This Was A Home For Animals

Above: the result of five-hours work on a dead, dead, dead Univox 50-watt tube bass head.  This model was originally designated the E-3 or the U-1236 (click here for the schematic).   It was made in 1968, and it’s similar to a blackface Fender Bassman in many ways: fixed bias 50Watt output with a pair of 6L6s, 12Ax7 and 12AU7 preamp tubes, volume, bass, and treble controls, and a pair of switches marked ‘Sharp’ and ‘Deep.’  These switches are similar to the ‘Bright’ and ‘Deep’ switches that you would find on the two separate input channels of a Fender amp.  Unlike the Bassman, the Univox uses an unusual additional balanced voltage amp between the phase inverter and the power stage.  The Univox is also single-channel, it lacks middle and presence controls, and the power supply uses a voltage doubler.

Above, the Univox as I originally found it in a filthy old garage.  It came with an empty pro-quality 2×15 ported instrument speaker cabinet; $45 was the total price.  A bit steep, I admit, but I was feeling optimistic I suppose. I think a fair price for this thing would have been somewhere between ‘FREE’ and $25.  Oh did I mention that it had no tubes in it either?  I declined to take the speaker cabinet and it’s probably in a landfill now somewhere.  Just can’t save ’em all…

Hey there!  Here’s the horrifying scene I encountered when I removed the single (incorrect) screw that was holding the chassis into the cabinet.  What you are seeing is a heartwarming combination of acorn shells, spider eggs, and ants. Long after this thing ceased to be a device capable of amplifying audio, it had a second life as home for wayward animals.  Let the crack fox explain. (click text at left for direct link; full video below).

Oh yeah so – the horrifying part?  Look at how this thing is constructed: a (basically non-removable) circuit board!  With half of it obscured by sheet metal.  This ain’t gonna be easy.  Feeling a little less optimistic at this point…

Above: the reverse side of the chassis (which was remarkably clean) after allllll of the repairs were completed.  Here’s what it took:

*When I found an appropriate-sized fuse cap and tubed the thing up, I attached the test speaker and I got… hummmmmmmm.  That’s it.  Lots of hum.  Now as a younger fellow this would have discouraged me, but I’ve learned better.  Hum may suck but it is SOUND, so at least the power transformer, rectifier, and output stage is relatively operational.

*I measured the voltages throughout the power supply – they were way off.  One of the (factory-original) filter caps was so, so drastically underrated – 150v whereas it needed to handle at least 300v – so it’s a miracle this thing ever worked at all.  Replaced all the filter caps (and the bias cap) and voila – some audio, a tiny, tiny bit,  was passing from the input and the tone controls seemed to work.  The B+ points were now also reading within appropriate ranges.

*After replacing the bias cap, the bias was reading 20v whereas the schematic indicated -32v.  I was also getting two different readings at the 6L6 grids.  I suspected the coupling caps, and sure enough, they were shot.  replaced those and bias was now -28v and balanced.  I little lower than the schem, but so was the B+ overall so probably correct.

*Next: started checking the B+ at every plate.  ALL THREE 47K plate-load resistors were shot.  As was EVERY SINGLE coupling cap in the amp.  Replaced all of that stuff and the amp was working alright.  Some hum, the tone sucked, but it was loud.

*As I was checking some voltages around the chassis I saw some sparking inside the output transformer.  The O/T is a paper/wire wound unit with no potting or wrap whatsoever – the coils are completely exposed.  Not a good idea.  Anyhow, something, moisture, dirt, who knows, had gotten inside of it, and now that it was putting out some real current, it was starting to burn up.  I had a spare output transformer (vintage US made unit from a discarded PA head) with the same turns ratio, same size and weight, so I threw that in.  Now the amp sounded great!  Almost done…

*Last steps: removed the two-prong A/C cord and the ‘Death Cap,’ replaced the DPDT power/standby switch (the original switch was intermittent), and put in a better-matched pair of output tubes (a pair of old Sylvania 5881s).  I used 50s RCAs for the preamp.  Hum is totally gone, everything works great.  It’s really a great-sounding amp now – loud, versatile controls, and so small and light for a 50W head.  Pretty good, pretty neat…

As far as the cosmetics go: one of the original knobs was missing and I didn’t have an identical unit in stock, so I replaced all three with some vaguely period-correct Japanese knobs.  I also didn’t have a Univox logo plate to replace the missing original, so I added a name plate from the random-logo-drawer onto the front of the amp just because it looked a little odd with nothing there.  Univox is now ready to go, much better than it was from the factory: better caps, better output trans, and much better tubes too.  If this had been a repair for a customer, the bill would have been around $400 for parts and labor (assuming I used new Chinese or Russian tubes and a new O/T).  $400 is clearly too much for a sensible person to spend on repairing a piece like this, as the street value for a properly working unit is only about $350.  Had someone other than an amplifier serviceman purchased this thing, it probably would have gone into the trash or into another garage for the next 45 years.  Now, if the Univox had been wired and assembled like a Fender amp, on a turret board, the repair would have taken half as much time and the repair could have been vaguely cost-effective.   It was the goddamn awkward circuit board that made it take so long to re-fit. Proof yet-again that repairman-friendly construction practices do add significantly to the service-life of an amplifier.

