| Author |
Message |
Jim Strickland
Guest
|
 Posted:
Tue Dec 28, 2004 8:10 am Post subject:
Knight KG250 too hot? |
|
|
I've just dug into my father's old Knight Kit KG250 amp. It works just
fine, no detectable humming (good filters, yay!) , but of the 4 6BM8s
that make up the driver and power stages, the inner two get a lot
hotter than the outer two. Now, one of these tubes is a Radio Shack
lifetime replacement tube, and it's the hottest of the bunch at about
275degrees F, but its neighbor (which is original to this amp) is
running around 250, where the other two are about 50 degrees lower.
I've not opened this amp up yet, I know from experience (cleaning the
band switch) that this chassis is a pain in the butt to get apart, so
I don't know if any of the voltages are way out of range.
An additional data point is that all the tubes in this amp test good on
my emissions tester, and the Radio Shack tube was brand new when it
found its way into my junk box lo these many years ago, to be placed in
this amp to replace a bad tube, also Radio Shack.
So I'm looking for advice. Are these tubes running abnormally hot for
this amp? If so, what should I look at first for the cause?
As I said before, the amp sounds fine, no humming, both channels work,
until I started smelling something *really* hot from the tube bay in
the amp I was thinking all I needed to do was clean the volume control
and put the second pre-amp out jack in and call it a day.
I do have the schematic on this amp, so if I do need to dig into it,
I'm not going blind.
Thanks for any help.
--
-Jim Strickland
jim@DIESPAMMERSCUMcalico.litterbox.com
|
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|
 |
Jon Yaeger
Guest
|
| Posted:
Tue Dec 28, 2004 8:36 am Post subject:
Re: Knight KG250 too hot? |
|
|
in article 2004122720105616807%jim@DIESPAMMERSCUMcalicolitterboxcom, Jim
Strickland at jim@DIESPAMMERSCUMcalico.litterbox.com wrote on 12/27/04 10:10
PM:
| Quote: | I've just dug into my father's old Knight Kit KG250 amp. It works just
fine, no detectable humming (good filters, yay!) , but of the 4 6BM8s
that make up the driver and power stages, the inner two get a lot
hotter than the outer two. Now, one of these tubes is a Radio Shack
lifetime replacement tube, and it's the hottest of the bunch at about
275degrees F, but its neighbor (which is original to this amp) is
running around 250, where the other two are about 50 degrees lower.
I've not opened this amp up yet, I know from experience (cleaning the
band switch) that this chassis is a pain in the butt to get apart, so
I don't know if any of the voltages are way out of range.
An additional data point is that all the tubes in this amp test good on
my emissions tester, and the Radio Shack tube was brand new when it
found its way into my junk box lo these many years ago, to be placed in
this amp to replace a bad tube, also Radio Shack.
So I'm looking for advice. Are these tubes running abnormally hot for
this amp? If so, what should I look at first for the cause?
As I said before, the amp sounds fine, no humming, both channels work,
until I started smelling something *really* hot from the tube bay in
the amp I was thinking all I needed to do was clean the volume control
and put the second pre-amp out jack in and call it a day.
I do have the schematic on this amp, so if I do need to dig into it,
I'm not going blind.
Thanks for any help.
|
****
Try the amp out in pitch black darkness and see if the plates of the 6BM8s
in question are glowing orange. If so, stop immediately.
It sounds to me like the bias may be out of whack or if it has coupling
capacitors, some are leaky.
I don't know from the temp specs if the tubes are too hot but others may.
Jon |
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TubeGarden
Guest
|
| Posted:
Tue Dec 28, 2004 4:18 pm Post subject:
Re: Knight KG250 too hot? |
|
|
Hi RATs!
Temp is a little bit indirect.
Measure voltages.
Taking the amp apart will get easier with practice.
I find it prudent to just play mine open face, as I dream up "improvements".
As JY hinted, bias and/or coupling caps may be leaking ... as in letting DC
through, not spilling their guts on the chassis ;)
Happy Ears!
