by EWBrown » Thu Sep 09, 2004 5:43 am
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
Real Radios Glow in the Dark