• 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.

Help teach a CS guy to test a tube amp!

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Hello,

I am a long confirmed software guy looking to learn some more hardware. Since the stereo seems like a good place to play - I want to figure out how well it works (at least basics).

I would like to start with a few (hopefully) simple things:
1) Hook an oscilloscope up and see a wave on the screen
2) Push a sine wave through so the backend is clear

I would like to pick up a USB scope/signal generator to do this. Since it seems easiest to have a combo scope/signal generator, I would like 2 - 4 channels. Does anyone have any good recommendations for such a scope?

I'm also curious about how to divide the voltage on the output of the amp. I'm assuming most scopes can't handle more than a few watts?

Anyway - thanks for the help!

Anthony
 
Tube is where I want to start. I'm sure more will also be looked at.

If I get an integrated signal generator - won't it need an extra channel to transmit the signal?

Any recommendations on which to get (ideally reasonably priced) or how to reduce voltage as needed?
 
1) Hook an oscilloscope up and see a wave on the screen
2) Push a sine wave through so the backend is clear

Good for you, hardware experience will broaden your horizons. There are many possibilities, just one example.
Pico makes an inexpensive 2 channel USB scope adaptor.https://www.picotech.com/oscilloscope/2200/picoscope-2200-portable-oscilloscopes
The waveforms are measured as voltage, and low voltages can often be directly connected without attenuation.
Probes are available to attenuate the signal by x10 or x100. Bear in mind that you can only measure voltages wrt ground,
unless you purchase much more expensive differential probes.https://www.picotech.com/accessories/active-oscilloscope-probes/25-mhz-700-v-differential-probe
Many software waveform generators are also available. Signal Generator - Software: Signal Generator

If you have more serious ambitions, look for a full feature scope.http://www.tequipment.net/oscilloscopes/brands_rigol/#/oscilloscopes/brands_rigol/?F_Sort=3
 
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"I'm also curious about how to divide the voltage on the output of the amp. I'm assuming most scopes can't handle more than a few watts?"

Understand that you cannot normally just put the output of the tube amp into the oscilloscope. Tube amps do not cope without the load of the speaker and may be damaged.
For silent testing you will need a dummy load and have the oscilloscope in parallel.
 
What is a dummy load? Do you just string a few resistors together?

Usually you use an 8 Ohm non-inductive load resistor, of 50W-100W.
You can buy these as a standard catalog item.
http://www.parts-express.com/8-ohm-100w-non-inductive-dummy-load-resistor--019-020

Again, most testing will use an unbalanced x10 attenuator probe to protect the oscope input stage.
This measures a voltage relative to ground only, but not a floating signal between two ungrounded nodes.
Some amplifiers do not have an output ground (balanced or bridged types).
 
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So if I put an 8 ohm 100w dummy load on an amp rated for 85wpc max, I will be safe.
Then if the scope can handle 10w, I just need an x10 probe.

Yes, a 100W load is ok for an 85W amp. However, the scope measures voltage, not power, so this is a separate issue.
You calculate the power from the output voltage into a defined load with Prms = Vrms x Vrms / Rload
Determine the maximum peak output voltage of the amplifier by calculating (Vrms out) squared = Prms rated x Rload.
Then Vpeak out = Vrms out x 1.414. This should be less than the maximum peak input voltage rating of your scope.
If it's larger, you must use an attenuator probe, usually a x10 type that reduces the voltage by a factor of ten.
 
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Yes, a 100W load is ok for an 85W amp. However, the scope measures voltage, not power, so this is a separate issue.
You calculate the power from the output voltage into a defined load with Prms = Vrms x Vrms / Rload
Determine the maximum peak output voltage of the amplifier by calculating (Vrms out) squared = Prms rated x Rload.
Then Vpeak out = Vrms out x 1.414. This should be less than the maximum peak input voltage rating of your scope.
If it's larger, you must use an attenuator probe, usually a x10 type that reduces the voltage by a factor of ten.

Yikes - my EE classes during undergrad aren't coming back as quickly as I hoped. How do I get from Voltage to Vrms?

At this point it may help to describe the first system to test. One of my wife's friends owns a record store and sells used equipment. Over the years we have picked up components. The current iteration of the system has some cracks and pops. I would like to work through the components and figure out which is the offender. For the most part, I have no desire to get into board diagnostics with this system. Just find offending parts.

I think I have gone as far as possible diagnosing with differing sources. I first gave up on the record player and switched to a digital input from iTunes through an airport express. It cleared up a bit, which helped me figure out how to dial in the record player. The system sounds a lot better now. However is still has a little noise unaccounted for. I know it is noise, because if I leave the system plugged in without a source, a bit of noise pops up.

The current non-isolated stereo iteration is a Van Austin pre-amp plugged into a Jolida SJ-502a amp plugged into Mcintosh XR5 speakers. The amp appears to have a max power 85wpc. Okay - so 85w & 8 ohms is 10.625 volts; but how do I get to Vrms?
 
Back to Dummy Loads for a minute - should I get a 200W or 100W at 8 ohm load?
The price difference, compared to everything, looks fairly negligible. The 200w is $32 and 100w is $15.

I'd get one for each channel if you will be testing a stereo amp.
The advantage of a 200W resistor is that it will run cooler at 100W.
Note that some of these resistors need a heat sink, and some need open air.
 
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