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Im working on fixing up a L-4B, it has fried resistors
in the supressors. They use 3 x 15 ohms but I have none
in my junkbox however I have plenty of 10 ohm Rs, will
that do? 3 x 10 ohm Rs?
It doesnt seem to matter too much, I notice that the L-7
use 2 x 47 ohm Rs and the Henry 3K 2 x 150 ohm Rs, it
seems to be all over the place.

Im following VE7RFs advice to ground grid pins with
wide straps. Suppose I have to put in a fuse in the B-
return to be safe. Also Im swapping out the .82 R in
the PS for a better glitch R, also I put diodes across
the I meter.
Anything else I should think about?

73 Jim SM2EKM

Jim,

If you think you need a B- fuse, put a 500 ohm 10-25 watt resistor across
the fuse incase it opens. You should also have a 100-200 Ohm 25 watt B-
lead to ground in the power supply and the 1 ohm grid metering resistor in
the head. That way things don’t "run wild" if the fuse opens.

My large projects have 5 ohm 100 Watt glitch resistors in the PS.

What's a parasitic suppressor? OH no...

BOB DD

On Nov 6, 2006, at 7:03 AM, Jan Erik Holm wrote:

I?m working on fixing up a L-4B, it has fried resistors
in the supressors. They use 3 x 15 ohms but I have none
in my junkbox however I have plenty of 10 ohm R?s, will
that do? 3 x 10 ohm R?s?
It doesn?t seem to matter too much, I notice that the L-7
use 2 x 47 ohm R?s and the Henry 3K 2 x 150 ohm R?s, it
seems to be all over the place.

Hello, Jim. == In a VHF parasitic suppressor, when one sees
obliteration, it usually means VHF oscillation. The goals of a VHF
parasitic suppressor are to build one with a low VHF-Q that won't
melt-down at 29MHz. Typical factory-stock suppressors have a Q of c.
5 @ 100MHz. However, VHF suppressors can be built with a Q of 1.5.
This Q-decrease decreases VHF gain with the tradeoff of about 2%
power decrease at 29MHz.

I?m following VE7RF?s advice to ground grid pins with
wide straps.

please check grid resonance with a dipmeter before and after and get
back to us with the results.

- note - at and above a grounded grid's resonant frequency, the grid
no longer shields the cathode/input from the anode/output.

Suppose I have to put in a fuse in the B-
return to be safe. Also I?m swapping out the .82 R in
the PS for a better glitch R,

good idea. With a sturdy 10Ω- 20Ω glitch-R, why use a HV fuse?

also I put diodes across
the I meter.

good move, Jim

Anything else I should think about?

lower Q suppressors, step-start, high speed switching. 8160 final
amplifier?

73 Jim SM2EKM




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--- In ham_amplifiers@..., "Robert B. Bonner"
<rbonner@...> wrote:

Jim,

If you think you need a B- fuse, put a 500 ohm 10-25 watt resistor

across the fuse incase it opens.

### IF u install a fuse in the B-.... it's technically a "cathode
fuse"... and a 3agc fuse holder will suffice. Use a 750 ma fast
blow fuse.

## A 500 ohm 10-25 watt resistor across it is a throwback to
Orr's books.["low Z across cathode fuse"] We already tried that on
the 3x6 amp.. and other amps. We removed the 800 ohm 10 watt R...
and replaced it with a 100 K 3 watt MOF. IF Cathode fuse
blows... the V-drop across the 100 k resistor will bias the tubes
to cut off ASAP. Dissipation across the 100 K R is virtually zero
watts.



You should also have a 100-200 Ohm 25 watt B- lead to ground in
the power supply and the 1 ohm grid metering resistor in the head.

### What u just said doesn't make sense at all. Are u saying to
install the 100-200 R between B- and chassis ??? or in series
with B- ??? Both are bad idea's and more throwback's to Orr's
books. IF u install a 100-200 ohm R between B- and chassis....
and say u get a B+ to chassis arc.... fault current could travel
up the 100-200 ohm R..... which makes it now a way too high in
value...."glitch R" ... but only if you fried the 1 ohm grid
shunt..1st.

