I added Dave Gillespie's EFB modification for the output tubes. I regulated the driver tube B+. Now I'm looking to add a ccs to the splitter.

Has anyone had experience with this? I note that it is done in El Cheapo.
thanks.

 

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RE: Constant Current Source for Dynaco ST-35, posted on December 12, 2014 at 12:57:58
dhieber
Audiophile

Posts: 50
Location: Illinois
Joined: November 17, 2000
 The El Cheapo uses a long-tail pair as a phase splitter with a CCS in the tail, while the ST-35 uses a cathodyne splitter. I don't see how you can use a CCS in the cathodyne circuit.

 

You can't add a CCS to a concertina PS., posted on December 12, 2014 at 14:38:37
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005
 You would have to put a CCS load at both ends and they would fight over which one would do the job..
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: Constant Current Source for Dynaco ST-35, posted on December 12, 2014 at 15:38:21
Caucasian Blackplate
Industry Professional

Posts: 8313
Location: Seattle
Joined: June 18, 2004
 You can replace the 330K resistor with a CCS, which will raise the gain and lower the distortion of that stage.

IIRC, making the plate and cathode load resistors of the cathodyne equal values (27K), while at the same time removing that local feedback network may be worth listening to.

I would temporarily put in a 50K linear pot in place of the 27K feedback resistor and tweak that by ear/scope. I'd imagine reduced feedback may sound a bit better as well.

 

RE: Constant Current Source for Dynaco ST-35, posted on December 12, 2014 at 22:05:57
JKT
Audiophile

Posts: 612
Location: Midwest
Joined: November 26, 2002
 If you do as the previous poster suggests and change to a "long tailed pair" phase splitter you could also replace the plate load resistors with a CT plate choke (Lundahl makes a good one). You would then set the reg. B+ to the plate voltage you want (plate chokes and transformers "pull down" from the voltage they are fed). I use this arrangement in my power amps (input stage CT plate choke loaded direct coupled to driver stage with IT). It works really well.

You could also use CCS plate loads instead of resistors. This would require adding resistors from the plates to ground (somewhere in the 20-30K range; pick a value that will give you the plate voltage you want with ~5-10mA). The excess current in the plate load is shunted across this resistor. Ohm's Law then gives you the plate voltage. Also calculate the heat dissipation and size accordingly. By adjusting the plate load CCS you can change the plate voltage. While adjusting the cathode current source you can set the bias (if you change the bias you will need to readjust the plate loads). This is a nice arrangement if you want to experiment with different operating points. The input stage of my phono pre uses this configuration. So does the input stage of Kevin Carter's ST-70 kit over on the K&K Audio forum. He also sells CCS kits which are very handy for this.


"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln

 

RE: Constant Current Source for Dynaco ST-35, posted on December 13, 2014 at 12:28:01
Palustris
Audiophile

Posts: 2408
Location: Cape Cod
Joined: September 12, 2008
 I built the "El Cheapo" topology into a Dynaco SCA 35 using the chassis and iron. I have a separate phono stage so I removed the phono board and replaced it with a piece of sheet aluminum to have a spot for an inductor and a negative supply. I had Front Panel Express build an aluminum plate to go where the other PC board was and point to point wired in the "El Cheapo" circuit using a 6N1P-EV for the front end.

This amp sounds way better than it should and I am really thrilled with it. It kicks butt and at the same time is crystal clear, articulate and clean. The ST35 may not have enough room for the negative supply; it has been a while since I have seen one. I have built the "El Cheapo" topology into several amps with a variety of front end tubes but this is the best one yet. I am loving push pull!



 

Nice schematic!, posted on December 14, 2014 at 11:24:07
JJ Triode
Audiophile

Posts: 716
Location: Northwest
Joined: December 2, 2004
 It may be coincidence, but I have also found that 6 dB of GNFB is a "sweet spot" for UL-PP EL84 or similar output tubes with the Dynaco replica iron.

 

sure you can...:), posted on December 17, 2014 at 16:51:57
PakProtector
Audiophile

Posts: 12365
Joined: May 14, 2002
 and there will be no fighting.
cheers,
Douglas

Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

 

RE: Constant Current Source for Dynaco ST-35, posted on December 19, 2014 at 03:04:40
altecboy
Audiophile

Posts: 404
Location: Northern NJ
Joined: May 29, 2004
 Hi Pak. I hope you will show us how.

 

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