Supro Supreme Gtr Amp Overhaul

A customer (J) sent me this wonderfully-preserved Supro amplifier to overhaul.  It is a Supreme Model from circa 1950.  Long-time readers may recall that this is the same amplifier model that I scratch-built a few years back as part of my ‘Field Coil Guitar Amp’ article.  Click here for that earlier piece, which contains links to the schematic as well as lots of information what makes these early Supros such interesting pieces. J had purchased this amp sight-unseen from the original owner, based largely on its excellent styling and strong cosmetic condition.  It came with a matching lap-steel electric guitar.  It was sold to him as ‘working,’ but what does that really even mean as far as a sixty-year-old tube amp is concerned? Anyhow, he received the amp, it sounded terrible, and he sent it me.  Here’s what I did to bring it back to it good-as-new.  Probably a little better.

Above, a view inside the chassis after most of the work was performed.  Since this is a field-coil amp, there are several wires running to and from the speaker/field/output trans, so it was quickest to perform the work at this slightly awkward angle.  When I received the amp, it did in fact pass audio at a decent volume level, but there were some obvious deficiencies.  The volume seemed to be a little low; there were some intermittent crackling sounds; and the sound was distorted at any volume level.  The voltages were also all over the place.  There was an expected 350 vdc coming off of the rectifier, but the voltage fell to 150 after the first filter stage.

First things first: change the filter caps.  Even if the original filter caps were not the problem, they are sixty years old, and J bought this thing in order to use it live, so reliability is paramount, and filter caps are cheap so long as you use the small plastic single-units as I did above (as opposed to paying $40 for one of those identical-fit newly-manufactured multi-caps).  I disconnected all wires going to the original multi-cap and left it in place.

Next: that crackling sound.  In my experience, this is generally caused by aged pre-amp plate-load resistors.  These resistors drop a LOT of voltage (which they do by turning it into heat) and they don’t last forever.  Changed all of those out and the crackling went away.

Now about that low B+ reading and the constant distortion: I had noticed that the volume pot had a slight crackle to it.  This can be caused by one of two things: dust and/or mechanical failure in the pot itself, or there could be DC voltage present at the pot.  The coupling capacitor ahead of the pot is supposed to block this DC, but these are 60 years old ain’t they.  After a quick spray of contact cleaner the crackle was still there, exactly the same.  I put the DC meter on the input of the pot and sure enough: 2 volts were present.  This is not a lot (I’ve measured as much as 25 volts here in amps that are actually in-service) but it’s enough to cause some noise.  I replaced the coupling cap with a Sprague Orange Drop (the big orange thing in the photo above) of the same spec and done.  Noise was gone.  The pot actually worked fine and did not require replacement.

Anyhow, the fact that this coupling cap had failed lead me to believe that the other similar caps in the amp were also suspect.  And as it turns out, the two .02 coupling caps feeding the grids of the 6V6 power tubes were VERY leaky: I measured 22 volts+ present at the grids of those tubes.  Since this is a cathode-biased amp, I would expect to see 0 volts DC at this point and around 20 volts present at the cathode.  Instead what we had was an amp with a 100% bias error.  It’s amazing that it worked at all.  The 22 volts present at the grids of the 6V6s meant that they were drawing a TON of current, which explained the low B+ reading.

Replaced those paper caps with a pair of Spragues and that solved all the remaining problems.  The amp now had the expected voltage readings all around and it sounded great.

Above, the sweaty, disfigured, overall funky coupling caps that had gone so wrong.  I can’t tell you how many amps I see that have bad coupling caps in them.  It’s funny that so many guitar players, even those who couldn’t tell a filter cap from a resistor, will talk about replacing filter caps etc etc how much it improves an amp etc.  Now, this is true, but old coupling caps are just as likely to need replacement.  And unlike filter caps, it’s very easy to tell if a coupling cap has gone bad.  Does it pass DC?  Replace it.