Al
Alan J. Marcy
Phoenix, AZ
PWC/mystic/Earhead |
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Patrick Turner
Guest
|
| Posted:
Tue Dec 28, 2004 7:13 pm Post subject:
Re: Knight KG250 too hot? |
|
|
Jim Strickland wrote:
| Quote: | I've just dug into my father's old Knight Kit KG250 amp. It works just
fine, no detectable humming (good filters, yay!) , but of the 4 6BM8s
that make up the driver and power stages, the inner two get a lot
hotter than the outer two. Now, one of these tubes is a Radio Shack
lifetime replacement tube, and it's the hottest of the bunch at about
275degrees F, but its neighbor (which is original to this amp) is
running around 250, where the other two are about 50 degrees lower.
I've not opened this amp up yet, I know from experience (cleaning the
band switch) that this chassis is a pain in the butt to get apart, so
I don't know if any of the voltages are way out of range.
An additional data point is that all the tubes in this amp test good on
my emissions tester, and the Radio Shack tube was brand new when it
found its way into my junk box lo these many years ago, to be placed in
this amp to replace a bad tube, also Radio Shack.
So I'm looking for advice. Are these tubes running abnormally hot for
this amp? If so, what should I look at first for the cause?
As I said before, the amp sounds fine, no humming, both channels work,
until I started smelling something *really* hot from the tube bay in
the amp I was thinking all I needed to do was clean the volume control
and put the second pre-amp out jack in and call it a day.
I do have the schematic on this amp, so if I do need to dig into it,
I'm not going blind.
Thanks for any help.
--
-Jim Strickland
jim@DIESPAMMERSCUMcalico.litterbox.com
|
Each beam tetrode half of the 6BM8s should have no more than
about 8 watts of anode and screen dissipation.
With the amp placed upside down on some wooden blocks, and with 8 ohm R
connected to the outputs.
Measure the DC resistance of each 1/2 of each OPT between the anode and the
CT
which is connected to a B+ voltage of about +250v somewhere.
Write down in an exercize book which 1/2 primary is connected to each
tetrode anode.
Turn the amp on with no signal input
Wait 2 minutes for some warm up, and measure the voltage across each 1/2
primary
of each OPT.
Measure the DC voltage between anode and cathode for each tetrode.
At this point, neglect making any measurements of screen voltages and
currents.
Turn off the amp.
Work out what the anode current is for each anode,
Ia = Ea-k / R which is an application of Ohm's law.
The heat power dissipated in each tetrode section
= Ia x Ea-k, and this should be under 7.5 wattsfor most class AB amps
If that is the case and the tubes all give the same anode dissipation
within 10%,
not too much is wrong, but while you have the cover off, check
the screen voltage; it should be about 200v, or near the same if the amp
is connected in UL with screen taps.
Many amps with 4 little output tubes have one bias voltage, or all the
cathodes are commoned and taken to 0V via just one RC.
if there is a single or multiple cathode R check the values when the power
is off,
and check the currents by measuring voltages.
Commoned cathodes cannot be measured separately, so with aged tubed the
anode currents of each may be very different, so tubes won't run with the
same temp.
measure the tetrode grid voltage between the grid and the bias supply,
which may be 0V
in the case of an amp with RK and Ck fitted.
The DC voltage across each bias resistor, probably a 470k,
should be under 0.5 volts, with the more positive end of the R at the grid
pin.
If a grid measures say +4v at the grid higher than the far end of the grid
bias R
then you may have a stuffed tube or a leaky coupling cap.
After the amp has been on for 1/2 an hour, the grids that were a little +ve
after turn on may be more
+ve when the tubes have fully heated, and the +ve grid v and heat causes
more Ia to flow.
If the amp has the original R&C components unchanged since it was made,
replace **all** capacitors with modern polyprops and electros rated at the
same or higher voltages than the originals.
Measure all resistor values. You may find many are more than 10% away from
the wanted value
shown in their colour code bands.
Replace all the resistors.
Be carefull when unpicking the lead wires around socket lugs.
When you have finished this work after a couple of days, satisfied no
mistakes have been made,
and that the standard of wiring is neat, tidy, and well soldered,
make sure the pin grippers in the tube sockets are all tight, so
get an awl and gently press the grippers together to make better contact
with tube pins.
Don't break anything.
Remeasure the output tube electrode voltages again to see if it makes any
difference.
Measure all other electrode voltages and reconcile what the schematic says
they should
all be, if there is no info, work out what to expect, and work out if
any of the minor tubes are stuffed.
