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Gent's

Some poor fellow on the 'other' amp reflector smoked the 25 ohm 25
watt wire wound glitch R on his Henry 3K .

What was not mentioned was that Henry installs the glitch R in
the B-..... NOT the B+.... no big deal... cept the glitch R + all
wiring in/out of the glitch R has to use 10 kv wire.. and
glitch R well insulated from chassis... on stand offs.

He also smoked the safety diode between chassis and one end of
the glitch R. He had an arc between base of plate choke and
chassis.

Fault current simply flows from chassis up through under rated
puny glitch R.... through the glitch R... back to B-...... whereby u
guessed it.... the safety diode shorted out [wouldn't handle the
surge]... then the 4 kv is sitting right across the 25 ohm glitch
R... blowing it to Kingdome come !

The fellow replaced the glitch R... amp works.. BUT his grid meter
is off the scale ! As u can see if u trace it... with a shorted
safety diode.... the plate AND grid meter are now directly in
parallel ! [neg of grid meter bonded to chassis... flows along
chassis... through shorted safety diode back to B-... then to neg
of plate meter] 1/2 plate current flows through EACH meter..
then the grid meter get's smoked.

The fix is of course.. 2-3 x 6A diodes in parallel between
chassis and in his case.. glitch R.... OR B-.... if glitch R is in
the B+ lead ["normal amp"]

IF he had a HV fuse in the B+.... he would NEVER have smoked the
glitch R. The slow breaker in his 240 V line is just that... way
too damned SLOW.


Also... IF he had a FAST grid fuse between chassis and neg of
grid meter/shunt... u would not smoke the grid meter. He shoulda
also had safety diodes across both the grid + plate meter. On a
8877... it really needs electronic fast grid overcurrent
protection..... which I doubt would even work with the safety
diode shorted between chassis and his glitch R/ B-

To stop the arcing between base of plate choke and chassis...
install a teflon /plastic disc or square, between chassis and
bottom of plate choke. On a Henry.. with a choke in it... make
sure the bypass caps at the base of the plate choke are good for
10 kv. .... in case a wirewound let's go in the bleeder string...
otherwise.... no load plate V will skyrocket... as the HV supply
now looks like a C input filter.

later... Jim VE7RF

On Nov 3, 2006, at 5:10 PM, pentalab wrote:

Gent's

Some poor fellow on the 'other' amp reflector smoked the 25 ohm 25
watt wire wound glitch R on his Henry 3K .

What was not mentioned was that Henry installs the glitch R in
the B-..... NOT the B+.... no big deal... cept the glitch R + all
wiring in/out of the glitch R has to use 10 kv wire.. and
glitch R well insulated from chassis... on stand offs.

Not only that, but the filament transformer must to be able to withstand the full anode V. This is why Henry Radio amplifiers need circuit improvements before they are fired up -- sometimes quite literally.
Another problem is that this particular resistor usually has an adjustable tap / slider -- and that makes it not suitable for glitch service.
A friend named Jim who repairs and builds amplifiers once asked Henry why they use VHF parasitic suppressors that exhibit a high-Q (c. 5.5 @100MHz) . The answer: Because we have always used them.

He also smoked the safety diode between chassis and one end of
the glitch R. He had an arc between base of plate choke and
chassis.

Which is another reason why the glitch R needs to go in the HV+.

Fault current simply flows from chassis up through under rated
puny glitch R.... through the glitch R... back to B-...... whereby u
guessed it.... the safety diode shorted out [wouldn't handle the
surge]... then the 4 kv is sitting right across the 25 ohm glitch
R... blowing it to Kingdome come !

The fellow replaced the glitch R... amp works.. BUT his grid meter
is off the scale ! As u can see if u trace it... with a shorted
safety diode.... the plate AND grid meter are now directly in
parallel ! [neg of grid meter bonded to chassis... flows along
chassis... through shorted safety diode back to B-... then to neg
of plate meter] 1/2 plate current flows through EACH meter..
then the grid meter get's smoked.

The fix is of course.. 2-3 x 6A diodes in parallel between
chassis and in his case.. glitch R.... OR B-.... if glitch R is in
the B+ lead ["normal amp"]

IF he had a HV fuse in the B+.... he would NEVER have smoked the
glitch R. The slow breaker in his 240 V line is just that... way
too damned SLOW.


