ampage Tube Amps / Music Electronics	For current discussions, please visit Music Electronics Forum.

ampage archive

Vintage threads from the first ten years

will this work?

:

4/25/2006 7:16 PM

will this work?

i build pedals for a living. and one of the tools we use to diagnose a broken pedal is a homemade probe type tool.
see pic: http://i43.photobucket.com/albums/e374/methodofcontrol/PROBE2.jpg

the cap is fully insulated w/ heatshrink or electrical tape to a chopstick (save for the end that serves as the poker) or such device that can safely be used as a pointer.
the idea is that you plug a guitar into the broken pedal and plug your probe into an amp, ground the negative side of the probe on the pedal then you strum the strings on the guitar and poke around in the pedal w/ the positive end of the probe starting at the input. the guitar will ring through the probe into the amp. when you place the probe at a point on the pedal that the signal is no passing, you won't hear the guitar through the amp anymore and then you havea good idea of where the pedal needs fixing.

ok....anyway....can i apply this same principle to an amp? i'm looking to find a crackly type noise in a traynor amp. (see my other post for details.)
i would use an attenuator as the load on the broken amp. and i would plug my guitar into the broken amp and plug my probe into a working amp and try to find the bad part of the circuit.
see pic for details on probe use here:
http://i43.photobucket.com/albums/e374/methodofcontrol/probeuse2.jpg

thanks for the help guys! i just want to know if this is safe for all amps involved, and whether or not i will even help me.

4/25/2006 7:52 PM
loudthud	It will work in principle but you will really need a 1meg audio pot wired as a volume control after the probe because the signals in most of the amp will be too big and overload the monitor amps input.

4/25/2006 8:19 PM

methodofcontrol

thanks for the quick reply. that was my biggest concern really...i don't want to break a working amp. what if i stuck the probe into my volume pedal input, and then ran the volume pedal into the monitor amp?
i think my volume pedal is only 25k. too small?
or what if i used the ext.amp jack on my 70's ampeg V4 as the monitor? that jack is placed in between the pre and power amp so when you daisy chain multiple V4's together you're only using the pre-section of the amp your guitar is plugged into. the other amps are just for volume.
once i got past the first pre-tube inthe traynor, it would essentially be like running the tranoy pre-amp into the ampeg power amp, right?

or should i just grab a 1m pot from work tomorrow?

4/26/2006 1:17 PM

I would suggest that you use a small amp as your monitor amp rather than the V4. I often use a little 9V battery powered practice amp to do tests. Some of the advantages of this are:
1) If (when) you accidentally send a large signal to the monitor amp you don't get blown out of your socks. This includes the click/pops when you are moving the probe.
2} The monitor amp is cheap and expendable
3) the monitor amp is totally isolated from the power system that the test amp is connected to. This may not be a factor with a pedal but it can be an advantage when troubleshooting other equipment.

Food for thought anyway. HTH.

Regards,
Tom

4/26/2006 1:22 PM
loudthud	The 1meg will work the best because it won't load down the amp being tested. The Ampeg V4's input will be OK for some points but not all. The main point is to have control over the volume and not being subject to input overload. Does the Traynor sill crackle with the load attenuator turned down ?

4/26/2006 3:20 PM

What you are proposing is called "signal tracing" and is as old as amplifier circuits. You do want a fairly large resistor to ground on the tester side of the cap so the cap can't charge up and present the monitor amp a large DC voltage.

They actually make commercial signal tracers. The Heathkit and Eico ones were popular 40-50 years ago. I think you can still find them even new.

A little battery powered amp would be ideal for this, but I usually have a few discarded little practice amps to press into duty. You know the ones, they are not worth fixing, but I do anyway.

A signal tracer is the audio equivalent of a scope. I don't use them much because I have the scope in front of me already.

Certainly you can use it on an amp, an audio circuit is an audio circuit.

Adding a volume control to your tracer is a good idea, since that 30 or 40 volts of signal on some plate will not make the input circuit of the monitoring amp very happy.

4/26/2006 5:08 PM

*since that 30 or 40 volts of signal on some plate will not make the input circuit of the monitoring amp very happy.*

This should not ordinarily damage a tube monitoring amp, however. It is usually some solid state inputs that are risky because this much signal voltage is higher than their rails.

You will need a high voltage cap, and everything else to be more well-insulated than you use for pedal work, be careful and don't forget.

Sometimes the best procedure by far is to connect probes to the circuit before powering it up, then take the reading, then power down and bleed off stored voltage before reaching back in there. High voltage is not supposed to be as convenient to work on as battery power.

Mike

Page 1 of 2