On Jul 19, 2006, at 5:16 PM, Jim Thomson wrote:
> #### see my comments below.... Jim VE7RF
>
> "I usually stay out of this sort of discussion and am sure that I
> will regret these comments. I may even get kicked off the list (at
> least threatened),
Jim -- If getting jackbooted off AMPS turns out to be your fate,
please e-mail your comments direct to me and the rest of the folks in
this thread and we will continue this debate for as long as it takes.
> but, statements/questions like "Why would you fuse a grid?" make my
> hair stand on end. Obviously, the gentlemen who asked this question
> dose not repair amps.
... OR PAY FOR THE PARTS TO REPAIR THEM!
> This is just about as bad/uninformed as the gentlemen who
> advertised for the original anode parasitic suppressors for the
> TL-922 with the idea of replacing modern, Lo Q nichrome
> suppressors, with the original "parasitic supporters" that came in
> the amp. I guess many of us are as blind and closed minded about
> amplifier theory as we are about politics.
Excellent comparison, but dangerously OFF TOPIC.
> We may have to pay $3.00 ++ for gasoline, after all, we did elect
> (or did we) an oil man to the White House,
Chortle. We elected TWO of them, and I for damn sure wouldn't go
quail hunting with one of them.
> but why pay $200 ++ for a 3-500Z when a 6 cent, 1/4 watt resistor,
> can/will save them from certain demise from several very common
> faults? "
>
> ### The original, STOCK parasitic suppressor in the TL-922
> works just fine ... ROCK STABLE..... PROVIDED you remove the 6
> x 200 pf caps and 2 x RF chokes on both tube
> sockets. ............ and then install 3 x wide copper
> straps... one per grid pin to chassis..... per socket.
Sounds plausible until one takes a dipmeter in hand and finds out
that there is minimal change in grid resonance between using 200pF
caps and using Cu straps. In fact, the bad news is that the grid
resonates about 1MHz Higher with caps instead of straps although the
purpose of the straps was to increase the grid resonance freq. and
improve stability.
-- Editorial --- As I see it, two of the most chortlesome beliefs in
Amateur Radio are: that a wide Cu strap has zero inductance, and
that there is such an animal as a non-inductive R.
>
> ### The drake L4B.... TL-922... SB-220 SB-221 + several Henry
> amps all used the identical 6 x caps + 2 x choke nonsense. It
> was supposed to be
>
> a " NFB" circuit using a V divider..... consisting of grid
> to cathode C in one leg..... and those silly 200 pf caps
> making up the 2nd leg. IT doesn't work... never did.
Good point, but since it was the idea of a beloved Amateur Radio
publisher who's now a SK, telling the truth that it never worked is
sure to be about as welcome as a dozen squealing piglets in a Mosque
during Friday prayers.
>
> ### everyone who has directly grounded the grids in the above
> 2 x 3-500Z amps all report rock stable operation..... with
> the stock suppressors... esp the TL-922 owners. In
> addition..... the fellows with the SB-220/221's report their TVI
> problems (satellite TV) in neighbours sets vanished after
> doing the direct grid grounding mods.
Excellent chortles, but you forgot to mention that bread rises faster
in the kitchen after direct grounding.
>
> ### also.... directly grounding the grids results in an actual
> measured REDUCTION of drive power by 20-25 watts !! You will
> measure that in all the above listed amps.... after the mod !
> In addition.... xcvr's like the Kenwood 870 etc, all
> experience a huge redux in TX imd when output of xcvr is reduced
> from 100-85-50 watts. Less drive into a linear means the
> xcvr is running cleaner to start with !
guffaw
>
> ### The measured TX IMD of the modified 3-500Z linears alone...
> ( listed above)... IMPROVES.... after the mod. The original
> "NFB" never worked.... in fact.... it made it WORSE !!
>
> ### Re grid fuses. On rinky dink tubes like
> 3CX-800A7's.....complete with 4 watt floozy grids......
> electronic protection IS required.
The trouble with electronic grid protection that I've seen is that
the small transistor that is commonly used to interrupt the flow of
grid current C-E shorts due to the large grid current that flows
during a VHF parasitic -- thereby providing zero protection during a
parasitic oscillation. For example, according to it's
characteristic curves, the cathode of an 8877 can produce 50A-peaks
24/7, and my guess is that it can do>60A-peak during a parasitic
event. Thus, the right size transistor for grid-protection duty is
probably Not one with a 2A rating.
