Any tips for scoping an amp

[fishy]

It is a new area for me so I was wondering if there are any tips or data anyone would like to share.
More specifically, signal levels and frequency, accepted methodology.

If anyone can point me to any useful stuff I can read up on, that will be appreciated also.

Pete

[Backlash]

Hi Pete

Electric Guitar Amplifier Handbook by Jack Darr has a quite a few examples on how to use a scope to help troubleshooting. It also has a lengthy discussion on amp repair and troubleshooting in general. A must have book in my opinion.

Edit: It is possible to get a new reprint in paper, but you can also try this http://www.pacificrecone.com/JackDarrBook.html


PeterM

[fishy]

Thanks, I'll check it out.

I was interested to scope my Express and Rocket as part of a blueprint.
Seems like a good good opportunity to learn something.

Pete

[Fischerman]

I don't go about it very scientifically.

First, I usually play the amp so I'm familiar with the vol/tone settings and then I'll usually come up with 'default' settings that I use for scoping the amp. The volume and tone controls have a big effect on what you might see on the scope. I record those settings on my voltage sheet.

Then I measure/record the incoming AC wall voltage and measure/record all the DC voltages.

Then I input a 100mV/880Hz signal into the amp. Then I measure the AC voltage (i.e. the signal swing) at every plate/grid (or cathode in a cathode follower) and at the output jack. Those are measured in true RMS using my Fluke DMM. Sometimes I'll also measure the DC voltages again while the amp is playing (they will change when the amp is played).

Then I go through the amp with a scope probe. Then I'll usually adjust the volume and see what happens in the amp. Like I'll try to find which stage clips first and at what volume setting...then what clips second and so on. Or I'll look at the PI plates and see if one side is clipping way more than the other (and if it is...then determine if I like it that way or if I like it better when the PI is more balanced).

I'm really just poking around and measuring stuff. I'm not really sure how helpful it is...but it couldn't hurt! And you learn some things...I was surprised that the preamp can have a huge amount of voltage gain but with a LTPI we only need a tiny, tiny bit of it to reach full power.

[Plague]

Gonna use some scope before I go to sleep tonight.

Thats about all I got.

Night Fish!

Ty

[fishy]

I don't go about it very scientifically.

First, I usually play the amp so I'm familiar with the vol/tone settings and then I'll usually come up with 'default' settings that I use for scoping the amp. The volume and tone controls have a big effect on what you might see on the scope. I record those settings on my voltage sheet.

Then I measure/record the incoming AC wall voltage and measure/record all the DC voltages.

Then I input a 100mV/880Hz signal into the amp. Then I measure the AC voltage (i.e. the signal swing) at every plate/grid (or cathode in a cathode follower) and at the output jack. Those are measured in true RMS using my Fluke DMM. Sometimes I'll also measure the DC voltages again while the amp is playing (they will change when the amp is played).

Then I go through the amp with a scope probe. Then I'll usually adjust the volume and see what happens in the amp. Like I'll try to find which stage clips first and at what volume setting...then what clips second and so on. Or I'll look at the PI plates and see if one side is clipping way more than the other (and if it is...then determine if I like it that way or if I like it better when the PI is more balanced).

I'm really just poking around and measuring stuff. I'm not really sure how helpful it is...but it couldn't hurt! And you learn some things...I was surprised that the preamp can have a huge amount of voltage gain but with a LTPI we only need a tiny, tiny bit of it to reach full power.

Hey, that helps. I think it is a great opportunity to learn something and hopefully correlate what you see with what you hear. Thanks.

[Fischerman]

Gonna use some scope before I go to sleep tonight.

Man-up and use Listerine.

[mss1]

Gonna use some scope before I go to sleep tonight.

Man-up and use Listerine.

Real man-up and gargle with Buckley's... Ahhhhhh.

M

[Plague]

HAH! So many other dorks on here besides myself.

Love it.

[RJ Guitars]

Hey All,

Happy Thanksgiving to you!

I've got the green light to add an O'Scope to my Chritmas list... within limited parameters anyway. It looks like there are several choices for less than a couple hundred bucks. Wow, technology has changed things a bit!!

I am curious what the experience level is with one of these BK Precision or Madill scopes is? You can buy these things new for less than that $200 i mentioned.

In addition to the comment on the cheap stuff (you get what you pay for I assume), How many channels do I need and what kind of probes?

Then ultimately after I get a scope, I'm kindy there with Fishy wondering what can I learn with one of these?

rj

[gahult]

Hi all,

Happy Thanksgiving.

Fishy, rj, anyone else, try this site, may help using a scope.

https://www.cs.tcd.ie/courses/baict/bac ... scope.html

Gary

[RJ Guitars]

Thanks Gary.

Couldn't help but snicker a bit when I saw the icon with your last post... just since I mentioned Christmas on that last post... huh could it be? Don't you live somewhere up north?

Anyway, about O'Scopes. I assume I need two channels so I can see the various in phase and outa phase components simultaneously?

rj

[RJ Guitars]

Doh..... slow internet and I got impatient and hit the send button too many times....

[RJ Guitars]

not just two times but three... sorry!

[Wayne]

Definately two channels are handy, and it sounds like you already know why. Other handy features that I have seen in the past are the ability to invert one channel, as well as the ability to add the channels together. Using both of these features at the same time allows you to make differential measurements.

I don't know the market these days - my B & K is so old that the owners manual proudly brags about the number of transistors used in the circuit (much the same way we list every tube when stating specs about our beloved amps). The word "new-fangled" isn't stated in front of "transistor", but it's implied. I guess the fact that it still lights up is a vote for B & K.

W