Peavey Ultra Plus Resonance circuit

Unlike the newer Ultra Plus amps, my Peavey Ultra 60 amp project doesn't have a NFB resonance control. I could experiment with any NFB loop filter, but why not start by reversing the Ultra Plus circuit? There isn't much info available on it, so it makes for a little detective work...

The Ultra Plus resonance control is a three-position switch type, labeled TIGHT, MED and LOOSE. The filter setting are preset at the factory. There's no adjustable POT like some resonance controls utilize.

One fact is known from the start--the Ultra and the Ultra Plus are VERY similar in nearly all aspects. The point on the phase inverter--where the feedback is routed--and the transformer tap (the 8 ohm secondary) are identical in both amps. The Ultra Plus merely adds some addtional components between the two points.

Off to the web to research. First off, it looks like Peavey went to some length to hide / obfuscate the feedback loop circuit. There are versions of the Ultra Plus schematic that omit both the feedback control schematic and it's small circuit board, too. And some experienced Peavey Techs even commented on that fact on the interwebs. There's also a little note on the "sanitized" schematic how "THIS CIRCUITRY IS PROTECTED BY U.S. PATENT NOS..." bla bla bla. Look up the patents if you wish, they are easily found. The 5,197,102 patent seems somewhat applicable. But I don't see anything groundbreaking there (i.e., anything unique) that applies to the Ultra Plus resonance circuitry... 

After more searching, eventually I did find a schematic with the NFB loop filter intact:

OK--for all the fuss, it's not a very complicated design. And not that different from other NFB filters, such as the Fender Bassman AC568 for one, and several older Gibson amps.

But the switch diagram and the switch description are confusing. The switch, tagged S3, certainly looks like a standard DPDT switch on the schematic, but the parts list says SPTT (or Single Pole Triple Throw). The drawing is not a single pole switch in the normal sense. More obfuscation, or just in-house terminology?

Moreover, it doesn't "decode" correctly as a standard DPDT switch, either. That is, it wouldn't function correctly using a standard switch. Too bad I don't have an Ultra Plus to poke around it's insides...

After several failed sketches and more research, I found the best candidate for the switch; a DPDT on-on-on switch, where the contacts on each pole are opposed in the middle position. It works like this:

(Of course, one wonders why this isn't termed a DPTT switch...)

Anyhoo, with this type of switch all three functions work correctly (as I read the schematic). If you'd rather not trace the paths yourself, I drew each setting individually:

Position 1) bypasses C10
Position 2) parallels C10 with an 82K resistor 
Position 3) parallels C10 with a 330K resistor

(..there's also a 68K resistor between the filter and the PI, as noted below..)


And that's the resonance control for the Peavey Ultra Plus. Could this be added to the Ultra 60 and Ultra 120? Sure! To replicate it exactly, order the switch from Peavey directly.

Can it be modified? Yup, easily! Replacing the switch and the fixed resistors with a POT, say 500K or 1 Meg would work. The bypass switch would effectively remove the resonance circuit completely, which would be a "stock" Ultra 60 NFB loop.

For comparison, below is the resonance / presence circuit for the Peavey 5150 ii. I've drawn a simplified version--this amp has dual controls, one for the clean and one for the lead channel, and the image below has been reduced to a single set of controls.
 

Note that the Peavey Ultra (and the Plus) has a fixed feedback resistor of 68K in the loop between the transformer and the phase inverter (and the Ultra Plus inserts the resonance control between the resistor and the transformer), while the 5150 has a 39K resistor between the transformer secondary and the filter circuitry, probably due to the addition of the Presence control--or the different sonic characteristics of the amp itself.

So if adding both Resonance AND Presence is your thing, give it a go...

One additional note: the aforementioned patent (5,197,102) seems more applicable to this circuit than the Ultra Plus... But I'm no lawyer, of course, so don't take my word on it.   Wink.