by EWBrown » Thu Mar 04, 2010 9:43 pm
I've had excellent results in the 1 watt or less class using 12B4As and 6S4As (not to be confused with Russian 6C4C / 6S4S triodes) as well as 6BL7s SET amps, these are good with both triodes connected in parallel.
For 12B4As, just copy the basic "Darling" SET circuit, but use 12B4As in place of the 1626s.
6S4As have a relatively high internal plate resistance, around 3700 ohms, so a minimum of 11K ohms prmary is needed, a 5K OPT with these tubes will sound like "something that is found in large quantities in a cow pasture" ;-) 22 to 24 K is even better sounding, very sweet, but at a somewhat reduced power output. I use the vintage 11K or 12K : 4 ohm German radio SE OPTs (generally made by Telefunken in the 1950s). These are frequently available on e-bay from "testedtubes"in Germany.
With an 8 ohm load, they behave as 22 or 24K primary OPTs, and are a "match made in heaven" for 6S4As, even moreso than with 12B4As. Sorry, Jef Larson, but I found something to surpass the "Abraxas" design, even though excellent in its own right, the 6S4As go one step beyond...
6S4As seem happiest with a B+ around 325V, cathode biased with 390-430 ohms, and 22-25 mA plate current. They need a fairly modest 8.5 to 10V P-P to drive them and any low or medium mu triode is a good driver for them. 12AT7 or 12AX7s have far roo much gain for most intents and purposes.
6AH4s are very nice, for 1 to 1.5W out, and I still believe that the much maligned 6CK4s can be "tamed" and produce good SET sound.
/ed B in NC
Last edited by
EWBrown on Fri May 30, 2014 7:52 am, edited 3 times in total.
Real Radios Glow in the Dark