Tag Archives: schematics

Audio Anthology: Collected Hi-Fi articles from AUDIO ENGINEERING in the 1940s

PS dot com reader Paul R. was kind enough to send us a scan of “Audio Anthology,” (ed. C.G. McProud) a 124pp softcover published in 1950.  “Audio Anthology” (hf. ‘AA’) is a collection of project-construction articles aimed at hi-fi (rather than pro audio) enthusiasts.  All of the material had been previously published in Audio Engineering magazine during its first four years of publication (1947 -1950).  See here, here, here, here, here, and here for previous 1940’s AUDIO ENGINEERING MAGAZINE coverage on PreservationSound. 

Above, an interesting boost-EQ stage that could be helpful in an instrument or mic amplifier.  Can any of our helpful readers advise which caps or resistors in this circuit could be modified (and within what range) to alter the turnover frequency of the two filters at work here?  I am guessing that the answer is: all four caps that follow the input DC blocking cap… which would basically get us back to this piece, which i swore i would never build again…

Above, the most complicated tube amp I have ever seen.  Kinda feel like Shadow Hills Engineering must have come across this image at some point.  Dynamic noise suppressor, phono pre, two eq controls, visual feedback of high and low frequency drive, variable metering, tons of input switching… talk about the kitchen sink.  This project is organized around the very interesting 6AS7G dual-triode power amplifier tube: basically two 2A3s in one glass envelope with a 6V heater supply.  This is potentially some useful information, especially when you consider that a 6As7G is only about $13 from AES (priced out 2A3s lately?). I think that there could be a unique new guitar amplifier in the works soon…

My lord.  At least they didn’t try to thrown in an AM/FM tuner.  Follow the link at the end of the this post for schematics and parts lists for some 6AS7G amplifiers.

‘Audio Anthology Vol 1’ is available at Amazon dot com.

FOLLOW THE LINK BELOW for 6AS7G project schematics…

Continue reading Audio Anthology: Collected Hi-Fi articles from AUDIO ENGINEERING in the 1940s

The Williamson Amp, part one

The original Williamson hi-fi amplifier schematic as published in “Wireless World” magazine (UK) May 1947.  

The Williamson amplifier is considered one of the earliest hi-fidelity audio amplifier designs.  It is certainly one of the most popular audio circuits ever developed for DIY’rs.  Without fail I seem to turn up at least one home-brewed Williamson every year at the local estates+fleas.  A PS Dot Com reader from the UK sent us the original articles from “Wireless World” as published in 1947.   I have yet to build a pair of these myself, and the idea of starting ‘from the top,’ as it were, with the original design, is appealing.  A few things to note: check out the provision to balance the driver stage, and separate bias level and balance controls for the output stage.  Also: check out R25: the formula for determining the feedback loop resistor.  I wish every schematic included this notation.   NB: the ‘L63’ valve is simply a 6J5 – aka, one half of a 6SN7.  the ‘U52’ rectifier is a 5U4 or equivalent.  ‘KT66’ is a better-performing 6L6; feel free to use 6L6 or 5881 if necessary.

Mullard 520 Power Amplifier c.1956

Download a four-page article from “Radio & Television News” 4/1956 regarding the Mullard 520 power amp:

DOWNLOAD: Mullard_520_amp

American industrial titan RCA offered schematics for a variety of tube-audio equipment in the back pages of their many “receiving tube manuals.”  Mullard, a prominent British maker of vacuum tubes, similarly published a book entitled “Mullard Tube Circuits For Audio Amplifiers”  (h.f. “MTCAA”). The designs are quite different from RCA’s, as Mullard promoted different tubes:  EL34 rather than 6L6/5881; EL84 rather than 6V6; GZ34 rather than 5U4; and EF86 rather than 5879.   The MTCAA also offered extensive plans for the fashioning of the actual sheet metal cabinet and transformer-cover.  The four-page article I am offering here is quite different from the one in MTCAA, but either will get you on yr way to building this unit.

This design promises 35 watts from a pair of cathode-biased EL34s.  It does require an ultralinear (IE, with screen taps) output transformer with a 16ohm winding for the negative feedback loop (such as this EDCOR model), but other than that it’s all very basic parts.  Now if I could find some good cheap EF86s…  Anyone try the new $17 Electro-harmonix EF86?

BTW, The Mullard book is still readily available as a reprint; well worth the $17 cover price IMO.  There is a circuit for a 3-stage ‘mixing preamp’ featuring EF86 pentode inputs with a 12Ax7 on the back end, the second triode of the 12ax7 wired as low-impedance cathode follower…  pretty tempted to try that one…  anyhow, you can buy ‘MTCAA’ at Amazon Dot Com or at Antique Electronics.

