--- In ham_amplifiers@..., Tony King - W4ZT <w4zt-
060920@...> wrote:
pentalab wrote:
<snip>
The trbl with teflon sheeting, etc... is u gotta
anchor it to the chassis. At least with my original Straight
up and down stove pipe chimney..it was 1/4" thick teflon,
and so heavy, it's weight alone held it to the chassis. As
noted b4... it restricted the intake air too much.
TONY SEZ.... Jim, have you considered sub mounting the tube? Current
methods dictate cutting lots of holes around the tube to allow the
air to flow. If you sub mount the tube by an inch, mount it on a
solid aluminum or, better still, copper plate, you can get LOTS of
air up around the seals and up to the anode without a huge wide
pattern of holes around the tube. Then a straight chimney will work
and you're back to a nice Teflon chimney that's easy to make though
a little expensive.
### YC-156 user's do this. Since the YC-156 has a 4.94" OD
cooler... but the built in grid ring is a whopping 5.25" OD...
obviously a straight up/down teflon chimney won't work in a normal
config.
## here's the problem though. You submount the grid flange 1"
below the chassis... it's GOTTA be mounted on aluminium/copper
standoffs. Two potential problems. One is.. the grid isn't as
well grnded. The other is... since you lowered the tube down one
inch.... the stray C from anode to chassis will INCREASE. The
chassis will also be getting closer to the lower anode. I haven't
measured it.. yet.. but I'm betting the stock 55 pf anode to chassis
C will increase by another 10-25 pf, when submounted 1". Maybe
not... since this sub mount scheme uses a bigger hole in the
chassis to start with 4.94" vs aprx 3-4".
This works the other way too. Reid at Eimac told me fellow's
will raise the tube up on a pedestal.. like a hollow piece of
aluminium thick wall pipe.... drilled and tapped on both ends...
this then gets the anode up AWAY from the chassis.. lowering the
anode to grid C.
In the submount case, the tube will also have to be inserted from
below ! IE: stuff the top of the anode UP through a min 4.94"
diam hole in the chassis.
Out of interest here. I couldn't initially see why a YC-156 had
36 pf of anode to grid C... which rises to 50-55 pf when bolted
to chassis..... while a 3000A7 is only 24 pf... rising to 33
pf..... and a 6000A7 is 24.5 pf... rising to 38 pf. Looking at
a 6000A7 closer tells the real story... they have shoved the fins
up higher on a 6000A7, right to the top.. compared to a 3000A7....
then on the underside... they sliced the lower fins at a steep
upwards rising angle.... to get as much anode away from the
chassis... which minimizes stray C. The 6000A7 has a much bigger
OD cooler too 6.125" vs 4.94" for a YC-156.
Usually, when I cut the holes in the chassis.. instead of using uni-
bit's, greenlee punches... I just cut it out in the shape of a
maltese cross.... with my Bosch jig saw. A lot faster,,, and loads
of airflow. Use what ever works.
I got the 1/4" thick custom made Teflon chimney from Arnold
Howell, of Howell tube sales in Ohio. He had tons of em custom
made in Cleveland. They cast em 1st... then machined out the
insides to a precise fit. They made em extra Tall... higher then
the tube itself.... since the 11m ops all use a fixed vac cap.[for
a plate block cap]...and stand it on end vertically.... then they
can cool it too.
The chimney is HEAVY... and it's weight holds it to the chassis.
Not cheap... about $115.00 new. I'm sure he could [probably
allready does] a smaller 4.94" chimney for a YC-156/179
tube. Howell mebtioned to me about submounting the YC-156 yrs
ago... then using a straight up/down 1/4" thick teflon chimney.
Some guy in W6 land wants me to design an amp around 3 x YC-156's
in parallel, GG... low band stuff. I figure with 600w of
drive.. it should do 30-36 k out.
later... Jim VE7RF
<snip>
73, Tony W4ZT