### If it's just to... "hold the B- close to chassis potential...
and not screw up the metering"... well , the B- is ALREADY at
chassis potentail... via the grid shunt. A better way to do all
this is to just use RVS connected 1N5408's BETWEEN chassis and B-

### This will ensure the B- can never wander more than +/- .7 V
Also, THEN... if u ever get a B+ to chassis arc... fault current
is up one side of the RVS connected diodes... then DIRECTLY to B-
completeing the loop... shorting the HV supply... and the 50 ohm
glitch will limit the Fault current to 53 A.. while the .82 ohm HV
fuse blows asap. [I have NEVER tripped the built in breakers in the
L4PS HV supply. ]





That way things don't "run wild" if the fuse opens.

### Do it with a 100 K across the cathode fuse... and nothing
happens.... just auto shuts down. BUT the CATHODE fuse HAS to
have A resistor across it at all times.... you don't want this
part of the circuit to ever open up... otherwise the cathode will
try and assume full plate V... and stuff like .01 uf bypass caps
between chassis and cold end of fil choke will start exploding.
make sure all the wiring from B-... all the way through to the CT of
the fil xfmr is rock solid.

My large projects have 5 ohm 100 Watt glitch resistors in the PS.

#### 5 ohms is not big enough value to limit current. 4 kv /5 ohms
= 800 A of fault current. 2 A of normal plate current [say a 2 x
8877 amp] thru a 5 ohm glitch will diss just 20 watts.

What's a parasitic suppressor? OH no...

### The stock suppressor in the L4B is fine as is... runs fine
with RTTY /FM/A0/AM on 10M too.

Later... Jim VE7RF

BOB DD

Bob,

I think I ditched the idea with a fuse. What the heck,
if my 8171 box doesnt have a fuse why bother with this
toybox.

73 SM2EKM
-------------
Robert B. Bonner wrote:

--- In ham_amplifiers@..., "Robert B. Bonner"
<rbonner@...> wrote:

--- In ham_amplifiers@..., "Robert B. Bonner"
<rbonner@> wrote:

Jim,

## A 500 ohm 10-25 watt resistor across it is a throwback to
Orr's books.["low Z across cathode fuse"] We already tried that

on

the 3x6 amp.. and other amps. We removed the 800 ohm 10 watt

R...

and replaced it with a 100 K 3 watt MOF. IF Cathode fuse
blows... the V-drop across the 100 k resistor will bias the tubes
to cut off ASAP. Dissipation across the 100 K R is virtually zero
watts.

***You would be surprised how little it takes to bias off an amp.

Currently

on one project I have a 3000 ohm 10 watt resistor biasing the tube

during

standby. If you lose a fuse do to some reason other than a HV arc

the 500

Ohm resistor does a sort of controlled slow shut down. 100,000 is

way up

there, to high in my opinion. You already used a 1000 ohm, and

mine is

playing with a 3000 ohm, and Hank uses 10K to bias the tube in

just about

everything.

### Anything from 500 ohms to 2 megohms works just fine. Rich's
electronic bias scheme used 1 meg. Depends on what tube u are
trying to cut off... and plate V. A 3x3 needs 45v to get it down
to just 1 ma idle current. [zsac] A lot of eimac specs will say
XXX V will produce 1 ma of zsac. IMO... somebody found a 25 k-
25 watt in their junk box... used it in a linear for cut off
bias.... on RX.... then it got copied into the next 25 hand books.

### IMO a 100 k or any value of MOF will be more reliable than a
wire wound.

However I have a 5 OHM 100 watt in the B+

You should also have a 100-200 Ohm 25 watt B- lead to ground in
the power supply and the 1 ohm grid metering resistor in the head.

### The grid meter in my case is a separate meter 0-1000Ma... shunt
is built into it... abt .82 ohm I think. Can't stand
multimeter's. I want to be able to see all the metering all the
time.. all at once... including fil V and current, plate V, grid
I.. plate I.. plate xfmr pri v and I... etc.

The 100-200 ohm resistor is for WHEN the fuse blows, anything over

10X the meter resistor doesn't effect the meter calibration. A 100
Ohm keeps the B- close to ground if the fuse blows. That's what its
for.

### That 100-200 ohm resistor from B- to chassis just keeps B-
from straying to far. IF ur cathode fuse BLOWS in the CT of a
thoriated tungsten fil tube like 3-500Z or a 3x3.... the B- is
STILL at chassis potential... via the grid shunt !!! A better
method is to just use RVS connected diodes between chassis and B-
.... then B- can never float more than +/- .7 V Dunno abt oxide
tubes like a 8877... with a separate cathode.