1957: A Few Bits From The End Of The Mono Age

From some 1957 issues of “Radio and TV News,” an electronic-serviceman’s publication, comes this collection of American Hif-Fi home audio kit of the era.  Notice the fact that…  it’s all mono. Commercial recordings released on stereo magnetic tapes were available as early as 1954, but it was the release of the first -ever stereo LP record in November 1957 forever turned the tide towards two-channel ‘Stereo’ recordings as the norm for recorded musical performances/productions.

Heathkit!  I have one of these A7s and damn it is a good-sounding little amp.  Anyone have a spare they wanna sell me (for stereo…. naturally…)?

I tend to think of Newcomb as more of an industrial-sound/PA sound company, but it looks like they made some home units too.  I have one of those huge glass-covered KX25 PA heads as shown here; it’s always been a little flakey but I can’t bring myself to part with it cos it came from a Catholic church and the knobs are labeled “Pulpit,” “Choir,” and “Sacrament Table.”  Take that, Kick/Ld Vox/ Bckng Vox.

Dude went to prison in 1974 for lying about the value of a music-collection that he donated to a university.  Tried to get Mancini and Bernstein to back him up and they would not.  Life is long….

Electro-voice home hi-fi drivers c. 1957

Eico home hi-fi amps and pre-amps circa 1957.  Eico was essentially the ‘other’ Heath(kit).  Eicos could be purchased either wired or as kits.  Here’s a two-page article on their flagship HF60, a beautiful amp with EL34 output tubes and an ultralinear Acrosound output transformer.

UDPATED (4): Presto Recording Corp: Pioneers of ‘Instant’ Analog Disc Recording

Download the fifty-page 1940 PRESTO RECORDING CORP catalog:

DOWNLOAD (part 1): Presto_1940_Cat_1

DOWNLOAD (part 2):Presto_1940_cat_2

Products covered, with text, some specs, and photos, include: Presto Model A, Model B, Model F recording installations; Presto Model C, Model Y, and Model K portable recorders; Presto type 8-A, 8-B, 28-A, 6-D, 6-E, 6-F, 26-B, 75-A, 75-B, 75-C, 9-A, and 9-B  recording turntables; Type 62-A transcription turntable; Automatic Equalizer 160; blower system 400; 150 and 151 pickups; Microphone Mixer type 130-A, B, C; Preamplifier type 40-A and 40-B; Presto radio tuner 50-A and 50-B; Recording Amplifiers  85-A, B, 85-E, 87-A and 87-B; plus a range of parts and accessories including Green Seal discs, Orange Seal discs, and Blue Label discs.

Above, the Presto Model A, their top-of-the-line system circa 1940.

Presto Recording Corp was a pioneer of coated-disc ‘Instantaneous Recording.’  From 1933 through the end of WWII, Presto was the US leader in providing high-quality recording equipment to broadcasters, schools, studios, and government.  There is a detailed history of the Presto Corp provided at this website, so no need to re-tread those waters.  Basically, what Presto offered was a way to make good-sounding LP and 78 recordings that could be played back instantly on any home turntable.  Unlike earlier commercial recording technologies, there was no intermediate submaster required.  Presto was able to do this by having designed an aluminum (later, glass) disc that was coated with a special cellulose-based compound (featuring 51 ingredients!).

At right, the Presto 200-A Electronics package.  This was a complete system of microphone preamps, cutting amps, patchbay, and AM radio tuner that was designed to accompany the Model-A pictured above.  Presto’s ‘instant-disc’ technology was basically rendered obsolete by the development of magnetic tape recorders in the late 1940s, most notably, AMPEX (and to lesser degree, Magnecord).   The specs for the better Presto systems weren’t awful: 50-8000hz frequency range, 50db signal-to-noise ratio; but this paled in comparison to the German Magnetophon technology that AMPEX built on, with a high-frequency response to 15,000hz.

On a more basic user-level: you could always record-over a piece of magnetic tape; but cutting into a lacquer-coated disc (at $16/unit in today’s money) was a commitment.