Also... IF he had a FAST grid fuse between chassis and neg of
grid meter/shunt... u would not smoke the grid meter. He shoulda
also had safety diodes across both the grid + plate meter. On a
8877... it really needs electronic fast grid overcurrent
protection.....

The problem with 8877s is not grid damage from too much HF grid-I, it's gold-evaporation from the grid caused by occasional UHF parasitic oscillations that causes large bursts of UHF grid-I that evaporates off thin layers of gold, which in turn forms gold melt balls that create leakage paths in the vacuum. It is my opinion that the way to stop gold sputtering is to limit peak discharge current from the HV filter C with a sturdy glitch R in the HV+ lead.
As I see it, 8877s have so much UHF gain that they can be a pain.

which I doubt would even work with the safety
diode shorted between chassis and his glitch R/ B-

agreed

...

R L Measures, AG6K, 805.386.3734
r@..., , rlm@..., www.somis.org

--- In ham_amplifiers@..., R L Measures <r@...> wrote:

On Nov 3, 2006, at 5:10 PM, pentalab wrote:

Gent's

Some poor fellow on the 'other' amp reflector smoked the 25

ohm 25

watt wire wound glitch R on his Henry 3K .

What was not mentioned was that Henry installs the glitch R in
the B-..... NOT the B+.... no big deal... cept the glitch R +

all

wiring in/out of the glitch R has to use 10 kv wire.. and
glitch R well insulated from chassis... on stand offs.

RICH SEZ.... Not only that, but the filament transformer must to

be able to withstand the full anode V. This is why Henry Radio
amplifiers need circuit improvements before they are fired up --
sometimes quite literally.

### Point well taken !! I drew it out on paper... ur right...
the fil xfmr will spike to full B+.... with the glitch R.. in
the B- lead. What were these guys thinking ? Easy to fix.


Another problem is that this particular resistor usually has an
adjustable tap / slider -- and that makes it not suitable for
glitch service.

### Why install an adjustable wirewound ?? Plane nuts. Try
this for an experiment... if u are brave enough. Install a 25 ohm-
25 watt wire wound wound... in a metal bomb proof box... fed
with 10 kv Red /Black Belden HV wire.... Hook the other ends of
the HV wire DIRECTLY to the B+ and B- of ur favourite HV
supply..... like a 3-K, etc....[NO HV fuse installed] Install a
heavy duty vac relay or better yet.. a vac contactor in the B+
lead. Turn on the HV supply.... stand back... and activate the
vac relay..... applying 3-8 kv across the glitch R...... then
lemme know IF the glitch R remains intact.

He also smoked the safety diode between chassis and one end of
the glitch R. He shoulda
also had safety diodes across both the grid + plate meter. On a
8877... it really needs electronic fast grid overcurrent
protection.....

RICH SEZ.... The problem with 8877s is not grid damage from too

much HF grid-I, it's gold-evaporation from the grid caused by
occasional UHF parasitic oscillations that causes large bursts of
UHF grid-I that evaporates off thin layers of gold, which in turn
forms gold melt balls that create leakage paths in the vacuum. It
is my opinion that the way to stop gold sputtering is to limit peak
discharge current from the HV filter C with a sturdy glitch R in the
HV+ lead. As I see it, 8877s have so much UHF gain that they can be
a pain.

which I doubt would even work with the safety
diode shorted between chassis and his glitch R/ B-

agreed

### Here's the deal..Now this IS important. . A buddy of mine
brought over a QST article a few yrs back... in the stray's
dept... it depicted ur typ "electronic grid overcurrent"
protection scheme.... seen in many handbook projects... and also
used in many commercially produced linears. In many handbook amp
designs.... you will see a pair of diodes REVERSE connected...
between the chassis and B- Only one diode is really needed...
the worse thing that can happen usually.. is a B+ to chassis
short.

