I am the second owner of a Heathkit SB-1000 that was built in the late
1980s. It was working great until I accidentally tried to tune it into a
mistuned antenna and I saw a flash through the ventilation screen on the
left side somewhere around the rectifier/filter section and there is now no
power output. The amp powers up, the fan runs, the meter lamps light and the
tube lights up. The amps multimeter shows 3400 volts on the high voltage
position (a little high maybe but it has always been like that). With no
drive, when the amp is keyed there is no plate current and no grid current.
Keyed with drive there is grid current, but no plate current and no power
output. These conditions are the same regardless of which band the amp is
set to. I had thought that the zener diode might have failed so I replaced
it, but nothing has changed. I would appreciate any thoughts as to what may
be the problem and how to fix it.

73
Chris
VE9ZX

Christopher Hall wrote:
I am the second owner of a Heathkit SB-1000 that was built in the late
1980s. It was working great until I accidentally tried to tune it into a
mistuned antenna and I saw a flash through the ventilation screen on the
left side somewhere around the rectifier/filter section and there is now no
power output. The amp powers up, the fan runs, the meter lamps light and the
tube lights up. The amps multimeter shows 3400 volts on the high voltage
position (a little high maybe but it has always been like that). With no
drive, when the amp is keyed there is no plate current and no grid current.
Keyed with drive there is grid current, but no plate current and no power
output. These conditions are the same regardless of which band the amp is
set to. I had thought that the zener diode might have failed so I replaced
it, but nothing has changed. I would appreciate any thoughts as to what may
be the problem and how to fix it.

Measure the plate, grid, and cathode voltages with a meter. Compare with
the manual. Work back from there.

A failed tube is entirely possible. Inspect it for internal damage.
--scott


--
"C'est un Nagra. C'est suisse, et tres, tres precis."

On Fri, 08 Apr 2011 09:20:18 -0400, Christopher Hall wrote:

I am the second owner of a Heathkit SB-1000 that was built in the late
1980s. It was working great until I accidentally tried to tune it into a
mistuned antenna and I saw a flash through the ventilation screen on the
left side somewhere around the rectifier/filter section and there is now
no power output. The amp powers up, the fan runs, the meter lamps light
and the tube lights up. The amps multimeter shows 3400 volts on the high
voltage position (a little high maybe but it has always been like that).
With no drive, when the amp is keyed there is no plate current and no
grid current. Keyed with drive there is grid current, but no plate
current and no power output. These conditions are the same regardless of
which band the amp is set to. I had thought that the zener diode might
have failed so I replaced it, but nothing has changed. I would
appreciate any thoughts as to what may be the problem and how to fix it.

Some basic things to do before you start trying to measure 3500 volts,
with equipment that's either inadequate or expensive:

One: Something obviously burnt. So look for something that's obviously
burnt. If the tube isn't flowing current then there's a good chance that
the current path between the power supply and the tube is broken. So
look carefully at every component between the power supply and the tube.
Trace the path from the transformer secondary to the tube, and check for
char. Because you're reading voltage at the meter, you can start by
going from the meter pick-off point to the tube.

Two: If you don't see anything, start measuring for continuity (still
with power off). Measure the resistance from the meter pick off point to
the tube anode -- it should be low (probably less than one ohm, because I
think those things used a coil wrapped around a resistor for UHF
suppression).

Three: Maybe you did blow up the tube. I'm not familiar with the
SB-1000. If the tube has a glass envelope, inspect the area from the
anode to the anode cap -- look for things that appear to be burnt and/or
melted.

Four: (really, this is part of one). Sniff around. Did you notice "that
expensive smell" after the bang and flash? If so, it should still be
lingering, and it should be strongest at the burnt component. Use your
nose to find the burnt part.

If none of this locates the problem, then go ahead and power it up and
start taking live measurements -- but do observe all precautions. 3500V
will have you in a philosophical discussion with St. Peter about the
meaning of the phrase "suicide by stupidity" before you ever realize that
you're dead. I'd have to dive into my ARRL handbook to learn about
safety at those voltages, but I would imagine that you'd start with a
decent high voltage probe, and go from there.

--
http://www.wescottdesign.com

Christopher Hall wrote:
I am the second owner of a Heathkit SB-1000 that was built in the late
1980s. It was working great until I accidentally tried
to tune it into a mistuned antenna and I saw a flash
through the ventilation screen on the left side somewhere
around the rectifier/filter section and there is now no
power output. The amp powers up, the fan runs,
the meter lamps light and the tube lights up.
The amps multimeter shows 3400 volts on the high voltage
position (a little high maybe but it has always been like that).
With no drive, when the amp is keyed there is no plate current
and no grid current.
Keyed with drive there is grid current, but no plate current
and no power output. snip

Hi,

Perhaps an open RF choke in the plate circuit?

