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Old 5th April 2003, 03:10 AM   #1
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Default Dynaco Sca-35

Dyna Sca-35
Ok! the amp part is OK. for a little push pull pentode. But are there any easy mods to the the phono preamp. Yea! I have
one of Dr. Bottleheads phono Preamps that I am about to
build but if there is a simple mod that helps the Sca-35 please
let me know.

regards,
Woody
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Old 5th April 2003, 04:06 AM   #2
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I removed the tone controls, balance pot and all the front panel switches except the tape monitor. I also installed my capacitor board, and replaced every cap and resistor in the thing. It's a different amp and quite capable now. I also replaced thevolume pot with a noble conductive plastic pot.


Sheldon
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Old 5th April 2003, 04:25 AM   #3
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If you have a new Bottlehead phono coming, clone the Foreplay, if you need the gain, and junk the SCA for the output iron. Then build the better sounding ST35 design (or maybe an Acro 270)with a powertrans that won't melt like the Dyna.
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Old 5th April 2012, 09:30 PM   #4
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Default Re: Dynaco SCA-35

Hi, I also forgot to mention that a very knowledgeable audio engineer by the name of Dave Gillespie has developed a patented design of a EFB
enhanced fixed bias circuit that makes the SCA-35
effortless at 29% gain in wattage per channel and 43% less distortion. It has been said that by adding higher voltage premium coupling caps improve the sonics as well. Sonicap's brand I heard is oustanding as well as Mundorf's silver/gold's. using metal film resistors I would go with caddocks and polystyrene in the pF region. volume control with a log/taper Audio Note or TAD would also be good.
I also know that audioregenesis sells a module to replace the can caps as well. Get creative use the large Blackgate electrolytic caps to replace the can caps, and wow would be the sound you will get if you can do all of these you would have a great sounding vintage amplifier without possibly having to bypass all the controls and I have seen photos of a re-work and it would be a job to do that right! the person wanted to use the filter switch to take out the tone controls, but to switch the other way to add them back in.... go wonder. anyway my 2cents worth. I love talking with dave he sure knows all about dynaco amplifiers.
Lydia

Last edited by pianolydia; 5th April 2012 at 09:37 PM.
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Old 5th April 2012, 09:49 PM   #5
SY is offline SY  United States
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Quote:
Originally Posted by pianolydia View Post
Hi, I also forgot to mention that a very knowledgeable audio engineer by the name of Dave Gillespie has developed a patented design of a EFB enhanced fixed bias circuit that makes the SCA-35
effortless at 29% gain in wattage per channel and 43% less distortion.
Do you have a number for that patent? Looking at the circuit (a bypassed CCS to provide cathode bias), it seems like there's a lot of prior art and I'm unable to find any patents issued to David Gillespie relevant to this. Thanks!

edit: On careful reading, he does have a novel feature (at least novel to me) of varying the idle current with B+ sag. I still don't see any patent, though, or any mention of one in his writeup. Dave was good enough to at least provide some back-up measurements, so kudos to him for that. He clearly doesn't like my LED bias scheme, but missed a few of the essential elements of it.
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Old 6th April 2012, 11:30 AM   #6
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Default enhanced fixed bias

SY, I don't think he has claimed a patent. He uses a negative three terminal regulator to establish a fixed voltage (not constant current) relative to the B+ by using the B+ rail to feed the adjusting terminal. I used this approach in a triode strapped el84 amp and it does work very well, and is a compact, easy to implement improvement over a simple resistor/cap combination. My decision, when I was comparing it to an array of leds was based on ease of implementation. I still bypass it with a cap (some bad habits die hard), but he (Dave G) says cap isn't needed. I am going to try this with a 7591 based amp somewhere in the future (have tubes and trannies laying around) as they seem to fit the bill as suitable candidates as well (low bias voltage).
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Old 6th April 2012, 12:04 PM   #7
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I don't see where he does, either; I would think Dave would have mentioned it. Lydia says it's patented, thus my question.

Your bypass cap may be a good idea- the output of a 317 is inductive.
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Old 6th April 2012, 05:44 PM   #8
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Default noise

Yes, I had always heard that these regs (317 and the like) generated some "artifacts", that I though cap might smooth out.
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Old 6th April 2012, 05:51 PM   #9
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Would that bypass cap around the 317 need to be a large value (as in electrolytic)? I would think it could be a small film type, but then again, I'm undoubtedly missing something...
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Old 6th April 2012, 06:47 PM   #10
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The output impedance of an LM317 (the text above says negative regulator so it would actually be an LM337) rises sharply with frequency. So if you want a neutral use (or lack there of) of local cathode feedback, you won't get it with just a LM317 alone. At low frequencies, the LM317 acts like a short and thus provides no local feedback on the output tubes, but at high frequencies, it can be several ohms. So I'd bypass it with a healthy sized cap to counteract the rising impedance with the falling impedance of the cap. As always, I'd also bypass the electrolytic cap with a film cap.

BTW, this thread is pretty old, I guess they never die...

Sheldon
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