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In Reply to: RE: Maybe I'm missing it posted by petercapo on November 09, 2014 at 12:16:14
I have read similar reports about the ST-35 being Dynaco's best sounding amp, and I agree with that opinion. I'd attribute that to the 7247 input stage, along with the excellent Z-565 output transformers. I have never read the same thing said about the sound quality of SCA-35 though. Again, I'm not saying it has never been said, only that I've never read or heard that.
"I have the Dynakitparts Stereo 35 that is the same power amp as in the SCA-35"
I think this is where I will also respectfully disagree with you. The two amps use the same output tubes, and output transformers, but that is where the similarity ends. The ST-35 uses the very good sounding 7247 tube, while the SCA-35 uses the 7199 in a different circuit topology, as you know.
If I went to the Dynaco Forum and told Roy and Bob that their VTA ST-70 was the same amp as a stock 1965 Dynaco ST-70 because it uses the same output tubes and output transformers, do you think they'd agree with me? The VTA (which I own) uses 12AU7s with a different circuit than a ST-70 with 7199s, and sounds quite different.
My point is that your ST-35 is an excellent sounding amp (yes, even without the EFB) but the SCA-35 was not to my liking at all.
As to the condition of my SCA-35, it was bench tested to spec prior to me buying it from a friend of mine, who had two of them. We tried both of them on a variety of speakers, including Altec 604s, original Quads, Advents, NHT SuperZeros, etc. Our consensus was that using either SCA-35 sounded like a wet blanket had been thrown over the speakers.
I tried many mods, including disconnecting the tone controls, upgrading the coupling caps, upgrading the power supply caps, etc. Nothing I did made me like the amp any better, and I put it on a shelf for many years. If anyone's curious why I bought it in the first place, it was for the output transformers. I always intended to turn it into a ST-35 eventually, but found another pair of Z-565 transformers to use for that project.
I started reading about the EFB mod, and figured I had nothing to lose, given that it was about $5 worth of parts to try it. I've posted elsewhere that the EFB took the SCA-35 from boring to musical. I now rotate it into my system on a regular basis. That's why I said the EFB was "mandatory". Given that it offers longer output tube life and greatly improved sound for a few dollars, why not?
My intent in posting certainly wasn't to disagree with you. It was with the concern that the original poster might not be aware that the 7199 tube is rather scarce and getting expensive. So is the 7247, for that matter. The difference being, the 7247s are worth it!
Follow Ups:
You piqued my curiosity, so I pulled up the schematics for the Stereo 35 and the SCA-35. The power amp sections seem pretty similar between the two, with the phase inverter topology the same as far as I can see – a form of cathodyne phase inverter in both cases, correct?A difference is that the first section of the 7247 uses a straight triode that I believe is for some voltage gain (or is it more of a buffering role?), while the first section of the 7199 is a pentode used for the same purpose, though in the SCA-35 schematic it looks like the 7199 is hooked up in a way that does not use all five electrodes, rather, it looks like it is running in some sort of triode mode(?) with the screen grid connected in some kind of feedback, no?
So, the first section of the 7199 has some differences in the associated circuitry, but this looks to be just what was needed to accommodate the inherent operating characteristics of the first section of the 7199 vs. the first section of the 7247. There are also some differences in the feedback schemes, but the distortion specs are very close – I don't think I could tell the difference between 0.1% and 0.2% IMD "at average listening levels."
I confess that I am not an engineer, but these two circuits look far closer to each other than the difference between the original Stereo 70 two-tube gain plus cathodyne phase inverter vs. the three-tube gain plus long tail inverter Roy uses in his Stereo 70 circuit.
Edits: 11/09/14 11/09/14
Nor am I, not even close.
Now is as good a time as any to say that I follow your posts, both here on the Dynaco Asylum, and at the Dynaco tube audio forum. I always enjoy reading what you write. My point being, you seem to know your way around a tube amp, and are willing to help others.
That being said, and admitting once again that I'm not an engineer, I'm going to switch to subjectivity here. It has been my experience that a ST-35 is a better sounding amplifier than a SCA-35. As to the "why" of that, I'll leave up to those more technically qualified to explain. I've always been told (and read) that it was due to the 7247 being a better sounding tube than the 7199.
I wish the original poster good luck in finding what you are looking for. I didn't intend to take your post off topic.
Thank you, and I appreciate the discussion.
Best regards,
Peter
That is all perfectly fine. Disagreements are okay, too.
I remember reading some of your posts related to your SCA-35. I was and am still kind of surprised that in spite of everything you did it still didn't sound good (until you added the EFB). I rebuilt one a long time ago (no EFB) and remember it sounding transparent enough. Go figure.
In any case, I had suggested to the OP that if he buys an old one that it would likely need rebuilding. I am sure if he sends it to Sal or Roy that he would get back a pretty good sounding amp, hopefully something in line with his taste.
The SCA-35 was a nice little integrated amp. Cost and budget conscience kit for those that wanted a little tube amp, but didn't want to spend a bundle.
But in the big picture the SCA-35 was simply a nice, not great, but fair amp. If the original poster is going to spend a grand on this, I think he would get a much bigger bang for the buck by stepping up to the (new) ST-70 kit.
If he wants to at a later date he can find or build a PAS. I don't remember if he needs vinyl? But the PAS properly upgraded was a very nice preamp.
And besides, the ST-70 sounds just great with a passive attenuator. I used one for years. Lordy I miss that amp!!!!!!!!!!!!!
charles
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