Amp Stethoscope

ampgeek · Post by **ampgeek** » Wed Nov 28, 2007 1:58 pm

The discussion here prompted a few PM questions about my embodiment of subject.

http://ampgarage.com/forum/viewtopic.php?t=3983

Obviously, the safe use of this devices requires an understanding of tube amplifiers and their inherent high voltages and associated dangers. The stakes get a little higher as two amplifiers are potentially exposed to damage if the device is built or used improperly.

I came across the device here. Spending a few moments studying Hoffman's description is highly advisable.

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

I configured mine as shown here:

http://tinypic.com/view.php?pic=6slxjbt&s=1

Any decent, high headroom guitar amp makes an adequate "listening" amplifier. I have used this device flawlessly on numerous occasions and it has become an essential tool in my box. The spring loaded test clip is great for quickly A/B'ing tone changes across signal path components.

Cheers,
Dave O.

drhulsey · Post by **drhulsey** » Wed Nov 28, 2007 2:27 pm

ampgeek wrote: Any decent, high headroom guitar amp makes an adequate "listening" amplifier.

Does "decent" exclude a SS amplifier

(There must be some use for those things)
Mounting the "safety" components in an inline project box eliminates the need for modifying the listening amp itself. This is a VERY handy addition to making this device

Thanks for sharing, Dave

ampgeek · Post by **ampgeek** » Wed Nov 28, 2007 4:44 pm

My pleasure Tim! Just trying to add back whatever small contributions I can to the community from which I have taken so much.

By my way of thinking, the selection of the "optimum" listening amp depends on the task at hand. If you are doing some very serious tone shaping, you wouldn't want your listening amp/speakers to significantly color the output of the amp being testing.

For example: if you were tweaking a Dumble'esque amp that is being mated with a 1X12 speaker enclosure and you plugged the stethoscope output into a JCM-800 with a 4X12 cab, what you hear would be colored by the Marshall's circuitry/speakers so you might be somewhat mislead.

Way OK though if you were troubleshooting gross problems like the noisey grid stopper resistor I described in the other thread.

So....for super serious tone shaping tasks, I would lean toward a broad frequency response, high fidelity listening amp/speakers which could include SS home stereo systems, studio monitoring systems and, in a pinch, a decent PA system or SS guitar amp (acoustic maybe?).

Gross troubleshooting tasks could use any well functioning amplifier as a listening amp. I have a Mesa Boogie DC-30 combo with a 1X12" Celestion V-30 set up on my bench just for this purpose!

Dave O.

drhulsey · Post by **drhulsey** » Wed Nov 28, 2007 5:14 pm

ampgeek wrote:... you wouldn't want your listening amp/speakers to significantly color the output of the amp being testing...

I hope I haven't thrown out my old, out-dated, but functional stereo amplifier

I would at least have an answer to my wife's perennial question, "Why don't you throw that away? You're never gonna use that!"

skyboltone · Post by **skyboltone** » Wed Nov 28, 2007 5:55 pm

drhulsey wrote:
ampgeek wrote:... you wouldn't want your listening amp/speakers to significantly color the output of the amp being testing...
I hope I haven't thrown out my old, out-dated, but functional stereo amplifier I would at least have an answer to my wife's perennial question, "Why don't you throw that away? You're never gonna use that!"

There must be about a million Radio Shack SS amps on Ebay in the $1-$10 range eh? Ampgeek, could a guy use a CD player (with impedence matching transformer) to provide the sound sample I wonder. I don't have any friends.....

gearhead · Post by **gearhead** » Wed Nov 28, 2007 5:59 pm

Just get yerself and extra-extra large thumb pick and put it on your toe. Do not try slide work while doing this test.

skyboltone · Post by **skyboltone** » Wed Nov 28, 2007 6:07 pm

gearhead wrote:Just get yerself and extra-extra large thumb pick and put it on your toe. Do not try slide work while doing this test.

ampgeek · Post by **ampgeek** » Wed Nov 28, 2007 6:11 pm

I just knew it.......We are all married to the Stepford Wives!!!

Now Skybolt...no friends?!? I doubt that very much.

Seriously though, if you use the little spring loaded, one handed operation, test clip thingy that I used, you can very quickly and easily move it around while holding a chord/note on a guitar. The business end of it fits perfectly and securely on our typical conductor and component lead wire diameters.

Forgive me for not knowing the correct nomenclature or having a part number for it.

Cheers,
Dave O.

skyboltone · Post by **skyboltone** » Wed Nov 28, 2007 6:25 pm

ampgeek wrote:I just knew it.......We are all married to the Stepford Wives!!!

Now Skybolt...no friends?!? I doubt that very much.

Seriously though, if you use the little spring loaded, one handed operation, test clip thingy that I used, you can very quickly and easily move it around while holding a chord/note on a guitar. The business end of it fits perfectly and securely on our typical conductor and component lead wire diameters.

Forgive me for not knowing the correct nomenclature or having a part number for it.

Cheers,
Dave O.

No, you're right. Clip thingy. I looked it up in Mouser!!

drhulsey · Post by **drhulsey** » Wed Nov 28, 2007 6:40 pm

ampgeek wrote: I just knew it.......We are all married to the Stepford Wives!!!

Or are we the STEPFORD HUSBANDS

"Honey! Come in the kitchen and help me! There's no time for that AMPLIFIER nonsense right now!"

skyboltone wrote:There must be about a million Radio Shack SS amps on Ebay in the $1-$10 range eh?

Yes, but those don't provide me an "I told you so!"

skyboltone wrote: ...I don't have any friends...

Let's all gather around him for a group hug

briane · Post by **briane** » Wed Nov 28, 2007 10:03 pm

very cool info. I'm gonna build one!

Just get yerself and extra-extra large thumb pick and put it on your toe. Do not try slide work while doing this test.

You thought that was a silly comment I suspect. The funny thing is one of the most talented players I ever saw ACTUALLY did that!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!bit OT. And yes he did bass slides!

He played 3 guitars at once. Lead in front: left hand, rhythm behind the back: right hand, killer bass: feet. This guy rocked, the bass playing was incredible...and oh he sang really well too. The bass was mounted on a special stand with a footboard. He was backed by a drummer.

Saw him in NO on a street corner. Suspect he was just one of those guys that was too talented to take it on the road. a 2 man full band act!

mchauck · Post by **mchauck** » Thu Nov 29, 2007 12:22 am

I'm so happy with y'all I could just bust!!!!!

Brian

Bob-I · Post by **Bob-I** » Thu Nov 29, 2007 1:02 am

I just soldered a .1uF 600V cap onto a test lead and connected the other end to a 1/4" in-line jack. I use the stereo in my little room as a listening amp.

drhulsey · Post by **drhulsey** » Thu Nov 29, 2007 3:40 am

Bob-I wrote: I use the stereo in my little room as a listening amp.

What is the wattage on your stereo amp

mchauck · Post by **mchauck** » Fri Nov 30, 2007 2:40 am

Just a quick update. I built my listening amp. I listened and I found.........a 1.5K 5W power resistor between my B+ and my output tube screen voltage @ the 2 x 32uF can on my 18 watt. It was burned out causing no voltage to get to the screens. I really am grateful to you all for the help

Brian

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