Any non-tube working scope sold on Ebay of 20 MHz of better BW -Dual trace
with a return-if-not-satisfied option!!

I hate to say this on RAT but I stay away from Tubes when in comes to test
equipment, unless they are simple pieces like Audio Signal Generators, etc..

Tubed scopes can be a hassle to restore!

"Jack A. Zucker" wrote in message
om...
Recommendations?

In article 3f2ea52b_1@newsfeed,
"Robert M. Braught" wrote:

Minimum requirements IMO for beginners who want don't want to hassle:
15 Mhz bandwidth vertical amplifier (5 Mhz will do 'OK')
Triggered Sweep (unless you like keeping one hand on a knob...)
Solid State (except the CRT, duh! An older all tube or hybrid scope can
be fine, but hassle???)
Owners manual can be a real plus

Expect to spend ~$50-$100 at a HamFest or swap meet...

Dual trace... if you keep at it, you'll wish you had it...

Plug it in and make sure it has a good, stable trace before walking away...

Cheers,
-Robert
QTS
http://www.Braught.com


To this good wish list, I'll add that if you're SERIOUSLY going to keep
at it, a delayed sweep is a Real Good Thing. Lets you zero in on parts
of the waveform which you want to concentrate (like crossover
distortion, leading edge fuzzies and other anomalies.

--Mike

--------------------------------------------------------------------
Mike Schway | [Picture your favorite quote here]
|
--------------------------------------------------------------------

Hi RATs!

I use an old Heathkit OL-1 - 3" diameter round display, 500Kcycles, single
trace.

Works great. Was $40.00, with manual, at antique mall years ago.

Happy Ears!
Al


Alan J. Marcy
Phoenix, AZ

PWC/mystic/Earhead

TubeGarden wrote:

Hi RATs!

I use an old Heathkit OL-1 - 3" diameter round display, 500Kcycles, single
trace.

Works great. Was $40.00, with manual, at antique mall years ago.

Happy Ears!
Al

Alan J. Marcy
Phoenix, AZ

PWC/mystic/Earhead

I am using a Labtech solid horror, one channel has ceased,
and the input attenator pot on the other has gone dodgy.
The cowboys in Taiwan forget they are making test gear,
and think thay are making low grade consumer electronics....

Then I have a collection of dysfunctional CROs, some SS and a couple of
tubies,
and one day, when I get desperate, I will restore the oldest which has a 5"
dia screen,
and is tubed, to make it give wider BW.
Trouble is, I'd have to get the Labtech to work to do the work,
since I'd need a CRO to fix another CRO.

Dual trace is nice for phase comparisons,
and for very approximate measurements of distortion 3%,
where you have the input wave on one channel, just filling the screen,
output on the other, just filling the screen,
then you add them, null out the fundemental by fine adjustment and display the
difference,
which is the distortion. The amplitude of the distortion is then a fraction
of screen height, say one division line, and you have say 20 divisions,
so thd = 1/20 = 5%.

Also not bad for sweep response, but I don't have a good sweep,
with calibration "pips" or markers at certain frequencies,
so the display can be interpolated.
For transfer curves, using a triangular input wave
with nice straight sides will show you what linearity you have.
bends to the straight lines indicate distortion, and or phase shift.

So I get by with a single trace CRO. Its 20 MHz, full of
14 pin chips and disctrete parts, and is difficult to service.
I once put a pulse of 2,000 volts into it, and blew most of the chips and many

transistors, and it cost $400 to fix, more than I paid for it, as a second
hand item.

Patrick Turner.

Lots of old HP453's for 50 bucks are around....George



Patrick Turner wrote in message
...


TubeGarden wrote:

Hi RATs!

I use an old Heathkit OL-1 - 3" diameter round display, 500Kcycles,
single
trace.

Works great. Was $40.00, with manual, at antique mall years ago.

Happy Ears!
Al

Alan J. Marcy
Phoenix, AZ

PWC/mystic/Earhead

I am using a Labtech solid horror, one channel has ceased,
and the input attenator pot on the other has gone dodgy.
The cowboys in Taiwan forget they are making test gear,
and think thay are making low grade consumer electronics....

