-----Original Message-----
From: Carl
Sent: Monday, June 03, 2013 11:26 AM
To: Jim Thomson ; amps@contesting.com
Subject: Re: [Amps] Source of Mica Caps?
I have 2 TL-922's on the bench as I type. Both had a tube arc that opened a
grid choke and prevented any further damage with their original 1989 Eimacs.
Since those are among the troublesome date codes a replacement tube will get
them back to their owners while directly grounding could have cost a lot of
money.
### whoa. Tube arc from anode to grid ?? A good 3-500Z will hi pot test
to well
over 8 kv. Why would it arc from anode to grid in the 1st place ? If it
arcs from
anode to grid, the tube is probably toast. Put a 50 ohm, 50 watt wire
wound in series
with the B+ lead. Put a high speed HV fuse in series with the 50 ohm, 50
watt wire wound
glitch resistor. Put a 2nd HV fast fuse between one leg of HV AC
sec...and input to FWB.
Put a rear panel 3agc fuse holder, with a super fast grid fuse. Wire grid
fuse, so it is
between chassis and negative terminal of grid meter....or grid shunt. Toss
the 6 x grid caps
and both chokes away. Bond the 6 grid pins to the chassis with wide cu
strap.
## If tube arcs from anode to grid, the 50 ohm glitch resistor will LIMIT
the fault current
to a low value. 3000 v divided by 50 ohms = 60 A of fault current.
The 60A of fault current
will then blow open the fast HV fuse in under 2 msecs. That has been
verified on the bench with
test gear, scope etc. So LIMIT fault current, then INTERRUPT the fault
current. This same scheme
not only works on 3-500Z amps, it also works on any other GG tube like
GS35B, 3x3 and 3x6 tubes etc.
## draw too much dc grid current, and 3agc grid fuse blows open. No path
for dc grid current means
the amp can't be driven! Power output of amp drops to zero watts. SWR
between xcvr and input of
GG amp rises to infinity....and xcvr shuts down asap due to high swr. If
either HV fuse blows open, or the
dc grid fuse, simply replace and you are back in business.
I'll let amp owners decide which choice they want and hopefully not be
swayed by a lot of published nonsense from a tiny minority. If an amp
becomes unstable replace the parasitic suppressors with something resembling
the up to 40+ year originals and not voodoo nichrome crap. Learn how to
diagnose and fix a problem and not a symptom.
## The reason the CC resistors increase in value over time is cuz they
are subjected
to a ton of heat from the HOT anode ! The heat will travel down the
lead...and cook the
crap outa the 2 watt CC. Then you have the additional extra heat from the
normal suppressor
action, since most suppressors will run hot on 10m..and also 6m anyways.
Using something
more than 2 watts will allow some leeway and headroom in the design of the
10 or 6m
suppressor.
Ive been converting amps to 6M since the mid 60's and have gone thru several
suppressor designs; if an old one failed then replace the resistor.
Grounding the grids "may" work and they "may" also take off with new tubes;
my experience is it "will" happen at times especially with Chinese tubes. I
leaned to the side of stability and consider suppressor resistors as
consumables just as spark plugs and modern resistors last longer just as do
ignition system components.
## what various 6m suppressor designs are you talking about now....or is
this
just more trade secret stuff ?
Stop constantly whining about the SB-220 choke voltage drop. It affects
nothing and if it bothers you that much put an electrolytic across the
zener.
## 150 ma of grid current ..per tube is a lot. 300 ma in total on my L4B
amps.
Your SB-220 uses grid chokes that have a 25 ohm DC resistance. .150 x 25
ohms = 3.75 vdc
of V drop across each grid choke. You only have a 5 volt zener in there
for bias to begin with.
On peaks, you will now have 5 + 3.75 = 8.75V of bias on each tube. I call
that lousy bias regulation.
BTW, putting a lytic across the 5 V zener will buy you nothing. The zemer
is already regulated!
## You claim the V drop across the 25 ohm resistor provides NFB. And that
bonding the grids to the chassis
will reduce the NFB to zero, and simultaneously degrade IMD....and also
increase gain. Heath claims the NFB
comes about by the V divider action from the tubes grid to cathode C +
grid caps. Which is it ??
## the drake amps use grid chokes that have LESS than 1 ohm DC resistance.
I measured .91 ohm on 8 grid chokes
from 4 different drake L4B amps. .150 x .91 = .1365 vdc V drop across
each drake grid choke. Drake amps run ZERO bias, so the
bias will fluctuate between zero and a max of .1365 V. These days, I
bond the grids to chassis, but use regulated bias.
Like 3-10 x 6A10 diodes, switchable in 3 steps using a DPDT-center off
toggle on rear apron.
Plus a huge value lytic across the entire string. Bias doesn't budge on
ssb/cw.
I used calibrated watt meters on both the input and output of each
amp....and did
a before and after grid grounding with wide cu strap test. 20 - 25 watt
reduction in drive power when the grids are directly
bonded to the chassis ! So much for the 25 ohm dc resistance of the
heath grid chokes providing NFB, they don't. The heath
chokes + grid caps don't improve linearity either.
## with a 20-25 watt redux in drive requirements, the XCVR will run a lot
cleaner, IMD wise. ARRL extended lab results for the kenwood
870, the xcvr was tested at 50-85-100 watt pep out for the 2 tone imd
test..on 20M. The imd @ 100w pep out was lousy, -30 db pep.
it really cleans up when run at 85 w out....and really clean at 50 w out.
With a 20-25 w redux in drive requirements, the same 870 would be operating
at 75-80 w pep out.....with lower imd. Lower xcvr imd means lower imd
from the kw linear amp that is being driven by the 870. GIGO concept.
Jim VE7RF
Carl
KM1H
----- Original Message -----
From: "Jim Thomson" <jim.thom@telus.net>
To: <amps@contesting.com>
Sent: Monday, June 03, 2013 12:42 PM
Subject: [Amps] Source of Mica Caps?
Date: Sun, 2 Jun 2013 19:58:58 -0400
From: "Carl" <km1h@jeremy.mv.com>
To: "Jim W7RY" <w7ry@centurytel.net>, "Randall K Martin"
<rkmassoc@comcast.net>, <amps@contesting.com>
Subject: Re: [Amps] Source of Mica Caps?
I still dont understand the claim made by a few that increasing gain and
power output increases stability when hard grounding the grids.
I would take various web sites with a grain of salt and try to separate
the
good from bad.
The 115pf caps were chosen by Heath at the end of the SB-220 run and a
bulletin suggested changing them. The SB-221 and HL-2200 use the 115pf but
Ive replaced with whatever 110 and 120pf I come across in bulk. Ive yet to
have instability in a well built and updated SB-220, even on 6M, with the
Heath design.
Carl
KM1H
## TL-922 are rock solid when oem suppressors used and grids directly
grounded, and chokes tossed.
Ditto with drake L4B..and just about every other amp that floats the grids
with caps... including SB-220.
## Sb-220 with grids directly grounded to chassis works superb on 6m. I
have seen KM1H SB-220
6m conversions, where the grids were grounded to chassis...after the fact
by the owner..works great.
## The 25 ohm DC resistance of the chokes used in the SB-220 isnt doing
you any good..just more
unwanted V drop. The DC resistance of the chokes used in the Drake
L4B is less than 1 ohm.
I still tossed the chokes and caps in all 4 of my L4B amps.
Jim VE7RF
.
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