### The tech note went on to show what really happens... IF
reverse connected diodes are installed from chassis to B- AND
electronic grid protection is used. They re-drew the entire
schematic.... and what is now depicted is the 2nd diode [not
needed].... ends up DIRECTLY in parallel... with.. u guessed it....
the small value resistor installed between grid shunt and
chassis. This small value resistor, [typ 5-15 ohms] has DC grid
current flowing through it all the time. It also has a V drop
across it.. typ 1-3 volts... which is fed to the wiper of a
pot... used as a V divider... then applied to a transistor...
activates a relay.. etc.. kicks amp offline, lights LED...
latches... etc. Trbl is... If the un needed 2nd safety diode
is used.... the V drop across the resistor is directly in parallel
with it... and turns on the diode !!!! DC Grid current is then
partially or fully diverted through the un-needed safety diode....
and u guessed it... the dc grid meter is now reading too low !

### The fix is... to remove the un-needed safety diode.

### On my 3000/6000A7 designs... NO electronic grid overcurrent
protection is used. A simple fast 3agc fuse is installed
between neg of grid meter and chassis. [with nothing across the
fuse either]. RVS connected 6 A diodes ARE used [I use 2-3
6A diodes in EACH direction... 4-6 in total].. wired between
chassis and B- I don't install any 10-1000 ohm resistor
between B- and chassis in either RF deck.... OR HV supply. The
RVS connected diodes will never allow the B- to float more than
+/- .7v from chassis potential[grnd]...(grid fuse open). With
the grid fuse intact... the B- ends up at chassis potential
anyway... since the neg of the grid meter is fed via grid fuse...
to chassis.

### Originally, we installed a 100 K 3 watt resistor across the
grid fuse holder... thinking IF the grid fuse opened up... the V
drop across the 100 k resistor would bias the amp off.... it
doesn't ! With 800 w of drive applied on a 3x6... and the grid
fuse OPEN... all of a sudden you get 60 w reflected power on the
input of the amp [normally zero]... and abt 20% of normal power
output... [2500w instead of normal 12 kw]. We removed the 100 K
resistor... leaving a wide open (if fuse opens up).... and now with
drive applied.. and an open grid fuse.... u get ZERO watts
output.... and of course... ZERO DC grid current. [desired effect]

### The BIG question is.... WHERE the hell does this 800 watts of
drive end up ??? It's still being fed to cathode. Does it
just sit there and cook the cathode ??

## In any event the grid fuse concept works superbly..
everytime. Some reason's for the grid fuse blowing are.. excess
drive with a dead cxr... HV fuse blown... underloading, etc. I'm
now thinking that since all bugs are out... and all steady state
grid/plate current/ fil V/ plate V measurements have been done....
.. and now we just pulse tune it... then talk.... OR just dial up
the vac tune load caps... "by the pre-set numbers".. we can
probably REDUCE the size of the fast grid fuse from the current
750 Ma... down to something like 400-500 ma.

### Also.. my scheme of paralleling a PAIR of 50A magnetic-
hydraulic breaker to make one big 100A breaker's works quite
well.... one such assy is installed in EACH leg of the 240V line.
The kicker here is... we removed the tie bar on each assy. With
steady state dead cxr stuff... all 4 x poles are turned on.... with
ssb.... u can safely shut one pole off per assy. [now it's a 50 A
breaker] THEN.. if something goes amuck... with a lower value
grid fuse.. and pri breaker [ HV fuse remained same value.. 3
A] ... while on ssb.... both the pri breaker and lower valued grid
fuse will blow even faster.

Later... Jim VE7RF

R L Measures, AG6K, 805.386.3734
r@..., , rlm@..., www.somis.org

On Nov 5, 2006, at 2:06 AM, pentalab wrote:

--- In ham_amplifiers@..., R L Measures <r@...> wrote:

On Nov 3, 2006, at 5:10 PM, pentalab wrote:

Gent's

Some poor fellow on the 'other' amp reflector smoked the 25

ohm 25

watt wire wound glitch R on his Henry 3K .

What was not mentioned was that Henry installs the glitch R in
the B-..... NOT the B+.... no big deal... cept the glitch R +

all

wiring in/out of the glitch R has to use 10 kv wire.. and
glitch R well insulated from chassis... on stand offs.

RICH SEZ.... Not only that, but the filament transformer must to

be able to withstand the full anode V. This is why Henry Radio
amplifiers need circuit improvements before they are fired up --
sometimes quite literally.