So what else did this lil Supro get?  New matched JJ 6V6s, to start with.  I auditioned several NOS 6SC7s in the phase inverter position and none of them made any difference, so I left that tube and the rectifier tube alone.  The 6J7 preamp tube sounded fine, but I was curious how much improvement could be gotten there, so I put in a 1620 (special selected low-noise 6J7 variant) and there was a marked reduction in white noise and less hum too (hum level had been low after the re-cap but now it was nearly gone).  I added a grounded, 3-wire AC cable, removed the ‘death cap’ from the AC primary, and bolted it all back together.  The amp sounds fantastic.  Really, really great.  Much better, in all honesty, to my DIY’d version, although how much of that is due to the speaker I’ll never know.  I’ll end on this note: people seem to be under the impression that these old Valcos and Supros are somehow low-budget, cheaply made, etc., but I found quite the opposite to be true.  This piece was extremely well-constructed, with very high-quality components throughout, and the soldering work was nearly flawless.  For a very small amp that only weighs about 15 lbs, it’s loud, clear, and dynamic, and overall just a very cool alternative to a tweed fender deluxe for about 1/4 the price.

It’s very easy to service a previously-working amplifier that has ‘suddenly stopped working.’  I plug it in and start measuring the voltages, starting from the power transformer primary, on to the secondaries, then to the B+ supply, etc, the tube plates, the cathodes, etc., until I find a voltage that looks off.  The component failure will likely be somewhere at that point.  On the other hand, an old amp that ‘works’ but which has numerous deficiencies (crackles, hums, noise, distortion) can seem a little more daunting, but if one employs a similarly systematic approach, all issues will eventually reveal themselves.  Even if a technician doesn’t have the service data on a particular amp, they have become familiar (through studying many amp schematics that do have test voltages indicated) with the kind of readings that one would expect to see on the grids, plates, and cathodes of various tube amp circuits.  That’s a good place to start.  The rest of it – learning which components are likely to fail, and what the symptoms of these failures are – learning all that stuff just takes time.


Astatic Microphones: Full Catalog Scan: Circa 1975

Download the complete circa 1975 (actual date unknown) Astatic Microphones Full-Line Catalog (22pp)

DOWNLOAD: Astatic_Catalog_197*

Models covered include: Astatic 810, 811, 820, 840, 850, 857, 860, 77, 335, 333, 332, 337, 551, 10M5A, 331, 400, 511, 513H, 525DL6, 531, and 530 microphones, plus various gold-finished and pedestal-mount sub-variants; the iconic Astatic D104 amplified microphone, DN-50, T-3, 150, 151, and JT-30 ‘harp’ microphone; plus stands, cartridges, and numerous other accessories.

Above: the Astatic model 77, the ‘other Shure 55S.’

Above: the Astatic 810 series of ‘ultra-cardiod’ mics

Above: The Astatic D104, their most iconic and most widely-available model.

Above: artists’ rendering of Astatic corporate headquarters circa 1975

Astatic Corp was based on Conneaut Ohio for many years. Although they primarily produced microphones for voice-frequency communications work, they also made higher-fidelity models which are much less common.  At some point Astatic re-branded/was bought out/I have no idea/someone wanna fill me in? as the CAD microphone brand, known for their inexpensive condensor mics.  Anyone out there using any of the higher-fidelity vintage Astatic models for music production work?  Drop us a line and let us know…

Click this link for my previous report on some of the various ‘heads’ available for the D104 base.

Click this link for my previous posting of the earlier 1964 Astatic Microphone Catalog.

Rare Tony Schwartz Interview/Profile From “Better Listening Through Hi-Fi” July 1957

Tony Schwartz is a towering figure in the worlds of audio production, advertising, and media studies.  I recently came across a very obscure, AFAIK never-before-reproduced (feel free to prove me wrong here guys…) profile on Schwartz from the July 1957 issue of “Better Listening Through High Fidelity” magazine (h.f. ‘BLTHF’). As far as I can tell, BLTHF was an advertorial publication distributed gratis via electronics retailers in the 1950s.  The author of the piece is Robert Angus.


