22 Apr
2005
22 Apr
'05
7:23 p.m.
From: "Bob Shannon" <bshannon at
tiac.net>
I've not been into R/C stuff, but I am into 'submini' tubes.
I recently built a 5-tube regenerative receiver for the 49 meter band using
5678's for the 2 stage tuned RF amp, detector, and first audio amp. A 5676
serves as the final audio driver.
Performance is awesome, and it runs on batteries. Warm-up time is at most
3 seconds, nearly instant-on operation.
---snip---
Hi
The direct heated cathodes turned on fast enough that
one didn't have a noticeable delay. It wasn't until the
indirect heated cathodes that things changed. That was
because it was too hard to build an AC radio with the
directed heated tubes. There was both a hum problem
and it was more difficult to get each tube biased correctly.
Once they had selenium rectifiers, direct heated became
practical for low power applications ( like the Zenith TO's ).
They still had to do some tricks to get the biasing
working right for each tube. Some clever engineering.
Dwight