Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => AmpTools/Tech Tips => Topic started by: Ambugaton on February 14, 2017, 05:20:21 pm
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I have read through the listening amp section in Doug's library and feel like it is something I should have. I think I understand how it is constructed, and how to operate it with his description...
I just wanted to know a few additional things:
1. I have a blackstar HT-40 that I was planning on using as the listening amp, and just putting the cap and pot in a pedal enclosure. If I use the clean channel on the blackstar, is that clean enough?
2. I understand I will be probing through the input signal to identify issues... I just don't have any experience with this. What exactly am I looking for? A drop in signal, nasty distortion, a change in frequency response? What does that tell me if I find it?
3. The shielded cable coming from the cap/pot going to the listening amp... should I use RG174 grounded to the output jack or just a standard instrument cable?
Thanks
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yes to all your questions
What exactly am I looking for?
You'll know it when you hear it. Lets say you have a buzzy hissy build. bust out you tool, start at input, at the point when your "sound"/signal goes south, starts buzzy hissy, you are "close". sounded good at V2a grid, sucks at V2a plate. pull V3, still suck?.......
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I understand I will be probing through the input signal to identify issues... I just don't have any experience with this. What exactly am I looking for?
The Listening Amp, a/k/a Signal Tracer, is a diagnostic tool. It is used to isolate the cause of an issue with a "DUT" -- a device under test. An amp might be making a bad noise, or passing weak or no signal. The question is: where specifically in the amp does this issue arise? The diagnostic/repair process is to put signal into the amp; then probe at logical points along the signal path to locate the point of the problem. The Listening Amp allows you to do this aurally. Alternatively, an oscilloscope could be used which gives precise visual info. But a 'scope is more expensive, harder to use and has a steeper learning curve.
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Thanks! Yeah I figured I wasn't ready to take the plunge into ocilliscopes yet but thought this will be a useful tool to have in case I need it. Appreciate it.
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> is that clean enough? .... What exactly am I looking for?
Sometimes you don't know what you are looking for. But anything "odd" may be a clue.
We are tracing a many-stage system looking for a problem. Let us say there is poor water flow in the upstairs bathroom. You would check for water at the street, at your meter, at your downstairs faucet, etc. If the complaint is "no" water upstairs, you could drill holes (or loosen fittings) to check for water at intermediate points. If "here" makes your shoes wet and "over here" leaves them dry, you have narrowed the problem to that length of pipe. If the complaint is "weak water" you may need a "better" gauge than just wet shoes, you might use something that can tell 40psi from 20psi.
I have been getting ice-cream hard as a rock. I thought it was my freezer and power failures. Then I found a tub from 2013 which was fine! So my freezer isn't the problem. I should backtrack to the supermarket, try a tub from the case. Then a tub from the back room. (I suspect the stock-clerks leave the stuff out while re-stocking.) Of course if the back-room tub is hard I should track the truck and warehouses all the way to Ben and Jerry's factory. SOMEWHERE the ice-cream is good and then bad at the next stage. (I assume B&J don't make this much bad ice-cream. But always doubt your own sources. Saw a report of a "pedal problem" traced to *two* part-bad guitars.)
If the complaint is "precision", like a lab process needs 40psi and 39psi won't do, then you need a real good gauge. Your Blackstar may not be "clean" enough to resolve 0.1% from 0.2% distortion. But most guitar amp troubleshooting is "gross" problems, sound goes from "good" to "echh!".
It would be wise to track-through a "good" amplifier first to get a sense where to poke and how the signal gets louder through amplifier stages and softer after tonestacks and other knobs. And how even a good amplifier can be "made" to sound bad if pushed too hard.)
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Which a nice explanation PRR :thumbsup:
Franco