--- In ham_amplifiers@..., R L Measures <r@...> wrote:
RICH SEZ... The 3cx3000A7 has 0.6 pF of feedback C. That's 6x more
than the 8877 and 4x more than a 3-500Z. At its max-f rating of
110MHz, the 3cx3000A7 has an output to input feedback path with
2500-ohms of XC.
### agreed... so what ?? IMO... the grid is better grnded on a
3000A7... than a 3-500Z.
### In MOST cases... on a HB
3000A7... u can just toss the suppressor.
RICH SEZ...... especially if you just love to repair amplifiers.
### Build a 3000A7 linear.. and try it yourself. The one turn
chrome suppressor from the 8-K is essentially doing...nothing.
### I'm convinced... 98% of stability is how well the control
grid is grounded. A 3-500Z has only ONE internal grid pin....
RICH SEZ... It's a 360? cone shaped collet.
#### Which in turn splits into 3 x extremly thin grid pins !
They should just fed 1 x WIDE strap from the cone collet.. down
to the 3 base pins.... ditto with tubes like 811-A's, 572B's, etc.
RICH SEZ...If the 3-pins are grounded directly, the resonance at the
grid is in the 80MHz range. If the 3 grid pins are grounded
through 200pF caps, the grid resonance is in the 80MHz range. In
my experiences it is > better to measure resonance with a dipmeter
than to guess at it.
### Who cares ??? All I know is that when you directly strap all
6 x grid pins, with wide strap, to the chassis, the SB-220, SB-
221, TL-922 become rock stable... with the OEM
suppressor's.......and that's good enough for me. My only guess
as to why it works is perhaps the grids are now really grounded...
which also makes for 20-25 w LESS drive power.
### Now the AL-80BX and Ameritron with the 2 x 3-500Z's do have
the grids directly grnded.... and STILL spit, arc, etc. [the
guys on the east coast em.. the "firecracker's " ]. The AL-80BX
is a poorly built amp imo. My buddy had so much trbl with his
brand new one... he tossed it... and bought a used L4B... I
modified the L4B.. with the usual mods... 100% reliable.
### W8JI once modified a small GG metal triode... improved the
grid grnding... and built the worlds only [as far as I know] 160-
6m linear.... with NO parasitic suppressor !!
RICH SEZ.... So why does the AL-1500 - that he apparently designed -
have a reputation for sudden 8877 failures? As I see it, an
amplifier that appears to be stable during an hour or so of testing
is not guaranteed to be stable in the long run.
### dunno. I do know a lot of amateur's with the AL-1500.... and
they run flawlessly... some pound out 2500 w PEP on ssb.... for
literally 5 hrs every night.... and almost 365 days a yr... for
the last 4 yrs. IMO,Perhaps some simple protection would
help... like adding grid fuses, high speed grid + plate
overcurrent protection.. a FAST HV fuse..a cathode fuse.... and
a real 50 ohm, 50-100 w glitch resistor. Toss in some fast
high swr protection, and also dump the stock mech T/R relay
scheme.. and replace it with a high speed vac relay. Use of a
digital delay for cw + ssb will help immensely..... these
modern xcvr's crank out globs of RF instantly... even if the sped
up vac relay does operate in < 2msec. Also, xcvr's like the
ICOM's.. will have peak overshoots. That's easy to fix......
just use fixed dc volatge.. fed to the ALC jack at all times !
### In any event...in yrs gone by.. I always start with a 50 ohm
globar.... and keep adding turns[ 1 1/2... 2...2 1/2....3]
till the parasitic goes bye bye. If using a real small globar...
and u run outa room.... reduce the width of the strap.
RICH SEZ... Smooth move. Reducing the width of the strap increases
RF- resistance, which lowers the L/R suppressor's VHF-Q, which in
turn lowers VHF gain and reduces the chance of VHF oscillation.
#### Whoa nellie ! Believe me... plan A was to use 1" wide
strap.... which would mean using a 6" long - 150 w CCS globar...
which I didn't have on hand. At 15 kw... on the high bands, the
RF current is just unreal through that suppressor strap !!!
Another way of increasing RF-resistance is to choose a conductor
that is more resistive than copper. Example -- The 8169 /
4cx3000A amplifier at:
appears to have no parasitic suppressor whatsoever, however the
anode to blocking-cap connector strap, as well as the connection to
the Bruene-bridge neutralizing cap is made out of a nickle-
chromium alloy.
### Here's my problem with Nichrome. It GLOWS ORANGE on the
higher bands. It runs so hot... this thermal heat in turn cooks
the plate blockers !!! IMO, nichrome belongs.... in a toaster
oven. Use wide copper strap on globars = zero problems. ANY
SP type Globar will handle 350 deg C heat CCS... and when used
with copper strap... run just fine. Even a big metal
triode/tetrode will never get hotter than 225 deg C.. when maxed
to the peg.
### Stainless steel is bad too... it will turn BLACK.
### Rich, on the high bands, the tube C makes up a huge chunk of
the total C1 tune cap... which means extremely high RF currents on
17-10m......... all flowing through the parasitic strap
material ........which is NO place for nichrome. Rich, when u
finish ur 4 x 10... fire it up.... on 10/12m... with a dead
cxr.... for just a few seconds.... then report back to us. I wanna
know whether the bagel came out right.... or charcoal !
According to (W6HW) - a friend who worked for Collins Radio Co. -
the 30S-1 amplifier used the same technique.
### Called "L-101" on my 30S-1 schematic.
Later...... Jim VE7RF
