by erichayes » Thu Jul 01, 2004 11:06 pm
Hi All,
A couple of footnotes...
Shannon, I found that the EL34s in my prototypes of the (very slightly modified versions) HF-89, were happiest at 60 mA, also. Taking them up to the max didn't improve the sonics, and just made everything run hot. I also found (through some hints from Bob Neil and Bud Wyatt) that 72 mA was the magic number for 6550s. And I concur that bias contributes more to how an amplifier sounds than any candyassed coupling caps or titanium oxide resistors.
Bud and I got to talking about the evolution of the 6L6 one evening. We're both hams, so we started comparing the 6L6 to the 6BG6 and the 807, its big brothers. I mentioned that it seemed that all the early (1948~1953) televisions with CRTs larger than 7" used 6BG6s as the horizontal output tube. Bud said that the RCA engineers originally tried to use a 6L6 as the HO, but had internal arcing problems. So they made an octal version of the 807, which was a highly regarded transmitting tube based on the 6L6, and used extensively in WWII equipment, along with its 12 volt counterpart, the 1625. Well, turns out the added current clout the 807 provided didn't help with the arcing problem. It was a voltage-power thing, not a current-power one. So they made some minor mods to the 807 (bumped the EPmax from 600 to 700V; the 6L6 was a paltry 360V) and beefed up the internal insulating properties so the tube could withstand 6.6 KV for 10 µS of every horizontal line generated. That was pretty impressive engineering, but we all know that things electronic kinda went nuts in general in that era.
Frank McIntosh used the 6BG6 as the output tube in the first generation of the MC-30, and, in my opinion, did not improve the amplifier by going to the 6L6 in its later iterations. I'm not disparaging the 6L6 by any means; it's a noble tube. The 6BG6 just kicks its ass.
Eric in the Jefferson State