Thanks much to Dave for the detailed info and the time he's devoted to addressing my bonehead questions and concerns.
For Dave and anyone else out there who's reading this, I have all the parts on order and have been doing advance planning for doing the EFB upgrade on three amps--I picked up a pile of original Dynaco gear at an estate sale several years ago and much of it has been gathering dust--an ST35, an ST70, four Mark 4's, two Mark 3's, an SCA35 and a pair of PAS preamps. I got suckered into shelling out $450 for the trunk-load of gear. I've picked up two more SCA35's since then and have completed three diytube ST35's with the pirated parts.
My bench is stacked deep with works in progress. I've finished a stock rebuild of the ST35, a VTA-80 ST-70 upgrade, a pair of Mark 4's with Shannon's Poseidon driver and a plain-vanilla diytube ST35 using the stock BOM. I rarely finish a project then leave it alone; I've played around with voltage regulated power supplies, CCDA designs and other gizmos trying to learn more about what's audible and what ain't in amp design.
I stripped all three of my SCA-35's to pirate the iron and chassis', thinking that the original circuit was so dated and compromised that it wasn't worth rebuilding. However after reading Dave Gillespie's comments and corresponding with George Ronnenkamp at Pacific Audio Regenesis I plan to order a set of SCA35 replacement boards and rebuild one of the SCA35's to spec (with EFB, of course). George took the time to write me at length and I was surprised to learn that the PECs in these half-century-old amps survive the test of time. I had taken one look at these stone-age spider-like proto-integrated-circuits and decided they were trash but George says that's not the case.
Those projects are in the future: My major project at hand is a diytube ST35 with a two-channel EFB module. Before learning about Dave's mod I had already started on a pimped-out version--tube rectifier, beefed-up PT, PRP/Takman/Mills resistors, Auricaps, a stepped-ladder attenuator and a matched quad of near-new Mullard EL84's I discovered in one of my garage-sale amps. With EFB I've gotten more excited about the possibilities of a maxxed-out ST35.
I know that the ST35 has adequate gain to function with a 100K volume pot and line level inputs, but I'd also like a balance control due to a crappy listening room. I thought I could simply include a balance control or separate volume pots for each channel, but after reading Dave's articles and learning that the original circuit design causes one channel to leech gain from the other, I would probably have gotten some weird and frustrating results.
I apologize for the lengthy lead-in, but from the beginning I intended to build my upscale diytube ST35 as an integrated amp...for starters I'll just be including line inputs and a 3-way source selector switch but I still listen to vinyl and will add a phono section if the amp meets my expectations.
I'd appreciate any opinions on how to handle the input--Maybe I could probably get by with a passive preamp, or no preamp at all. Since I have other Dyna gear sitting around I considered using the original SCA35 or PAS3 preamp sections in conjunction with the diy ST35, but that presents some problems.
I've built a couple of items from Glass Audio, checked out their site the other day and spied their ACF-2 (Aikido Cathode Follower), a highly customizable, simple and inexpensive line preamp ($35 for the PCB). It's a unity-gain buffer for folks who don't need any voltage gain, but do need more current than a passive line stage provides. It has a 4"x6" footprint, modest power requirements and accommodates a variety of tubes--I'd probably keep it simple and go with a pair of 12AU7's. You can see it at: http://tubecad.com/2011/01/blog0198.htm
I'm a believer in KISS and I don't want to clutter up the signal path any more than I need to. I'd greatly appreciate feedback from builders who've tried different methods for preamplification and input selection with the ST35. In a sense the diy ST35 has a built-in line preamp already but if using more than one input (or considering a phono input) a unity gain buffer seems like a clean, simple and inexpensive solution for matching inputs and eliminating noise.
To ask a slightly different question, if you choose to use the ST35 with a single dedicated input device--a CD player--is there ANY reason to use a passive or active preamp or will a simple volume control provide clean and balanced sound?
Thanks for any thoughts you may have,