If you are not familiar with Schwartz’ work, read this basic profile here.  Schwartz ranks alongside such luminaries as Brian Eno, Janet Cardiff, Morton Subotnik, and not too many others in my personal  pantheon of audio greats.  Schwartz is widely recognized to be the first person to successfully conceptualize and properly exploit the possibilities of tape-recording-as-art.  Not music recording or music production, mind you, simply documentary tape-recording. Schwartz then took this hobby, essentially, and went on to create one of the most significant careers in the history of advertising and media production as a producer and director of commercials.  His most famous production, which I teach in not one but two of my classes at the Uni, is the spot known colloquially as ‘Daisy.’ If you have never seen this short piece of film, take sixty seconds to watch it below.  You will see a no-budg spot that many media pundits acknowledge as having possibly decided one of the most crucial presidential elections in US history, an election that, had it gone the other way, could (and yes this is a stretch) have significantly altered the course of all human history.  Watch the spot:

NEways…  so, so much has been written about ‘Daisy’ that there is literally no way I can add anything new to the conversation.  Which is partially why I was so thrilled to uncover SOMETHING about Schwartz-in-his-salad-days that seems to have been largely overlooked.  As far as ‘Daisy,’ a quick google search will reveal much more than I could offer at this point in the evening, two-drinks-in as-it-were.  One more point before I go… in case you were wondering what brand of tape machine Schwartz was using for his editing work in the 50s:

Yup that’s a Magnecord.  If you are interested in learning more about Schwartz, let the man himself speak to you.  I recommend first his excellent book “The Responsive Chord.” For a buck-o-five you got pretty little to lose and maybe a lot to gain…

Welcome To The Sears World Of Sound! Let’s Install A Dashmate! (197X)

I think they left out Phase Six: smoke grass.  Jesus my car stereo really started fukkin up y’day.  The CD player (which is an enormous, inscrutable six-disc changer built into the dashboard) started spewing out nothing but insane digital hash.  E. suggested that perhaps some force was trying to direct me to the New Sound. Before I could get a recorder to sample it, all returned to normal.  S’pose it’s just a matter of time before the doorway opens again.   And then: The New Sound.

Add Vibrato (?) To Any Tube Amp! (1962)

Download a short article from 1962 by one F.H. Calvert on the subject of adding a vibrato circuit to any vacuum-tube audio amplifier:


Above, the schematics.  These are not plans for a stand-alone device: rather this circuit (the schem on the left) is intended to be added to any resistance-coupled voltage amplification stage (for instance, the circuit on the right).  It requires an extra single hi-mu triode section.  The author suggests 1/2 a 6SC7 or 1/2 a 6SL7, but it would presumably work just as well with 1/2 a 12AY7 or 1/2 a 12AT7, with maybe just a slight change to the 2.2K cathode bias resistor (can anyone tell me what the single-triode sub-miniature equivalent of the 12AY7 and the 12AT7 are?  Do they even exist?)  I have not built this circuit yet so no promises.   A few observations tho:  I find it hard to believe that this is actually a vibrato device; it seems like it’s likely a tremolo circuit.  It looks very similar, in fact, to the trem circuit in the ole 18watt Marshall combo. Also: if it’s worth building, it’s certainly worth adding the speed variation pot.  Contrary to what the author suggests, my best guess would be to replace the left-most 2M resistor with a 2M pot PLUS a fixed 470K resistor in series. Def gonna try adding this to the next Recycled Champ that I turn out.

Absurd And Brilliant 1980s Consumer Electronics Commercials

If you’ve been reading this website for a while, you’ll know that I post a lot of old print adverts and catalogs featuring 1920’s – 1980’s audio equipment.  I collect this sort of paper and I have 1000s of pieces of this stuff that I am slowly bringing online for y’all.

Larger consumer electronics chains also produced adverts for cable and local television broadcast.  Most of them were negligible affairs driven solely by budget concerns, but every field will have its mavericks.  My good friend GJ turned me on to the collected television advertising of the Federated chain of stores. GJ: “…the Federated spots…were done by Shadoe Stevens when he was a young unknown playing a guy named Fred. They were small time/ cheap and weird and made a little shop really popular. Developed a cult following…some are amazing and inspiring.”  No small praise coming from GJ, a fine director himself.  You can watch his latest production, a music video for the fantastic group Peaking Lights, at this link.

I won’t offer any analysis or commentary on the Federated spots, as one could quite literally write a book about this series of spots:  there is that much going on as far as the highly intertextual and media-aware nature of these little narratives, the smart visual language, and the savvy use of minimal production bucks to create memorable advertising that really does relate well to actual consumer-benefit of the products offered.  So get ready for a journey through time and space:

Yard Sale Speakers Spotter’s Guide: 3: Dynaco

Above, the Dynaco line-up circa 1974: A10, A-25, A-35, and A-50 speaker systems; Stereo 120, Stereo 50, and Stereo 400 amps, PAT-4 Preamp, FM-5 and AF-6 tuners, SCA-80Q integrated amp.  Dynaco components were offered pre-assembled (and badged as Dynaco), and in kit form (badged Dynakit).  The speakers, AFAIK, were all pre- assembled.  Dynaco had an earlier line of tube components that are more sought-after, but you can find plenty of all types out there… the speakers are not as common but apparently not-too-bad.