### Point well taken !! I drew it out on paper... ur right...
the fil xfmr will spike to full B+.... with the glitch R.. in
the B- lead. What were these guys thinking ?

They probably weren't.

Easy to fix.

indeed

Another problem is that this particular resistor usually has an
adjustable tap / slider -- and that makes it not suitable for
glitch service.

### Why install an adjustable wirewound ?? Plane nuts.

It was part of the over-current relay ckt. -- something that
thoriated-tungsten filament tubes do not need because their emissive
layer (di-tungsten carbide / W^2C) can not be damaged by too much
cathode current - as is the case with oxide cathode tubes such as the
8877.

Try
this for an experiment... if u are brave enough. Install a 25 ohm-
25 watt wire wound wound... in a metal bomb proof box... fed
with 10 kv Red /Black Belden HV wire.... Hook the other ends of
the HV wire DIRECTLY to the B+ and B- of ur favourite HV
supply..... like a 3-K, etc....[NO HV fuse installed] Install a
heavy duty vac relay or better yet.. a vac contactor in the B+
lead. Turn on the HV supply.... stand back... and activate the
vac relay..... applying 3-8 kv across the glitch R...... then
lemme know IF the glitch R remains intact.

A 25W ww resistor is c. 2" long. 3000v is on the ragged edge of too
much V-gradient for a 2" resistor. Since 10w ww resistors are also
c. 2" long, it would be preferable to series two of them for glitch
service at 3000v or greater. The ultimate solution is to use a surge-
rated - in Joules - type resistor. The glitch resistors we current
supply with our low-Q suppressor retrofit kits are rated at 120J,
which is slightly more than what a SB-220 HV-PS is capable of during
a serious glitch.

He also smoked the safety diode between chassis and one end of
the glitch R. He shoulda
also had safety diodes across both the grid + plate meter. On a
8877... it really needs electronic fast grid overcurrent
protection.....

RICH SEZ.... The problem with 8877s is not grid damage from too
much HF grid-I, it's gold-evaporation from the grid caused by
occasional UHF parasitic oscillations that causes large bursts of
UHF grid-I that evaporates off thin layers of gold, which in turn
forms gold melt balls that create leakage paths in the vacuum. It
is my opinion that the way to stop gold sputtering is to limit peak
discharge current from the HV filter C with a sturdy glitch R in the
HV+ lead. As I see it, 8877s have so much UHF gain that they can be
a pain.

which I doubt would even work with the safety
diode shorted between chassis and his glitch R/ B-

agreed

### Here's the deal..Now this IS important. . A buddy of mine
brought over a QST article a few yrs back... in the stray's
dept... it depicted ur typ "electronic grid overcurrent"
protection scheme.... seen in many handbook projects... and also
used in many commercially produced linears. In many handbook amp
designs.... you will see a pair of diodes REVERSE connected...
between the chassis and B- Only one diode is really needed...
the worse thing that can happen usually.. is a B+ to chassis
short.

... which would be the case if the glitch-R was not capable of
limiting I-pk to the diode's peak-current ability. (typ. 200A-pk in
3A diodes or 400A-pk in 6A diodes)

### The tech note went on to show what really happens... IF
reverse connected diodes are installed from chassis to B- AND
electronic grid protection is used. They re-drew the entire
schematic.... and what is now depicted is the 2nd diode [not
needed].... ends up DIRECTLY in parallel... with.. u guessed it....
the small value resistor installed between grid shunt and
chassis. This small value resistor, [typ 5-15 ohms] has DC grid
current flowing through it all the time. It also has a V drop
across it.. typ 1-3 volts... which is fed to the wiper of a
pot... used as a V divider... then applied to a transistor...
activates a relay.. etc.. kicks amp offline, lights LED...
latches... etc. Trbl is... If the un needed 2nd safety diode
is used.... the V drop across the resistor is directly in parallel
with it... and turns on the diode !!!! DC Grid current is then
partially or fully diverted through the un-needed safety diode....
and u guessed it... the dc grid meter is now reading too low !

Use a 0.5Ω grid-current meter shunt-R and one can read up to 1A of
grid-I without exceeding a Si diode's 0.5v threshold point.