Then I have a collection of dysfunctional CROs, some SS and a couple of
tubies,
and one day, when I get desperate, I will restore the oldest which has a
5"
dia screen,
and is tubed, to make it give wider BW.
Trouble is, I'd have to get the Labtech to work to do the work,
since I'd need a CRO to fix another CRO.

Dual trace is nice for phase comparisons,
and for very approximate measurements of distortion 3%,
where you have the input wave on one channel, just filling the screen,
output on the other, just filling the screen,
then you add them, null out the fundemental by fine adjustment and display
the
difference,
which is the distortion. The amplitude of the distortion is then a
fraction
of screen height, say one division line, and you have say 20 divisions,
so thd = 1/20 = 5%.

Also not bad for sweep response, but I don't have a good sweep,
with calibration "pips" or markers at certain frequencies,
so the display can be interpolated.
For transfer curves, using a triangular input wave
with nice straight sides will show you what linearity you have.
bends to the straight lines indicate distortion, and or phase shift.

So I get by with a single trace CRO. Its 20 MHz, full of
14 pin chips and disctrete parts, and is difficult to service.
I once put a pulse of 2,000 volts into it, and blew most of the chips and
many

transistors, and it cost $400 to fix, more than I paid for it, as a second
hand item.

Patrick Turner.

"Jack A. Zucker" wrote:

Recommendations?

Here is one I picked up recently, works fine.
http://cgi.ebay.com/ws/eBayISAPI.dll...ategory=46 77

This looks like a "what's your scope?" thread... so who am I to stop
a natural progression in a thread

Tek 475, *free* from a very generous friend (but he has no intention
of returning to electronics). Dual channel, additive, dual trace,
X-Y graphing, sweep up to 1ns/div (with 10x zoom) with two timebases
allowing all sorts of freaky and neat scrolling, zooming, chopping
and cutting of the waveform.
I don't like using it though because the CRT sounds like arcing.. smells
like ozone...

Tim

--
In the immortal words of Ned Flanders: "No foot longs!"
Website @ http://webpages.charter.net/dawill/tmoranwms

"TubeGarden" wrote in message
...
Hi RATs!

I use an old Heathkit OL-1 - 3" diameter round display, 500Kcycles, single
trace.

Works great. Was $40.00, with manual, at antique mall years ago.

Happy Ears!
Al


Alan J. Marcy
Phoenix, AZ

PWC/mystic/Earhead

Hi Tim,

That is a common 475 problem. They have an electrostatic lens built
into the CRT to give it a large beam deflection with a very small change
in vertical plate amplitude. They use a very high anode voltage to give
you that nice bright trace at high sweep speeds.

Anyway, Tek used wire that has poor insulation for the 21KV anode lead.
The lead is potted right into the tube's anode terminal. Clean up all
the black gook that the anode lead sucked out of the air with isopropyl
alcohol (91%), and coat the offending section of the anode lead with
a good grade of clear silicone RTV...preferably the kind that doesn't
smell like vinegar. Anywhere there is black gook, is where there is a
problem. Corona dope will work too, but it is brittle, and won't take
any flexing.

-Chuck

Tim Williams wrote:
This looks like a "what's your scope?" thread... so who am I to stop
a natural progression in a thread

Tek 475, *free* from a very generous friend (but he has no intention
of returning to electronics). Dual channel, additive, dual trace,
X-Y graphing, sweep up to 1ns/div (with 10x zoom) with two timebases
allowing all sorts of freaky and neat scrolling, zooming, chopping
and cutting of the waveform.
I don't like using it though because the CRT sounds like arcing.. smells
like ozone...

"george craig" wrote

Lots of old HP453's for 50 bucks are around....George


Right, tell him, george.

Patrick, buy a decent s/h dual-trace scope and throw all that junk
out, you mean old bugger.

cheers, Ian

Thought I might add that you should consider probes when you buy.
Decent ones can be hard to find cheap if they don't come with the
scope. For a dual trace obviously you need two.