To do this properly, full measurement of the signal voltages at each
electrode needs to be done
so the gains of all the stages can be worked out.
That's just a little of what needs to be done to get the best from some
ancient old banger
that has hardly ever been serviced.
If all that sounds like too much, since high voltages can kill the
unpractised novice with ease,
then do what sensible folks do, take the critter to the Toobe Vet,
and have someone who knows all the tricks to give your amp a few singing
lessons.
I have seen 4 tube stereo amps with a quad of 6GW8 ( similar but more
powerful then 6BM8 ),
which have only been able to make 6 watts of distorted power instaed of 12
healthy
watts since the tube emission was down after 35 years in the amp.
A tech with a CRO and meters will be able to check the power and thd.
He will also spot hot running resistors, and see mods done after the amp
was purchased
as well as bodgy repair efforts, noisy volume controls, crackly wrong tone
controls and eq,
inaccurate phono amp eq, intermittent switches, wrong tube types plugged
into
input stages, and the list of things wrong with some old amp
just goes on and on.
The tech should provide 4 separate RC cathode bias circuits for the output
tetrodes.
Just about every stereo amps brought to me by dudes
who have aquired something on ebay or from their dad, brother, cousin or
mate
has a litany of faults to fix.
My oldest client of 92 had me help me fix his amp with lots of bothers.
He could barely see what he did.
He originally built it "not so long ago" as he said, which was
1964.
How he never electrocuted himself is a favourable act of God.
Unlike SS amps, tube amps try to keep running with badly biased circuits,
exausted tired tubes, leaky caps, etc, etc, etc.
Patrick Turner. |
|
| Back to top |
|
|
Jim Strickland
Guest
|
| Posted:
Wed Dec 29, 2004 1:17 am Post subject:
Re: Knight KG250 too hot? |
|
|
Darn, I was hoping to avoid recapping the beast, since it was built in
the early 1960s with a better class of capacitors than the cheap 1940s
radios I normally work on. There aren't many caps in it, at least. As
I said, the filter caps seem to be doing their job nicely, there's no
noticeable hum coming from the speakers when I tried it out, which is
good, since the caps for all 5 stages are in one can. Obviously I need
to check my voltages out of the power supply, since that electrolytic
can is nearly 40 years old. (My father put this kit together the year
or so before I was born.)
On the subject of resistors, do they also degrade with age? I know
even with capacitors there's debate. In the radio restoration world, I
found one book (Fixing up Nice Old Radios - Ed Romney) that says even
if you're looking at an old wax and paper capacitor, if it's still
doing its job, leave it alone, after this many decades it's not going
to undergo any further changes. I replace them anyway, I trust my
orange drops more. I've seen many fewer sources suggesting replacing
the resistors as well, and have never done so in the radios I've worked
on except where voltages were seriously out of line, or as in one case
where the resistor was destroyed when the radio went up in smoke.
Thanks to everyone for their kind suggestions, my work with tube
electronics to date has been of the "recap everything, make sure
voltages are what the schematic says they should be, see if device
works" variety. I've been studying the theory a bit recently, and your
comments are of great help tying the theory to the hardware.
-Jim
On 2004-12-28 07:13:08 -0700, Patrick Turner <info@turneraudio.com.au> said:
| Quote: |
Jim Strickland wrote:
I've just dug into my father's old Knight Kit KG250 amp. It works just
fine, no detectable humming (good filters, yay!) , but of the 4 6BM8s
that make up the driver and power stages, the inner two get a lot
hotter than the outer two. Now, one of these tubes is a Radio Shack
lifetime replacement tube, and it's the hottest of the bunch at about
275degrees F, but its neighbor (which is original to this amp) is
running around 250, where the other two are about 50 degrees lower.
I've not opened this amp up yet, I know from experience (cleaning the
band switch) that this chassis is a pain in the butt to get apart, so
I don't know if any of the voltages are way out of range.
An additional data point is that all the tubes in this amp test good on
my emissions tester, and the Radio Shack tube was brand new when it
found its way into my junk box lo these many years ago, to be placed in
this amp to replace a bad tube, also Radio Shack.
So I'm looking for advice. Are these tubes running abnormally hot for
this amp? If so, what should I look at first for the cause?