### The fix is... to remove the un-needed safety diode.

### On my 3000/6000A7 designs... NO electronic grid overcurrent
protection is used.

Does the 3cx6000A7 have a gold-plated grid?

A simple fast 3agc fuse is installed
between neg of grid meter and chassis. [with nothing across the
fuse either].

I would put a transient suppressor diode across the fuse. They are <
a dollar.

RVS connected 6 A diodes ARE used [I use 2-3
6A diodes in EACH direction... 4-6 in total].. wired between
chassis and B- I don't install any 10-1000 ohm resistor
between B- and chassis in either RF deck.... OR HV supply.

good

The
RVS connected diodes will never allow the B- to float more than
+/- .7v from chassis potential[grnd]...(grid fuse open). With
the grid fuse intact... the B- ends up at chassis potential
anyway... since the neg of the grid meter is fed via grid fuse...
to chassis.

### Originally, we installed a 100 K 3 watt resistor across the
grid fuse holder... thinking IF the grid fuse opened up... the V
drop across the 100 k resistor would bias the amp off.... it
doesn't ! With 800 w of drive applied on a 3x6... and the grid
fuse OPEN... all of a sudden you get 60 w reflected power on the
input of the amp [normally zero]... and abt 20% of normal power
output... [2500w instead of normal 12 kw]. We removed the 100 K
resistor... leaving a wide open (if fuse opens up).... and now with
drive applied.. and an open grid fuse.... u get ZERO watts
output.... and of course... ZERO DC grid current. [desired effect]

### The BIG question is.... WHERE the hell does this 800 watts of
drive end up ??? It's still being fed to cathode. Does it
just sit there and cook the cathode ??

How can you measure P if the input Z goes bananas?

## In any event the grid fuse concept works superbly..
everytime. Some reason's for the grid fuse blowing are.. excess
drive with a dead cxr... HV fuse blown... underloading, etc. I'm
now thinking that since all bugs are out... and all steady state
grid/plate current/ fil V/ plate V measurements have been done....
.. and now we just pulse tune it... then talk.... OR just dial up
the vac tune load caps... "by the pre-set numbers".. we can
probably REDUCE the size of the fast grid fuse from the current
750 Ma... down to something like 400-500 ma.

If it works, why fix it?

### Also.. my scheme of paralleling a PAIR of 50A magnetic-
hydraulic breaker to make one big 100A breaker's works quite
well.... one such assy is installed in EACH leg of the 240V line.
The kicker here is... we removed the tie bar on each assy. With
steady state dead cxr stuff... all 4 x poles are turned on.... with
ssb.... u can safely shut one pole off per assy. [now it's a 50 A
breaker] THEN.. if something goes amuck... with a lower value
grid fuse.. and pri breaker [ HV fuse remained same value.. 3
A] ... while on ssb.... both the pri breaker and lower valued grid
fuse will blow even faster.

Is faster better?

Later... Jim VE7RF

R L Measures, AG6K, 805.386.3734
r@..., , rlm@..., www.somis.org

Yahoo! Groups Links

R L Measures, AG6K, 805.386.3734
r@..., , rlm@..., www.somis.org

--- In ham_amplifiers@..., R L Measures <r@...> wrote:

### Why install an adjustable wirewound ?? Plane nuts.

RICH SEZ.... It was part of the over-current relay ckt. --

something that thoriated-tungsten filament tubes do not need
because their emissive layer (di-tungsten carbide / W^2C) can not
be damaged by too much cathode current - as is the case with oxide
cathode tubes such as the 8877.

### Apples and oranges. The 25 ohm 25 watts wire wound R in the
B- lead is fixed..NOT adjustable. It IS the glitch R. The
variable one you are refering to is I believe, part of the FIL
step start ckt. I believe the slider is also used to set the
correct fil V... so they can kill 2 x birds at once.

