Univox U305R ... May Have Been Played By Elvis!

capnjuan

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Earlier this year, I bought this '67-'69 Univox U305R: twin 7591s, 6CA4 rectifier, two channels; 3X12AX7s; input/volume, tremolo oscillator, and output driver, and a 6AN8 and a 6AV6 for reverb drive and recovery. The reverb, like an Ampeg, is resistor/capacitor-coupled not transformer (Fender/Gibson) or speaker (several Guild models) coupled.


Front.jpg



Schematic - Shows 6973s but 7591s in this model:

schematic.jpg


Tube deck view - note exposed lugs on output transformer in the left-center, a tell for off-shore gear. US mfrs always have bell ends on transformers with leads coming out. Reverb to / from pan in lower left; we appreciate the big cut-out that the transformer straddles because .....

tubedeck.jpg


Without it, there would be no access to the printed circuit board traces below. These were the achilles heel of many of the later 60s amps. Even if a bad part was identified, getting it off the pcb and a replacement in depended on getting lucky; that you could get a soldering iron on the trace side to disconnect and replace the defective part. If that wasn't possible, then all of the lateral connections on one side had to be disconnected to flip the pcb up on its side. Not hard to imagine how many repair estimates were refused because of several hours of labor being needed to replace a $.15 part. Note open lugs on power transformer at right of pic.

chassis01.jpg


Bonus! 15" Jensen/Muter C15PS Special Design speaker. The frame side was coated today with Metal Ready; a solution containing phosphoric acid to prep rusty metal for painting; in this case, some mild rust on the up-facing side of the frame, otherwise in fairly good condition. These were standard-grade commerical speakers of the day; popular with many manufacturers; dates to 15th week of 1967, probably, but not necessarily, original. Cone and gasket in good condition.

speaker.jpg



Right now, you're probably asking yourself 'why on earth would anybody screw with one of these?' ... it's a reasonable question. Our BBer matsickma is a big fan of the 7591 tube as are Gilded and Mad Dog who both own/have owned Ampeg 7591 amps at one time or another. The thing is that there are no Guild or Ampeg 7591 amps here in the rock 'n roll wastelands of south FL which means that, in addition to buying an amp unplayed, it'd cost another $100 in shipping just to spin the wheel of fortune.

This one showed up on Craigslist and, in addition to the 7591s, I got reverb and a 15" speaker for an hour's drive up the coast to Fort Pierce. I also lucked out in that, right after I got it here, a dual Univox footswitch showed on eBay so it's got all its pieces and parts and, although you can't really tell by the pics so far, it's in pretty good condtion ... be nice if it sounds good too. I've had it running and it passes a signal; needs a reverb pot, both reverb tubes, and the unavoidable power supply and cathode caps; all the parts here, no biggie but but but but but.....

May have been played by Elvis!!

petty.jpg


That's right folks! Tom Petty was born in N. FL in 1950; was 17 or 18 about when this amp was manufactured. Somebody ... maybe Petty-Daddy ... used one of those electric etching pencils to inscribe the name 'Petty' into the back chassis panel. The seller told me that the guy he bought it from told him that the amp had been Tom Petty's.

Being a Guilder, I discounted the claim but, at some point, I'll probably see if I can contact him ... who knows, if it was his, he might like it back ... In the meantime, I'll have something cleaner and brighter for the Rickenbacker to work with; ok w/ the Gibsons at low volume but when pushed ... the Rickenbacker bite gets lost in the Gibsons' distortion ... it sort of mushes out... so ... we'll see ... anyone got Tom's name in their address book? :lol: CJ
 

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Couple of multicaps above and below the deck, cj?

I wonder how many techs mickey-rigged replacement caps and resistors, by clipping the bad parts close to the body of the parts and bending the still-soldered-into-the-board old leads straight up to wrap the new leads around. It would save flipping the board over, even though you'd need to use a gator clip to keep the old lead from loosening from the pcb.
 

Walter Broes

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I had no idea printed circuit boards appeared in guitar amps that "early".