### IF u got a B+ to chassis short.... and u didn't have diodes
across ur meter's... and only a 200 ohm r between chassis and B-
...... u would smoke both meter's.... IF the meter's blew wide
open.... the only other alternate path is through ur 200 ohm
resistor from chassis to B-. IF the meter's were protected,
the B+ would come out of the chassis.. thru both meter'... back
to B-.... shorting out the B+ supply.

*** That way things don't "run wild" if the fuse opens. Yes

without a resistor or if that resistor opened up there could be a
safety issue.

### agreed. IMO the MOF resistor would be less likely to open up
than a wire wound. For redundacy... a PAIR of 100 k 2-3 w mof's
across the fuse... or a pair of wirewounds would be the ultimate
answer... problem solved.

There are many ways to design and or build amplifiers all are just

fine and values can be variable within reason.

### Partially agreed. some designs are flawed... some are just
outright dangereous. Some older ARRL designs put the HV meter
between B+ and chassis... instead of B+ and B- ... no R between
B- and chassis.. and no meter protection. IF grid shunt ever
opened up... or no connection via multi meter switch... HV meter
would drop to zero.... when in fact.. HV is still present... and
lethal.

I use 1 ohm grid shunt. A 100 Ohm in the PS B- to chassis,

Usually 10K to bias the tube during standby and have used as little
as 3000. A pair of 20 OHM step start resistors and 100K bleeder
across the power supply.

### You only need step start resistance in one leg of the 240 v
line... and only one spst relay/ contactor to short it out. Only
thing of interest in a HV step start is loop resistance in the
primary. Having said that... I tried 50 ohms... and also 25
ohms. 50 ohms will limit 240 v inrush to 4.8 A 25 ohms will
limit 240 v inrush to 9.6 A I found with 50 ohms... that plate
V would not rise as high as 25 ohms.... and when shunted, the
secondary surge was WAY higher. That was with 6800Vdc supply...+
100 uf filter cap.... and 14 second delay. A 0-50 A ameter in one
leg of the 240 V primary will give one a real eye opener...esp that
sec surge.

### 25 ohms works great.. and any relay/contactor will handle
9.6A. Plate V starts at zero.... just sails right uo.... then
slowly keeps climbing... then it jumps that last little bit. Energy
in joules goes to the square of the voltage. A cap that only
has 1/2 V on it.... is only 1/4 charged up..... u want the V up as
high as u can get... b4 shunting... and a 25 ohm resitor works
better than a 50 ohm. [for a 6.8 kv + 100 uf filter]

I have always placed 500 across the cathode fuse, however the bias

size would also work. The 500 will drop amp gain to nothing almost
instantly and you've still got power applied at that point. That RF
needs to go somewhere.

I haven't experimented with my expensive tubes by removing the

cathode fuse and applying power... I've only had 1 cathode fuse
POP in 35 years, and that was during power up for some reason.

### Try it... tube shuts down.... works great.

I have never used a 100K for a standby resistor. I would

personally prefer to just cut off the tube rather than bury it in
cutoff.

### Anything from 600 ohms to 2 megs works fine for the RX cut
off... and no, u are not burying it into cutoff. IF it actually
was really toatally 100% cut off.... and no current flow... u
would also have NO v drop... and NO cut off bias developed. Fact
is...it reaches equilibrium.... it's never quite cut off..... just
enough current flows to almost cut it off ... but not quite. EG: to
get 45 v of cut off bias.... you will have .45ma of current flowing
through the 100 K resistor. [diss =.02 W]

#### Haven't tried anything above 2 meg.... I'm sure even 10 meg
would work.... u just don't EVER want a wide open.... so a PAIR
of resistors... either MOF OR wire wounds would be better... then
u have redundancy.

later... Jim VE7RF

Yeah who needs fuses?

Current limit with the power company.

Hello? Yes this is NUKE STATION #6, yes this is SM2EKM I'm coming on line,
pull the rods out.

Hee hee.

Thanks Bob, nice to start the day with a nice laugh.

73 Jim SM2EKM
--------------

Robert B. Bonner wrote: