Pentalab wrote:
--- In ham_amplifiers@..., Tony King - W4ZT <w4zt-
060920@...> wrote:
pentalab wrote:
<snip>
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.
Actually that's not so. The grid ring OD is 5.030" to 5.090" per the data sheet and verified measuring here. The anode is 4.812" to 4.936". That isn't a problem. Stock PTFE tube is available that can be turned easily to fit.
## here's the problem though. You submount the grid flange 1" below the chassis... it's GOTTA be mounted on aluminium/copper standoffs.
That is what some have done but doing it just like I do my GS-35B mounting fixtures <> will eliminate the problem. The standoff could be made from 1/4" flat stock with a pattern of holes. You get the sub mount without giving up the good grid grounding and have great cooling.
Two potential problems. One is.. the grid isn't as
well grnded.
see above
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".
I don't think any appreciable increase in stray will result. the straight chimney will also save lots of horizontal space on the chassis as well as a little vertical space if that was a consideration.
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.
Nothing you can do external will lower internal anode to grid C. Any external effects caused by lowering the anode by an inch will probably not impact anyone using it below 25 MHz. The only part of the anode cooler getting closer to chassis is the edge. There's not a large flat surface getting closer (except the vertical).
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.
Not in the case of a YC-156 or YC-179. It works just fine inserting it from the top.
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.
That's true. The physical shape of the bottom of the cooler has everything to do with it but as you said earlier, if the hole is cut out then the closeness is at a diagonal to the bottom of the cooler. The direct impact on the external anode to grid C would have to be measured but I suspect it would be minimal.
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 agree... don't let the absence of a particular tool hold you back! I have to wonder about suspending the tube out there in the middle of the cross. There's a bit inductance gained grid to ground by doing that. Plus there's a significant amount of loss in the conduction of heat away from that flange.
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.
I use stock PTFE tube from McMaster-Carr and machine it on the lathe as necessary. The stuff just can't be beat!
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.
Unless you really needed the chimney to go high up on the anode, it doesn't take much PTFE tube to make a chimney. Yes, it is a little costly but once done, you never have to worry about it again.
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.
Wow... well if one tube isn't enough... you need a BIGGER tube! I can't imagine dealing with the problems of paralleling 3 YC-156's. He needs a 15k or 20k!
<snip>
73, Tony W4ZT
