1 Mar
2010
1 Mar
'10
10:15 p.m.
On 1 Mar 2010 at 20:49, Tony Duell wrote:
Philips (IIRC) even made a transformerless valve
output stage. 2
output pentodes (EL84 and UL84, the latter because of the good
heater-cathode insulation IIRC) as a totem pole, with a high impedance
speaker (800 ohms) from the centre tape to ground through a capactior.
The speaker was the main problem, it was wound with very thin wire
(one reference says 0.3 thou diameter) and it was prone to go
open-circuit.
There were a few audiophile hobby designs that used multiple
paralleled beam tubes in a cathode-follower arrangement to directly
drive an 8 or 16-ohm speaker. An interesting project for a time
because the WWII-surplus 1625 (7 pin 12.6v version of the 807) could
be had by the caselot for very little. A friend built a grounded-
grid 160m final using them, powering the series filaments directly
from the line. At higher frequencies, it was a common
practice to
hacksaw or drill into the base and fish out the beam-forming
electrodes and bring them to a spare pin for GG operation.
--Chuck