Hum is down to a few reasons.
1/ Power supply inadequate and ripple on smoothing caps.
2/ Poorly screened input signals.
3/ Long tracks on input signals.
4/ Mains pick up from nearby transformers.
5/ Poorly laid out power supply. Input to power supply on one side of it and output on the other. Dont mix audio grounds with power supply ground as charging impulses into smoothing caps will modulate the ground line and get into the audio.
6/ Bad earthing or no earthing. Only one earth in your audio system. More than 1 will cause an earth loop. No earths will allow in RF and coupling across transformer windings.
1/ Power supply inadequate and ripple on smoothing caps.
2/ Poorly screened input signals.
3/ Long tracks on input signals.
4/ Mains pick up from nearby transformers.
5/ Poorly laid out power supply. Input to power supply on one side of it and output on the other. Dont mix audio grounds with power supply ground as charging impulses into smoothing caps will modulate the ground line and get into the audio.
6/ Bad earthing or no earthing. Only one earth in your audio system. More than 1 will cause an earth loop. No earths will allow in RF and coupling across transformer windings.
ok - so maybe a punt on an old scope makes some sense ....£50 or so.
what spec (do I need what Mhz?), and there are lots of tektronics on ebay, is that the brand to go for - or is that irrelevant.
I guess I want probes etc with it or that will add up?
Thanks
what spec (do I need what Mhz?), and there are lots of tektronics on ebay, is that the brand to go for - or is that irrelevant.
I guess I want probes etc with it or that will add up?
Thanks
I'd say 20 MHz minimum and probably 60-100 MHz maximum. My 400 MHz scope spent most of its time with the 20 MHz bandwidth limiter turned on so I could measure without seeing the local FM radio station on the scope. My daily driver scope is now a Keysight DSOX1102G (70 MHz).
If you want to use the scope you will need probes. One is mandatory but I often have two channels hooked up to see the timing between two signals. Having the ability to trigger on one signal while measuring another is another common use for two probes.
For tube work, you may want an 100x (really a 100:1) probe as well. Many 10x probes are limited to 250 V.
Tom
If you want to use the scope you will need probes. One is mandatory but I often have two channels hooked up to see the timing between two signals. Having the ability to trigger on one signal while measuring another is another common use for two probes.
For tube work, you may want an 100x (really a 100:1) probe as well. Many 10x probes are limited to 250 V.
Tom
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If you still have any contacts at your uni, maybe there's a way: at the uni of Geneva, staffers are allowed to have decommissioned gears on a 99 years loan basis. Possibly, they've got some similar policy in your country.
My house is a new build and since I designed it and acted as the architect I got a dedicated mains supply to the music room, and each socket has it's own spur into the consumer unit with large capacity cable - could this itself be a problem, my brain at the time thought of it as star earthing?
Have you any experience with Ferrite cores?
This is a very useful and interesting list, thanks!
see notes above
If you have different items of audio equipment connected to different spurs and if that is equipment is grounded (3 core mains leads rather than double insulated) then you could possible create an issue as you then connect them via their interconnects.
I guess the proper order to buy this kind of gear is screwdriver -> soldering iron -> DMM -> oscilloscope -> function generator -> audio analyser -> better screwdriver -> better soldering iron -> better DMM ....... ad infinitum.
This is interesting, so I can experiment with removing an earth maybe?
I THINK I did this already but it won't hurt to try again
For tubeamp usage 5mHz would do just fine.
Dual channel is importent as you can compare 2 signals
external trigger is useful as you can trig on mains and identify hum easily
A 1:100 probe that can withstand 500Volt is also good to have
not a good idea for a first scope.
A scope is a complex bit of gear, it's hard to advise you on what to get, but I'd say stay away from any old digital scopes or the more complex Tek scopes,unless these have been serviced recently your buying something that may not last long and will be a sod to fix.
If you look for an old Hameg, Philips or one of the older Tektronix scopes you should be ok,make sure you can find a good service manual for your potential scope.
Finding hum is a bitch, yes old tech's can fix gear with nothing but an old T bag and a bit of string, but they have 50 years experience, the OP doesn't.
If your preamp is not a DIY jobbie then finding hum is simpler. First off what sort of hum is it, 50/60hz or 100/120hz,the SC scope I mentioned will tell you this,as will the SA in Audacity.
If 100/120hz then check PSU caps, PSU de-coupling caps etc.
If 50/60hz check grounds for dry joints, loose wires etc,pull the first valve, does hum get better? If your preamp is non DIY then you need to check your work and any work the phantom fiddler has done in the past. Xraytony on Utube has a good saying, " show us where you've been (EG where you did work/made changes) and I'll show you where the problem is" I paraphrase.
If it's a DIY amp Nigel's post #21is a good starting point. good luck, Andy.
GOS-622G-Product-GW Instek
20MHz Bandwidth, Dual Channel (GOS-622G)
High Sensitivity 1mV/div
But unfortunately is discontinued.
You can buy similar one at Aliexpress:
Gwinstek GOS-620FG 20MHz, 2-Channel
Caltek CA9040D dual-channel
They are more expensive than the same general purpose oscilloscope with 5mV sens.if that sensitive is more desirable than other features.
20MHz Bandwidth, Dual Channel (GOS-622G)
High Sensitivity 1mV/div
But unfortunately is discontinued.
You can buy similar one at Aliexpress:
Gwinstek GOS-620FG 20MHz, 2-Channel
Caltek CA9040D dual-channel
They are more expensive than the same general purpose oscilloscope with 5mV sens.if that sensitive is more desirable than other features.
I'd say that 20MHz is fine for a first scope and will cover anything you need to do audio wise.
As with most things, specs improve over the years. We used 20 to 50Meg scopes for TV work and I remember one of the techs being amazed that he could see the IF output from a tuner with the Hitachi scopes.
Dual trace, 20Meg (or higher) and it will cover everything you need to do.
As with most things, specs improve over the years. We used 20 to 50Meg scopes for TV work and I remember one of the techs being amazed that he could see the IF output from a tuner with the Hitachi scopes.
Dual trace, 20Meg (or higher) and it will cover everything you need to do.
Thanks, it's an old well regarded pre amp, and yeah it's had a few things replaced in it's 35+ years of service...I'll look at the fresher solder joints and when I get a scope have a proper sniff about
Those are radar frequencies, greater than 30 Mhz is radar! Could have used it for that purpose.
Nothing beats the old Tektronix. Proof is a 465 still goes for good money, but slightly newer ones, still good old 100Mhz, 2 channel, delay CRT scopes go for less that $100 though shipping can kill you unless you find one local. The $12 probes that are all over e-bay are pretty good. Not a P6-106, but 1/10 the price. HPs go a little cheaper, but then again we all hated HP scopes.
Read a lot on the "digital" scopes. I may buy a new storage scope, but holding off.
I run Zelscope ( $10) on my PC, but of course it does not do DC. You have to make probes with safety limit diodes in them. It was handy for some audio transient stuff I was working on. Of course, I do have an old Tek. 🙂
Read a lot on the "digital" scopes. I may buy a new storage scope, but holding off.
I run Zelscope ( $10) on my PC, but of course it does not do DC. You have to make probes with safety limit diodes in them. It was handy for some audio transient stuff I was working on. Of course, I do have an old Tek. 🙂
Here is one of many examples on e-bay I noticed. 2000 options.
Tektronix 2215 Analog Oscilloscope | eBay
Tektronix 2215 Analog Oscilloscope | eBay