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Signal Generator and Scope

multichamp12

multichamp12

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I'm looking to purchase a signal generator and oscilloscope so I can measure the performance of my home built amplifiers and adjust the feedback loops with different output transformers.

Can anyone point me in the right direction as to what scope or what signal generator would be good for a first purchase. I'm fine with buying used.
 
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For our hobby, most of the cheaper DSO's on the auction site are too low of a voltage (< 50v with a 1x probe) if you work with it inside the amp.I think the cheap signal generators would work fine,unless using for a distortion analyzer, or signal injection inside the amp, as I don't they are protected against HV .
Most Tektronics and Hp scopes will easily do the job, there are usually some on BT
 
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I have a Siglent SDS1052DL -- you can get them for around $250 (there is actually a SDS1052DL+ model now; the original non-+ model has been discontinued). It's a decent little scope for the money for sure. It only has 50MHz bandwidth, so it's not really useful if you have to fiddle with FM radios, but for audio and the sort of RF work I do with AM radios, it's perfectly adequate. It has a max input voltage of 400V, which is handy when you're dealing with tube stuff.

As far as function generators go, I have an old B&K 3010 that I use for basic testing, and I have a couple of Keithley 2015 THD multimeters that also have nice audio-range sine wave generators in them (that are used as the reference signal for THD measurement). These 3 items I picked up second hand.

The Analog Discovery 2 (https://analogdiscovery.com) is a nifty looking little item, and I've been thinking of picking one up ... it pairs up with software than runs on our Mac or PC, and has a 'scope function, a waveform generator, a spectrum analyzer, etc. The software can do sweep analysis and generate bode plots, etc. which is handy. Only downside that I see is the input / output voltage limits. But if you can live with the limits, then it's certainly a cost-effective option.
 
As of right now all I want to do is test my amps performance (distortion, clipping) and help me diagnose problems as well as the adjust NFB as I have a few pairs of nice Williamson transformers (acrosound to300, stancor a8072, parmeko Mullard 5-20 ) that I have been dying to implement!

The analog discovery 2 looks interesting! I can get it cheaper with a student discount since I'm in college. Will it be able to perform working on tube amplifiers with high voltage?
 
multichamp12 said:
As of right now all I want to do is test my amps performance (distortion, clipping) and help me diagnose problems as well as the adjust NFB as I have a few pairs of nice Williamson transformers (acrosound to300, stancor a8072, parmeko Mullard 5-20 ) that I have been dying to implement!
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If that's the case, and you have a compatible Mac or PC, then the Analog Discovery 2 might be the ticket. It has a +/- 25V input range (with over-voltage protection to +/- 50V), which should be sufficient for many / most output transformer secondaries. You'll probably want to spring for the BNC adapter board as well so can just you regular oscilloscope probes.

If you want to poke around inside the amp itself, and want to protect the input from the high voltage DC, you can always connect the lead through an appropriately-sized-and-rated film capacitor.
 
multichamp12 said:
Will it be able to perform working on tube amplifiers with high voltage?
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For the stuff you said you wanted to measure, for the most part you're concerned with what's on the appropriately-loaded tap of the output transformer, so the voltage rating of the AD2 would be sufficient for may applications. 50Vpp is 17Vrms, and that would be sufficient for 70W on a 4 ohm tap, 35W on an 8 ohm tap, and 18W on a 16 ohm tap. As for other parts of a tube amp, if you AC-coupled the probe, you'd be protected from he DC, but you'd have to be concerned about the voltage swings in play, if you're wanting to probe anything other than the OT secondary.

And, of course, you can always use probes that have built-in voltage dividers (e.g. a 10X probe actually divides the voltage going to the instrument by 10), and many scopes have settings that scale up the trace so that you don't have to do the math in your head. I dunno if the AD2 has such a setting, but you can always do the math in your head :-)
 
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multichamp12 said:
As of right now all I want to do is test my amps performance (distortion, clipping) and help me diagnose problems as well as the adjust NFB as I have a few pairs of nice Williamson transformers (acrosound to300, stancor a8072, parmeko Mullard 5-20 ) that I have been dying to implement!

The analog discovery 2 looks interesting! I can get it cheaper with a student discount since I'm in college. Will it be able to perform working on tube amplifiers with high voltage?
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Clipping you can see easily, distortion is not easy to be seen. Distortion has to be really bad before you can see on the scope.

I think any scope with 50V/div ( after the 10x probe) should work for tube amps. Any 100MHz scope should be good enough. You don't need a good generator. This is the two I have:

https://www.ebay.com/itm/25MHz-Dual...Counter-Kit-/351825634342?hash=item51ea712426

https://www.ebay.com/itm/Wavetek-18...0-240-Volts-/332384940704?hash=item4d63afb2a0
I use two generator because I use the first one to gate the output of the second one to get a burst of signal for testing.

For testing distortion, it's going to be expensive. I use a Quant Asylum QA401( cheapest that can do the job), it's over $400. This is to measure THD and provide you FFT plots on all the harmonics and their amplitudes. It has it's own very low THD signal generator, the normal signal generators is not even close to the low THD it needs, don't even try.
 
multichamp12 said:
I'm looking to purchase a signal generator and oscilloscope so I can measure the performance of my home built amplifiers and adjust the feedback loops with different output transformers.

Can anyone point me in the right direction as to what scope or what signal generator would be good for a first purchase. I'm fine with buying used.
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A Tec scope is never wrong ! Look at ebay.
As for signal generator, a large number of generators with ramp, square and (almost)sinus is available
An example : https://www.ebay.com/itm/GW-Instek-...648487?hash=item2ef2c76167:g:dd8AAOSwHoFXqP57

as scope, https://www.ebay.com/itm/HITACHI-DU...839150?hash=item239122e76e:g:J4oAAOSwEotZ3JPJ
( not a tec but it would do )
 
I snagged a Heathkit IG-18 signal generator for small change. Works fine. Not the lowest distortion producing sine wave but good enough for entry level work. I found an upgrade kit for it on eBay that reduced sine wave distortion by a factor of 10 or something. Produces sine and square outputs.