As I said before, the amp sounds fine, no humming, both channels work,
until I started smelling something *really* hot from the tube bay in
the amp I was thinking all I needed to do was clean the volume control
and put the second pre-amp out jack in and call it a day.
I do have the schematic on this amp, so if I do need to dig into it,
I'm not going blind.
Thanks for any help.
--
-Jim Strickland
jim@DIESPAMMERSCUMcalico.litterbox.com
Each beam tetrode half of the 6BM8s should have no more than
about 8 watts of anode and screen dissipation.
With the amp placed upside down on some wooden blocks, and with 8 ohm R
connected to the outputs.
Measure the DC resistance of each 1/2 of each OPT between the anode and the
CT
which is connected to a B+ voltage of about +250v somewhere.
Write down in an exercize book which 1/2 primary is connected to each
tetrode anode.
Turn the amp on with no signal input
Wait 2 minutes for some warm up, and measure the voltage across each 1/2
primary
of each OPT.
Measure the DC voltage between anode and cathode for each tetrode.
At this point, neglect making any measurements of screen voltages and
currents.
Turn off the amp.
Work out what the anode current is for each anode,
Ia = Ea-k / R which is an application of Ohm's law.
The heat power dissipated in each tetrode section
= Ia x Ea-k, and this should be under 7.5 wattsfor most class AB amps
If that is the case and the tubes all give the same anode dissipation
within 10%,
not too much is wrong, but while you have the cover off, check
the screen voltage; it should be about 200v, or near the same if the amp
is connected in UL with screen taps.
Many amps with 4 little output tubes have one bias voltage, or all the
cathodes are commoned and taken to 0V via just one RC.
if there is a single or multiple cathode R check the values when the power
is off,
and check the currents by measuring voltages.
Commoned cathodes cannot be measured separately, so with aged tubed the
anode currents of each may be very different, so tubes won't run with the
same temp.
measure the tetrode grid voltage between the grid and the bias supply,
which may be 0V
in the case of an amp with RK and Ck fitted.
The DC voltage across each bias resistor, probably a 470k,
should be under 0.5 volts, with the more positive end of the R at the grid
pin.
If a grid measures say +4v at the grid higher than the far end of the grid
bias R
then you may have a stuffed tube or a leaky coupling cap.
After the amp has been on for 1/2 an hour, the grids that were a little +ve
after turn on may be more
+ve when the tubes have fully heated, and the +ve grid v and heat causes
more Ia to flow.
If the amp has the original R&C components unchanged since it was made,
replace **all** capacitors with modern polyprops and electros rated at the
same or higher voltages than the originals.
Measure all resistor values. You may find many are more than 10% away from
the wanted value
shown in their colour code bands.
Replace all the resistors.
Be carefull when unpicking the lead wires around socket lugs.
When you have finished this work after a couple of days, satisfied no
mistakes have been made,
and that the standard of wiring is neat, tidy, and well soldered,
make sure the pin grippers in the tube sockets are all tight, so
get an awl and gently press the grippers together to make better contact
with tube pins.
Don't break anything.
Remeasure the output tube electrode voltages again to see if it makes any
difference.
Measure all other electrode voltages and reconcile what the schematic says
they should
all be, if there is no info, work out what to expect, and work out if
any of the minor tubes are stuffed.
To do this properly, full measurement of the signal voltages at each
electrode needs to be done
so the gains of all the stages can be worked out.
That's just a little of what needs to be done to get the best from some
ancient old banger
that has hardly ever been serviced.
If all that sounds like too much, since high voltages can kill the
unpractised novice with ease,
then do what sensible folks do, take the critter to the Toobe Vet,
and have someone who knows all the tricks to give your amp a few singing
lessons.
I have seen 4 tube stereo amps with a quad of 6GW8 ( similar but more
powerful then 6BM8 ),
which have only been able to make 6 watts of distorted power instaed of 12
healthy
watts since the tube emission was down after 35 years in the amp.
A tech with a CRO and meters will be able to check the power and thd.
He will also spot hot running resistors, and see mods done after the amp
was purchased
as well as bodgy repair efforts, noisy volume controls, crackly wrong tone
controls and eq,
inaccurate phono amp eq, intermittent switches, wrong tube types plugged
into
input stages, and the list of things wrong with some old amp
just goes on and on.
