Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => AmpTools/Tech Tips => Topic started by: HumBuzzThing on February 16, 2024, 12:44:02 pm
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Hello,
I have a quick question regarding the 1 Meg Pot used in the stethoscope listening amplifier circuit.
I am going to build it into a stomp-box.
I have some Alpha 16mm A1M, (1 Meg Audio taper) in my parts bin.
Would those be suitable, or should I use a pot with a higher wattage rating?
I have not been able to find specs that list voltages for these pots so I am not sure
what the voltage rating is on the ones I have. I am guessing 1/2W maybe less.
Just being cautious.
I know that the capacitor would block any DC but if using it to probe... say an ampeg svt power section.
Maybe add a pad switch to the circuit as well... :dontknow:
Cheers.
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You need to provide the guys with a link to this so they can see what you are talking about
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Hi,
Sorry, should have thought about that.
Here is a link to the Amp Tools section on the website you can find the stethoscope listening amplifier at the bottom of the page.
https://el34world.com/Hoffman/tools.htm
Cheers.
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(https://el34world.com/charts/images/tool15.gif)
> I have not been able to find specs that list voltages for these pots so I am not sure
what the voltage rating is on the ones I have. I am guessing 1/2W maybe less.
Voltages? or Watts? or you dunno yet?
Google "Alpha 16mm A1M, (1 Meg Audio taper)" found a likely part on Mouser. Mouser's links usually break but you might find it by "RV16AF-20-15S1-A1M-LA". The specs are in a PDF
https://www.mouser.com/datasheet/2/13/alpha_taiwan_08162019_RV16AF-20-15S1-A(resistance_-1627828.pdf
(yes that looks wrong but works)
Snips below.
What Watts/Volts do you think you need? (Hint: this is for poking VERY small signals, under 1 Volt.)
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Thanks for the pdf.
In the last paragraph before the schematic diagram
>With this device you can listen to the input grids of pre amp tubes and power tubes.
You can listen to the cathodes and the plates also.
You can listen to the plates of output tubes and you can even listen to a speaker jack.
There is nothing like your own ears for finding where a signal goes bad in an amp.
I was hoping to use this to listen and locate problems along a signal path, but if as you say...
> (Hint: this is for poking VERY small signals, under 1 Volt.)
Maybe adding a PAD switch? I'm not so sure now.
Guitar input signal avg 150mV but after a couple of tube stages wouldn't the signal be over 1 Volt?
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I think you are over-thinking. Just build it and try it.
If listening amp expects 100mV to 500mV, then an Audio taper pot can accept 10V to 50V without being too twitchy. And not all problems are at maximum level.
The pot is worth, what? Three bucks? And 0.06 Watts in 1Meg is 245 Volts. Fine for testing.
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The 1ma pot is just adjusting the input signal level
Listening to a V1A grid level is way less than listening to the V6 grid power tube level
You need to adjust the input level so the listening amp is not swamped
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Thanks guys.
Your right, I do tend to over think some things.
Will take your advise and just build it.
Cheers.