Presto Model C, their top-end portable system of 1940 ($20,000 in 2011 dollars; 138 lbs)

Looking through this catalog, the most fascinating aspect is the large range of mechanical devices and accessories recommended to insure the fidelity of the audio.  Nowadays almost all audio control happens electronically; once the room is treated and the microphone carefully placed, our work as recording engineers leaves the realm of physical manipulation and enters a world of electronic control.  In the era of analog disc recording, though, a careful recording engineer needed blowers…

…to efficiently remove the bits of cellulose material that the cutting needle carved out the the recording blanks;

viscous-oil-filled dampers to regulate vertical movement of the cutting head (a mechanical audio compressor, I would imagine);

…an optical microscope to examine the grooves that you just cut for quality-control purposes…

fresh sharp needles to do the actual cutting work…and, if you wanted the ultimate in convenience, an ‘automatic equalizer’ to automatically boost the treble frequencies as the cutting head moved closer to the center of the disc (since discs spin at a constant rate, as the needle gets closer to the center of the disc, the actual linear speed of the needle relative to the surface medium gets slower, and as we know well in all types of analog recording, slower equals less high-end).

Above, the Presto 40-A microphone pre-amplifier, the one piece of equipment in this lengthy catalog that could still be of potential use to modern recordists.  It uses two 1221 tubes to deliver 55dbs of gain (from what I can gather, 1221s are interchangeable with 6C6, the 6C6 being the predecessor to the 6J7, likely making these 40As likely very similar to RCA BA1/2/11 series mic preamps).  If anyone has the schematic to the Presto 40 mic preamp, please send it to us…  coincidentally I built a preamp with 6C6s a few years ago (based on a schematic from an ancient UTC catalog) and I liked the results.


Thank-you to reader EL for sending us the schem to the Presto 40-A.  Here ’tis as a download: PRESTO Type 40-A preamp schematic

…And here as well:

This must be a slightly later version of the 40-A, as the 2nd tube is a 6SJ7, which is a variant of the 6J7 that has the input grid connection in the base rather than on the top.  Other things to note: the input transformer spec’d is an ‘LS-10,’ which I can only assume means the UTC LS-10…  circuit-wise, we have the first 6J7 connected in pentode, coupled by a .1uf cap to the 2nd 6SJ7 stage, this time wired in Triode in order to more easily drive the output transformer.  A ’50M’ resistor (or as we know them, 50K ohms) provides negative feedback from stage 2 to stage one.  Thinking that this circuit could be nice as the back end to a 3-stage pre, maybe with a something low-gain like a 76 or 6J5 on the front end with a volume pot following.


EL also provided some images of his particular 40A units… check ’em out…

Pretty amazing that these things made it all the way to Australia way back when… my lord can you imagine how much these things must have cost in their day?








Update Sept 2013:

E.L. directed us to this eBay auction; a Presto 40a in nice condition (Nashville, TN) sold for $510.  Here are some images:


Presto_circuit Presto_inside2 Presto_Interior1 Presto_label Presto_output_trans************



Anyone out there using any Presto equipment in their work?  Drop a line and tell us about it….

 UPDATE: a friend has alerted us to The 78 project, a series of new recordings of notable musicians made using just a Shure 51 mic and a Presto disc recorder.  They sound great, and in the videos you can hear both the modern production-sound of the session via the camera audio-track and the actual 78 playback.  Very interesting contrast…


Multi-use vacuum-tube audio mixer circa 1960

Download a three-page article from Electronics World “Hi Fi Annual & Audio Handbook 1960” which details the construction of a 4-channel tube mixer:

DOWNLOAD: Tube_Mix_It_Box_1960

“Electronics World” Magazine published these ‘hi-fi annuals’ in the late 50s/early 60s; from what i can gather, they seem to be collections of reprints of the notable audio-content that EW published in the prior year.  I picked up this 1960 edition a few years ago in the excellent Antiques Mall of Madison; some excellent vinyl LPs and rare paper dealers as well.

Anyhow.  This mixer has some interesting features; nice simple circuit for calibration pot for DB meter; the best part though is the output stage: a 6AK6 tube is used into a 10k ohm load and promises a .8 watt output.  I am not aware of any other hi-fi line output stages with 6AK6s; def want to try it.  .8 watts output would very welcome in many of my builds…  can’t get enough of that clean headroom…

Here’s the schematic (included in the download as well).


QST Magazine in the 1940s

QST magazine is the monthly publication of the American Radio Relay league (h.f. ARRL).  ARRL has published QST since 1915.   The ARRL is the main membership organization for ‘Hams,’ otherwise known as amateur radio operators.  We discussed Hams a bit in this previous post on vernacular graphics.  I am not a Ham radio-operator, and I know next to nothing about radio-frequency broadcasting equipment.  But, since most Ham radio broadcast-chains begin with the human voice and a microphone, and it is largely a DIY-type activity, there is plenty of relevant content in these old magazines.