Also, I would be interested to hear from others about differential
probes. I am about to go to the extreme (for me) of making one coz
I can't find s/h. My dual trace scope has an xy function but no
scalar addition. Does everyone have a diff. probe, and if not, how
do you manage without? Or is it usual for dual-trace scopes to
include diff. operation?

I think my ex-school 1Mhz scope is sufficient. I don't need to see
higher frequencies. I can measure them with my voltmeter.

cheers, Ian

"Jack A. Zucker" wrote in message
om...
Recommendations?

Chuck Harris wrote:
Hi Tim,

That is a common 475 problem. They have an electrostatic lens built
into the CRT to give it a large beam deflection with a very small change
in vertical plate amplitude. They use a very high anode voltage to give
you that nice bright trace at high sweep speeds.

Anyway, Tek used wire that has poor insulation for the 21KV anode lead.
The lead is potted right into the tube's anode terminal. Clean up all
the black gook that the anode lead sucked out of the air with isopropyl
alcohol (91%), and coat the offending section of the anode lead with
a good grade of clear silicone RTV...preferably the kind that doesn't
smell like vinegar. [...]

The kind that doesn't release acetic acid on curing is usually labelled
"Silicone II" on the tube.

Cheers,
Fred
--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects, Vacuum Tubes & other stuff: |
| http://www.dogstar.dantimax.dk |
+--------------------------------------------+

Ian Iveson wrote:

Thought I might add that you should consider probes when you buy.
Decent ones can be hard to find cheap if they don't come with the
scope. For a dual trace obviously you need two.

Also, I would be interested to hear from others about differential
probes. I am about to go to the extreme (for me) of making one coz
I can't find s/h. My dual trace scope has an xy function but no
scalar addition. Does everyone have a diff. probe, and if not, how
do you manage without? Or is it usual for dual-trace scopes to
include diff. operation?

I think my ex-school 1Mhz scope is sufficient. I don't need to see
higher frequencies. I can measure them with my voltmeter.

cheers, Ian

"Jack A. Zucker" wrote in message
om...
Recommendations?

Most Dual Trace Scopes do not perform well in the Diff Mode
if they have one. That is because the combination of a pair of
single ended amps in the scope will not have much in the way
of common mode tolerance. To do that you need a front end
that is a true differential amplifier. Several of the older TEK
plug-ins did exactly that. To get good performance you also
need to have some way of adjusting the gain of one side of
the diff amp or one of the probes in order to cancel the
common mode. Cheeeeeeers, John Stewart

"Chuck Harris" wrote in message
...
Hi Tim,

That is a common 475 problem. They have an electrostatic lens built
into the CRT to give it a large beam deflection with a very small change
in vertical plate amplitude. They use a very high anode voltage to give
you that nice bright trace at high sweep speeds.

Yup, heard about that. And by needing a small deflection they make it
easier to have a vert amp so fast...

Anyway, Tek used wire that has poor insulation for the 21KV anode lead.
The lead is potted right into the tube's anode terminal. Clean up all
the black gook that the anode lead sucked out of the air with isopropyl
alcohol (91%), and coat the offending section of the anode lead with
a good grade of clear silicone RTV...preferably the kind that doesn't
smell like vinegar. Anywhere there is black gook, is where there is a
problem. Corona dope will work too, but it is brittle, and won't take
any flexing.

I take it I can find the RTV at the hardware store?

Tim

--
In the immortal words of Ned Flanders: "No foot longs!"
Website @ http://webpages.charter.net/dawill/tmoranwms

"Ian Iveson" wrote in message
news

I think my ex-school 1Mhz scope is sufficient. I don't need to see
higher frequencies. I can measure them with my voltmeter.


** If you can't see them on the scope how do you even know when they are
there ?

If you don't know they are there then why would you use an (RF)
voltmeter ??




.............. Phil

Phil Allison wrote:

"Ian Iveson" wrote in message
news

I think my ex-school 1Mhz scope is sufficient. I don't need to see
higher frequencies. I can measure them with my voltmeter.



** If you can't see them on the scope how do you even know when they are
there ?

If you don't know they are there then why would you use an (RF)
voltmeter ??

