Saturday, January 26, 2013

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.

Keith Richards arrested for stealing Telecaster (wink, wink)

Keef, No!

And things were going so well...

OK, not that Keith Richards. From the Crime Report in today's Record Courier daily newspaper, Portage County, Ohio, Jan. 26, 2013:

Keith L.Richards, 22, with a last known address of 8937 S.R. 88, was arrested Jan. 22 and charged with theft, a first-degree misdemeanor. Richards allegedly stole a Fender Telecaster electric guitar from its owner on July 31, 2012, with intent of selling it, according to the Portage County Sheriff's Office.

The RC Crime Report is pay-walled so I can't provide a direct link, unfortunately.

Saturday, January 12, 2013

Some Cool Guitars (Part One)

Here are a few of my guitars, at least some of the funky ones. I have others guitars that are more ...functional... and modern, of course, but these are great fun.

The first is a Norma "Barney Kessel" model. It doesn't actually say that on the guitar, but a prominent website that features vintage guitars (My Rare Guitars) refers to the guitar by that name. Quote: "A very rare Norma split p/u Barney Kessel design." His guitar is black, and I don't see a bridge cover in the photo; otherwise they are twins. It has a big "baseball bat" neck, but it's a blast to play and it's great for a rockabilly type sound. The split-pickup setup is pretty unusual, and actually functional.

It has another cool feature--there's an inductive "varitone" type filter built in to it. It uses a small transformer as the inductor. The filter switch (L,M,H) is down near the lower (and oddly placed) F hole cut-out. It might be a resonant filter (should be), but it sounds like a high-cut to me.

Norma Barney Kessel
The Norma "veritone" inductor and switching for the split pickups

The split pickup switching plate for the Norma

The green burst is my Sekova--coolest headstock ever! 

Next is a red-burst Kingston thin-line hollowbody guitar. It has gold-foil pickups and an extra long tremolo bar...but it feeds back like mad. Looks cool, but wouldn't play it above living room levels.

Kingston #I
This violin style Kingston is a surprisingly great guitar! The action is excellent, the intonation good and it has a cool voice.  

Kingston #II

Two project guitars--a Kimberly Phantom 22 thin-line longhorn model. This guitar has the same pickups as a Teisco May Queen, and the tremolo and bridge also are the same. That's gotta say something about the date (and location) of manufacture. The other is a Maxitone something or other. Someday I'll finish fixing both up. 

The Maxitone is a step up from most MIJ thin-line guitars. It's definitely heavier, more solidly built. Another with a uniquely weird headstock. I tried fixing the one bad pickup, which kinda worked, but was too noisy. Haven't found a replacement yet...

Kimberly Phantom 22 and a Maxitone

All the funky tremolos work fairly well on these MIJ oddities / beauties. Better than many stock Fender-style trems. Not the thing for dive-bombs, but still pretty cool.

Finally, here's an unbadged lap steel but I think it's a National. That is, I've seen identical guitars advertised as such on the 'bay.

National lap steel (?)

Peavey Footswitch Conversion--Triumph to Ultra

Converting a Peavey Triumph footswitch to a functioning Peavey Ultra 60 footswitch

After trolling ebay for a Peavey Ultra footswitch, and realizing the cost would be somewhere between $50 and $85, it was time for the DIY route.

I had one advantage--I'm the semi-proud owner of two Peavey Triumph footswitches; someday I'll detail the "tale of two Triumphs." Both are the 6 pin DIN type (the Triumph PAG uses the same schema as the Ultra, but I'm not that lucky). Might as well use the one with the broken Switchcraft DIN plug...

Busted Switchcraft plug

Some go-getter added big stick-on labels to the switch...

The Triumph and Ultra FS layouts are identical: three switches, four LEDs and the functional setup is the same. But the Ultra uses a 7 pin DIN connector, while the Triumph is the 6 pin DIN type. 

I crack it open, and I'm happy to discover another bit of serendipity--while the Triumph connector is 6 pin, the cable itself has seven wires, and one is unused. Nice. So converting it to the Ultra 60/120 footswitch is very do-able.