Try
this for an experiment... if u are brave enough. Install a 25

ohm-

25 watt wire wound wound... in a metal bomb proof box... fed
with 10 kv Red /Black Belden HV wire.... Hook the other

ends of

the HV wire DIRECTLY to the B+ and B- of ur favourite HV
supply..... like a 3-K, etc....[NO HV fuse installed] Install

a

heavy duty vac relay or better yet.. a vac contactor in the B+
lead. Turn on the HV supply.... stand back... and activate

the

vac relay..... applying 3-8 kv across the glitch R...... then
lemme know IF the glitch R remains intact.

RICH SEZ...

A 25W ww resistor is c. 2" long. 3000v is on the ragged edge of

too

much V-gradient for a 2" resistor. Since 10w ww resistors are

also

c. 2" long, it would be preferable to series two of them for

glitch

service at 3000v or greater. The ultimate solution is to use a

surge-

rated - in Joules - type resistor. The glitch resistors we

current

supply with our low-Q suppressor retrofit kits are rated at 120J,
which is slightly more than what a SB-220 HV-PS is capable of

during

a serious glitch.

### agreed... or just use a 50 w wirewound.. if it will fit. A
50 ohm -50 w wire wound fits perfectly on the Millen on the L4B RF
deck. The glitch R on the new HV supply under construction
consists of 4 x paralled 275 w, 200 ohm globars. These are the
old CX type... built in the 60's... can't be immersed in oil... and
are 1" diam x 12" long. With A0.. they will dissipate 450
W... so a pair of 6" fans will suck air outa the Separate HV
cab.... everything is positioned in such a fashion so the plate
xfmr and caps and diode board plus glitch R's get cooled.
Each glitch R is mounted on standoffs, horizontal, just behind
front panel... and 4 x high... so they eat up a 6" x 12" space...
hence the 2 x 6" diam fans side by side on front panel. With
speed control... not much noise.

He also smoked the safety diode between chassis and one end

of

the glitch R. He shoulda
also had safety diodes across both the grid + plate meter.

On a

8877... it really needs electronic fast grid overcurrent
protection.....

RICH SEZ.... The problem with 8877s is not grid damage from too
much HF grid-I, it's gold-evaporation from the grid caused by
occasional UHF parasitic oscillations that causes large bursts

of

UHF grid-I that evaporates off thin layers of gold, which in

turn

forms gold melt balls that create leakage paths in the vacuum.

It

is my opinion that the way to stop gold sputtering is to limit

peak

discharge current from the HV filter C with a sturdy glitch R

in the

HV+ lead. As I see it, 8877s have so much UHF gain that they

can be

a pain.

### agreed..... plan B is to use a REAL tube. In fact.. that's
also Plan A.

which I doubt would even work with the safety
diode shorted between chassis and his glitch R/ B-

agreed

### Here's the deal..Now this IS important. . A buddy of mine
brought over a QST article a few yrs back... in the stray's
dept... it depicted ur typ "electronic grid overcurrent"
protection scheme.... seen in many handbook projects... and also
used in many commercially produced linears. In many handbook

amp

designs.... you will see a pair of diodes REVERSE connected...
between the chassis and B- Only one diode is really

needed...

the worse thing that can happen usually.. is a B+ to chassis
short.

RICH SEZ..... which would be the case if the glitch-R was not

capable of limiting I-pk to the diode's peak-current ability. (typ.
200A-pk in 3A diodes or 400A-pk in 6A diodes)

### agreed. 8 kv /50 ohm glitch = 160A Those surge ratings
for diodes are only 1/2 cycle ratings... or just 8.3 msecs. The
100+ uf filter cap's I use, PLUS the 50 ohm glitch R form a RC
network.. dragging out the DURATION of the surge. To avoid
stressing the safety diodes.... I use 2-3 paralleled 6 A
diodes.... + a HV fuse. 160 A will open a HV fuse <2 msecs.
With no HV fuse... you require some big surge rated diodes... and
6 A diodes are dirt cheap..... unless you are Ameritron... and can
only aford one x 1 cent 30 A rated diode.

