Integrating Baby Blue

[Bottlehead]

I keep getting suckered into lousy deals...I built my current ST35 when I came across a half-completed DIY project on Ebay more than a year ago--it included a gutted SCA-35 (except the owner had replaced the speaker and RCA terminals with custom gold-plated ones and left them there) plus an uncompleted point-to-point ST35 copy with a nice chassis, gold-pin tube sockets, mounted transformers and more gold terminals. I blew $85 on the works. To top off that rip-off, a few weeks ago I bought a badly-listed SCA35 for $153--it works, but it's got old Dynaco-labelled 7199's and EL84's that were made in England by some company called Mallard or Dullard or something, and I believe in buying American.

Both the first chassis and the new complete SCA35 are excellent cosmetically. I bought the latest one intending to lift the transformers but my eyes got to looking at them side-by-side and something occurred to me....if I went with a quad cap board and dumped the aluminum electrolytics, something I'd do as a matter of course anyhow, the output transformer at the front of the chassis would drop in right where the cans now sit. That would leave room for a Shannon Parks special at the front of the chassis with several inches left over.

While the power amp section of the SCA35 is pretty similar to the original Dynaco ST35, the pre-amp section bears no resemblance to any stand-alone Dynaco product. It's half a PAS3 basically, using two 12AX7's rather than four.

In any case it got me thinking about installing Baby Blue plus a pre-amp PCB under the original SCA35 hood and using the existing switching and tone controls. I've got an ALPS/Noble stepped pot for the volume control, I blew $6 on THAT scam.

SCA35's by themselves are becoming more popular and pricey, though it's best to bypass the tone controls entirely--they're based around those primordial IC's called PECs (Packaged Electronic Circuits), small collections of resistors, diodes and caps in ceramic-cased centipedes. However I've found schematics for them and they could be easily breadboarded using premium components.

I'm mostly thinking out loud, but hopefully someone sees where I'm going here. Why gut an SCA35 and go to the trouble of building a custom plate and chassis when Baby Blue will tuck right in there with room left over (not much, about 3"x4") for a preamp circuit? I know little about the original Dynaco gear and how things tied together, but could I use the original 1/2-PAS SCA35 PCB with Baby Blue? Any suggestions for other compact preamp circuits, active or passive? I'm just an aged English major who usually remembers which end of a soldering to grab. The nice thing about this idea is that I've already got most the parts and it'll be simpler to build than putting together a second ST35 clone from scratch. Prepping the SCA35 chassis for the mods might take 20 minutes.

Thanks for any input from y'all, I'm sure I'm not the first one to think of this but I couldn't find anything posted.

And what should I do with all these antique tubes with a name that sounds like a duck?

[TerrySmith]

Hey Bottlehead, if you don't want those nasty brown stained Mullards, I'll take 'em!

Actually, those Mullards are the BEST EL84 you can have. They are probably still good with many years left to go! :drinking:

[Shannon Parks]

Bottlehead,
You've got a Rev B board, right? Certainly that can't squeeze into a SCA35 chassis, can it?

Shannon

[EWBrown]

I know from experience that a rev B board won't fit within, but a rev C should do so pretty easily.

My first pass at refurbing an SCA-35 was to gut out the original boards,tone control/balance/filter stuff, and then I replaced the PA boards with fiberglass ST-35 boards from "triman" and added mre filter capacitance (keeping the original two metal can electrolytics.

I also added a C-354 choke to replace the 50 ohm power resistor, the added caps were a 32+32 525V JJ and a 470 uF 450V panasonic cap scrounged out of a dead switcher power supply, which really makes a great improvement, this is connected after the choke, across the original cap, and straight to the B+ red leads on the OPTs. The two extra electrolytics were mounted on a piece of fiberglass circuit board, cut to fit the original preamp board cutout. The choke was chassis mounted.

Final touch was to install two isolated/insulated RCA jacks on the back, and a 100K Alps stepped attenuator, change the original silicon rectifiers to UF4007s and place a CL-90 in series with the fuze, this avoids the startup "thump" as the full AC voltage meets those discharged electrolytics head-on. I didn't bother with a preamp, as this was to be driven by a Rotel RCD-855 CD player, which was a nearly perfect match
for this amp (and DIY-35s).

The rev C board came along after all this was done, but it does look like it will fit, but al the taller components will have to be mounted on the tube socket side of the PCB, as there is not much below-chassis clearance.

This re-worked SCA/ST35 serves very nicely here in the lab at work, along with another RCD-855 and a pair of Optimum-7 (rat shack/RCA) speakers.

That original SCA-35 tone control circuitry really mangles and tortures the audio, with the long complex and very messy signal path, and attenuation that it causes. It really had to go. I even took out the balance pot, and original volume control and added the Alps attenuator ($15 on e-bay) for the cleanest overall sound that this unit could deliver.

