• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

amp freq response with o-scope

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Can anyone give me a location that I can get simple instructions on how to use an o-scope to measure freq response. I am quite new at this and would like to find out how to measure low end and high end limits of my 6bm8 tube amp.
Thanks in advance for any suggestions!
n0plb said:
Can anyone give me a location that I can get simple instructions on how to use an o-scope to measure freq response.

You also need a signal generator or a test CD with 1/3 octave tones and CD player with flat FR. You feed the amp with the signal from the above and then use the scope to measure the amplitude of the output at different frequencies.
Do not forget to check the input to the amp as well.

And make sure the o'scope can handle the voltage you are checking. Start with the highest voltage possible and work your way down to a nice & easy to read waveform. Tubes distort into sine waves, so unlike a sand amp, which will start cutting off things, the tubes will start smoothing them out.

Good luck.
you will find some nice sweep generators (Wavetek, Tektronix and the like) for less than a hundred bucks on the bay (maybe $50) set the trigger on the scope to begin at the beginning, duration of sweep, periodicity etc. it isn't rocket science.

One of the nicest and least expensive devices to use with a scope is the Hewlett Packard HP3581 analyzer -- this has a sweep generator built in -- you can pick off the output right from the device -- the 3581 has X and Y outputs. these are one of the great bargains of western civilisation at current prices. Some of the manual can be found here:
http://www.tech-diy.com/HP3581/Hewlett Packard HP.htm
How about the computer programs (there should be some free/shareware I quess) which use a RAM and a soundcard (a good one, with flat freq. resp. and at least 96kHz sample rate) as digital signal generator and scope in one? Any experience? I would like to set up such a test jig.
I have used a software generator program, a cheap freebie tone generator for my work & found the THD to be minimal. It works great! I tested the THD via a Minilyzer ML-1 from NTI. A very nice unit that is, too. I have the luxury of using high end audio generators (NTI’s NT-1) in my line of work, too. A plus with this hobby!

Still, you have to deal with the crappy 1/8' jack, or get a decent soundcard with at least RCA outputs. Balanced XLRs are even better, of course, but put you into a whole different league. The soft-synths that are available now are very powerful & have extremely clean waveforms. So much so that many musicians spend their time muddying them up.

Give a software generator a try. You will not be disappointed.
Abreviated Instructions:

Assuming you have a signal generator and an oscilloscope
1) First connect the oscilloscope across your output load and apply a 1kHz signal to the input from your signal generator.
2) Turn up the signal generator level until you see clipping on the output and then back of the signal generator until the output voltage is 0.7 of the clipping level - this means that you are doing your frequency response at half maximum output power and will get sensible results.
This output level is your REFERENCE level.
3) With the input signal level constant turn down the signal generator frequency until the output level drops to 0.7 of the REFERENCE level.
This is the -3dB lower corner frequency.
Note: sometimes with valve amps you won't be able to measure an exact lower frequency because severe waveform distortion sets in due to output transformer saturation before you reach the true -3dB frequency.
4) Now turn up the signal generator frequency until the output drops to this same 0.7 of the REFERENCE level.
This is the -3dB upper corner frequency.

If you make mods and want to check the change in frequency response it important to set the same output REFERENCE level, otherwise the results are meaningless. You can prove this for yourself by checking the frequency response at different REFERENCE levels. I find that if I stick to the half maximum output power (0.7 of the clipping level voltage) its usually a good choice for a REFERENCE level.

Aside: Be very wary of those adds for valve amps which quote the frequency response at 1 Watt Output. The frequency response quoted this way will be far in excess of its true "useful" frequency response.

Using half maximum output power is just a suggested REFERENCE level which I find gives me meaningfull results. Some other like to use Full power (just below clipping) and as intimated above the "If you can't blind them with science then baffle them with BS" brigade use 1 Watt as the REFERENCE level. (this is OK if its a 2 watt amp).

Hope this makes sense to you.
I want to thank everyone for the helpful information that they have given and will be trying some of the software that was mentioned. Right now though I think I will try the instructions on using a function generator. I will keep you all updated on how easily I can become confused!:)
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