Here's the original wiring. Note the current-limiting resistors for the LEDs. In the Ultra series, those resistors are in the amplifier itself, not the footswitch. Also, note the unused red wire zip-tied to the cable...

Original wiring
The wiring conversion in the footswitch itself was super easy. There are several documented Peavey footswitch diagrams on the web. Here's one that was very helpful. I've converted it to an image, but the original is a PDF, and you can find it if you look...

And here's the FS rewired. I've clipped out all the resistors, and wired the switches, LEDs and wire colors exactly as shown in the diagram. The switch tabs in the diagram correspond exactly with the footswitch itself. Even the colors of the LED wire leads (red and black) correspond:

Converted to seven pin, Ultra style

But that's only half the story. I ordered several 7 pin male DIN plugs, courtesy of an ebay supplier. Obviously, these go on the end of the cable.

In conjunction with the wiring diagram above, I've added a drawing with the color codes for DIN pin connections. The colors correspond with the wire colors internal to the footswitch--assuming the switching diagram is followed exactly. The wire to the ground pin (4), labeled "common" below, doesn't have a color; it's unclad stranded wire. This drawing made wiring the plug easier for me; maybe it'll help someone else:

Ultra DIN pin wire colors

Helpful tip: don't buy cheap ebay DIN plugs. These things are the devil to solder. I gave up the first time. While the solder side ends have small recesses for the wires, they aren't of a consistent depth...and the plastic pin holder melts easily.

Tiny, inconsistent soldering recesses

Cheapo DIN plugs
So save yourself the headache and buy good Switchcraft DIN plugs. It's worth it. I eventually succeeded, but it took plenty of flux, a lot of squinting...and swearing... Sorry, there's no photo of the finished plug. Once it worked I was reluctant to disassemble it for a photo. 

I'm psyched to have all three channels functioning, and the reverb switch working as well. The clean channel on the Ultra is beginning to grow on me. And I'm hearing the "Crunch" channel for the first time. It's cool! You can really dial it down to contrast with the "Ultra" channel. Great for bluesy stuff. Or punch it up for a more Marshall-y sound.

I have doubts as to the "robustness" of the connector, and I don't think it would last long while actually gigging. But if it comes to that, I'll buy the Switchcraft plug.

Oh yeah--Helpful Hint #2: Save yourself some grief; the "Channel" button must be pushed, or the footswitch won't function correctly. I read that in the Ultra manual (online at a last ditch effort to discover why the pedal didn't work...after about two hours of aggravation.

Also, I did find a nice DIN plug locally (at Philcap electronics) and it has the larger solder collars on the pins. It's not Switchcraft, but it's an older MIJ part. Mid priced--about $4.50 USD; maybe half the cost of Switchcraft (minus the shipping). I'm holding it in reserve, 'cause the current plug is bound to fail...

Monday, January 7, 2013

Resurrecting a Peavey Ultra 60, Part 2

Reverb Repairs

The Peavey Ultra 60 amplifier didn't include a reverb pan, or the connector cable, so I needed to reverse-engineer the reverb connector and replace the reverb tank. Pretty easy to figure out the connector with the schematic and a digital VOM in hand.

Based on my poking about, here's a drawing of the Ultra 60 reverb connector pinout:

Peavey Ultra reverb pinout

Viewed from the top, front of the chassis, left-to-right. The pins are:

1) Return GND
2) Return signal (from the reverb output)
3) NC
4) Send GND
5) Signal to reverb input

A 5-pin female MOLEX connector works great here. The center pin is unused. It slides right on the the pins. I've fitted the connector with two RCA jacks, and use a pair of pre-made audio cables, short ones.

Here's the connector in situ, with the cables that run to the reverb tank:

The pan type is "EB" ("FB" type works also), i.e., a tank with a somewhat higher input impedance than a standard Fender-type reverb. The 9EB2C1B is an example. 

