On 04/13/2011 09:37 PM, Edward Knobloch wrote:
Hi, Chris

Very good. I'm glad your 3-500Z survived OK.

People avoid the rectangular shaped resistors
(which look like cement) for this application - I don't know
if they feel that the insulation is insufficient
for the 3 KV to be encountered, or if they feel the resistor
won't shatter enough to act as a fuse.

Enjoy your SB-1000!

73,
Ed


On 4/13/2011 9:11 AM, Christopher Hall wrote:
Thanks for the info Ed, I have learned a lot and you and everyone else
has
been a great help. I removed the blown resistor and reconnected the
lead and
the amp works FB again, so problem solved. Im glad it turned out to be so
simple. I will install another 10ohm 10W resistor to replace the blown
one
in the near future before I put the amp back on the air for real.

Thanks again all and 73
Chris
VE9CEH
I would test any wire would resistor with a grid dip meter to make sure
it ISN'T self resonant on any ham band, otherwise it will fail again.
OR You can prevent the self resonant problem by putting a .01 uf 1000v
ceramic capacitor in parallel with it.

On 4/14/2011 8:40 PM, Kenneth Scharf wrote:
I would test any wire would resistor with a grid dip meter to make sure
it ISN'T self resonant on any ham band, otherwise it will fail again.
OR You can prevent the self resonant problem by putting a .01 uf 1000v
ceramic capacitor in parallel with it.

Hi, Ken

I have to disagree with you about the added capacitor you suggest.

The added resistor is intended to act as a fuse,
on the output side of the HV supply. If it opens (by exploding),
the parallel capacitor will have 3 KV across it, assuming
the short condition is still present downstream, so a 1KV rated
capacitor is inadequate.
Plus, we want all of the voltage spike to be across the resistor,
so it blows up "real good". A capacitor across the resistor
would have the effect of slowing the resistor failure time.

There should be little or no rf current in the resistor,
so no need to test it with a grid dipper.
The resistor is isolated from the 3-500Z by the plate choke,
and bypass capacitors shunt the rf to ground in the HV line.

73,
Ed Knobloch

On 04/15/2011 01:55 PM, Edward Knobloch wrote:
On 4/14/2011 8:40 PM, Kenneth Scharf wrote:
I would test any wire would resistor with a grid dip meter to make sure
it ISN'T self resonant on any ham band, otherwise it will fail again.
OR You can prevent the self resonant problem by putting a .01 uf 1000v
ceramic capacitor in parallel with it.

Hi, Ken

I have to disagree with you about the added capacitor you suggest.

The added resistor is intended to act as a fuse,
on the output side of the HV supply. If it opens (by exploding),
the parallel capacitor will have 3 KV across it, assuming
the short condition is still present downstream, so a 1KV rated
capacitor is inadequate.
Plus, we want all of the voltage spike to be across the resistor,
so it blows up "real good". A capacitor across the resistor
would have the effect of slowing the resistor failure time.

There should be little or no rf current in the resistor,
so no need to test it with a grid dipper.
The resistor is isolated from the 3-500Z by the plate choke,
and bypass capacitors shunt the rf to ground in the HV line.

73,
Ed Knobloch
If there IS an rf bypass capacitor between the 'cold' end of the plate
choke and ground then my suggestion isn't needed. If the only bypass at
the cold end of the choke is though the power supply, then one should be
added. In some amplifiers a second rf choke is placed in series with
the main plate choke and a bypass capacitor to ground is placed between
them. Such a bypass capacitor should be one of those 'doorknob' types.

One thing that has not been mentioned is that an amp should not be driven
with grid current and no plate current. If you see no plate current, the
full power of the transmitter is absorbed in the grids, which will destroy
them. As soon as you lose plate current, shut down and find out why.

Ceramic resistors are not used simply because when they explode, you have
nasty particles of ceramic all over. Somewhat like a slightly docile
grenade. Wire wound resistors do not do that - generally.

73, Colin K7FM

On 4/16/2011 10:20 AM, COLIN LAMB wrote:
One thing that has not been mentioned is that an amp should not be driven
with grid current and no plate current. If you see no plate current, the
full power of the transmitter is absorbed in the grids, which will destroy
them. As soon as you lose plate current, shut down and find out why.

Ceramic resistors are not used simply because when they explode, you have
nasty particles of ceramic all over. Somewhat like a slightly docile
grenade. Wire wound resistors do not do that - generally.

73, Colin K7FM



Good points - thank you, Colin.

I figured the ALC from the amp would crank back the exciter
to prevent grid damage in the amplifier, in the absence
of amplifier plate voltage, but I was probably mistaken.

73,
Ed Knobloch

On 4/15/2011 8:27 PM, Kenneth Scharf wrote:

If there IS an rf bypass capacitor between the 'cold' end of the plate
choke and ground then my suggestion isn't needed. If the only bypass at
the cold end of the choke is though the power supply, then one should be
added. In some amplifiers a second rf choke is placed in series with the
main plate choke and a bypass capacitor to ground is placed between
them. Such a bypass capacitor should be one of those 'doorknob' types.

Thanks, Ken, good points.

Basically the added 10 Ohm 10 Watt resistor
is intended to interrupt the energy from the power supply
electrolytics in the event of an arc-over in the final tube.
By the time the primary fuse kicked in, the damage would be done.

They do sell HV fuses suitable for the purpose, used
in microwave ovens. I see some on eBay for around $8 each,
rated for 5KV interruption.

73,
Ed Knobloch

On 04/16/2011 10:20 AM, COLIN LAMB wrote:
One thing that has not been mentioned is that an amp should not be driven
with grid current and no plate current. If you see no plate current, the
full power of the transmitter is absorbed in the grids, which will destroy
them. As soon as you lose plate current, shut down and find out why.

Ceramic resistors are not used simply because when they explode, you have
nasty particles of ceramic all over. Somewhat like a slightly docile
grenade. Wire wound resistors do not do that - generally.

73, Colin K7FM


The 3-500Z is a fairly robust tube and can take a bit of abuse like that
before it is destroyed. (At least the Eimac built tubes could, I don't
know about the Chinese clones). OTOH ceramic metal external anode tubes
usually DON'T have a grid structure that can tolerate such abuse and
WILL suffer 'meltdown' if not treated correctly. This is why the 4cx250
series CANNOT be used in grounded grid operation. They CAN be used as
cathode driven tetrodes in AB1 (NO GRID CURRENT) however.