The tech should provide 4 separate RC cathode bias circuits for the output
tetrodes.
Just about every stereo amps brought to me by dudes
who have aquired something on ebay or from their dad, brother, cousin or
mate
has a litany of faults to fix.
My oldest client of 92 had me help me fix his amp with lots of bothers.
He could barely see what he did.
He originally built it "not so long ago" as he said, which was
1964.
How he never electrocuted himself is a favourable act of God.
Unlike SS amps, tube amps try to keep running with badly biased circuits,
exausted tired tubes, leaky caps, etc, etc, etc.
Patrick Turner.
|
--
-Jim Strickland
jim@DIESPAMMERSCUMcalico.litterbox.com |
|
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|
|
Wbittle
Guest
|
| Posted:
Fri Dec 31, 2004 1:28 am Post subject:
Re: Knight KG250 too hot? |
|
|
Hi,
I am assuming you are talking about the actual temperature of the
glass bulb. This is not an indication of a problem. Different brands of
tubes will run at different temperatures. But, what you need to worry
about is weather or not any of the old components are going south on
you. What worries me is that from your statement about the chassis being
hard to open and your reluctance to pop it open, I must assume this amp
has not been properly restored. Lack of hum means little. If this amp
has been sitting around chances are it's coupling capacitors are all bad
or going bad. This can cause the bias onthe output tubes to be off and
cause things like shortened tube life, overheated transformers and the
like. All are very bad for the amplifier. This amp really needs to be
rebuilt by a qualified tube technician before placing it back into
regular service. If this amp is fixed bias, and if the bias rectifier is
an old selenium type, you are operating a time bomb. I would really take
the time to open this amp up and replace the plate feed resistors,
coupling capacitors and bias rectifier at the very least.
Bill Bittle
Archangele sound: We design, build, and repair all tube hifi and guitar
amps. |
|
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|
|
Jon Yaeger
Guest
|
| Posted:
Fri Dec 31, 2004 1:47 am Post subject:
Re: Knight KG250 too hot? |
|
|
in article 2004122813171316807%jim@DIESPAMMERSCUMcalicolitterboxcom, Jim
Strickland at jim@DIESPAMMERSCUMcalico.litterbox.com wrote on 12/28/04 3:17
PM:
| Quote: | Darn, I was hoping to avoid recapping the beast, since it was built in
the early 1960s with a better class of capacitors than the cheap 1940s
radios I normally work on.
|
*** What's the old saying: if you wanna dance to the music, you gotta pay
the piper?
There aren't many caps in it, at least. As
| Quote: | I said, the filter caps seem to be doing their job nicely, there's no
noticeable hum coming from the speakers when I tried it out, which is
good, since the caps for all 5 stages are in one can. Obviously I need
to check my voltages out of the power supply, since that electrolytic
can is nearly 40 years old. (My father put this kit together the year
or so before I was born.)
|
*** Electrolytic cans sometimes heal themselves well enough to carry on.
Yet in restoring gear of this vintage I've discovered multi-cap cans with
bad (e.g. open) sections, horrible leakage, reduced capacity, etc. that may
not have gone to the point where there is noticeable hum. I'll also touch
the can to see if the cap gets warm, possibly indicating leaking current.
It's prudent to install new devices, yet I often leave the old cans in place
if they test OK (but I'm not amazed if one day they fail). Exact
replacements are often scarce.
*** Coupling caps are another matter. I will ALWAYS change the output tube
coupling caps and any cathode bias electros, as in the former case, a cap
failure can fry the output tubes and trannies. The only way to know for sure
is to test each cap at the rated voltage and check for leakage, etc. It's
usually simpler and easier just to replace them, and you get added peace of
mind.
| Quote: |
On the subject of resistors, do they also degrade with age?
|
*** They sure can, especially carbon comp devices. For some reason the
high values (e.g. >100K) seem to drift upwards. This is easy to test: if
your ohmmeter shows 1.5 megs across a 1 meg resistor, it's time to change
it.
*** I have good faith in Allen Bradley types and zero faith in some
Japanese ones. I recently restored a Kenwood 1100 receiver and had to
change a pocketful of resistors that went bad.
I know
| Quote: | even with capacitors there's debate.
|
*** Debate as much as you like; the odds are a brand new capacitor is going
to outlast a vintage one.