### The tech note went on to show what really happens... IF
reverse connected diodes are installed from chassis to B- AND
electronic grid protection is used. They re-drew the entire
schematic.... and what is now depicted is the 2nd diode [not
needed].... ends up DIRECTLY in parallel... with.. u guessed

it....

the small value resistor installed between grid shunt and
chassis. This small value resistor, [typ 5-15 ohms] has DC grid
current flowing through it all the time. It also has a V drop
across it.. typ 1-3 volts... which is fed to the wiper of a
pot... used as a V divider... then applied to a transistor...
activates a relay.. etc.. kicks amp offline, lights LED...
latches... etc. Trbl is... If the un needed 2nd safety

diode

is used.... the V drop across the resistor is directly in

parallel

with it... and turns on the diode !!!! DC Grid current is

then

partially or fully diverted through the un-needed safety

diode....

and u guessed it... the dc grid meter is now reading too low !

Rich SEZ...Use a 0.5? grid-current meter shunt-R and one can read

up to 1A of grid-I without exceeding a Si diode's 0.5v threshold
point.

### Apples and oranges. The v drop across the grid shunt was not
the issue. It's the small 5-15 ohm resistor in SERIES with the
shunt.... and the Vdrop across this 5-15 ohms is used for the
electronic grid overcurrent ckt. With the electronic grid
current protection removed... u are only left with the normal tiny
shunt resistance of a typ 0-1000 ma grid meter... which typ
consists of a 50 ohm internal resistance 0-1 ma meter in parallel
with a small value shunt... like .082 ohm.

### The fix is... to remove the un-needed safety diode.

### On my 3000/6000A7 designs... NO electronic grid overcurrent
protection is used.

RICH SEZ... Does the 3cx6000A7 have a gold-plated grid?

### point well taken... no it does not... it's a "real deal 225 W
CCS grid"... the kind that can be used as a dummy load for ur FT-
1000D. I didn't want to mess with oxide tubes with floozy 4
watt grids in em... like 3CX-800A7's. With those kind of tubes you
have two problems.... the fragile grid... AND the oxide cathode.
One too many times with spikes from overshoots from Xvcr's...
and you will eventually fry the oxide cathode's. It's a nice
tube... but for 2-3 of em... they are NOT cost effective. IMO...
3x3 and 3x6/YC-243's are the way to go.... huge bang for the
buck... zero warmup... can be rebuilt countless times.... and
reasonably priced. Both the 3x3 and 3x6 are used in FM
broadcast around the globe. Those tubes will be around for a
LONG time.

A simple fast 3agc fuse is installed
between neg of grid meter and chassis. [with nothing across

the

fuse either].

RICH SEZ... I would put a transient suppressor diode across the
fuse. They are a dollar.

### I'm assuming you mean a MOV ? Why put a MOV across the
fuseholder ??? IF so... what value MOV? 10-50 V ???
The RVS connected 6 A diodes between chassis and B- are
ALREADY in parallel with the grid fuse holder !.... so no MOV is
really required. We also used RVS connected 6A diodes across
both grid + plate meter's.

RVS connected 6 A diodes ARE used [I use 2-3
6A diodes in EACH direction... 4-6 in total].. wired between
chassis and B- I don't install any 10-1000 ohm resistor
between B- and chassis in either RF deck.... OR HV supply.

RICH SEZ... good

The
RVS connected diodes will never allow the B- to float more

than

+/- .7v from chassis potential[grnd]...(grid fuse open). With
the grid fuse intact... the B- ends up at chassis potential
anyway... since the neg of the grid meter is fed via grid

fuse...

to chassis.

### Originally, we installed a 100 K 3 watt resistor across the
grid fuse holder... thinking IF the grid fuse opened up... the V
drop across the 100 k resistor would bias the amp off.... it
doesn't ! With 800 w of drive applied on a 3x6... and the grid
fuse OPEN... all of a sudden you get 60 w reflected power on the
input of the amp [normally zero]... and abt 20% of normal power
output... [2500w instead of normal 12 kw]. We removed the

100 K

resistor... leaving a wide open (if fuse opens up).... and now

with

drive applied.. and an open grid fuse.... u get ZERO watts
output.... and of course... ZERO DC grid current. [desired

effect]


######## Ok, here's what I think happens when a 100 K R is
installed across the grid fuse holder. When Grid fuse blows
open... dc grid I now flows through the 100 K R... and V drop is
applied to the cathode... adding cut-off bias. Trbl is... the v
drop across the 100 K R is also in PARALLEL with the safety
diodes [rvs connected between chassis and B-] The UN-NEEDED
safety diode between chassis and B- will turn ON.... allowing
DC grid current to flow though it! .. and at the same time... the
V drop across the 100 k resistor is adding bias to the
cathode.... it all reach's equilibrium... and what u end up with
is not enough bias added to cut the tube off.... and at the same
time u have an alternate path for DC grid current.

