FFT Spectrum Analyzer Analysis of Guitar Tube Amplifiers

[ZeusMC]

I've recently become interested in making tube guitar amplifiers,. In tube circuit design  I'm thinking an FFT spectrum analyzer might be useful to have.
I've read about and seen several YouTubes on the HP 3582A Spectrum Analyzer.
I've also watched some YouTubes showing using the FFT spectrum analyzer function in digital scopes.
I'm wondering what digital based equipment is available,including usb types, that would have the spectrum analyzer functions and features, that I might find useful in tube circuit design?
Thanks.

[DaJMasta]

More specifically, you'd probably want an audio analyzer for such a task, but it likely doesn't have to be one of the more expensive dedicated units.  Basically these are FFT analyzers that focus on low frequencies and analyzer their full bandwidth continuously, whereas swept spectrum analyzers cover a broader range and selectively catch the frequency area they are measuring with each point on the curve.  An audio analyzer will also be optimized for low noise performance, typically offering more dynamic range at lower frequencies and offering audio friendly terminations, connectors, and drive levels.  An audio analyzer also includes at least one source, so that you can effectively perform network analysis on your DUT, which is probably what would be most beneficial in your designs.  Being able to plot the frequency and phase response of the device under test should tell you a lot about the unit you're working on.  The 3582A is a dynamic signal analyzer, which is basically the same idea as an audio analyzer in terms of operation but without the integrated source.


This all said, you can get good performance doing all of this using a conventional PC and sound card with a couple of caveats.  First, you'd need to select software suitable to the measurements you were making, there are some choices available.  Second, there is no real input protection and not much level ranging on a sound card, so the signal you'd be measuring would have to be at the right general level to get good measurements and if you were just probing around in a tube circuit (with high voltage present) there would be a risk of damaging your sound card.

Another option would be a high resolution scope and signal generator, something with 10 or 12 bits of ADC input would be high enough detail to show you most of what could be heard, especially if using high resolution mode, and it would include the input protection and ranging that is useful for general measurements.  Any modern scope also includes an FFT mode and some include a signal generator, the two of which can be used to measure frequency and phase though not always on the unit itself.  Standard 8 bit scopes will still work fine, but there will be fine detail in the signal that you can definitely hear that doesn't show in the limited dynamic range of the converter, even with high resolution mode.  There are some other things in this general category which could work the same, like the often recommended analog discovery, but the reality is you have quite a few options, so unless you need very high resolution or specific features, you likely already have something that can do the job in some configuration.

[jonpaul]

ZeusMC: Suggest to Forget the FFT, spectrum or audio analyzers.

As many tube amps have a lot of 2HD, and it may even be desirable for the sound, the measurements are not very useful.

Your best analyzer is between your ears, LISTEN as you make changes and test with many types of guitars, pickups, etc.

The classic Fender tube amps are   good models.

Suggest to  do a bit of reading..... histories of Fender, Les Paul, Peavy etc  are great reading!


Bon Chance


Jon

[RoV]

I absolutely agree with DaJMasta.
I had an hp3582a and was very happy to sell it to a collector, because it had (and still has) a good value if in good conditions. But it is a very old, large and heavy instrument: it has a nice user interface, but performances that were astonishing in 1979 and are easily overcome today by any smartphone. It is slow at computing the FFT and the ADC has limited resolution: I don't remember if it reaches 12 bits or less.
As DaJMasta suggested, I also recommend an audio card with a suitable software, using suitable input attenuators. There are external cards with 24 bits and 192 kHz sampling rate that can give very good results. I purchased at least ten years ago an E-MU 0202 that already had 24 bits/192 kHz.

[oPossum]

https://quantasylum.com/

[radar_macgyver]

As others have said, PC sound-cards (especially external USB ones) with appropriate software will give very good measurements and a low noise floor. For a more turn-key option, consider the Dayton Audio DATS v3. It comes with software that you can use to measure T-S parameters for loudspeakers, and can also function as an LCR meter. No affiliation, just a happy user.

https://www.daytonaudio.com/product/1650/dats-v3-computer-based-audio-component-test-system
https://www.parts-express.com/Dayton-Audio-DATS-V3-Computer-Based-Audio-Component-Test-System-390-807

[TimFox]

Of course, instrument or guitar amplifiers are not designed for "high-fidelity reproduction", they are part of the sound generation.
A valid reason to do harmonic analysis of the amplifier output is to emulate previous amplifiers that are well-regarded for guitar use.
(If I remember correctly, the original Fender pre-war amplifier designs were taken directly from the circuits in the RCA receiving-tube manuals intended for public-address applications.)

[ZeusMC]

DaJMasta - Thanks for your reply, the Analog Discovery 2  USB oscilloscope seems a likely candidate. Before I read your reply I also found the PicoScope 2000 Series USB scopes. There are various models at different price points on Amazon. The best price,  for the Analog Discovery 2 is £310.64 Includes vat tax and free delivery. Some of the lower priced Picoscopes might have the functions and features I need. Will see what their tech support come back with.

This is what Picoscope say about their FTT spectrum analyzer:

" Spectrum analyzer. With a click of a button, you can open a new window to display a spectrum plot of selected channels up to the bandwidth of the oscilloscope. A comprehensive range of settings gives you control over the number of spectrum bands, window types and display modes.PicoScope software allows you to display multiple spectrum views with different channel selections and zoom factors, and see these alongside time-domain waveforms of the same data. A comprehensive set of automatic frequency-domain measurements can be added to the display, including THD, THD+N, SINAD, SNR and IMD. You can even use the AWG and spectrum mode together to perform swept scalar network analysis. "

I emailed them, saying what I wanted to do and asked if their spectrum anaylyzer would have all the functions and features I might need.

