magic smoke pilot aa903

[coyotesgator]

I just let the magic smoke out of one of my pilot AA-903 amps. Does anyone have any specific information/advice on inproving these amps?
5Y3, 2X12ax7, 6c4, Ecc82, PP-6v6
cgater

[DigitalJunkie]

I hope it wasn't transformer smoke.

[coyotesgator]

Not transformer smoke thankfully, was resistor between 5y3 and capacitor. Pretty sure 5y3 is toast now too, considering it lit the room up when it happened.

[TubeHead Johnny]

You should have this amp refurbished, completely if possible. At the very least, you're going to need filter capacitors, (sounds like one or more of the old caps have shorted) plus you will need the rectifier tube. You're lucky the PT didn't go south as well! I would invest in a Variac and reform the old caps for the next amp you buy, but for this one, it's too late.

~~John~~~

[coyotesgator]

Your're right Johnny, one cap had shorted. Been running strong for last year pushing Klipsch (right side).

[Tom Bavis]

If you want a scan of the Sams folder, give me a buzz. tbavis(at)rochester(dot)rr(dot)com

[coyotesgator]

Thank you for the offer Tom, but I have a schematic I made (with actual and indicated values), and photofact. Do you have any suggestions that might take some of the "smear" out of the bass during heavy passages?
Rob

[Tom Bavis]

Bass distortion can be due to mismatched output tubes - even a small current imbalance will reduce the primary inductance of the transformer, and bring the core closer to saturation. Of course, even a properly working amp will run out of headroom with inefficient speakers...

[zobsky]

Originally posted by coyotesgator
Thank you for the offer Tom, but I have a schematic I made (with actual and indicated values), and photofact. Do you have any suggestions that might take some of the "smear" out of the bass during heavy passages?
Rob

I own a pair of the same amplifiers, . both decided to quit on me over the last year. Need to refurb them.

I recapped one of them (signal caps only) a few years ago, but the smear you speak of remains (though I'd describe it as tubby bass) . It only really bothered me on inefficient speakers though (less than 90dB)

Two things I could think off
1. beefing up the power supply
2. implementing the williamson circuit mods. detailed on the chimera labs page http://www.chimeralabs.com/williamsmod.html .


Some audio enthusiasts have noticed that the amplifiers sound better when they are connected to efficient speakers. In fact when they are connected to less efficient speakers, two design flaws surface. At higher output levels they exhibit high levels of distortion and instability. In 1961, Talbot M. Wright published an insightful analysis of the circuit in Electronics World, highlighting these two problems. Fortunately he also told us how to fix them and his recommendations form the basis of this article.

Hope that helps.

[Wavebourn]

Originally posted by zobsky



2. implementing the williamson circuit mods. detailed on the chimera labs page http://www.chimeralabs.com/williamsmod.html .

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