Does that power tube correspond more or less with a 6V6 in output power?
 

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Conventional wisdom says slightly more power, cleaner, more headroom. One of these days I have to a/b it with a deluxe reverb to verify that. I've been trying to get my degree taken care of, so I haven't played it too much lately, but I actually prefer the Mav over my Vibrolux. The Vibro is quite a bit louder, but the CG I had sounded best through the Guild. The Mav is pretty big for the volume. Whatever the amp was that had a single 12" would have been perfect for me, but I'm liking the 7591.
The Hilgen that I have originally had 7591s and I converted it to 6v6s. It had a real nice grind with the old tubes.
 

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Looks like a nice new project, capn--and its got pedigree. I'll have "my people" work on getting you that address. :mrgreen:

Walter Broes said:
I had no idea printed circuit boards appeared in guitar amps that "early".
Who knew?
 

capnjuan

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Default said:
Couple of multicaps above and below the deck, cj? ... I wonder how many techs mickey-rigged replacement caps and resistors, by clipping the bad parts close to the body of the parts and bending the still-soldered-into-the-board old leads straight up ...
Hi Steve: yes, the can underneath is a conventional 3-section filter cap but the one above ... that's a different story. Not certain yet but I think the 2-section on top is the two caps identified on the left (5uf/150V). Whether I've got it right or not, what's a little more confusing is those caps in the schematic. I admit I don't completely understand what's going on but the image lower left is the phase inverter from a Fender Super 5E4-A which I don't fully understand either but Dave Hunter's book (pgs 55,56) identifies it as a split-load or cathodyne inverter. Except for the Univox's leg from the stage one cathode and these capacitors, they're very similar.

phaseinverter.jpg


Circled on the right is the dried-out electrolytic paste ejected when at least one section of the cap failed. Whatever it's function, the wacky mounting arrangement looks like it's a result of running out of space underneath.

As far as replacing components on the pcb, I see that you know mickey too ... other than replacing the board, that's the only work-around for the de-solder/re-solder exercise. As they say in basketball; 'good if it goes'. It can compromise the joint between the original leg and the trace underneath and, other than turn the amp on and see what happens, there's no way to know for sure if it's ok. Since this isn't good practice, I know that neither you nor I would engage in it ......... :mrgreen:
 

capnjuan

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Walter Broes said:
I had no idea printed circuit boards appeared in guitar amps that "early". Does that power tube correspond more or less with a 6V6 in output power?
Hi Walter: In the pic below, the amp in the upper half is a Thunderstar Bass and it's a point-to-point amp but the amp in the lower half is a Thunderbass and you can see the small, separate audio boards; the power supply underneath is all point-to-point. Other than saying these are mid-late 60s designs, I don't know which was first.

twobass.jpg



The 7591 is a compromise between the low-power, distortion-prone 6V6 and the high-power, less-distorted 6L6 and typically puts out 28-30 watts Technical Data Sheet Here. They were used by Ampeg (Reverberockets, Geminis), Guild (Thunderbirds), Gibson (later GA19RVTs and GA30 Invaders) and I'm sure others. They were also common in hi-fi gear.

If you compare the sizes of the transformers in the Univox (7591) to the size of the transformers on the Thunderbass (lower half, 8417 or 6L6), you get some idea of the cost savings involved with making and selling 7591 amps compared to 6L6 amps. When the 7591 went out of production, at least some Ampegs were converted to 6L6 amps; maybe what happened to Mad Dog's Gemini II which was threaded here some time ago. There is also a report of a 7591 Hilgen being cowboyed into a 6L6 amp by a well-known ampista from Philadelphia :wink: . For the time being, EH and maybe Sovtek(?) have brought the 7591A back. I have no idea about its tone or performance characteristics but I'd guess that it's similar to a 6L6; just less volume.
 

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Thanks guys, very interesting all. Sounds like an amp with those tubes and a healthy output transformer would have the edge over a Deluxe Reverb on stage - those are a hair too quiet for me, usually.