In the radio restoration world, I
| Quote: | found one book (Fixing up Nice Old Radios - Ed Romney) that says even
if you're looking at an old wax and paper capacitor, if it's still
doing its job, leave it alone, after this many decades it's not going
to undergo any further changes. I replace them anyway, I trust my
orange drops more. I've seen many fewer sources suggesting replacing
the resistors as well, and have never done so in the radios I've worked
on except where voltages were seriously out of line, or as in one case
where the resistor was destroyed when the radio went up in smoke.
Thanks to everyone for their kind suggestions, my work with tube
electronics to date has been of the "recap everything, make sure
voltages are what the schematic says they should be, see if device
works" variety. I've been studying the theory a bit recently, and your
comments are of great help tying the theory to the hardware.
|
*** Good luck.
- Jon
|
|
| Back to top |
|
|
Patrick Turner
Guest
|
| Posted:
Fri Dec 31, 2004 8:25 am Post subject:
Re: Knight KG250 too hot? |
|
|
Jon Yaeger wrote:
| Quote: | in article 2004122813171316807%jim@DIESPAMMERSCUMcalicolitterboxcom, Jim
Strickland at jim@DIESPAMMERSCUMcalico.litterbox.com wrote on 12/28/04 3:17
PM:
Darn, I was hoping to avoid recapping the beast, since it was built in
the early 1960s with a better class of capacitors than the cheap 1940s
radios I normally work on.
*** What's the old saying: if you wanna dance to the music, you gotta pay
the piper?
|
And find some desparate old tart who'll accept one's brash invitation to dance.
| Quote: |
There aren't many caps in it, at least. As
I said, the filter caps seem to be doing their job nicely, there's no
noticeable hum coming from the speakers when I tried it out, which is
good, since the caps for all 5 stages are in one can. Obviously I need
to check my voltages out of the power supply, since that electrolytic
can is nearly 40 years old. (My father put this kit together the year
or so before I was born.)
*** Electrolytic cans sometimes heal themselves well enough to carry on.
Yet in restoring gear of this vintage I've discovered multi-cap cans with
bad (e.g. open) sections, horrible leakage, reduced capacity, etc. that may
not have gone to the point where there is noticeable hum. I'll also touch
the can to see if the cap gets warm, possibly indicating leaking current.
It's prudent to install new devices, yet I often leave the old cans in place
if they test OK (but I'm not amazed if one day they fail). Exact
replacements are often scarce.
*** Coupling caps are another matter. I will ALWAYS change the output tube
coupling caps and any cathode bias electros, as in the former case, a cap
failure can fry the output tubes and trannies. The only way to know for sure
is to test each cap at the rated voltage and check for leakage, etc. It's
usually simpler and easier just to replace them, and you get added peace of
mind.
On the subject of resistors, do they also degrade with age?
*** They sure can, especially carbon comp devices. For some reason the
high values (e.g. >100K) seem to drift upwards. This is easy to test: if
your ohmmeter shows 1.5 megs across a 1 meg resistor, it's time to change
it.
*** I have good faith in Allen Bradley types and zero faith in some
Japanese ones. I recently restored a Kenwood 1100 receiver and had to
change a pocketful of resistors that went bad.
I know
even with capacitors there's debate.
*** Debate as much as you like; the odds are a brand new capacitor is going
to outlast a vintage one.
In the radio restoration world, I
found one book (Fixing up Nice Old Radios - Ed Romney) that says even
if you're looking at an old wax and paper capacitor, if it's still
doing its job, leave it alone, after this many decades it's not going
to undergo any further changes. I replace them anyway, I trust my
orange drops more. I've seen many fewer sources suggesting replacing
the resistors as well, and have never done so in the radios I've worked
on except where voltages were seriously out of line, or as in one case
where the resistor was destroyed when the radio went up in smoke.
Thanks to everyone for their kind suggestions, my work with tube
electronics to date has been of the "recap everything, make sure
voltages are what the schematic says they should be, see if device
works" variety. I've been studying the theory a bit recently, and your
comments are of great help tying the theory to the hardware.
*** Good luck.
|
Teaching old amps to sing is a labour of love as well....
Patrick Turner.
|
|
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|
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