by EWBrown » Thu Jun 01, 2006 1:14 pm
300Bs should be quite happy in class A PP. Just keep in mind that the B+ PSU current requirements can be fairly high, we're talking a minimum of 300 mA continuous, with a safety margin, go at least 400 mA capacity.
I would imagine that the basic ST70 design can be used with 300Bs, just change the output sockets, tape off the UL taps, and mind the B+ voltages. Typically, 300Bs are happy with a 800 to 1000 Ohms, 20Watt (I like to use a power rating about 3X the actual dissipation, for safety margin) "cathode" resistor for each tube, and at 70 to 80 mA plate current, the filaments will "float" at around 70-80VDC above ground. For PP, the resistor can be half the value, shared between the two tubes, this requires well matched pairs. Since these are DHTs (directly heated triodes) they will need their own 5VCT filament transformers, and four will be required for separately "cathode" biased tubes, and two if each pair of tubes shares a common 400-500 Ohm "cathode" resistor. Fixed bias could also be used, or a combination of fixed and cathode (using lower value resistors). One thing about DHTs, is they can tend to be hummy at times, with AC filament voltage, unlike a power tube with an indirectly heated cathode.
This is mostly a power consideration, your mileage may vary. It all depends on personal preferences / taste, the speakers being used, and the desired sound pressure levels. With a pair of homemade "Karlson" speakers, running about 96-98 dB SPM/w/m efficiency, SET 300Bs can cause "pain level" loudness, and on an ST70, they're almost lethal... The Karlsons are a 50-something year old design, and goldurn, they really work. My application, I use a Pyle Pro / Eminence Kappa 12 inch woofer and old Altec 800 Hz horns, drivers and crossovers, mounted on top. This eliminates the infamous "Karlson Honk"
Backloaded horns are efficient, but they have their own unique characteristics, some love 'em some hate 'em, once again personal preferences, etc. I like them for acoustic, jazz classical, but they're not as great for rock, metal, hiphop (gasp!) or other bass-heavy material. BLHs can sound "congested" (or nasally) to some.
/ed B in NH (I've rambled enough for now) Yellow_Light_Colorz_PDT_02
Real Radios Glow in the Dark