On Jun 28, 1:59*pm, flipper wrote:
On Wed, 27 Jun 2012 22:06:46 -0500, John Byrns
wrote:
In article ,
flipper wrote:
For this application I tend to think of them as almost the same thing
but am beginning to realize that the 'extra' G4 screen is probably
what gives a heptode the higher plate impedance. Is that why, despite
the 'accidental' mentioning of dual control pentode, you seem to be
adamant about heptodes, or is there another reason?
The reason I am "adamant about heptodes", if that is what you want to call it,
is not because of the plate resistance, it is simply because I have no
familiarity at all with DC pentodes and their characteristics.
Oh, okay. Well, just saying you're not familiar with them was enough
I don't even know any DC Pentode type numbers and am not sophisticated enough to
know that the plate resistance of a DC pentode is lower than a heptode.
Assuming the plate resistance of a DC Pentode is lower than a heptode, is that a
bad thing?
Well, it affects plate tank Q.
6AS6, 6DT6, 6GY6, and 6888 are some of the more 'popular' ones for
broadcasters.
I am not "adamant about heptodes", in fact I don't particularly care for grid
modulation, preferring plate modulation instead, however it is hard to conceive
of how to build a plate modulated transmitter using only one single section
tube, leaving the heptode, or possibly the DC Pentode, as the only easy choice
for a single tube transmitter. *I do have a design for a single tube transmitter
using your 6ME8 that meets my design brief, however it would require me to build
my own well balanced push-pull RF antenna coupling transformer.
Yeah, a PP transformer is in some of my designs too but I'm skittish
about trying to wind one.
PP RF tranny is easy. You can have just one winding with CT to B+, and
double tuning gang with frame and moving plates at 0V with 0.01uF caps
from fixed plated to coil ends.
the thing is to allow the tubes to drive a nice high ohm RLa-a load,
and have a secondary wound over the middle part of the winding, ie,
the earthy part, and have this winding about 1/5 of the total P turns,
so you get a 25:1 impedance match so antena C has hardly any effect on
coil tuning. if the RLa-a in parallel with tube Ra was say 25k, then
Zout from sec is 1k0, quite low enough to get over effects of C across
OP terminals etc. probably an air cored coil is OK using 40mm PVC pipe
former with cat 5 strands of solid wire for coil. But maybe best if
you drill many holes as possible in PVC to reduce diaelectric losses.
Layout and keeping coil away from other metal and keeping leads tidy
assist in overall Q which need not be huge. Another way to get low Z
output is to have a tap on one side of PP coil, at say 20% of turns so
you can use 0.01 cap to take out SE signal. No sec winding, and you
still get benefits of the Z transformation and stray output C from
antenna won't worry the coil.
Patrick Turner.