SCA-35 need to know?

[TerrySmith]

I've been studying the schematic to a SCA-35. It looks like the line inputs go around the preamp board, directly into the tone controls then to the 7199. Am I correct to assume this?

Has anyone tried ST-35 replacement boards in a SCA-35?

Or should I yank the iron for other projects?

[erichayes]

Hi All,

Terry, in my humble opinion, the SCA-35 could be best described as an engineering mistake and a moderate marketing success. Dyna took snippets of the PAS, ST-70 and ST-35, threw them into a Cuisinart, and came up with the SCA-35. The phono preamp stage doesn't come even close to RIAA characteristics, the tone controls definitely alter the sonics of the amp, but not in a euphonic way, and the things ran hotter than Hell on an August afternoon.

They used a passive tone control network ahead of the power amp section and essentially transplanted the 7199 first audio/phase splitter of the ST-70 into the output section of the ST-35, relying on the gain of the pentode section of the 7199 to make up for the insertion loss of the tone controls, and the sensitivity of the EL84s to accept the lower output of the 7199 triode phase splitter.

The result was an integrated amplifier that supposedly combined the virtues of the PAS and ST-35 and shoehorned them into a box the size of the PAS. If listened to alone, especially back in the '60s, when speaker technology was still in Kindergarten, The SCA-35 sounded significantly better than a lot of integrated amps of comparable price. But if you put it up against a PAS/ST-35 stack (as my buddy, who had the latter, and I, who had the former, did back then) the difference was night and day.

My advice is to harvest the iron, case, knobs, controls and switches and SC the boards and wiring. But that's me.

[EWBrown]

Last year, I revamped an old "beater" SCA-35 (originally destined to be an iron donor) into something functionally very similar to a DIY-35.

All the original preamp and 7199 boards were removed, then I put in two of "the Triman" ST-35 replacement boards, which use a 7247/12DW7 and 2 EL84s. For Rs and Cs, I used the same as specified in the DIY-35 rev B parts list. I added a C-354 choke in place of the 50 ohm resistor, and added a LOT of extra capacitor on a small bpard cut to fit the original preamp PC board's cutout. This has a 470 uF/450 VDC (for the OPT's B+) and a 50/50 uF/525 VDC JJ cap mounted, (for the input stages). THese are in parallel with the original Dynaco electrolytics, which were in good condition. The added capacitance makes a huge difference, better low freq response and it eliminates any residual hum, as the original Dynaco scaps were pretty much a design minimum.

The tone control and "filter" circuitry did nothing but mangle the audio signal, so all of that was removed. I kept the original volume and balance controls, and installed two isolated RCA jacks on the back panel, and ran shielded wire from the RCAs to the vol and balance pots, and then to the
ST-35 board's inputs.

Later on, I took those two pots out, and installed a 100K ALPs "Chinese knockoff" stereo stepped attenuator (from an E-bay seller in Taiwan), and this is the present configuration. I also "borrowed" from Shannon's design and put an ICL CL-90 in series with the AC primary, for softer startups, and replaced the original Dyna silicon rectifiers with UF4007s.

I kept the original Dynaco 95 0hm 5W resistor for the 4 EL-84 cathodes, which does necessitate using a matched up quad. I connected a 1000 uF, 35V electrolytic in parallel with the 95 ohms and its original cap. THiis fits easily under the shallow chassis.

The unit sounds gerat, far better than the original SCA-35 (I have another "factory built" one for comparison).

I suppose that the original input jacks and selector switch could be used, if you want the convenience.

The 7247s have lower gain (and input sensitivity) than the 7199s, but then they do sound a bit cleaner, so it's a worthy tradeoff.


/ed B in NH

[TerrySmith]

Well, that gives me a direction to go. I have three SCA-35's, two of them are very nice cosmetically, one was a wreck.

The hip thing is to remove the tone controls, is there anyway to boost the bass and treble without the controls?

I've often thought of removing the orig preamp board, (and doing away with the phono-tape section) and fitting a PAS preamp board, to have a really nice amp.

[EWBrown]

If you keep the original volume and balance controls, there is a loudness circuit consisting of a resistor and capacitor (and switch) that connects to that "fourth tap" on the volume control. It does give some bass boost at lower volume settings, and becomes out of circuit once the volume wiper passes that point. The rest of the bass and treble controls are basically a frequency-variable attenuator, so "boost" is only relative. I left in the loudness, before the final change to the stepped 100K attenuator.

My own personal preference is to keep the signal path clean and simple, and that "rats' nest" behind the SCA-35's front panel is about as far from simple (or clean) as one can get.

I have a few used and NOS SCA-35 "7199" boards, I figure those will go into a "cheap amp" project I'm working on, using some 12AB5 or 6AQ5 / 6005 power tubes and Uncle Ned's soon to be released UL "fender" replacement OPTs (at around $20 each). Waste not, want not, as the old adage goes...

I'll endeavour to take some photos of the revamped SCA-35 and I'll send them along.

/ed B

[TerrySmith]

I got to inspecting my two good SCA-35's, one is factory built, and had a LOT of use, the boards are shot. The other was a kit, and was sloppily built.

I need to know where you can buy replacement boards, I really like the idea of using 7247's, so ST-35 boards will work I hope.

I'm not really a big fan of the 7199, it ain't nothing more than a 6GH8 that knows somebody!

[Shannon Parks]

I got to inspecting my two good SCA-35's, one is factory built, and had a LOT of use, the boards are shot. The other was a kit, and was sloppily built.

I need to know where you can buy replacement boards, I really like the idea of using 7247's, so ST-35 boards will work I hope.

I'm not really a big fan of the 7199, it ain't nothing more than a 6GH8 that knows somebody!

Here's John Sullivan's boards:

http://www.dslextreme.com/users/triman/triman.html
http://homepage.mac.com/%20triman023/PhotoAlbum1.html

Maybe keep an eye on Ebay, too.

Shannon

[EWBrown]

Those are the same boards I used in teh SCA-35 to ST-35 conversion, they are teh same layout as the originals, but on fiberglass rather than crispy phenolic. I have three ST-35 carcii that I plan to rebuild, the chassis and trannies are fine, the boards are in various states of "toast" or are cracked.

Occasionally he has them listed on e-bay (that's where I originally found him), but "direct" thru his site is probably better.

/ed B in NH

[TerrySmith]

Thanks fella's for the help,

Well, I replaced the caps on the output board, bypassed the tone and filter circuits, that really makes it sound like a high end amp! I did leave the loudness switch.

After playing awhile, one EL84 starting glowing, due to gross mis-match. I will install seperate bias resistors to help this.

[EWBrown]

The original ST-35 and SCA-35 really need matched quads to run properly, they cheaped out on the cathode resistor, but it costs more in the long run. Also, since JJs tend to be higher gain and bias a bit "hotter" than NOS US 6BQ5s, they will run hard in a stock '35.


Even the ST-70 should have matched pairs as they share a common cathode resistor for each channel pair.

/ed B in NH