Good point. While a 1 mHz 'scope is certainly adequate for most
situations, there can be times when a faster 'scope can be very useful,
even necessary.

For instance, the first cut of my "universal quasi-op-amp tube
preamplifier" oscillated at about 11 MHz. at unity gain. With a 1 MHz.
oscilloscope I *might* have seen some fattening of the lines, but more
likely I would have just wondered why the voltages were so weird and why
it sounded so "dry" and was prone to be easily overloaded.

But give it another order of magnitude (10 MHz) and I'd have to agree
with Ian.

Cheers,
Fred

PS - I'm with the other posters who recommend against low-end tube
oscilloscopes such as Heath or Eico. They're more trouble than they're
worth. Some things tube do wonderfully, others they suck at unless you
use a *lot* of them.... like Tek 535's or whatever those behemoths were
called.
--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects: http://dogstar.dantimax.dk |
+--------------------------------------------+

"Phil Allison" wrote

** If you can't see them on the scope how do you even know when
they are
there ?

I measure with a voltmeter. Also, the scope's trace goes fuzzy.

If you don't know they are there then why would you use an
(RF)
voltmeter ??

To check they are not there :-)

This is in the context of audio valve amplifiers. When I check an
amp I put a 25MHz voltmeter across the load. If I get a significant
reading with no input signal, it is oscillating. If I need to know
the frequency of oscillation then I use a frequency counter. The
scope only gets connected when I know the amp is not oscillating.
Both these instruments are decent, ex-military units and fairly
recently calibrated. I can read them without using a ruler.

To check for ringing above 2MHz, I suppose I could sweep and use the
voltmeter, but I don't bother. Perhaps for guitar amps, or other
things with complicated voltage stages and crowded wiring?

Do you often discover anything worth finding above 2MHz? Maybe I
should save up for a new scope?

cheers, Ian

Ian Iveson wrote:
Must be of limited value, surely?

Nope. Even "just" with guitar amps,
it's quite handy. With more complex
gear (esp. some digital stuff) it's
a necessity.

All analysis is based on the comparison of two things.

I'm not sure what limited world you live in,
but clearly it's a different one than many
of the rest of us.


Many things
can be compared only with reference to a single other thing, to
which they are compared one at a time.

And many can not.

So two traces may not always be convenient, but it will always be
enough, even if you are looking for a phenomenon that happens only
once. One-time events may create a need to store more than two sets
of data simultaneously, but you will still only need two traces for
display.

I'm not talking about event or data storage.

Just as an example, it can be extremely
useful to *simultaneously* compare what
a signal looks like at the input, after
the first preamp, after the second preamp,
and at the OT of a simple amp.

Is it *necessary*? Usually not. But it
can still make life a lot easier - and
it's very educational.

Fred Nachbaur wrote:

Phil Allison wrote:

"Ian Iveson" wrote in message
news

I think my ex-school 1Mhz scope is sufficient. I don't need to see
higher frequencies. I can measure them with my voltmeter.



** If you can't see them on the scope how do you even know when they are
there ?

If you don't know they are there then why would you use an (RF)
voltmeter ??

Good point. While a 1 mHz 'scope is certainly adequate for most
situations, there can be times when a faster 'scope can be very useful,
even necessary.

For instance, the first cut of my "universal quasi-op-amp tube
preamplifier" oscillated at about 11 MHz. at unity gain. With a 1 MHz.
oscilloscope I *might* have seen some fattening of the lines, but more
likely I would have just wondered why the voltages were so weird and why
it sounded so "dry" and was prone to be easily overloaded.

But give it another order of magnitude (10 MHz) and I'd have to agree
with Ian.

Cheers,
Fred

PS - I'm with the other posters who recommend against low-end tube
oscilloscopes such as Heath or Eico. They're more trouble than they're
worth. Some things tube do wonderfully, others they suck at unless you
use a *lot* of them.... like Tek 535's or whatever those behemoths were
called.

