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On Mon, 19 Nov 2018 16:22:22 -0800, John Robertson
wrote:

On 2018/11/19 4:01 PM, wrote:
Hey all youse 'scope gurus,
I'm gonna be getting a new and a used tube amplifier. The new one
is on the way and the used one is in my neighbor's garage. He still
needs to dig it out so I don't know anything about it except that has
tubes in it.
If the new one doesn't come with a schematic then right off the bat
I want to draw one so I can learn more about vacuum tube audio amp
workings. If I have to draw a schematic then I will put in the actual
values of the passives as well as the marked values. And I want to
measure the voltages in the circuit as well and put them into the
schematic. If the old one needs a schematic as well then if I can't
find one online I'll need to give it the same treatment as the new
one. If I do have schematics then all I will need to do is the
measurements and write them down. So....
I have a TEK 465B oscilloscope and it has served me well until now
diagnosing my CNC equipment. All the voltages I have needed to measure
have been below 50 volts. But looking at the 'scope it seems like at
the 10x setting on the probes the highest voltage I can measure is 200
volts. That's 4 divisions at 50 volts per division. And that's peak to
peak, not RMS. I do have one 20x TEK probe, a P5120, that I used for
measuring the mains power, but the voltages inside tube equipment go
much higher.
I have been watching YouTube videos about vacuum tube equipmet and
have learned a lot so far. "Uncle Doug" has several videos that have
taught me a lot. Maybe it's because he used to be a teacher I am
learning so much.
Anyway, do I need a different 'scope to to measure accurately what
is going on inside vacuum tube equipment? Should I just look for
different probes with even more attenuation? I certainly don't need
the 100 MHz bandwidth that the 465B has for working with audio
equipment.
If you all think another 'scope should be in my future I would love
some advice. Since this is a hobby I don't wanna spend a lot of money
on yet another toy, er tool. So a used 'scope is fine with me. I love
my used TEK 465B.
Now before everybody starts giving me advice about high voltages
and all that I need everybody to know that I am expert and know all
there is to know about working with high voltages. Don't waste my time
telling me to put down my drink before I start working around high
voltages. I always use plastic cups for my drinks when working with
high voltages. Oh, and I don't put any salt on the rims of my Bloody
Marys either. And no cans of beer either, only glass bottles. Safety
first. As an expert I know alcoholic drinks are good. They keep my
hands from shaking too much in the morning. And I don't like smoking
pot. I figure I'm just as expert as some other self professed experts
here who don't need any extraneous advice.
I just watched a video about isolation transformers, how to build
one in a proper enclosure and how to use it properly. So even though I
have the isolated variac I think I'll make a 1:1 isolation xmfr too
with a little more ampacity. And if anybody here thinks I could use
some more advice about working around high voltages I welcome it and
will certainly take heed.
Thanks,
Eric

Most tube amps are serviced in our shop with a digital voltmeter...not a
scope. Really all that you need to know are correct are the plate and
cathode voltages, and if they are off then you check the screen and
grids for proper bias - after testing the tubes for proper operation.

Most common failures are electrolytic caps, then inter-stage isolation
caps followed closely by plate resistors (they overheat with bias goes
wrong) and then cathode resistors, bias circuit, grid resistors, and
then - perhaps - a damaged output transformer.

Virtually all of that can be tested quickly with a voltmeter once you
know the average plate voltages. If the plate voltages are good then the
amp is fed a known AC signal of a set level and you then follow the AC
signal through the amp using your AC setting on the voltmeter. I check
first the input level, if good I then check the high side of the volume
control and if is on spec I then go to the output of the phase splitter
tube(s). Divide the amp in half, then halve the remaining sections to
find the problem.

If you are doing high end amp work where distortion is a factor then you
will break out a slow dual trace scope to find the issues, but for
regular tube amps like guitars and jukeboxes we usually don't bother.
Not that we don't have scopes (have almost ten in various sizes and
speeds), but they rarely help - at least for tubes!

John :-#)#
Thanks for the reply John. I think my tongue in cheek "expert" comment
fell flat but you saw through my poor attempt at humor. Since I will
be interested in looking at the AC signal through to the speaker to
see how it changes, when clipping starts, a scope will be necessary.
From what you say it looks like using a VOM for the DC and a scope for
AC would be a good way to see how the amp is working.
Thanks,
Eric

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