>
> ### On 3-500Z's.... 3CX-1200A7's... 3CX-3000A7's (has a 225 watt
> CCS real grid...... the joke being you can use it as a dummy load
> for your FT-1000-D),
That's a good one, Jim.
> a grid fuse works very well. "rectifier fuses" are very
> fast. "Semiconductor fuses" are even faster. I install
> a simple 3 agc bayonet fuse holder on the rear apron of all my
> linears..... and use the appropriate size grid fuse.
> Here's the kicker. The grids are DIRECTLY grounded to chassis...
> 1st. The fast acting grid fuse is simply wired between
> chassis and neg of grid meter..... or chassis to grid shunt.
>
> ### the mass of a fuse element is < 1 gram.... which is
> miniscule compared to the mass of a typ grid.
> #### When the grid fuse pops open..... there is NO Return
> path for DC grid current.... amp can't be driven..... effectively
> shut off.......... very simple.... works every time.........end of
> story.
But wouldn't it put more feathers in your war bonnet to use something
that's more sophisticated and high-tech?
>
> ### Install some 6A10 safety diodes ( I use 2-3 in parallel
> for more peak surge) rated at 1 kv - 6 A CCS - 400 A surge...
> EACH between chassis and B- and also across plate and grid
> meters etc.
>
> #### The problem, IMO.. with 30 ohm grid fuses... is.... with
> normal grid current flowing through em.... you get a V drop
> across each resistor..... which amounts to wildy varying UN-
> NEEDED bias... added to ur existing zener, etc. Also... with
> TWO 30 ohm grid resistor's... u could end up with just ONE of
> em blowing open.... and have the remaining tube still
> working. And no.... you don't want the grid(s) left floating
> in mid air after the 30 ohm resistor explodes.
So it would seem, but a buoyant grid apparently causes no problem, no
matter whether the grid/gnd circuit opens or the HV-neg/gnd path
opens. After all, they're in series.
>
> ### IMO... a high speed, "semi conductor fuse" will blow a
> helluva lot faster than a 1/4 watt , 30 ohm resistor.
The trouble with semiconductors is that they tend to fail shorted,
not MIA.
> Also, one high speed fuse will protect several tubes.... with
> just one fuse. Also.... a rear panel mounted fuse holder...
> makes replacing the fuse a snap.....fast.
Fast, but also vulnerable to being replaced with a 20A-slowblow.
>
> ### SIZE the real grid fuse.... so it just handles the grid
> current.... don't oversize it.
> ### as a side note.... some have installed a 100 K- 2 watt MF
> resistor across the fuse holder... and/ or a MOV.
> ### use the electronic grid protection for the 4 watt tiny
> grids...... and if you do use electronic protection schemes......
> not only kick the linear (hotswitch it ) into standby... also
> apply some neg voltage to the ALC jack on the xcvr.... and also
> remove the grnd from the TX inhibit lead (if u have a FT-1000-
> D.... all 3 of those schemes will shut off the
> amp............. I use em for high reflected power conditions.
What if the electronic protection's switch device C-E shorts?
>
> ### electronic plate current protection schemes work pretty
> good too... same deal..... shuts the amp off..... in addition to
> above...... some will open off the HV via a vac relay... vac
> contactor.... or a HV relay designed to open circuit...fast.
>
> ### I use globars for glitch resistors in the larger HV
> supplies ( 100-300 uf.) consisting of 4 x 200 ohm globars in
> parallel... each 1" diam x 12" long...... also use BUSS HV
> sandfilled fuses...5" lone x 3/4" diam............. use one fuse
> in the B+....... and a 2nd HV fuse in one leg of xfmr
> secondary.... to diode board. Use one HV meter in RF
> deck.... and a 2nd HV meter in separate HV box. That way.....
> when B+ HV fuse pops open..... the plate V meter in the RF
> deck reads zero.......... the 2nd HV plate Meter in the HV
> supply... still reads 7800 Vdc.
Globars work okay in glitch service providing that the J-rating is a
consideration. The most capable Globars for glitch service are the
SP type, Presumably SP stands for surge protection.
cheers, Jim
>
> Later......... Jim VE7RF
>
>
>
>
>
>
R L MEASURES, AG6K. 805-386-3734
r@somis.org
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