But I only have a 20Mhz cro, and I could miss seeing a 50 MHz
oscillation, but then I doubt anything has ever oscillated at such HF
when I didn't want it to.
I built my own 100 MHz FM transmitter which can be modulated by stereo signals
without a 200 Mhz capable cro.

In the case of audio amps, if you only have 1 MHz BW on the cro,
one MUST check with an RF voltmeter routinely.
perhaps the oscillation is of unknown F, in which case an F meter
is a nice thing to have.
But a wider BW cro is by far the easier more informative option.

Easy to construct RF probles which convert the RF to DC at the probe
could be built for $10. Its in the ARRL handbooks.

Patrick Turner.


--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects: http://dogstar.dantimax.dk |
+--------------------------------------------+

"Ian Iveson" wrote in message
...

"Phil Allison" wrote


** If you can't see them on the scope how do you even know when
they are there ?

I measure with a voltmeter. Also, the scope's trace goes fuzzy.


** That contradicts my "if" condition .

10 Mhz makes no effect on a 1 Mhz scope.



If you don't know they are there then why would you use an
(RF) voltmeter ??

To check they are not there :-)


** You are assuming continuous RF oscillations - more usually ones sees
bursts superimposed on a audio wave.



This is in the context of audio valve amplifiers. When I check an
amp I put a 25MHz voltmeter across the load. If I get a significant
reading with no input signal, it is oscillating. If I need to know
the frequency of oscillation then I use a frequency counter. The
scope only gets connected when I know the amp is not oscillating.
Both these instruments are decent, ex-military units and fairly
recently calibrated. I can read them without using a ruler.


To check for ringing above 2MHz, I suppose I could sweep and use the
voltmeter, but I don't bother. Perhaps for guitar amps, or other
things with complicated voltage stages and crowded wiring?


Do you often discover anything worth finding above 2MHz? Maybe I
should save up for a new scope?


** I started out ( as a teenager) with a home brew 3 inch, 2 MHz all
tube scope 35 years ago ( still have it too) then a 10 MHz single beam and
for the last 16 years a 50 MHz dual beam.

SS amps can have parasitic oscillations up to 40 MHz or more, especially
if they use MOSFETS.

Probably 2 MHz is enough for someone who works on tube amps exclusively.




.............. Phil

I'm not sure what limited world you live in,
but clearly it's a different one than many
of the rest of us.


Cripes! Actually you could be right...since I realised there are no
square waves in my world I have lusted after a 20MHz dual storage
scope, but that's another story.

Let's say you use your 4-channel scope to conveniently display
input, output, and two intermediate stages simultaneously. I
contend that when you look at your display, you compare traces in
pairs. This is not just a limitation of your eyes or your brain:
it is because there is no actual sense in comparing more than two
things. The question "What is the difference between these X
things?" only makes sense where X = 2.

There are (X-1)*X/2 differences between X things. You can sensibly
ask "What are the differences?" and these you may evaluate by
considering each possible pair. Hence, whatever comparison you
carry out with your 4-channel scope can be done with a maximum of 6
probe swaps with a dual-channel scope.

If you remember all your early maths, like long division and
multiplication, and even adding several numbers, that was all about
breaking the process down until you could deal with numbers in
pairs.

That is how our single dialectic universe is.

Many things
can be compared only with reference to a single other thing, to
which they are compared one at a time.

And many can not.

sorry, ambiguous. Replace my "Many things" with "A plurality of
things"

Is it *necessary*? Usually not. But it
can still make life a lot easier - and
it's very educational.

Never, actually. But yes, it may be worth the money to save those 6
probe swaps if you often need to compare more than 2 traces.
Whether it is educational or not is a tricky question. If I had a
student who claimed to understand a 4-trace display, and yet could
not map this claimed understanding onto several 2-trace displays,
then I would suspect a narrow concentration span or a lack of
primary education, or both. These are by far the hardest students
to deal with, and I would start by restricting them to 2-trace
displays until they grasped the plot.

Comms network technicians seem to need dozens of traces...I suppose
the avoidance of *extreme* inconvenience almost qualifies as
necessity in the commercial world.

cheers, Ian

The OP asked in the context of valve amps. I do work on valve amps
exclusively, and they are usually made by
me, and they have grid stoppers where they should have.

