by EWBrown » Thu Oct 02, 2003 11:02 am
From what I could gather earlier, the EF86 is similar to, but better than, the pentode section of the 7199 used in the Dynaco ST-70 and SCA-35.
The "Uncle Ned" ST-70 replacement driver board uses 2 EF87s, and either a 12AU7 or 12BH7 as the phase inverter, rather than 2 7199s.
The EF86s are usually a tad pricey, but I did find a source for then on E-bay, from the Boston area. He sells the "National" branded Russian made ones, about $40 for a sleeve of 5, which is less than most other sources.
The pentodes have a gain of 250-300 versus 100 for the 12AX7, this was quite noticeable when I was in process of modding the SCA-35 that I now use in the lab at work. First step was to bypass all the tone controls and "filters" which only served to mangle and mutilate the audio signal.
next step was to disconnect the preamp section altogether, and just go
direct to the volume and balance controls straight into the PA boards. Better, but the 7199s still didn't sound right. Next was to purchase two
replacement ST-35 PCBs and stuff them with left over "mouser" parts.
This improved audio quality nicely. I had considered trying to stuff a "blue board" in there, but it would have required more mechanical re-work than I was willing (or able) to do. Last remaining step will be to replace the original silicon diodes with the UF4007s, and put in more filter capacitance - I plan on adding two or 3 2X32 uF 450V J/J caps.
Back to the Eiclone: I notice in the original HF87 schematic, they use two different value resistors in the plate loads of the 6SN7 phase splitter, 33K and 28.750K, similar to how Dynaco used 33K and 27K in the ST-35 (and SCA-35) "cathodyne" phase splitter.
I like the idea of making the Eiclone configurable, I just happen to have two pairs of 6B4Gs I obtained recently, and a couple Chinese made Valve
Art 300Bs.
The next "blue sky" project will be SE amps using 211As, 845s or WE 276s, I am slowly gathering the parts for this, and am still in the process of deciding on a finalized circuit. Gotta have something to keep myself from going crazy with cabin fever during the endless NH winters...
/ed B in NH
Real Radios Glow in the Dark