Some Ameritron models have a series HV rectifier in the plate
circuit, to serve as a fuse. If present, perhaps it opened.

If you are not comfortable working around lethal voltages,
Ameritron may service this model Heathkit amp.
(It's basically a kit version of their AL-80, single 3-500Z amp).

73,
Ed Knobloch

These conditions are the same regardless of which band the amp is
set to. I had thought that the zener diode might have failed so I replaced
it, but nothing has changed. I would appreciate any thoughts as to what may
be the problem and how to fix it.

73
Chris
VE9ZX

On 04/10/2011 12:17 AM, Edward Knobloch wrote:
Christopher Hall wrote:
I am the second owner of a Heathkit SB-1000 that was built in the late
1980s. It was working great until I accidentally tried
to tune it into a mistuned antenna and I saw a flash
through the ventilation screen on the left side somewhere
around the rectifier/filter section and there is now no
power output. The amp powers up, the fan runs,
the meter lamps light and the tube lights up.
The amps multimeter shows 3400 volts on the high voltage
position (a little high maybe but it has always been like that).
With no drive, when the amp is keyed there is no plate current
and no grid current.
Keyed with drive there is grid current, but no plate current
and no power output. snip

Hi,

Perhaps an open RF choke in the plate circuit?

Some Ameritron models have a series HV rectifier in the plate
circuit, to serve as a fuse. If present, perhaps it opened.

If you are not comfortable working around lethal voltages,
Ameritron may service this model Heathkit amp.
(It's basically a kit version of their AL-80, single 3-500Z amp).

73,
Ed Knobloch

These conditions are the same regardless of which band the amp is
set to. I had thought that the zener diode might have failed so I
replaced
it, but nothing has changed. I would appreciate any thoughts as to
what may
be the problem and how to fix it.

73
Chris
VE9ZX



My first thought was the plate rf choke as well. It would take QUITE an
overload of plate current to blow that, but a high SWR into a bad
antenna might induce enough RF current in the choke to fry it. If you
are lucky the break will be on the surface and might be repairable.
Otherwise a new choke is needed. Another possibility is the parasitic
suppressor between the tube plate cap and the plate choke may have
opened. This is easier to repair/replace.

I REALLY doubt that you blew the tube that quickly. The only way to
destroy a 3-500Z in an instant would have involved frying much of the
power supply as well. If the tube filament is not open and the plate
isn't melted (hole burned though it!) or discolored unevenly or the
glass softened in one or more spots the tube is good! 3-500Z'z die
slowly or self destruct like a supernova!

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?

Thanks
Chris

"Christopher Hall" wrote in message
...
I am the second owner of a Heathkit SB-1000 that was built in the late
1980s. It was working great until I accidentally tried to tune it into a
mistuned antenna and I saw a flash through the ventilation screen on the
left side somewhere around the rectifier/filter section and there is now no
power output. The amp powers up, the fan runs, the meter lamps light and
the tube lights up. The amps multimeter shows 3400 volts on the high
voltage position (a little high maybe but it has always been like that).
With no drive, when the amp is keyed there is no plate current and no grid
current. Keyed with drive there is grid current, but no plate current and
no power output. These conditions are the same regardless of which band the
amp is set to. I had thought that the zener diode might have failed so I
replaced it, but nothing has changed. I would appreciate any thoughts as to
what may be the problem and how to fix it.

73
Chris
VE9ZX

On 04/11/2011 03:37 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?

Thanks
Chris

"Christopher wrote in message
...
I am the second owner of a Heathkit SB-1000 that was built in the late
1980s. It was working great until I accidentally tried to tune it into a
mistuned antenna and I saw a flash through the ventilation screen on the
left side somewhere around the rectifier/filter section and there is now no
power output. The amp powers up, the fan runs, the meter lamps light and
the tube lights up. The amps multimeter shows 3400 volts on the high
voltage position (a little high maybe but it has always been like that).
With no drive, when the amp is keyed there is no plate current and no grid
current. Keyed with drive there is grid current, but no plate current and
no power output. These conditions are the same regardless of which band the
amp is set to. I had thought that the zener diode might have failed so I
replaced it, but nothing has changed. I would appreciate any thoughts as to
what may be the problem and how to fix it.

73
Chris
VE9ZX



I'm betting that resistor (wire wound) was self resonant at the
frequency you were operating on, that's why it exploded. I would just
leave it out for now. If you DO replace the resistor make sure to use a
none inductive resistor and/or bypass it on the HOT end with a suitable
HV capacitor having of at least .001 uf.

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