For analog work I prefer an analog scope. I recently purchased a new old stock GW Instek 20 MHz analog scope for something like 60% off of list price. Was really looking for a 100 MHz scope, but found this one so cheap I couldn't pass on it. Works very well for most work.
 
kward said:
I snagged a Heathkit IG-18 signal generator for small change. Works fine. Not the lowest distortion producing sine wave but good enough for entry level work. I found an upgrade kit for it on eBay that reduced sine wave distortion by a factor of 10 or something. Produces sine and square outputs.

For analog work I prefer an analog scope. I recently purchased a new old stock GW Instek 20 MHz analog scope for something like 60% off of list price. Was really looking for a 100 MHz scope, but found this one so cheap I couldn't pass on it. Works very well for most work.
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The analog signal generator is way cleaner then the function generator ebay ad i posted. Both will work.
Problem with old analogue is that the need calibrating ( probably) , the "modern" function generators
will do their work well without any calibrating.
And an analogue scope is the best, but the modern digital tec's ( and similar) has the ability
to catch singleshot events ( that is not needed in this application but useful anyway)
 
Ha ha, I don't want to make any suggestion on scope........because I am an old fart!!!! I still believe in the good old industry standard Tek scope even if it is an old analog scope. I have two, one is 465 100MHz scope that has been around since the late 70s. My other one is a 2465A 350MHz scope that was the industry standard in the 80s. They last forever, both are as good as they just came out of the box new. Never have to question whether they work or not. I can't even say about their digital scope, I don't know. I know, these are from the stone age, but I just love it.

the 2465A is not cheap, I paid over $400 for that one, but no regrets. These are the Rolls Royce of scope even it's old.

I am not trying to make this as suggestion, I just can't help commenting on this.

I rather have a good reliable scope and spend more money on the spectrum analyzer like my QA401, that will give me a totally new weapon in testing and engineering.
 
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i'm a total ameteur - but i thought i'd just toss out that there's several free signal generator apps in the app store, very handy for my needs
 
Alan0354 said:
Clipping you can see easily, distortion is not easy to be seen. Distortion has to be really bad before you can see on the scope.
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This is why I suggested something like the AD2, because its spectrum analyzer function (coupled with its own reference signal generator) does have a THD function. And if you buy a scope that has an FFT function, then it's possible to calculate the THD from the FFT plot.

Alan0354 said:
For testing distortion, it's going to be expensive. I use a Quant Asylum QA401( cheapest that can do the job), it's over $400. This is to measure THD and provide you FFT plots on all the harmonics and their amplitudes. It has it's own very low THD signal generator, the normal signal generators is not even close to the low THD it needs, don't even try.
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THD is primarily why I picked up my Keithley 2015s -- those things definitely aren't cheap ($5000 new), but Parts Connexion was having a blow-out sale on them (refurbed and calibrated a while ago for < $400/ea. Sadly, they're out of stock now.
 
thorpej said:
This is why I suggested something like the AD2, because its spectrum analyzer function (coupled with its own reference signal generator) does have a THD function. And if you buy a scope that has an FFT function, then it's possible to calculate the THD from the FFT plot.



THD is primarily why I picked up my Keithley 2015s -- those things definitely aren't cheap ($5000 new), but Parts Connexion was having a blow-out sale on them (refurbed and calibrated a while ago for < $400/ea. Sadly, they're out of stock now.
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I looked into the scope with FFT, they are not nearly the resolution needed. I am looking for 0.001% THD or lower and at least -120dB noise floor. Maybe those scope are good enough for tube amps, it's just not even close to what I need for my SS amps.

The Quant Asylum QA400 and QA401 are very good, not the best in the world, but cheap. Too bad they don't sell the QA400, it's less than $300. QA401 is over $400. I can consistently measure down to 0.001% THD to 20KHz. That should be beyond the need of any audio amps.
 
s-petersen said:
I have 2 signal generators, one is a B&K Model E-310B, the other is a Heathkit IG-18 modded with a Morrey mod. The distortion meters are a IM-5248 intermodulation analyzer and a IM-5258 Harmonic distortion analyzer. I use a Tektronics 2213a o-scope currently
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I also have the older Heathkit IM-58 harmonic distortion analyzer. It's a nifty little unit, but at the mercy of it's own internal noise. I refurb'd the power supply and then discovered it needed a more thorough restoration. I'll get to it eventually.

I also have a Heathkit IM-22 audio analyzer in my to-restore queue.

I have a hard time resisting old Heathkit gear.
 
I just receive an email from Newark about promotion of Tek digital scope. I don't know anything detail, take a look and see whether it's worth while. I think the 70MHz one is only $352..............I don't know, you'll be the judge.
 

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It's more money then a used noname, but
a new scope, 5 years warranty , looks ok to me.

If you can spare the money you have an audio scope for life.
Don't forget to buy 100:1 probes that can resist 1000V if you ever will measure endstages plates.
 
peterh said:
It's more money then a used noname, but
a new scope, 5 years warranty , looks ok to me.

If you can spare the money you have an audio scope for life.
Don't forget to buy 100:1 probes that can resist 1000V if you ever will measure endstages plates.
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Wow, talk about ignorance on my part, I never knew there is 100:1 probe!!! Any suggestion on one, I want one too.

At work, we really worked on HV, we have 1000:1 probe. After that, we only have 10:1. I always had to adjust the trace position to look at signal when I worked on the guitar amp.
 
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