Local Current Feedback for the SCA35

for the DIY ST35, the Dynakit and every other PP EL84

Local Current Feedback for the SCA35

Postby Geek » Thu May 19, 2011 5:24 am

Hi folks,

Tonight I worked on a circuit for the SCA35 that eliminated the need for gNFB.

The SCA is such a simple amp to begin with, so I decided to keep it simple. Working on a suggestion by DeathRex here: http://www.diytube.com/phpBB2/viewtopic.php?t=4760 , I took the FB off the output tube cathode resistors.

An interesting effect that I did not expect was the constant F3, which was 11Hz to 29KHz clean across the output power... 100mW to full out.

Here's the circuit:
Image

Bias on the output tubes were best with 560R, but performance the best with 470. Hence the compromise in the output cathodes. Another advantage of this bias method is matched tubes aren't required.

The driver is a single 12AX7 in paraphase. The trimmer sets the symmetry for a wide variety of tubes of various health I tested. With the tweak on symmetry, you can gain an extra couple of watts before clipping over a single 10K or so resistor there (which may be fine for some, but not other 12AX7's I had in the bin).

Next step is to build a "speaker impedance simulator" to look for instability at resonances. I used a pretty inductive 8R2 load and had no oscillations or upper spectrum resonances.

Cheers!
-= Gregg =-
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Postby DeathRex » Sat May 21, 2011 10:39 am

I was looking at buying some Russian 0.068uf PIO caps, and found that the 100nf caps have a resistance of 79K at 20Hz (hope that's correct, 1 / 2πFC). With 220K to ground, the capacitor sees about 220K resistance at high frequencies and nearly 300K at 20Hz. I wonder if, without feedback, if this is noticeable, and how well this type of feeback would fix it, if it is noticeable? Could you use a 68nf cap or even a 10nf cap and have the feedback fix it?
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Postby Geek » Sat May 21, 2011 2:41 pm

Hi,

I'll have to give it a test!

I know 68n has little effect replacing 100n in traditional gNFB amps.... F3 in those amps is still below 20Hz.

You definately don't want to go larger here (or smaller on the grid snubbers).... that part is so designed so the phase shift to where this becomes a giant flip-flop lies outside of the passband by a good octave.

Cheers!
-= Gregg =-
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Postby rmyauck » Sat Nov 19, 2011 6:34 pm

Gregg

Is that the "Floating" paraphase? How does it compare in sound quality to concertina used more commonly now like on the ST-35? How does the LTP or Mullard compare?

Thanks in advance!

Randy
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Postby Geek » Sat Nov 19, 2011 9:28 pm

Hi Randy,

The topology selection was limited, due to the SCA/ST-35's power tranny. Plus the need for cathode access ruled out a LTP, unless you returned the FB to the grids, then you needed like 10V of drive =:o

Project was shelved for now, as this unit needs a zobel to remain stable at LF resonance... SCA/ST35 fans I talked to were incredibly allergic to the zobel idea :/

Cheers!
-= Gregg =-
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Postby rmyauck » Sat Nov 19, 2011 11:32 pm

Great! Thanks Gregg!
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Postby EWBrown » Sat Nov 19, 2011 11:35 pm

The PI circuit is not the "floating paraphase", that would require the two 220K grid resistor "ground ends" to be tied together, then connected to ground through a third 220K resistor, and the second triode grid (right hand side) would be fed by the junction of the three 220K resistors.

I know, the proper PP circuit is somewhere on this forum, but it might be like trying to find the proverbial needle in a haystack to locate it.

But, that would not work for the current feedback design, as would not the classic long tail pair PI.

This current feedback path approach is a very interesting one, though it can be a lengthy, reiterative process, to get it tuned just right.

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Postby Geek » Sun Nov 20, 2011 12:23 am

EWBrown wrote:....though it can be a lengthy, reiterative process, to get it tuned just right.


Uh huh, you can say that again (sick)

Cheers!
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