ST35 Vs SCA35 Power amp section

[erichayes]

Hi All,

Oboy . . . this is opening a subjective can of worms. I'll try to be as objective as possible, but I'm apologizing in advance if/when I piss someone off.

The pentode 1st amp/triode cathodyne phase splitter was developed to produce the most bang for the buck. Cathodynes need lots of gain to work properly, and a pentode stage preceeding it was one of those "duh" decisions. Heath was one of the first companies to use the configuration in 1958 with their W7, which used a TV IF amplifier/sync separator tube, a 6AN8, to perform the duty. Dynaco, and others, followed suit, and the "three tube" power amplifier was an accepted member of the hi-fi scene.

The most notable characteristic of the above circuitry, in my rather extensive listening experience, is that it is euphonic, but inaccurate. The midrange is somewhat "forward" sounding, with all the "tube" overtones, but there is a nebulous characteristic that prevents (or inhibits) the detection of individual instruments. I have referred to this phenomenon in the past as sonic soup.

Triode stages, and the long tail phase inverter in particular, have a more neutral sound that requires a little more dedicated listening to appreciate. The midrange is apparently more subdued, but it's actually because the low and high frequencies are more accurately represented. I call this sonic salad.

In designing amplifiers professionally, one has to subject oneself to listening to the same program material over and over and OVER and OVER in order to detect the subtle change a different cathode resistor or feedback capacitor value makes. It also takes a listening discipline that most DIYers (and virtually all audiophools) aren't willing to invest the time or effort to subscribe to. There's nothing wrong with that; if it sounds good, listen to it. But if you do decide to learn the discipline, you'll be rewarded (and cursed) with hearing, and recognizing things you never heard before.

[Shannon Parks]

In designing amplifiers professionally, one has to subject oneself to listening to the same program material over and over and OVER and OVER in order to detect the subtle change a different cathode resistor or feedback capacitor value makes.

Saturday while testing about five different cartridges on my turntable, I listened to the same blasted album side five times. Informative yet very boring. Yellow_Light_Colorz_PDT_29

[EWBrown]

Once can listen to only so much Osibisa fusion "outer space" jazz at a time Yellow_Light_Colorz_PDT_01


If you're running a sound source like a CD player or FM tuner straight into the power amp, then the additional gain of the SCA35 boards has its advantages, the 12DW7 / 7247 boards just do not reach full power capability with many sources. My test setup, I use an old Sony "Carman" CD player, it sounds decent, but the line level is rather anemic, so some additional gain is in order here. Straight into an ST35, this gives me perhaps 5watts maximim, but place a linestage inbetween, and the full capability blossoms forth.

Way back when, I modded an SCA35 wo bypass the preamp and tone controls completely (added an extra set of insulated RCA jacks, a Hong-Kong 100K stereo stepped attenuator, and eliminated all of that front panel rat's nest, long and treacherous "signal path") as well as beefed up the power supply to "Shannon Parks" specifications. The CD player easily maxed out the power capability.

Later on, I replaced the somewhat "crispy" phenolic SCA35 boards with aftermarket fiberglass ST35 boards, with better caps, and even though the overall gain was significantly lower, the sound just seemed a lot better to me. Maybe it's the all-triode input, maybe its the better coupling caps and the new boards, or perhaps it is just a "placebo effect", but whatever it is, I like the final results better, than the original SCA35, with its original weird and nasty internal wiring. Yellow_Light_Colorz_PDT_29
Yellow_Light_Colorz_PDT_31

A casual look inside the old RCA tube manuals, all three of their amplifier circuits use the 7199 pentode / cathodyne input curcuit, rather than 7247 or a more complex long tail PI stages.

As far as comparing triode vs pentode input, it's just a matter of personal taste, and I'd not say either one was right or wrong... :parp: Yellow_Light_Colorz_PDT_07

/ed B in NH

[EWBrown]

Back 40 some years ago, when Dynaco was selling these amps, I'm sure than nobody there ever thought that they would still be working and being duplicated / cloned en masse, forty years later...

I keep the original SCA-35 7199s, they test perfectly good, and I can still use them in my (mostly) original design ST70 (it has a new "old design" 7199 board, fiberglass instead of crunchy phenolic, otherwise identical circuit). That is one that my Dad built around 1968, and I want to keep it mostly original, rusty chassis and all... The Sovtek 7199s are OK, but just not the same as the olden tubes.

Not too long ago, SCA-35s went for relatively cheap money on E-bay, whereas ST35s went for their weight in gold, almost. Now everybody's figured out that they use the same iron, so the SCA-35 price went up and up...

If one is really good with metalworking, an SCA-35 chassis can be re-worked to fit a rev C or D board, with a little creative engineering. The preamp, tone controls and input selector switch have to go away, in order to get this all to fit inside. The board has to have all of the "taller" components on the tube socket side, resistors and smaler parts can go on the bottom side as needed.

/ed B in NH