I can see an Ampeg Reverberocket or Gemini in my future if the right one pops up over here at a price I can afford.
Lovely reverb, and that unique sound you can't seem to get from anything else.

I wondered for years what the amp could be the great Billy Butler used on Bill Doggett's "Honky Tonk", and I wasn't surprised to learn it was indeed...an ampeg!
 

capnjuan

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Walter Broes said:
... I can see an Ampeg Reverberocket or Gemini in my future if the right one pops up over here at a price I can afford.
I know you're not serious about this yet but you should know that there are several versions of both the Reverberockets and Geminis out there; some of the RRs have 6V6s and 7868s besides being fitted with 7591s and among the Geminis, the G-15/Gemini II and GV22 use 6L6s. The later versions of both amps use a opti-coupler ('roach' in Fender-speak) to connect the trem oscillator into the line TM-1 at Fliptops.net

Further, some of the schematics indicate 'PEC' or packaged electrical circuit for the tone stacks; typically resistors and caps in a molded epoxy jacket with their leads sticking out. One schematic identifies the PEC as a 'Factory' part which is all to say that, as with the Magnatones and their varistors, the continued good health of some of these amps depends on parts that are increasingly difficult to find.

Despite what probably sounds like bad-mouthing, I'd like to have an Ampeg too! CJ
 

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Thanks for the info! I've spent quite a bit on amps in the last year, so no, it won't be for real soon, but I do want one. I have a friend in TX I might be able to talk out of his Gemini too, we'll see. :D
 

capnjuan

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Re: Univox U305R ... May Have Been Played By Tom Petty!

Progress pics:

First things first; new 3-wire power cord, old power switch on its way out:

powercord.jpg



... and then these will go; the 'death caps' (RIP Keith):

relfcaps.jpg



Old and new 30uf/30uf/30uf @ 450V can caps:

oldnewcap.jpg



Tube-deck mounted 2-section cap not a part of the phase inverter but supplemental or replacement for original can cap; leads to can cap can be seen above emerging from under the pcb and draped across the corner of the old can cap:

twosectioncap01.jpg



New bias diode, the old one on the left is Selenium which, in anything over trace amounts, is poisonous. Silicon on the right. The wiring-strip mount is a workaround for the pcb-style constuction and no access to the traces below:

oldnewdiode.jpg



Finally ... an example of what default calls 'Mickey-rigging'; either the original resistor mounted on the board is broken or somebody wanted to change the value of the resistance in the circuit. Either way, this is the workaround.

mickeyresistor.jpg



The printed circuit board show above is one of the many fore-runners of motherboards in PCs and, at the same time, a reason why so few of these amps survive today. A dead motherboard today means a decision between sticking money in a older pc or, for the cost of repairs and then some, you can have a newer model with fender skirts and dual rear view mirrors.

Five years after this board was made, if something blew and it was messed up beyond patching, there weren't any more replacement boards in production and, if someone wanted it running, the only choice was cannibalize an intact board from another amp or throw it away.

In some respects, this U305R is a GAD of its day; US company has Japanese firm build these to their specifications and market them in the US v. Ampeg, Guild, other makers of mid-range amps; less cost with same functionality means ...

For those interested: read more about Univox Here.
 

capnjuan

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Yes Coastie, that would be he ... your and Jeff's 'Keith' caps were removed at the Studioz d'Juan ...
 

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capnjuan said:
May have been played by Elvis!!

petty.jpg


That's right folks! Tom Petty was born in N. FL in 1950; was 17 or 18 about when this amp was manufactured. Somebody ... maybe Petty-Daddy ... used one of those electric etching pencils to inscribe the name 'Petty' into the back chassis panel. The seller told me that the guy he bought it from told him that the amp had been Tom Petty's.

