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In Reply to: RE: Vintage amp or receiver for my Dynaco A10 speakers posted by mjcmt on April 22, 2015 at 05:44:59
My advice would be to stay away from an SCA-35. I have one, and it does not have enough power (maybe 9 watts per channel) to drive A25's let alone the less efficient A10's. For Dynaco tube, I would not go lower than a Stereo 70, which will really set you back. The good new is that Dynaco speakers work great with solid state, so any good 25 or 30 watt receiver, or amp will drive them well. Amps and receivers in this power range a common and not sought after, so it won't take much to get them up and running. Much more power than that and you run the chance of overpowering the woofers, but 50-60 watts should still be fine if you are judicious with the volume. No Telarc 1812!
Dave
Follow Ups:
Here's a chart of a working to spec SCA-35, measured and posted by Dave Gillespie.
The first (Before) column is a stock SCA-35, the second is after his simple EFB mod.
Even a stock, working to spec SCA-35 will average about 53% more power than you're giving it credit for. Add the $5 (or less) EFB mod, and it will produce even more power, with less distortion, and many (including me) say it sounds better. In addition, the power transformer runs cooler, and the 6BQ5 output tubes will last longer, both because the tubes are drawing less current.
The main reason I'd steer the original poster away from the SCA-35 is the unavailability of the 7199 input tube. As Eli Duttman says, "They've gone the way of the dodo". There are adapters available to convert 7199 amps to use other tubes, or the amp can be modded to use the 6U8A (link below), if he found one cheap enough.
The 17 WPC may still not be enough for his A10 speakers, but I'm betting that for FM radio (background music?) on a bedroom system, it could be.
"You won't come back from Fletcher-Munson curve"-Jan and Dean
I was talking stock, not modified. Given my experience with the SCA-35, Gillespie must of had that thing pretty optimized to get 14 watts out of it. Even in that case, we are talking a little over 1.5 decibels, which is very barely audible. Note also that distortion is pretty high in that chart. I believe the standard is 1% for tube gear, which is a distortion level that I still wouldn't want to listen at (YMMV). I would not expect the amp to be very listenable at those power levels.
Back to my original point, which is not negated by Gillespie's measurements. My fully functioning SCA-35 does not have enough power to drive my Dynaco A25's let alone the smaller, less efficient A10's, that is when driving modern stereo records. It could be drive them fairly loud listening levels, but the sound was compressed and distorted. At those levels it was unlistenable in my opinion. It was useless for classical or rock, although it did work for folk and small chamber music. YMMV, but from your dislike of the amp, it may not have. So I stand by my 9 watts, but I am talking about real world power in stereo, with tolerable levels of distortion, but I am willing to give or take a decibel or two. Yes, it was a estimate, comparing it to other amps like Mark IV's which I have also owned. I don't have power measuring equipment.
From Gillespie's article, "On the bench, the ambiguity of the "each channel" designation is shown for what it really is: each channel at a
time. Measured as such, the SCA-35 is generally capable of meeting its performance standards. That may have been fine for early stereo that often threw nearly all the sound to one channel for dramatic effect, but with the more balanced presentation of today's stereo sources, full performance is required in both channels at all times. The fact is, when a stock SCA-35 is tested under conditions of both channels driven, the specifications suffer significantly such that it becomes about a 27-28 watt RMS (total) amplifier, at greatly elevated distortion (typically 3-5 times that of single channel performance). To be very clear here, that means that when both channels are driven, the power output of each channel drops to about 80% of single channel performance, and at that reduced power, distortion is 3-5 times higher than what is produced at the higher power output of single channel operation."
Dave
It's not a very good amp. I subjectively believe it sounds better after the EFB mod (I can't measure power either), but even still, there's that 7199 issue.
It's noted for having great output transformers though. Eventually I'd like to build a good 6BQ5 amp around the second SCA-35 I own.
"You won't come back from Fletcher-Munson curve"-Jan and Dean
Another option is to go passive with the preamp and put a modern ST-35 board. The ST-35 is a much better circuit and you can buy boards that use available tubes. But then you still have that "marginal" phono stage!
Dave
thanks for the good info if I find one at a good price.
Everyone seems to agree that the EFB mod sounds good at high volume levels near clipping. How does the lower operating point at idle affect the sound quality at low levels?
Edits: 04/22/15
Everyone? I've never read that before. Can you provide a few links?
As I've posted before, I could never stand the sound of an SCA-35. It was like someone threw a wet blanket over the speakers, in other words, dull.
The EFB mod didn't "transform" the sound of the amp, but it certainly improved it for me. As for it only offering improvement at full power, my main amp is a First Watt F1J, which is 10 WPC. I'm not regularly clipping that amp, so I doubt I'm getting anywhere close to the EFB modded SCA-35s 17.6 WPC. It sounds better to me than the stock amp did, and that's at any volume.
"You won't come back from Fletcher-Munson curve"-Jan and Dean
Apparently you misunderstood my question. How does the lower operating point at idle affect the sound quality at low levels?
Thanks for the heads up. I've been recommended the HK330c solid state receiver at 20wpc. It looks cool enough to not disrupt the decor in our bedroom, but will it's power rating be satisfactory for modest volumes?
It's a surprising 20 watts! Not to get into the rating system. But 20 watts in the 70's was a different 20 watts today. My bedroom system is 22 watts and it can blast me out if I wish too.
Go for the lower HK unit. The SCA-35 is a fine little stereo. But you will need to invest some money to have it restored. And "it is what it is". A "nice" little stereo.
And if you really like the HK you can have it restored fairly easily and cheaper than a tube stereo. Don't sell these little babies short.
Also the HK receivers of this vintage were engineered to sound good as well as meet specs. HK's can deliver their power into real loads with some headroom to spare.
Dave
Yes, 20W should be fine for bedroom use.
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