[HAM] Beefy Phat.

[David Anderson]

I do find the AO-28 somewhat sensitive (not extremely) to different 12AX7s--not so much in terms of brands, but plate structures. To my ears, 12AX7s with large, flat plates tend to be richer-sounding, while those where the cathode goes through a small central box-shaped structure (i.e., later Sylvania and GE types) sound cleaner & brighter. (I find the same thing true of 6SN7 drivers in the earlier Leslies.) I'd imagine the effect has to do with slightly different inter-element capacitances.

One exception: I tried the JJ ECC83S frame-grid 12AX7 in an AO-28, and it was awful, although I've used them with no problems in RIAA phono preamps. 

The 12AX7 can also affect how much hum and hiss you get from the AO-28. It makes a good test bed for selecting low-noise tubes.

The only thing I've found in older ceramic capacitors is higher Dissipation Factor compared to more recent production, but I'm not sure if this is a result of aging or original manufacture. In a negative feedback position, increased DF could actually make the final signal brighter by lowering NFB at higher frequencies. 

Also, any capacitor that has no negative feedback loop around it will tend to color the end result more than those that have NFB around them. Examples: the .02/.033uF capacitor before the swell capacitor, the 30pF cap, and the 1uF output to the vibrato line.

This all comes under the heading of what I call "voicing": things that can nudge your final result just a little in one direction or another.

David A.

-----Original Message-----
>From: Scott Hawthorn <organfreak at donobi.net>
>Sent: Jan 30, 2008 2:16 PM
>To: The Hammond Forum <hammond at zeni.net>
>Subject: Re: [HAM] Beefy Phat.
>
>At 10:17 AM 1/30/2008, Jordan Kersten wrote:
>
>>Ah, yes! Failing electronics.   My questions is, what components shall I 
>>suspect to drag down by beef!?
>
>What David Anderson said. Secondly, it may well be that your AO-28 is just 
>fine and sounds like it is supposed to, and you were just used to the sound 
>of your AO-10.
>Then there are the mods, which I have not tested, posted by Kon, but you 
>didn't ask about mods.
>
>There are more than a few tone-shaping caps in the AO-28, but conventional 
>wisdom is that they seldom go bad, being ceramic disc caps that aren't 
>under much stress, in general. And like David said, if anything, they will 
>cause muddiness in failure mode, not a lack, uh, meat. What is commonly 
>replaced in an AO-28 that needs renewing:
>
>Power supply caps
>All electrolytics and cathode bypass caps
>Coupling caps (.03 or .02, depending on the year of the amp)
>Screen and grid caps and resistors that surround the 12AX& and both 6AU6s.
>Sometimes the 12AX7 itself, and I like to start with a fresh 12BH7, call me 
>superstitious.