Being a Guilder, I discounted the claim but, at some point, I'll probably see if I can contact him ... who knows, if it was his, he might like it back ... In the meantime, I'll have something cleaner and brighter for the Rickenbacker to work with; ok w/ the Gibsons at low volume but when pushed ... the Rickenbacker bite gets lost in the Gibsons' distortion ... it sort of mushes out... so ... we'll see ... anyone got Tom's name in their address book? :lol: CJ
You found it in Ft. Pierce? Isn't that on the way to Daytona? Might be a different Petty.
PettyGuild.jpg
 

capnjuan

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Jeez ... I gotta get out more; forgot about that guy ... but if I want to sell it later, I guess I can go offer it to the NASCAR crowd ... :wink:
 

capnjuan

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Sounds like a plan ... how did your Hilgen turn out?
 

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It sounds fine, it's a 2-12ax7, 2-6v6, gz34 line up. 15" CTS speaker.

I have to say that I liked the sound of it through a 10" speaker. It had a nice snarl, but the 15" was just too flubby for my tastes. It'll be easy to convert back. It's a question of switching two wires on each output tube socket and reverting back to a 125 ohm cathode resistor instead of the 250 ohm I put in.
 

capnjuan

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Default said:
... but the 15" was just too flubby for my tastes.
I may find the same thing. Not all speakers are created equal (famous political saying) ... so many variables; efficiency, voice coil health, cone condition, on and on. Speaking of speakers ....

The Bell & Howell-branded 16 ohm Jensen P12N is back from the speaker shop. Fished out of deBay and tried out in the GA8, its voice coil was rubbing pretty badly (Thanks eBay seller; I hope you get a flat tire :evil: ). Since the cone was going to go anyway, it was replaced and I had the voice coil replaced too to drop the impedance from 16 to 8 ohms. I don't have any 16 ohm amps and the 16 ohm dropping resistor across the coil was ok for testing but, in the end, not for listening. Its next try-out will be in the GA30 when it gets on and off the bench.

Jensen02-1.jpg
 

capnjuan

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Progress pics:

Left half shows the 'death caps' in place, the two wire power cord, and old power switch. The right half indicates how the power ground wire has been grounded to the chassis, death caps now gone, and new switch. Instead of two switches for power and standby a la Guild, this one is DPDT - On/Off/On; 110V power connected in either 'On' position firing the heaters but high voltage DC only on when switch is in the other 'On' position.

switch05.jpg



These are the existing connections to-from the reverb can; component side of chassis on left, tube deck side on right. Cables are cheesey audio style. Waiting on my boys in Westerly ... no not those boys ... the ones at Ocean State Electronics to send me a couple of sets of RCA / audio jacks. Also waiting <ahem> on a BBer to send me some blank phenolic board to cobble up a fitting similar to what's show on the left except with plugs; not hard-wired like you see.

reverbconn03.jpg



Unlike Fender, Gibson hard-wired their speakers, footswitches, and, as late as the tweed GA30RV, reverb cables to the chassis instead of jacking / plugging them like Fender; the cost of the hand-soldering is the same but the cost of the fittings is avoided. Any Fender owner who wants to export his signal only needs to unplug the speaker in the cabinet and connect an external speaker or cabinet. To do the same thing with a Gibson means drilling a hole in the chassis (five yards / loss of down from purists) or hanging a jack on a bracket at the edge of the back cabinet opening.

This will work except that provisions have to made for undoing it or else when the chassis comes out, the bracket, the speaker, and, if present, footswitch and reverb cables all follow it around because they are all umbillically connected. The same go-where-it-goes feature in this Univox is going to get changed so the chassis can come out without having to take the reverb can out of its bag. The reverb out/in cables are going to get replaced with mid-fi audio grade interconnects which gets around terminating new RCA plugs on the ends of the cables. Speaking of cable ends ....

I found an original Univox footswitch; end of good news. The better-grade button switch hanging out on the right is shot; the lesser grade on the left works okay. I have one of the two remaining working originals from the GA19 (Coastie has the other in his Silvertone's footswitch) and another switch from a Fender style, single action FS that came with the Fender 75.

footswitch01.jpg



The 1/4" plugs have been replaced / come loose from the cable; I have new plugs to replace these and will try and shrink wrap them to the cables but it's not going to look very good; may yet look around for a suitable cable that's factory-fitted with plugs. CJ
 
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