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 Post subject: Tube amp testing
PostPosted: 19 Jul 2020, 13:18 
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Joined: 01 Feb 2015, 13:41
Posts: 179
Location: Athens-Greece
This is a topic that has been discussed many times but I have not seen somewhere concentrated instructions for testing. I understand that there are different approaches but it would be very useful to the inexperienced ones (I am the first here).

I have a frequency generator and oscilloscope so some test could run. In the past I got some advises about square signals at different frequencies. But I am not sure if that applies to both SE and PP amps. Moreover, should we expect same behavior with preamps?

If someone could propose a set of tests including

1. The signal type (square, sinus etc)
2. Range of frequencies
3. Voltage applied etc
4. A few words about test set up

it would be highly appreciated

In my attempt with an EL84PP I tried 20 Hz, 200Hz, 1 KHz, 20 KHz 0.5V sinus wave
and with pre amps , 20, 200, 400, 800 Hz, 2KHz, 5 KHz , 1 Vpp square wave

I do not know if that is enough. Beyond hearing, it would be nice to have some data-results comparison among amps.

Your ideas or views on this subject are more than welcome

Thanks
Dimitris


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 Post subject: Re: Tube amp testing
PostPosted: 21 Jul 2020, 06:09 
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Joined: 23 Feb 2017, 02:02
Posts: 669
For amps one would want to see the square wave response first in any amps, at 20kHZ
This allows you to see if the amp is oscillating for a start

You want to apply a low voltage first if you suspect the amp may blow then go up to high power outputs,
also you need to test the amp with and without load, in some cases the amp oscillates without a load and is unacceptable.

Then you want to check the sine wave to see if it looks ok.
Then you can drive the amp near clipping to see how much power it can give out and you can normally see the distortion visiblly if the amp is poor quality.

Then your done.

Tube amps normally don't oscilalte but still you need to check.


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 Post subject: Re: Tube amp testing
PostPosted: 22 Jul 2020, 12:34 
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Joined: 22 Aug 2017, 21:57
Posts: 12
Location: Layton, Utah, USA
With your signal generator and scope you can determine the frequency response of your amplifier by plotting gain vs frequency, with the frequency varying from low (maybe 20Hz depending on your amp) up to a high of 20KHz or greater again depending on your amp. This is generally done with a sine wave signal and repeated at various input amplitudes (low, medium, and high for example). You can then determine the 3dB high and low cutoff frequencies which tell you the bandpass/bandwidth of your amp. If your scope has FFT capability you can also determine the harmonic distortion components at various frequencies assuming your signal generator generates a low distortion sine wave. As mentioned above just visualizing the sine wave on your scope gives you a rough idea if there is any distortion at various frequencies and input signal amplitudes. There are some software packages for a PC with a sound card which allow you to do much of this same testing with your PC i.e. frequency response, bandwidth, and distortion, etc. but are automated so the software does all the work. Some of these software packages may be free or low cost, I'm not sure, its been awhile since I checked, others here may know more. Google is your friend and there is a lot of info about this out there if you can find it. If you're interested you can read up on the theory of this type of testing in many basic electronics text books, and online as well.


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 Post subject: Re: Tube amp testing
PostPosted: 22 Jul 2020, 21:34 
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Joined: 04 Jun 2008, 20:59
Posts: 4212
Location: Arizona, USA
Hi, A complicated issue. For folks with minimal test gear the way I would do things is to use both square and sine waves and an oscilloscope. For tube power amps (any style output se, pp ul) you need to place a non-inductive resistor equal to the load value of the amp. If there are choices I use 8 ohm loading. I like to check for both high level and low level response and distortion. At a one watt level (2.83 volts into 8 ohms) test at spot frequencies from about 10HZ to 30KHZ using sine waves. This will provide a basic idea of how linear the response is. Then switch it to square waves at the same level. Beware with tube amps the output transformers are seldom anywhere close to linear on this test and the wave forms are almost certainly going to look weird at the lowest and highest frequencies. A generally accepted rule of thumb is that an amp will respond well at frequencies 5 times above and 5 times below the frequency where the square waves are clean. So for 20HZ response it needs to be clean at 100HZ. For 20K it needs to be clean at 4K. Yes some will exceed the top value by a relatively large amount, few however get much below the 100HZ clean frequency. Now for power output I use only sine waves. It is difficult to eye ball when distortion sets in but if you can see any wave form distortion it is probably about 5%. That is what I would call max power with this method. I would do this at spot frequencies of 50Hz, 1000HZ, 5000HZ and 15000HZ. Above and below spots are fine, but likely not terribly informative.