The "triman" boards are an exact layout copy of the original ST-35 boards, just on fiberglass rather than toasted phenolic, and they don't have the eyelets. They really do need a better groundplane which the Rev C definitely has. All the original SCA-35 power and output iron was kept intact.

/ed brown in NH (I'll add the URL for the "triman" boards after I find it on my other confuser)

[EWBrown]

Speaking of those brown-stained mallards or dullards tubes, here are some more on E-Pay....

http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... 85044&rd=1

I have a bunch of those in my "old but still functional, but really need to be properly tested" tube stash. All along, I had figured the brown stain was the result of a bad getter flash :oops:

/ed B

[Shannon Parks]

Speaking of those brown-stained mallards or dullards tubes, here are some more on E-Pay....

http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... 85044&rd=1

I have a bunch of those in my "old but still functional, but really need to be properly tested" tube stash. All along, I had figured the brown stain was the result of a bad getter flash :oops:

/ed B

'Chocolate Drops'. I like that. :coffee2:

Shannon

[Bottlehead]

Thanks to everyone for the input. Shannon, my current unit is a Rev B that's up and running but still in the tweaking stage and I was planning to purchase a C or D for this project--with the transformers re-arranged I would have a 9"x4" hole and could mount a Rev C on spacers slightly below chassis level.

I agree that the stock tone circuits suck, but I had figured on using a stepped ALPS unit for volume control, replacing the PECs with hard-wired components if necessary (Dynaco bragged about these high-tech minicircuits) and rewiring everything with 20ga silver/teflon. The stock pots seem to be good quality after a liberal bath of De-Oxit. The input selector isn't the greatest but a fiberglass replacement is available. OR I could just bypass the tone circuitry altogether--the only reason I'd like to keep it is because it's there. The output connections from the pre-amp board lead to pins 4 and 5 of the easternmost EL84 for each channel.

I was hoping someone knew of a replacement board. Curcio has both line stage and phono stage boards for the Dyna PAS preamps, I assume that tapping into the PA774 PT wouldn't be a problem--I'll do some more research for future reference, but I'd like to keep the cost down and for now I'll likely try component-upgrading the original circuit as best as I can. I don't see how I can really go wrong unless I fry something expensive. The chassis isn't doing me any good anyway, and will serve well as a straight enclosure for Baby Blue if I decide to drop the "integrated" scheme--I'll just fabricate a new face plate and use the gold RCA's for another project.

I'm a simple self-taught tinkerer and if any of you engineering types have reason to tell me I'm an idiot I won't be a bit offended--in fact I'd appreciate it. My only major concern is that one of the OPT's will be tucked tightly between the preamp PCB and the input terminals (where the cans are now) and on general principles that's the last place I'd like it to be, but if I route things carefully I should be okay. If inductive hum is a problem some small pieces of soft iron or mu-metal should shield it out, I'd think. Maybe it's not even a concern, it's just a problem I ran into once before.

The first link is the assembly manual w/schematics for the SCA35 and the second is an article that runs down some details on the preamp section.

http://www.curcioaudio.com/SCA35%20Manual.pdf

http://www.tnt-audio.com/ampli/dynaco-sca35_e.html

Now I just have to find decent replacements for those Hershey's Kisses with hyperactive pituitaries, the foreign Mallard duck tubes. They might even be okay, they nearly top out the scale on my Dyna-Jet 606.

Any additional advice, information, guidance or questionable reflections on my parentage is appreciated. Hey, where my people come from first cousins have been getting hitched for generations and it ain't caused no problems so far, except maybe for the time when Bubba Earl escaped from the root cellar and gave them tourists heart attacks. Me, I just stay indoors and mess with lectrical gizmos. I'm working on a time-travel thingamabob right now and I've just about got it figgered out.

[tubes4hifi]

Hi Bottlehead,
I'll give you $20 apiece for all the ducks you've got, and I'll take 100 of those ALPS stepped volume controls for $6 apiece.
What else you got you don't want??
Send me email !!!

[EWBrown]

I have some ST-35 replacement boards coming from tubes4hifi, those will be used to restore some old Dynaco ST-35 carcii that I have on hand, two of which are in excellent condition for the chassis, iron and cages, and one which is a little bit banged up and grungy, but some TLC should get it back to as good as new. That one (at least) will get the 6GK6 modification.

I'll put in the new driver boards (PC-13s) and beef up the original rather wimpy PS capacitors as there is space on top of the chassis for a C354 and a couple of hefty 'lytics.

The input RCAs and speaker terminal strips will be replaced with something a bit more up to date.

So many amps, so little time...

/ed B in NH