Audioromy fu-29 - push pull!

[milosz]

I just got an Audioromy FU29 which was advertised by it's seller as being a SINGLE ENDED amp. I dug around in the thing and from my ohmmeter it sure looks like the 2 plates of the 829 are at opposite ends of the output transformer primary winding, with B+ fed to the tube via the center tap. That is a classic PUSH-PULL topology- NOT single ended!!!!!

Anybody here have any comment?

(It's a nice sounding amp, really, but it's advertised as SINGLE ENDED and it appears it is actually PUSH PULL!)

[costis_n]

Well, since this FU29 is not widely used, I would look up the datasheet to see how many watts would be expected from it in Single-Ended and Push-Pull Operation, and compare these with the advertised power output of the amplifier. I have never seen your amp, neither the datasheets, but from the look on it I can bet it makes 40-50 wpc in Push-Pull and about 12 wpc in Single Ended configuration.

In all, it is really not the end of the world. Just enjoy the greater power and efficiency. Don't even mention it to your friends it is a Push Pull amplifier if you feel you have been cheated. They will be impressed how strong 12 Single Ended watts are.I am sure it sounds pretty sweet as-is.

[kuroguy]

If the output transformer has 3 wires (center tapped) on the primary, it is push pull.

[milosz]

The folks selling these amps on eBay are now (mostly) listing them as PUSH-PULL, which they are.

They are not "ultralinear" as far as I can tell, no output transformer tap for the screen grid. Also, feedback seems minimal- so they have a distinctly "tubey" sound. Sounded great on a pair of bargain Polk RT25i speakers- the interaction between these speakers and the amp was kind of magical, I suspect it would be the same on many good 2-ways. It really made the little Polks sound warm and musical.

Build quality is not great, but it is "OK." Cosmetics are very good. Paired with the right speakers, it's a very musical ~25 watt/channel amp available for a very reasonable price.

[aardvarkash10]

yep - the '29 is not a candidate for u/l since the g3 is common to both sections... 25wpc is a tad optimistic, but not horrifically so.

[costis_n]

25 watts ONLY? One would think that fu29 in push pull would make more.

[milosz]

LOW FEEDBACK WILL RESULT IN LOWER POWER LEVELS>

[costis_n]

I'm not sure what you mean, how so? maybe lower voltage?

[DF96]

"Single terminal Class-A" - no idea what single terminal means, but I can understand that many people might assume it means single ended. Buying anything from China is a gamble, so only use money you can afford to lose or plan to rebuild it properly.

[Jeb-D.]

Originally Posted by milosz

LOW FEEDBACK WILL RESULT IN LOWER POWER LEVELS>

Not really. The dynamic output impedance varies with feedback, but the max power that the tube is able to deliver per given load does not change.

I seem to recall seeing a schematic for this amp. Push pull, tetrode wired, cathode biased. You can squeeze more power out of these tubes but it requires a relatively lean bias to not exceed max plate dissipation. You give up quality for quantity.

The Audioromy amp is probably biased near its max plate dissipation in lean class A. Neither section reaches full cutoff, but it comes really close and the current conduction waveform of each section is highly asymmetrical (though most of this asymmetry cancels out by the time it reaches the output). It seems that back in the old days they would refer to this as a type of opperation as Class AB for cathode bias, but nowadays they teach if neither device cuts-off during part of the cycle; its class A. Either way, this type of operation is a good balance between power, quality and efficiency. 25W out seems about right to me for this tube. I bet with an audio signal it will put out 60W peaks.