B+ Ripple

[frus]

[SIZE=2]
The philosophy of a class A amplifier (note 1) breaks down when the amplifier clips if it has negative feedback (which the AC30 doesn't). Even without negative feedback, there is usually enough drive voltage available to drive the output device(s) to cutoff the current. The AC30 is not a perfect class A amp, but it's pretty close. There are flaws in the arguments presented against the AC30 IMHO. Nobody disputes that a single ended amp is class A (ie 5F1), but current in the output tube is cutoff when the output clips and the supply current could go down depending on bias point and how much current the speaker is demanding when the tube is saturated.

but the point is, when the tube is going into cutoff in AC30, the output waveform is NOT going into clipping. it is going to stay "clean" until the tubes start going into saturation (and they are biased asymmetrically, ie much closer to cutoff than saturation). Class A design (single ended or not) will clip the output as soon as any tube goes to cutoff or saturation (and we would normally bias it so that in idle, it's as far from cutoff as from saturation, to get maximum power out)

[loudthud]

I sold my AC30 in the early 90's, but I still have a 5E3. I installed 1 ohm resistors between the cathodes of the 6V6s and the shared cathode resistor. It's a little tricky measuring differentially across the 1 ohm resistors, I had to use a Tektronix 7A13 plugin to acquire the signal DC coupled. That's the top trace in the photo. The bottom trace is the output across an 8 ohm dummy load just below clipping, about 12.3 watts at 1.1% distortion. You can clearly see the tube cuts off, the zero reference is the center graticule line verified by shorting the two probes together at the DC cathode voltage of 22.4V.

There are additional photos of the probes connected to the 1 ohm resistor, the probes plugged into the scope and me showing off in front of my newest toy, an Audio Precision System 1. Also there is a distortion graph of the 5E3 generated by the AP system.

[loudthud]

Enzo commented on diyAudio I think it was the Fender made a big deal about measuring their power output at 10% distortion. So what does 10% distortion look like? In the photos below the first is 10% distortion, 68 watts. Next is the same signal expanded out and line synced so you can see the ripple. Sorry, the shutter speed cuts the scope trace off. That's a single ended amp so the ripple is only on the top of the waveform. The last pic is the signal reduced to 1% distortion, 54 watts. The power and distortion readings come from the AP System 1. It takes a true RMS reading of the voltage including the distortion and converts it to power using, in this case, a 4 ohm load.

[Steve Conner]

You got an AP System 1? I'm jealous! Actually make that insanely jealous.

I make the distinction between design class and operating class. From my point of view, it makes perfect sense to say that a heavily overdriven amp is operating in Class D.

If it was designed to sound good like that, maybe its design class is D as well!

[trobbins]

Did you go back to fitting the polaroid bellows attachment for those last three photos ;-)

Does the distortion curve change with frequency much? Maybe the 5E3 OT core contribution can be seen coming in.

[loudthud]

Actually that's the same camera in still mode, 640x480. I shoot the X-Y pics as a 5 second movie, Then use Windows media player to pause where I like the display and use screen capture to export to Paint for cropping. The movies are lower resolution (320x240?), the camera uses a floppy disc for storage. Have you tried to buy floppys lately? None of the office supply places stock them anymore. But they still have VHS tapes!

I've had the System 1 for a while, but didn't have the interface to connect it to a computer. The guy I knew at the company never returned my emails. So I did a search of members on diyAudio and found him. That system is very old and a lower than low configuration, just a single channel and no digital analyzer. Can't do FFTs. I was planning to use the RS232 interface but it turns out that interface is only supported by the oldest DOS version of the software. Some former employees of the company sold me a lunchbox computer with the last version of software that supported System 1, a copy of Windows 98, and a driver they wrote that allows the printer port to talk to the AP bus. I paid a little less than what you can find used System 1s on ebay these days. Some of the new AP systems run over $20K USD, ouch!

The software's print function only outputs printer files, no way to convert to a jpg or gif that I know of. I use screen capture and Paint on that computer, but have to save the file as bit map, then use Paint on an XP machine to convert to jpg or jif. It's a pain. There are also some hardware or software bugs that cause the watts graphs to shift to the left about half a screen. Still trying to figure that one out. The software is quite complicated.

Going to be busy with taxes this week, maybe next week I can do some more tests.

[tedmich]

detail from OP

[trobbins]

Yes, Win7 should've come out with a DOS emulation as well as well as XP ! See if you can print to Adobe Acrobat - you can then save as tif, and then convert further.

That's an old camera - are you talking hard floppy or 5 1/4 floppy-floppy! It's hard to get polaroid film too it seems. Maybe you can use your mobile phone!!

Perfect - Tedmich.

[defaced]

I don't want to be a kiss ass, but I really dig these threads. I don't have much to offer in discussion, but they're a really nice addition to all of the other materials out there to learn from.

It's hard to get polaroid film too it seems.

Expensive (IMO), but their service was great and I got half a brownie point for finding polaroid film for the miss's thrift store find (5 dollar camera, 30 dollar film, go figure): The Impossible Project

[Gingertube]

Loudthud,
I found it interesting that you found that a cap coupled output had some of what you were looking for.
I built / modded/ tested most of the options in that Self Paper "Distortion in Power Amplifiers". Tried all sorts of circuit mods and "audio jewellery" parts (Blackgate caps etc.) and built 5 or 6 amps.
Now I know he was talking about Class AB Amps AND I was using them as HiFi Amps, but the one thing which I took away from all of those experiments is that for a little "tubiness" and gorgeousness in the music presentation I kept comming back to the capacitor bootstrapped VAS load. I tried Current Source loading, deliberately compromised current sources etc. etc. BUT nothing was sonically better (more pleasing) than the capacitor bootstrapped VAS load. I also found using a 2:3 ratio on the resistors (2 to rail and 3 to VAS collector) was the best. I think this comes down to balancing odd and even harmonic distortions and keeping both low order. Keeping the harmonic distortions low order also limits the amount (number not magnitude) of intermodulation products AND (getting to the aim of your thread) the main intermodulation signal source is the residual power supply ripple.

Of-course I finally ditched the SS power amps in the HiFi and went with all tube designs but the experience and learning curve was worthwhile. Some of what I learned has been directly applicable to tube amp designs.
Hope this is of interest.

O.T. - I did my electronics training in a major public hospital - I once ordered 3000 ASA Land Film for the polaroid oscilloscope camera - about 3 weeks later I had a visit from ASIO (Aussie version of the CIA) wanting to know why I wanted 3000 ASA polaroid film.

Cheers,
Ian

[trobbins]

I'm pretty sure I used to use 667 film to get reportable results many decades ago - but I was no technical Mapplethorpe. We used to keep the film in a fridge.

Tek also had a great current probe (A6302 AM503 plugin) which made it a cinche to get current signal traces - now that would be good for plate current sensing - no need to solder in a sense resistor, and you get plate signal only!

[J M Fahey]

I *still* use my old trusty DOS PCB design software (Tango PCB and Protel Autotrax )
It runs well (with some tweaking) up to XP but not higher.
The perfect solution for us old timers in Windows Vista and 7 is called "Dos Box"
Try it, it works.
I have it installed everywhere.
It emulates DOS completely, including old Soundblasters, direct write to disk and all the stuff, same as if you had a 486 based machine on your desk.