I would like to make a 4 channel oscilloscope to view all 4 audio outputs of a quadraphonic amplifier. I can cheaply get 4 of the same type of oscilloscope but would like to have four CRTs side by side in a custom made enclosure. What I want to know is could I simply wire the four CRTs to one oscilloscope's electronics or would this require beefing up whatever output transistors there are? Edit: Obviously I would use the four seperate vertical amps for the individual inputs. I meant can the horizontal drive circuit be expected to cope with driving four tubes at once. My understanding is that not much current is involved, only high voltages which should be the same for all the tubes if they are Identical.
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\$\begingroup\$ No, you can't simply wire them in parallel because, even if the drive circuit for one was powerful for four CRTs, you'd get the same display on all four. Build a box is my recommendation or get some cheapo far-east LCD o-scopes. \$\endgroup\$– Andy akaCommented Dec 11, 2022 at 12:23
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5 Answers
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The part you'd probably want to be independent (deflection) might work sort-of okay (reduced performance, no big deal for audio) if connected together because loading of electrostatic deflection is capacitive, however there would be no way to adjust the sensitivity. Even tubes of the same part number from the same manufacturer have a tolerance in sensitivity.
However, focus voltage adjustment and brightness adjustments also needs to be independent to each CRT or they would not look the same brightness and you'd only be able to focus one of the four.
High voltage power supplies are probably not capable of 4x the current, and heater power supplies (typically in the 1A range at 6.3VAC) would almost certainly not be.
There are a number of potentially lethal voltages/current capabilities within analog oscilloscopes, so extreme care is called for.
answered Dec 11, 2022 at 12:55
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2\$\begingroup\$ Thanks, yours is the most intelligent answer. The need for independent focus and brightness is the biggest problem that I hadn't thought of. \$\endgroup\$– gazmonoCommented Dec 11, 2022 at 13:01
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\$\begingroup\$ During the horizontal retrace period the beam current is turned off to avoid an ugly visual artefact. It is not so easy to feed this blanking signal to the 3 other cathode drivers if the circuit is unknown. \$\endgroup\$– JensCommented Dec 11, 2022 at 19:32
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There’s no reason to do it since you already have all the drive electronics in quadruple. Paralleling connections, especially for anode supply, will be way more work than using those scopes as-is.
The filament supply will be overloaded, and potentially the anode supply as well.
As others said: the tubes need some of the bias and focus voltages adjusted per-tube, since the tubes aren’t actually identical. Mechanical tolerances affect electrical operating point.
You still need the vertical amps… which come in the scopes. You’d have to copy the design or design your own. Seems like way more work than just using those scopes as-is.
Now, if you were making a product out of it, even if only a small production run, it would make sense to design your own electronics for the display, and then you would have no problem handling four CRTs, since the design would be specified for such purpose. Due to the low frequencies involved in audio, it would be a bit easier than designing for a wider bandwidth scope.
It is not exactly clear that having four tubes running off a common horizontal deflection makes sense. If the signal is truly quadrophonic then some channels may be mute, or run at wholly different frequencies (even if for testing purposes), so a common sync running off one channel, or even a sum of all channels, may not give the best results.
If I were doing it, I’d use the four scopes as-is. This is almost guaranteed to provide the best results for least effort. What’s not to like about that!
answered Dec 11, 2022 at 13:45
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1\$\begingroup\$ Just set the triggers to be the same, or slave the trigger. \$\endgroup\$– DKNguyenCommented Dec 12, 2022 at 1:46
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Most oscilliscopes these days use LCD displays rather than CRTs.
If you use a large LCD display (think iPad or Android tablet), you should be able to display multiple waveforms on it.
answered Dec 11, 2022 at 16:45
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You can't simply put the CRTs in parallel, even assuming you mean just the time base circuitry (horizontal drive) rather than the the time base and the vertical amplifiers. I mean, you could but it would be a lot of work with high voltage stuff to amplify the horizontal drive circuitry. You'd also have to work in some way to individually adjust the drive output for each tube - they won't be perfectly identical even if they are the same model.
Some oscilloscopes have a trigger output, and almost all oscilloscopes have a trigger input. You could use the trigger output from one oscilloscope to drive the trigger inputs of the other three oscilloscopes so that they are all synchronized. That wouldn't do for nanosecond accuracy, but should be more than adequate for looking at audio signals. Putting them "all in one box" would be difficult, though.
You could, alternatively, just buy a four channel oscilloscope. That would put all four traces on one screen, though.
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Of course you can't parallel them.
For many reasons.
For starters, let's imagine at a high level that it is even possible to parallel the CRTs.
The electronics built for driving one CRT load would have a quadruple load of 4 CRTs, but it is not designed to handle a load of 4 paraller CRTs.
Even if it could handle it, the single electronics could still show the same waveform of 1 input channel, because you only have electronics of 1 scope, one input signal driving the four CRTs identically.