If I use overall nfb, then ringing for a power amp is likely to be
around 80kHz and 1MHz. Never thought through why this always seems
the case. The former is dealt with by snubbing the OPT primary
somehow, and the latter by power-valve grid stoppers usually.

If I built pre-amps with feedback then a better scope might attract
me more. As it is I rely on sensible precautions.

cheers, Ian

"Phil Allison" wrote in message
u...

"Ian Iveson" wrote in message
...

"Phil Allison" wrote


** If you can't see them on the scope how do you even know
when
they are there ?

I measure with a voltmeter. Also, the scope's trace goes fuzzy.


** That contradicts my "if" condition .

10 Mhz makes no effect on a 1 Mhz scope.



If you don't know they are there then why would you use
an
(RF) voltmeter ??

To check they are not there :-)


** You are assuming continuous RF oscillations - more usually
ones sees
bursts superimposed on a audio wave.



This is in the context of audio valve amplifiers. When I check
an
amp I put a 25MHz voltmeter across the load. If I get a
significant
reading with no input signal, it is oscillating. If I need to
know
the frequency of oscillation then I use a frequency counter.
The
scope only gets connected when I know the amp is not
oscillating.
Both these instruments are decent, ex-military units and fairly
recently calibrated. I can read them without using a ruler.


To check for ringing above 2MHz, I suppose I could sweep and use
the
voltmeter, but I don't bother. Perhaps for guitar amps, or
other
things with complicated voltage stages and crowded wiring?


Do you often discover anything worth finding above 2MHz? Maybe
I
should save up for a new scope?


** I started out ( as a teenager) with a home brew 3 inch, 2
MHz all
tube scope 35 years ago ( still have it too) then a 10 MHz
single beam and
for the last 16 years a 50 MHz dual beam.

SS amps can have parasitic oscillations up to 40 MHz or more,
especially
if they use MOSFETS.

Probably 2 MHz is enough for someone who works on tube amps
exclusively.




............. Phil

Ian Iveson wrote:

The OP asked in the context of valve amps. I do work on valve amps
exclusively, and they are usually made by
me, and they have grid stoppers where they should have.

If I use overall nfb, then ringing for a power amp is likely to be
around 80kHz and 1MHz. Never thought through why this always seems
the case. The former is dealt with by snubbing the OPT primary
somehow, and the latter by power-valve grid stoppers usually.

The reason for oscillations in this 80 kHz to 1MHz region is due to the
accumulated phase shift of the voltage amp, and that of the OPT,
which acts like a second order LPF, due to the series leakage
inductance,
and shunt capacitance.
As one rolls the F upwards, the open loop phase shift might be say
180 degrees at say 150 kHz.
Even though the gain has rolled off a lot, its sufficient that when FB
is applied,
the fed back signal is positive FB, and without an output load, gain is
at a
a maximum, and maybe it just oscillates. With a resistance load,
maybe it stops oscillating, but will still oscillate if a cap load of
typically
0.22uF is added. The 0.22 causes a further phase shift.
The use of just the right values of R&C connected across the load of V1,

and the OPT half primaries, will usually reduce the open loop gain at
HF,
but not below 20 kHz.
Also, the zobel network at V1 reduces the phase shift between V1 and V2,

whatever they may be, and although phase shift at 20 kHz is slightly
increased
in the open loop response, the stepped response of V1 due to the
load value being dropped from say 47k down to say 4.7k means the miller
effect between
V1 and V2 is reduced, and the full 90 degrees of ultimate phase shift
will be moved from around say 100 kHz to say 500 kHz, by which time the
V1 gain
has been reduced say 12 dB by the zobel.
This is critical damping for a bandpass filter with feedback, and all
amps are bandpass
filters, as well as amplifiers.
The cap across the Feedback Resistor to the cathode of V1 from the
OPT secondary, if this is fitted, is chosen with caution.
As the F rises, the phase of the fed back voltage applied to the V1
cathode
is advanced, to compensate for the lag in phase by the time one gets to
say 150 kHz,
The fedback voltage thus is more in phase with the input voltage,
so the feedback remains effective up to a higher F.
Too big a compo cap, and phase shift at say 500 kHz becomes positive,
and voila, the amp oscillates at around 500 kHz.

