Tube Theory...True RMS meter to detect signal? Renamed

[mchauck]

Yo....can I use the Freq % setting on my DMM to detect a signal going through my amp (or not going through as is the case)? If so, what is the easiest way to generate a signal at the input?

Brian

[tele_player]

The easiest way is with a signal generator. For instance, a BK 3001, for about $75.

Used signal generators might be found on eBay for less...

[ampgeek]

Hey Brian,
As another option, check out the amp "stethoscope" a couple of frames down here:

http://www.el34world.com/Hoffman/tools.htm

It is, without a doubt, the slickest little tool in my toolbox!

I attached one of those spring loaded, test probe thingys to mine so I can clip it anywhere into the amp being tested and still strum a guitar. Any decent, high headroom guitar amp can be used as the monitor.

The "resolution" with this device is outstanding. I had been toiling with a Dumblesque type build for months. I whipped one of these things up and started walking through the signal path and within minutes identified the problem. A noisey grid stopper resistor!

Good Luck,
Dave O.

[drhulsey]

I whipped one of these things up and started walking through the signal path and within minutes identified the problem. A noisey grid stopper resistor!

Do you hear the same noise you were getting through the test amp

[ampgeek]

Yes! A pleasant and smooth tone on the input side of the resistor and an un-pleasant, ragged, slightly distorted tone on the output/grid side. The difference was as clear as a bell. Nearly identical (e.g., minus tone coloration from downstream signal path componants) to what I was hearing right from the git-go in the test amp. I recognized it the second that I heard it.

Replaced the resistor and all is well now.

And to think.....I was just about to gut the whole chassis and start over!

You can also hear the effect on tone from coupling/tone caps in the signal path with this device. Clip the test lead on the upstream side and strike a note and then quickly switch the lead to the downstream side for an instant A/B comparison. I heard subtle difference in some locations. More obvious in others.

Although I didn't go all the way to the "end", I assume that you could also evaluate OT health/tone coloration by slapping it right onto an output tube plate. Of course, in a PP, AB plan you would only be hearing slightly greater than ~1/2 of the wave form but I think that this would be enough to get the idea with even a novice's ear.

Getting instant feedback on how individual signal path elements effect the tone and having a high resolution troubleshooting technique at hand are the true advantages of this device!

Dave O.

[drhulsey]

Getting instant feedback on how individual signal path elements effect the tone and having a high resolution troubleshooting technique at hand are the true advantages of this device!

I could be wrong, but this sounds like one of those "Oh, my God!" things
Like, "Oh, my God This is a device that can be DIYed, tell you tons of things about your amp circuit, troubleshoot, not cost a lot...." It sounds like a great device for those without an O'scope or knowledge of how to use one.
Am I wrong

[tele_player]

It sounds like a great device for those without an O'scope or knowledge of how to use one.

I just wonder what such people are doing inside an amp.

[mchauck]

Gosh ampgeek, I have to change my underwear!!!!
That sounds like just the ticket!!!!

Brian

[mchauck]

BTW tele_player....Einstien failed high school math!!!!

Brian

[drhulsey]

I just wonder what such people are doing inside an amp.

I've read a lot of disclaimers about LETHAL HIGH VOLTAGE, etc., but I've never seen one that said, "If you don't know how to use an oscilloscope, don't have one, or don't have an EE degree you should not undertake this project." Are you ready to take a poll of all those who have built an amp to see who has one and who doesn't Some of us just build amps for ourselves

[tele_player]

BTW tele_player....Einstien failed high school math!!!!

Brian

I suspect he had ADD...

[mchauck]

OK....I took a small mini-twin (like a small pignose) and made a probe to test with. I plugged the probe into the mini input and attacheda ground from the mini to the big amp. I began tracing along the signal path starting at the input jack of the big amp an low and behold, it works! I quit (ala tele_player) to learn a bit more about signal path and how to follow it. I am confused about one thing though. In the amp I am checking, the signal seems to go to the grid of the first PA tube. Then it exits at the plate. Then following the "Normal" line, it enters the next PA stage at the grid and exits at the cathode. Why does it switch like that?
Then when it enteres the Power tubes, it works from grid to plate again?
I know I am showing my ignorance here, but I'm just trying to learn something.

Help

Brian

[mchauck]

Ouch man, that "ADD" hurt! Was it meant that way or am I just being overly sensitive?
Wait....what was I talking about?

Brian

[Lonely Raven]

Ouch man, that "ADD" hurt! Was it meant that way or am I just being overly sensitive?
Wait....what was I talking about?

Brian

He was in fact Dyslexic and ADD. I think Tele was just pointing out a fact.

[mchauck]

Whoooo Hoss....just kidding ereh !!!!!