I used one of the larger EB pans as a replacement. It's 16 3/4 in. long. I had a large and a small reverb tank, tested them both and thought the larger was superior. The pan is screwed to the top of the head case--there is just enough clearance so the pan doesn't touch the power transformer. The recess in the case top is also just wide enough that the pan stays clear of the large vent on top.

Having a working reverb is nice, but it's not pure kandy. It suffers a little in comparison with other reverbs I've heard (and own). The Ultra's reverb is somewhat darker, and "plinkier" than I personally prefer. It does work fine with the gain channels as well as the clean, so that's a plus; no feedback issues. It certainly sounds very good, but it doesn't have that "bright" reverb sound, so it's not the amp for surf music. I'm certainly spoiled by my Ampeg Gemini II :-). At least the larger reverb tank is considerably less "plinky" than the small one. Unfortunately, I don't have a clue as to how it sounds with the stock tank...

EDIT: I might need to rethink the "not for surf music" comment, after getting the "Crunch" channel working (see the footswitch conversion entry). Man or Astroman was playing in my car for two days. I got home I turned Crunch on, with the reverb up to max and the bass way down.  With that channel, the reverb has a nice bright sound--really...not the equal of the Ampeg Gemini II, but not bad at all...

Sunday, January 6, 2013

Resurrecting a Peavey Ultra 60, Part 1

A Needed Fix; Repairing an Ultra 60

Peavey Ultra 60, sans case
Keeping with what's apparently becoming a minor obsession with Peavey gear, I picked up a Peavey Ultra 60 amp (91B, made in 1991) a couple weeks ago for $100. The seller said it worked fine, but for the price I had my doubts. Still, I've done well with malfunctioning multichannel amps. Usually the fix is fairly simple; some bad caps or a disconnected wire, cracked resistor, etc.

While missing power tubes, it came with a full compliment of working preamp tubes (four 12ax7's). So I popped in a pair of 6L6's and down the road I go...

OK, it works, but there's a problem. At low volume levels the amp behaves. Push it a little, and one power tube starts to redplate, big-time. Swapped tubes, and it's definitely not the tube itself.

After my misadventures with a Peavey Triumph, I'm gratified to see that the Ultra 60 has only three circuit boards (OK four, if you count the footswitch and send/return board, but that board would be easy to eliminate). A main board with the preamp, power supply, etc., a board for the triodes (preamp, PI) and a power amp board. It's a PAG design, very similar to my Peavey Bravo (head conversion), with all the 12ax7 triodes inside the chassis. But there are WAY fewer inter-board connectors, MOLEX and otherwise, than the Triumph.

So I pull the output board--easy, yes? NO. The output sockets are riveted to the chassis. An application of Dremel tool and a power drill, and the board is free.

And the problem is immediately evident. A torched resistor has burned carbon deposits all over the board, and that's a ticket to redplate city. The Ultra 60 schematic is a bit harder to find in the wild than most other PV schematics. But it is out there. The toasted resistor is part R80 (10K) and there's also part CR12, a 1N4003 diode right beside it, covered in carbon.

From the inspector's marks,this board was assembled in April, 1991.

Arcing is bad

Carbon deposits on the chassis from the arcing

I removed the fried components first. Then the whole output board was submerged for a good wash-up in my ultrasonic cleaner. After that, a thorough scrapping and sanding removed all the remaining carbon deposits. I replaced the 10K resistor with a 1/2 watter, the diode with a 1N4007; both replacements somewhat more robust than the originals.

Test it once, looks OK. Reinsert the output board, and try to bolt it in place... No go. Not enough clearance; the screws and nuts required prevent the tubes from seating well. In frustration, overnight I research all sorts of speed nuts, etc. that might work. In the end, I simply re-rivet the sockets to the chassis, just like the factory.

The Power Amp board repaired

In the photo above it's easy to see the larger 1/2 watt resistor near the center of the board. The new diode sits right beside. The little pale green thingies are 100pF caps masquerading as resistors. Also visible are the screws & nuts I tried in my failed attempt to replace the rivets.

With the board in place, and the connectors reattached, it was time to fire it up, give it some juice....

Success! (and WOW--sounds really good!)