With SS amps, there is no OPT, and no leakage inductance,
but there is miller effect in the driver amp, and phase shift in the
whole amp, but usually the the first HF pole is at 1 kHz, and open loop
response falls at
6 dB/octave above 1kHz, so the open loop response is down 60 dB at 1MHz.

if 40 dB of NFB is applied, then the response is flat to say 100 kHz.
a zobel network across the main gain stage of the SS amp is still
required,
to prevent early phase shift, and lower the open loop gain,
so when FB is applied, you don't have an RF transmitter.
I am simplifying what happens here, and most SS amps have
inductive zobel networks operating between the output and the speaker
terminals,
and feedback loop does NOT include this passive network,
which prevents the amp from ever suffereing the phase shift due to being
directly
connected to a capacitive load.
Sometimes SS amps oscillate, but usually at much higher F than tube
amps.

All amps using NFB have to be set up carefully if they are to remain
stable under all load conditions, including connection of a 0.22 uF
across the output, with no R load.
This makes many tube amps oscillate violently.



If I built pre-amps with feedback then a better scope might attract
me more. As it is I rely on sensible precautions.

In the fullness of time, if you do want to see more of what happens with

amps that have NFB, or oscillate by means of stray capacitance between
an output and an input someplace, then a 20 MHz cro is
probably all you really need for most tube circuits.
Occasionally, one does see RF oscillations at higher F than 20 MHz,
and a sixth sense is needed to detect them, or a meter.
A cascode preamp I recently built did oscillate at some high RF,
and just touching parts where the voltage was supposed to be fully
bypassed
with a screw driver caused a loud click at the output speaker.
the screw driver changed the effective circuit at RF, which is quite
different
to what you have at audio, and the oscillation stopped, and the
stop and start of a stream of oscillation will cause a click, that of
course shouldn't
be heard.
Plenty of info about simple RF detectors in ARRL handbooks.

Patrick Turner.



cheers, Ian

"Phil Allison" wrote in message
u...

"Ian Iveson" wrote in message
...

"Phil Allison" wrote


** If you can't see them on the scope how do you even know
when
they are there ?

I measure with a voltmeter. Also, the scope's trace goes fuzzy.


** That contradicts my "if" condition .

10 Mhz makes no effect on a 1 Mhz scope.



If you don't know they are there then why would you use
an
(RF) voltmeter ??

To check they are not there :-)


** You are assuming continuous RF oscillations - more usually
ones sees
bursts superimposed on a audio wave.



This is in the context of audio valve amplifiers. When I check
an
amp I put a 25MHz voltmeter across the load. If I get a
significant
reading with no input signal, it is oscillating. If I need to
know
the frequency of oscillation then I use a frequency counter.
The
scope only gets connected when I know the amp is not
oscillating.
Both these instruments are decent, ex-military units and fairly
recently calibrated. I can read them without using a ruler.


To check for ringing above 2MHz, I suppose I could sweep and use
the
voltmeter, but I don't bother. Perhaps for guitar amps, or
other
things with complicated voltage stages and crowded wiring?


Do you often discover anything worth finding above 2MHz? Maybe
I
should save up for a new scope?


** I started out ( as a teenager) with a home brew 3 inch, 2
MHz all
tube scope 35 years ago ( still have it too) then a 10 MHz
single beam and
for the last 16 years a 50 MHz dual beam.

SS amps can have parasitic oscillations up to 40 MHz or more,
especially
if they use MOSFETS.

Probably 2 MHz is enough for someone who works on tube amps
exclusively.




............. Phil

"Patrick Turner" wrote in message
...


Occasionally, one does see RF oscillations at higher F than 20 MHz,
and a sixth sense is needed to detect them, or a meter.
A cascode preamp I recently built did oscillate at some high RF,
and just touching parts where the voltage was supposed to be fully
bypassed with a screw driver caused a loud click at the output speaker.


