Finally An Oscope In My Hands

[Justa]

Well I never did get the old scope with the bad sync to play with.   But my brother popped by recently and gave me an old AWS 15 Mhz dual trace scope.  Not a Techtronix but the dang thing seems to work and has a bright display that focuses well.  I only know the very basis stuff about oscilloscopes.  Is 15 Mhz fast enough for working on tube amps?  I suspect it is but really have no idea.

Thanks 

[DummyLoad]

Is 15 Mhz fast enough for working on tube amps?

more than adequate...

[tubesornothing]

Congratulations!  What's the model number?  Dual channel? 15Mhz is tons of bandwidth.

If you have a signal generator, put a 500Hz sine wave into the amp input, around 200mV (measure it using your new scope!).  Then use you scope to measure the signal throughout the amp.  See how the signal changes as you crank the amp.  Soon you will become familiar with various patterns of distortion, compression, oscillation, the desired "shark fin", the dreaded "leading edge spike".

If you dont have a signal generator, pick up a cheapy, or build a simple single frequency one .  Keep in mind if you purchase one, try to get one with very low harmonic distortion.

CAUTION: do not touch your scope probe directly to a tube plate or the B+ - the voltage will be too much for the scope.  But hooking it up after a coupling cap will work just fine.

Cool!  I LOVE scopes.

[Justa]

Opps, forgot to include the model number.  It is a 315P.  I found a schematic for a very simple signal generator that can generate 1Khz and 2Khz signals and has adjustable amplitude.  I slapped it together and it does seem to work properly.  I really am not sure what a good frequency would be to trace a signal through an amp but I have these two frequencies available.

Something that I forgot to specifically ask is if the 15 Mhz is fast enough to pick up oscillations that might occur during an amp build.  I don't know the range of frequencies that might occur.

Thanks Guys!   

[tubesornothing]

15Mhz is more than fast enough.  Sule of thumb is 10 times the max signal you will capture.  That means you can see oscillations up to 1.5Mhz - WAY more than enough.  The oscillations I typically see are anywhere from 4kHz to 40Khz.

1Khz singal is fine enough for now.  I like to use 500Hz, cause its more in the midrange of the guitar.  But wait until you can pick up a cheap signal generator.

It looks like that scope will work just fine - 2 channel - good deal!  Put one channel before the gain stage, and another after.  Or the same idea over the tone stack.

[Justa]

I am impressed that you could find images of the AWS 315P.  I can find almost no references to this scope on the net.

I will change a few parts on the signal generator and so I can generate a 500Hz signal.

Cool, the x10 factor of scope speed to what you want to capture will be easy to remember and I now know where oscillation frequencies might lurk.

Can't wait to trace a signal through an amp.  No more funky probing to hear the "pops" end to find a problem.

Thanks Tubesornothing!!

[bnchwrmr]

  Congratulations on your great scope.

  It looks like those images show that your
  maximum volts per verticle division on your
  CRT screen scale is 50 volts. Even the 20
  volts per division will be adequate for
  checking preamp plate signals and most work.
  And the 50 volt setting should let you fully
  observe large signal swings on tremelo circuits
  and even output tube plate signals.
 
  A lot nicer to have than just a 5 volts per
  division limit as on a lot of scopes out there.

  bnchwrmr

[Justa]

I have  a switchable 1x, 10x probe that i guess will be pretty useful.  Hope I don't smoke my probes.  LOL

[PRR]

> if the 15 Mhz is fast enough to pick up oscillations

You get midrange squeal, slightly supersonic squeal, and very rarely some horribly high MHz oscillation.

99% of my time, a 20KHz 'scope is all I need or want. (I actually built a 19KHz 'scope once: enormous savings in cost.)

The last time I ran into MHz trouble, my 15MHz 'scope would not resolve the actual MHz wave, but the "bursts of fuzz" were reasonably visible. That was a too-tricky chip-BJT amp with (in retrospect) a real urge to sing 14MHz, but only when excited by a lower tone driven near clipping. The 'scope trigger was so hung on the lower tone that the 14Mhz wave would not stand still.

Tube guitar amps "are designed" to pass the audio band... and not much more. You can have high gain or wide bandwidth. Gain is vital. The designer allows "just enough" bandwidth to do the job, so the trade-off favors gain where it is wanted. So MHz oscillations are unlikely. And practically impossible on 12AX7 gain stages.

But possible on power pentodes. They have gain in reserve, and low parasitics. A length of wire looks like a short for audio, but can be a tuned-circuit inductance in the radio bands. Wire-wound resistors of low to medium value are resistors for audio, but can be kinky at high frequencies. I once had a big power-amp go crazy, and traced it to a plate lead run close to a wire-wound cathode resistor. While the OT would barely pass audio, the lead to it could sing much higher, and the coiled-resistor coupled enough energy back into the tube to (apparently) sustain radio oscillations. It took weeks to work out the (possible) theory, but only minutes to solve the immediate problem: Keep Outputs Away From Inputs! (And yes, a cathode is an input.)

> Hope I don't smoke my probes

I've done it a lot. Many probes use 600V or even 300V trimmers; maybe today they give you 100V trimmers.

Most of the time, when you are working above 300V, you are woring on a POWER stage. You really should not be sticking fingers OR probes in power stages. But when you do, build your own probe. Use a 1Meg and a 10K resistor. This gives 100:1 division, so even cheap 'scope inputs can be used with quite hi-volt amps. The 1Meg is no sweat on a 8K plate load. This un-compensated probe is flat "only" to 100KHz, which is less than the 'scope's 15MHz but more than you are ever likely to want to see.

[Justa]

Wow PRR,

Thanks for taking the time to post such a comprehensive post!  I've been wondering about amplifier oscillations for a long time.  I am loving the scope already as I have so much to think about and learn.  Now that you mention oscillations in audible frequencies I am realizing that I definitely have heard them before and sure remember shutting off the amp as fast as I could to save the speakers.

I will build up a 100 factor probe like you mentioned.  I am sure I will toast a probe or two learning and so it sure would be nice to have a couple of fixable probes to play with.

I have my scope set up in properly in the living room.  Appropriate don't you think?