## So it's EITHER remove the un needed safety diode... OR remove
the 100 k resistor across the fuse. I prefer removing the 100 k
resistor. Reason is.. even though the un needed safety diode
isn't needed for normal faults... like B+ to chassis shorts... it's
STILL required to stop the B- from floating more than .7 V from
chassis potential. BTW... the RVS connected diodes between
chassis and B- are STILL in parallel with the grid fuse.... so
a MOV across the grid fuse isn't really required. Some
paralled 6 A diodes with a 800A surge rating will fair better
than any small common MOV anyway.

### The BIG question is.... WHERE the hell does this 800 watts

of

drive end up ??? It's still being fed to cathode. Does it
just sit there and cook the cathode ??

RICH SEZ..... How can you measure P if the input Z goes bananas?

### good point. With a 100 K resistor across the fuse holder... u
have an alternate path for dc grid current vi the un needed
safety diode. You also end up with partial added bias on the
cathode.... so input Z changes.. and reflected power goes up...
and tube can still be driven.

## with the 100 k R removed.... and fuse open.... input Z should
skyrocket.... and IPA should shut down. We will recheck this, and
report back.

## In any event the grid fuse concept works superbly..
everytime. Some reason's for the grid fuse blowing are.. excess
drive with a dead cxr... HV fuse blown... underloading, etc.

I'm

now thinking that since all bugs are out... and all steady state
grid/plate current/ fil V/ plate V measurements have been

done....

.. and now we just pulse tune it... then talk.... OR just dial

up

the vac tune load caps... "by the pre-set numbers".. we can
probably REDUCE the size of the fast grid fuse from the

current

750 Ma... down to something like 400-500 ma.

RICH SEZ.... If it works, why fix it?

### sorta along the same lines as your 1/4 ...1/2 watt 30 ohm
resistor you use as a grid fuse in the SB-220. Some user's have
smoked the 1/4 watt 30 ohm resistor. Dunno how much a SB-220
pulls for normal Grid I.... but on a L4B... on ssb plate V [2500 v
@ 800ma] normal grid I with 110 w of drive is aprx 300 Ma... or
150 ma per tube. 150 ma through a 30 ohm grid resistor = .675 W !!
[on key down A0] On ssb/cw... AFTER pulse tuning up the amp... u
can probably get away with a 1/4 W resistor. Average grid I is
probably 1/2 of keydown grid I... or 75 ma. 75 ma through a 30
ohm resistor = just .167 watt.

### Also.. my scheme of paralleling a PAIR of 50A magnetic-
hydraulic breaker to make one big 100A breaker's works quite
well.... one such assy is installed in EACH leg of the 240V line.
The kicker here is... we removed the tie bar on each assy.

With

steady state dead cxr stuff... all 4 x poles are turned on....

with

ssb.... u can safely shut one pole off per assy. [now it's a 50

A

breaker] THEN.. if something goes amuck... with a lower value
grid fuse.. and pri breaker [ HV fuse remained same value.. 3
A] ... while on ssb.... both the pri breaker and lower valued

grid

fuse will blow even faster.

RICH SEZ.... Is faster better?

### yeah.... faster is always better. It all boils down to duty
cycle.... which is mode dependent. If ur a RTTY/ A0 type.... then
fuse accordingly. If ur a ssb/cw op.... then you can install
smaller sized fuses/breakers. The kicker here is... with rear
panel mounted 3 agc fuse holders... plus a pair of 2-3 pole
breakers on primary 240 v line [with tie bars removed].. it's a
snap to either change the 3agc fuse to any size you want.... or
ditto with the primary breakers. IF the Grid fuse inadvertantly
blows from op error.... no big deal..... 5 seconds to replace it
with a spare.

Later... Jim VE7RF

R L Measures, AG6K, 805.386.3734
r@..., , rlm@, www.somis.org