Date: Tue, 23 May 2017 23:26:55 -0500
From: "Jeff AC0C" <keepwalking188@xxxxxxxx>
To: "Bill Turner" <dezrat@xxxxxxxxxxx>, "Amps group"
<amps@xxxxxxxxxxxxxx>
Subject: Re: [Amps] new amp race
<A single band amp, 3-500z glass tube - that's not even a beginners
challenge by comparison. the Cout is about 7x higher plus the monster anode
on the YC156 makes a hell of a good capacitor with the enclosure where the
effect on a glass tube is token.
When you hang the L on the YC156, you get releif on 10m (for example) but it
pushes the Cin zero point on the pi-net down in frequency so that you may
get it working on 10m but now it's marginal on 12m and you have a monster Q
on 17m.
The bottom line is a YC156 makes a great beginners project for the low
bands. But if you want to make it work on the top end, my advice is to pick
another tube and save yourself a mountain of hours. Despite the somewhat
higher costs, it's actually pretty hard to beat an 8877 with a grid
protection circuit as a limit-capable all band tube when you consider the
entire project.
73/jeff/ac0c
## On a buddys YC-156, I designed the 80-10m , band switched PI net output
tank using GM3SEKs PI spread sheet. That took 2 of us a trillion hrs b4 we
nailed it.
## That YC-156 tube is a bitch to work right on 10m. The tube, on a wooden
test bench,
measures 36 pf from anode to grid..... which then rises to 53-55 pf, when grid
flange is bolted
to the chassis. That additional C is caused by the proximity of the lower
anode fins to the
chassis below them. The ceramic tune cap we used was a 10-375 pf @ 15 kv
type.
## So now you have 55 + 10 = 65 pf....with the tune cap fully UN meshed ! The
fix
is to introduce a TINY bit of uh...between anode and C1 tune cap. We used
silver plated
wide cu strap for both that tiny coil..and also the main 10M tank coil. If
you make the tiny coil
too big, the resulting plate load Z ends up low....which then requires a real
small main 10M
tank coil. Make the tiny coil b4 main PI net.... too small, then the Q will
rise on 10M, to perhaps
something you dont want. You want the tiny coil to be big enough to also work
on 15M band.
## we call that type of network an... L- PI. It works on the principle that
the tubes 55 pf, combined
with the tiny uh, forms a ..step down L network, which drops the plate load Z
of the tube WAY down
on 10M.....down low enough that the following PI net can handle..with a semi
normal loaded Q.
Which of course will require higher tune + load cap values....and a real
small main 10M tank coil
between em.
## As jeff found out, the optimum size of that tiny L coil, between anode and
C1 tune cap is different
between 10M..and 15M. So a compromise has to made, and here we are talking
about miniscule
amounts of uh, like .23 uh, .25 uh etc. So then you have to wind flat strap
coils..used since 1.125 inch OD tubing
is impossible to work with... using different IDs... for both the tiny L
coil..and also different IDs... for the
main 10M coil....such that you end up with the correct amount of uh on both of
em...and that their
overall lengths are also correct, to make the connections. Flat strap coils
have virtually zero C between turns,
since they appear like 2 x knife edges facing each other.
## Even with all of that, and several versions tried, and more software
employed, and also moving some of the
C2 from the 80-10m, bandswitched, and manually tuned...PI tuned input...to the
cathode.... the best we could get
on 10M was 7.5 kw. And 9 kw on all the lower bands. The high power PI
used a triple wafer model 85
bandswitch, with all 3 wafers in parallel, to handle the current. Tuned
input consisted of a pair of broadcast
air variables and a tapped solid 8 gauge coil..which was tapped with a small
single wafer bandswitch.
## That was extremely difficult to get right on 10M. If it was a monoband
10M, it would have been a lot
easier. The YC-156 is a 80-40-30-20-15-10m affair. Not used on 12m or 17m.
The loaded Q rises on 12M,
if the 10M position is re-used.
## I have seen one YC-156 monoband amp on 6M....and that was a trick and a
half...with eff not that great.
## On my hb 3CX-3000A7, I used a roller for 160-80-60-40-30 M..then .375
tubing taxcked on the end of the
roller for 20M. 15M tap on the 20m coil done with a modified HV solenoid
contactor. The L-PI trick was also
used...to get the 15M loaded Q way down....down to about 8. Then when 15M
position re-used on 17M band,
Q rises to 12. In this one off case, I had to use a .66 uh coil between
anode...and main PI net. It would also
work without the .66 uh coil, but the Q rose a lot higher, and could not be
re-used on 17M band.... Q was way too high on 17M.
## 3CX-3000A7 is 24 pf between anode + grid.....which rose to 33 pf, when
inserted into socket, same deal with proximity of lower
fins to the chassis. The 3CX-6000A7 measures 24.5 pf on the bench...and it
rises to 41 pf when in the socket.
## If I had to do this again, I would leave 10 + 12m out of the amp...and
instead build a monoband 10+12m amp. Since the
fil xfmr is external, and ditto with the B+ supply, no big deal to re-use
both on a monoband 10-12 amp.
## latest amp project is 160-80-40-20-17-15m.. band switched. I would not
use a roller again..pita, buys you nothing really.
RF parts used to supply a metal chimney for the 3x3 tube.....but it added
exactly 10 pf when installed. Useless chimney.
Their latest version uses clear acylic + teflon ring on top..then no
additional stray C.
## 8877 and 3-500Z is doable..and fairly straight forward. But these YC-156
metal tubes are no fun. Interesting to note,
the YC-156 is 4.94 diam..and the 3x6 is 6.125 diam..yet the 3x6 has way less
stray C. The ...ceramic stem on both the
3x3 and also 3x6 is way smaller diam than the YC-156. 3x3 and 3x6 both use
the same socket.
Jim VE7RF
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