I read some buyer reviews on the Analog Discovery 2. Said, " Comparable Products/Other parts you considered: Picoscope 2205A MSO, QuantAsylum QA101B, Analog Arts SL9XX series, Link Instruments MSO-28 " Is what put me on to the Picoscope.

TimFox - " (If I remember correctly, the original Fender pre-war amplifier designs were taken directly from the circuits in the RCA receiving-tube manuals intended for public-address applications.) " Yes your correct on that.

radar_macgyver, oPossum, RoV and  jonpaul, thanks for the input. Have a look at my other topic I'm about to start.

[ITTSB.Europe]

I've also watched some YouTubes showing using the FFT spectrum analyzer function in digital scopes.
I'm wondering what digital based equipment is available,including usb types, that would have the spectrum analyzer functions and features, that I might find useful in tube circuit design?
Thanks.

Frequency domain analysis by the use of Oscilloscope, this is now a sector that GW Instek shown fresh products with powerful capabilities and reasonably priced.
GDS2000A this is older series, frequency domain analysis this is not good enough, as is at newer series.

[mawyatt]

As a young kid I helped put myself thru school repairing, modifying and designing around the popular tube amps in the 60s. The Fender was the most popular (had a Twin Reverb along with a lower version of the Les Paul (Walnut finish, which I still have). Did not know the Fender was a copy of the RCA PA systems, thanks!! They had a very unique sound which complemented R&R at the time, the Marshall, and VOX were also popular. Most used the fragile 6L6 tube, which often blew and destroyed the PS and/or the output transformer. I recall putting in industrial metal case versions of the 6L6 for local bands, think it was 6510 or something, which lasted longer.

If you want to study the harmonic content of these old style guitar tube amps, some of the modern digital scopes likely have enough features (FFT & Bode Plots) and resolution to do a reasonable job.

Please keep us updated, very interesting to "see" what you find. If someone has (I don't) and old Fender Amp would be very curious as to what the Bode plots and output spectrum looks like. What makes the Fender Amp with the 6L6 soft limiting so pleasing

Best,

[TimFox]

The 5881 is a military-grade 6L6GB, in a stubby glass cylinder, which is common in guitar amplifiers.
The original 6L6 and a transmitting variant 1614 use black metal cylindrical envelopes.
The 6550 is a later, beefier tube in a glass envelope, similar to the KT88, both of which were used in hi-fi and guitar amps.
(The 6550 and KT88 have substantially higher heater currents, but all suffixes of the 6L6 have the same modest 0.9 A at 6.3 V.  The 25L6, 35L6, and 50L6 are different tubes.)
As the 6L6 family evolved into the final 6L6GC, the plate dissipation rating increased from the original 19 W to 30 W, with the same heater power, so they were backwards compatible.

[jonpaul]

Anyone try 6080 power triodes?

Jon

[TimFox]

Not an expert on guitar amps, but all that I have seen use beam-power tubes (e.g., 6L6 or KT88) rather than triodes.  The distortion (without feedback) is quite different.  All other things being equal, THD for triodes without feedback is quite a bit lower than for pentode or beam-power tubes.

[Bud]

What is the purpose of having a spectrum analyzer in a guitar amplifier?

[TimFox]

I believe he is designing and building a new amplifier and wants to test it on the bench.

[ZeusMC]

Picoscope usb oscilloscope Choice.
Looking at the various Picoscope oscilloscopes on Amazon, https://www.amazon.co.uk/s?k=Picoscope+oscilloscope&ref=nb_sb_noss  which would be my best choice, for guitar tube circuit development?
Thanks.

[Old Printer]

If you search YouTube for “Analog Discovery Audio Analyzer Suite” you will find quite a bit based around freeware software written by The Stuff Made. In particular a YT contributor named Tomtektest has done quite a bit of tube audio work with the AD. Unfortunately the new ADs have become a bit pricy, about $400 but occasionally a good deal can be found on a used one. I have two of them and use them all the time for basic benchwork.

I responded with info about a USB scope because that is what you seemed interested in. Most of the tube guitar amp guys I know of say a scope is not terribly necessary, but recommend a simple 2 channel analog scope of maybe 20mhz. Higher bandwidth is not really necessary for this kind of work. Of course these guys also have the skills to keep an old scope running too, not the simplest task if you are a beginner.

[ZeusMC]

Old Printer - Thanks for that, appreciated. I've been looking through Tomtektests vids, some interesting stuff there. Am actually playing his Transformers in Tube Circuits.  I have both books he refers to, the Radiotron Designers Handbook and Valve Amplifers by Morgan Jones. I also have Mr Jones more practical construction book. I'm also waiting for delivery of  Kevin O’Connors 6 TUT tube guitar amp builders books.

I've been looking at the Picoscope Oscilloscopes again and I've pretty much decided I'm going to buy  the PicoScope 2205A 2 Channel 25 MHz USB Oscilloscope. Best price I've found is £166 inc tax and shipping which is almost £40 cheaper than Amazon at £202.80, which was the next best price. I  saw Tomtektest also did a series on using analog spectrum analyzers.  " Tomtektest has done quite a bit of tube audio work with the AD " I did look through his videos but I couldn't find these?
Thanks again