OK that is the simple way, I use entirely different methods. As a serious designer I use much different gear and tests. I use a sweep generator that comes with one of my PC based digital storage scopes. It will sweep any range from 1HZ to 1MHZ. I get a graph of the response, plus detailed data on it. Slick, and such scopes are highly recommended for anyone serious in diy. I do it at high and low power. I can also look at square waves in the same manner. I use a HP distortion analyzer to see how much distortion I have at any frequency/power. For preamps and such I use similar methods but at the 1 volt output level and later determine the max un-distorted output level.

Now some hints. Nothing is 100% clean or linear. For power amps at the 1 watt level the distortion ought to be under 1% and preferably about 0.25% or less. Response for modest ones should be within +1/-3 db from about 30HZ to at least 15KHZ. For really high quality amps the response should be from below 20HZ to past 20KHZ within +0.25/-0.25 db. I consider the max power output of power amps that are not hugely controlled by negative feed back (I hate using any BTW) to be the level where the distortion reaches 2%. Ones with lots of NFB will be lower, but IMO it interferes with the sound in obscure ways. (YMMV). When using square waves there are some tell tale things that can be observed. Poor low frequency response (including phase shift) can be seen when the tops and bottoms of the waves are tilted. The more tilt the worse the response (remember the 5 up and 5 down bit on this). At high frequencies the waves will tend to get rounded. The rounding of the leading edge will indicate the reduction in high frequency response (remember the 5 up and down again). If the top on the leading edge (and bottom) have squiggles then it indicates some instability at higher frequencies and likely harmonic distortion. In all case the top and bottom should be mirrors of each other. If one is different then the amp is not linear in that direction (positive or negative going voltages). A tube amp that can deliver really clean square waves at 100HZ and 4-5KHZ is likely to be quite linear and low distortion. Most of my designs easily do this and minimally the waves at 50 and 10K are clean. This actually is a logic trap. Response, particularly well above the audio band is not really needed or desirable. I find that anything above about 35KHZ makes the amp sensitive to high frequency EMI and noise that can contaminate the sound and not be easy to diagnose. It just sounds "different". If you could build an amp or other device (analog) that went from 20HZ to 20KHZ and had brick wall stops it would be super for eliminating extraneous stuff you don't want. Such is the realm of digitals not analog. This is not an all inclusive set of directions or information, just some quick and dirty guidance.

EDIT: I didn't mention hum and signal to noise levels. IMO if you can see it on the scope at idle and it is over about 0.5 millivolts for a preamp and 1-2 mv in a power amp then it is too much. If you can hear it at a normal volume setting with no input signal then it is way too much. I personally design for -90dbv. The only exception is the phono preamp that because of the design is stuck at about -85 and -80 is more realistic. :) That BTW is really quiet.

Good listening
Bruce

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 Post subject: Re: Tube amp testing
PostPosted: 24 Jul 2020, 08:07 
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Joined: 01 Feb 2015, 13:41
Posts: 179
Location: Athens-Greece
Many thanks to all of you. Enough material for initial study. It is time for experimentation.


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 Post subject: Re: Tube amp testing
PostPosted: 31 Jul 2020, 08:16 
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Joined: 23 Feb 2017, 02:02
Posts: 669
Theres acctually free softwares and tutorials online which allows you to use your PC directly as a scope, but they recomend using an external USB sound card just incase so you don't burn your PC audio output.

But PC scopes without dedicated extenal hardware to convert the signal, may not be able to detect HF noise so is not good enough for anyone serious, or anyone buliding transitor amps.

What I typically do is crank up the frequency to see the distortion, allot of amps has HF distortion at low output levels and high output levels.
This can be clearly seen on the scope by the eye considering the user has good eyes.


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 Post subject: Re: Tube amp testing
PostPosted: 31 Jul 2020, 11:52 
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Joined: 01 Feb 2015, 13:41
Posts: 179
Location: Athens-Greece
I have the Quant Asylum QA400 192 Ksps 24-bit stereo audio analyser but I am not sure how to use and if it is working with KT88 and EL84 PP amps. Has anybody used it? It is assumed that it is better than PC sound cards but I might be wrong


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 Post subject: Re: Tube amp testing
PostPosted: 31 Jul 2020, 20:06 
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Joined: 23 Feb 2017, 02:02
Posts: 669
You should look at the tutorials online provided for the QA400 analyzer


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 Post subject: Re: Tube amp testing
PostPosted: 01 Aug 2020, 11:33 
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Joined: 01 Feb 2015, 13:41
Posts: 179
Location: Athens-Greece
Thank you. I am waiting the new QA401 with QA451 selectable load to do my measurements. But still have some doubts about the possibility to measure tube amps ( I am confused with impedances, amp out , QA401 in etc.)
I contacted them for further clarifications. The whole idea is to have a full set of measurements for each of my amps, including audio analysis with QA401 and waves (sin and square) analysis as previously mentioned


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