** A simple voltage regulator using a BD139 and 18 volt zener built for a
disco mixer proved effective at wiping channel 9 completely off the screen
of a TV set in the same room. No sign of trouble on a 10 MHz scope.

Power mosfets make nice HF and VHF oscillators as well - the Hitachi
2SJ /K ones - it pays to keep leads to them short with minimal inductance.



............... Phil

On Thu, 07 Aug 2003 13:34:34 +0100, Ian Iveson wrote:

Let's say you use your 4-channel scope to conveniently display
input, output, and two intermediate stages simultaneously. I
contend that when you look at your display, you compare traces in
pairs. This is not just a limitation of your eyes or your brain:
it is because there is no actual sense in comparing more than two
things. The question "What is the difference between these X
things?" only makes sense where X = 2.

You remind me of the people who insist
that nobody can visualize beyond three
dimensions, so there is no real use for
arrays of more than three dimensions.
Yet I've been both visualizing and using
arrays of more than 3d for decades. And
having no trouble teaching others to do
so. 8^)

I have to admit, I'm really more interested
in the number of inputs than the number of
traces. But the more the merrier. Even
with multiple traces (vs channels superimposed
on each other), it's often easier to grasp the
overall effect of what's happening, and see
how everything relates and impacts everything
else, if one has it all before the eyes at once.

There are (X-1)*X/2 differences between X things. You can sensibly
ask "What are the differences?" and these you may evaluate by
considering each possible pair. Hence, whatever comparison you
carry out with your 4-channel scope can be done with a maximum of 6
probe swaps with a dual-channel scope.

Which is an obscene amount of time. And if your
memory and visualization skills are less than
perfect, it's all to easy to forget exactly what
happened when, where and why, and thus miss
something crucial.

If you remember all your early maths, like long division and
multiplication, and even adding several numbers, that was all about
breaking the process down until you could deal with numbers in
pairs.

I coach people who dropped out of school,
and are going for their GED or into college,
on math. I think I get the concept! But
it doesn't apply that well here, IMO.

Just for grins, let's say you're trying to
show the effect of having two preamp
stages, with volume controls after each.
You can demonstrate how things work much
better, and much quicker, if you haev four
inputs. You can always kill two inputs to
focus in on any one stage, but you can also
show the overall picture *much* better with
those 4 inputs!

You're welcome to your opinion - but that's all
it is (just like mine 8^). And while you're busy
swapping probes, the guy with the 4 input scope
(soon to be me, I hope) will have resolved that
issue and gone on to something else.

Phil Allison wrote:

"Patrick Turner" wrote in message
...

Occasionally, one does see RF oscillations at higher F than 20 MHz,
and a sixth sense is needed to detect them, or a meter.
A cascode preamp I recently built did oscillate at some high RF,
and just touching parts where the voltage was supposed to be fully
bypassed with a screw driver caused a loud click at the output speaker.

** A simple voltage regulator using a BD139 and 18 volt zener built for a
disco mixer proved effective at wiping channel 9 completely off the screen
of a TV set in the same room. No sign of trouble on a 10 MHz scope.

Power mosfets make nice HF and VHF oscillators as well - the Hitachi
2SJ /K ones - it pays to keep leads to them short with minimal inductance.

I once built a 2 x 300 watt mosfet class AB amp,
http://www.turneraudio.com.au/webpic...et400w302h.jpg
It has 6 hitachi mosfets per channel, 2SK1058, 2SJ162,
and yes, I did have some trouble with oscillations, and with making sure the
rails were fully bypassed right up to RF.
Eventually, I got a very nice looking error signal,
even with a 20 kHz square wave, from a fast sig gene,
and this amp sounds indistinguishable from many decent tube amps.

Sloppily built mosfet prototypes usually always oscillate,
until you learn how to set things up as you say.
But there is fun getting it right, and going better than many commercial
amps.....
Sometimes one has to introduce inductance, just at the right place,
to obstruct HF signals.

Patrick Turner.



.............. Phil