On 4/11/2011 3:37 PM, Christopher Hall wrote:
Thanks to all who have posted regarding this issue, its been a lot of help.
I checked the HV to plate circuit and found a blown (actually totally
destroyed) wire wound resistor in series between the filter board and the
plate choke. The manual and schematic doesnt show this resistor as part of
the kit, so Im assuming it was a mod done by the original owner. I suspect
it may be there to lower the voltage on the plate to extend tube life or to
provide some safety a la Eds post regarding a RF choke to provide some sort
of fuse. The problem is the resistor is so damaged that there are no values
printed on it. Does anyone have any ideas as to what that resistors values
would be?


Hi, Chris

That resistor in series with the plate lead is a good, standard mod.
The usual value is 10 Ohms, 10 Watts, enamel coated wirewound.

Here's a picture of one added to a Kenwood TL-922 amplifier:
http://ve3fwa.ham-radio-op.net/tl922/mod7-an.htm

(The guy used a 12 Watter there, no problem, that's a bigger amp).

The idea is that should a 3-500Z arc over, the resistor will open
(and +stay+ open). Hopefully, this will happen fast enough
to save the tube and the plate rf choke.

73,
Ed Knobloch

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

"Edward Knobloch" wrote in message
...
On 4/11/2011 3:37 PM, Christopher Hall wrote:
Thanks to all who have posted regarding this issue, its been a lot of
help.
I checked the HV to plate circuit and found a blown (actually totally
destroyed) wire wound resistor in series between the filter board and the
plate choke. The manual and schematic doesnt show this resistor as part
of
the kit, so Im assuming it was a mod done by the original owner. I
suspect
it may be there to lower the voltage on the plate to extend tube life or
to
provide some safety a la Eds post regarding a RF choke to provide some
sort
of fuse. The problem is the resistor is so damaged that there are no
values
printed on it. Does anyone have any ideas as to what that resistors
values
would be?


Hi, Chris

That resistor in series with the plate lead is a good, standard mod.
The usual value is 10 Ohms, 10 Watts, enamel coated wirewound.

Here's a picture of one added to a Kenwood TL-922 amplifier:
http://ve3fwa.ham-radio-op.net/tl922/mod7-an.htm

(The guy used a 12 Watter there, no problem, that's a bigger amp).

The idea is that should a 3-500Z arc over, the resistor will open
(and +stay+ open). Hopefully, this will happen fast enough
to save the tube and the plate rf choke.

73,
Ed Knobloch

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

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 4/15/2011 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