Above is the ARRL’s mission-statement as published in 1947. Anyhow, over the next few days I will post a few interesting bits from QST in the immediate post-WW2 era.  There was a tremendous surge in amateur radio activity at the time, owing to the return home of the servicemen who had learned radio-technology in the war.

These men had been given an introduction to radio and electronics in the most intense possible situation -the life-and-death struggle of global warfare – and it’s no surprise that this powerful link would fuel an intense post-war peacetime interest in Ham activity.

Above: a Tom-Of-Finland-esque advert for Solar Capacitors from a 1947 QST.

We’ll start today with a couple of interesting schematics for push-pull audio amps: a 6F6 15 watt push-pull amp, and a cathode-coupled 6L6 40 watt amp.  I have never used a 6F6.  Anyone?  And I don’t recall ever having seen a cathode-coupled push-pull driver circuit.  Check ’em out…

Tomorrow: Turner Ham mics of the 1940s.



Building an (almost) RCA OP6 Mic Preamp

(image source)

The RCA OP-6 ‘Portable Amplifier’ is one of a handful of truly visually-iconic vintage mic preamps.   The OP-6 was designed as a “remote,” as-in, ‘on-location’ single-channel mic preamp for radio station broadcasts.  It uses three 1620 (6J7) tubes  – most mic pre designs of the period use two.  Furthermore, the 1620s are in pentode operation rather than triode.  The result: a ton of gain.  95db, apparently.  This is almost twice as much gain as the classic RCA BA1 and BA2 mic pres.  OP-6s are in high demand – click here for a seller asking $2900.   Further indication: Blackbird Rentals in Nashville has thirteen in stock as rental units.  The first time I heard an OP-6 in use was at Blackbird; I was producing/directing a live-in-the-studio performance for Martina McBride and John McBride was engineering; he was using the OP-6 for something… I can’t remember what exactly. Anyway, it caught my eye and we talked about it a bit; later I learned that he has an especially strong appreciation for these units.  A strong endorsement coming from the man who likely owns more vintage pro audio gear than anyone who ever has or ever will live.

The OP-6 schematic is pictured above (this file is readily available in high-res on the internet).  So anyhow, if we consider how in-demand the OP-6 is, it seems striking that no one offers a modern equivalent for sale, even on a small-scale level.  If you take a close look at the circuit, the reasons become apparent.  There are two big obstacles to re-creating even a semblance of an OP-6.  First is the input attenuator:

Sure, it’s a voltage divider; probably constant impedance; but what exactly are the values?  And what about that value of that feedback path issuing from the attenuator back to the input stage?  If I could get my hands on an original OP-6 and open up the attenuator, sure I could maybe sort it out.  But I imagine that re-creating that part on custom order could cost hundreds of dollars; frankly I have no idea.

The second obstacle to re-creating the OP-6 is the output stage choke.

The choke is designated L-1 in the schematic.  Curiously enough, it’s actually physically part of the output transformer.  Based on this fact, there is zero chance that this was an off-the-shelf choke, say a UTC for example, that we could track down.  OK – but the crucial value of a choke in a circuit like this is the inductance.  So long as the voltage an current handling values are sufficient, any choke of same inductance should give a similar result.  Now again, if I had an original OP-6, I could measure the inductance and maybe a current off-the-shelf part exists to satisfy the requirements.  But… I don’t think anyone out there is gonna send me their prized OP-6 to open up.  And $2900 is a pretty stiff R+D cost.  So what do we do?  Well, in the absence of any actual electrical engineering training, I looked for some good advice and then I guessed.

The very friendly+talented John Atwood sent me the diagram above; in response to what I can’t recall. This diagram explains why the choke is necessary in order to get the best possible performance from a single-ended vacuum tube line output stage.   Looks pretty similar to our OP-6 output stage, right?  Based on this… I made a wild guess.  I ordered the very inexpensive Hammond 156C choke.  150mh inductance with 8ma current capacity.  8ma might be a little low, but I have found that Hammond really undersells the specs of their transformers, so I’m not worried.

Alright so now we’ve got a choke that might work.  What about that input attenuator?  The best course of action would probably be to get a used 100k Daven T-pad, but without implementing the feedback path that the stock OP-6 attenuator has, it seems like this is slightly pointless vis-a-vis maintaining originality.  So instead: I used the input stage from my favorite, yes yet again, the RCA BA-2.

The BA-2 schematic is pictured above. The input stage uses a 1620 tube, wired as a triode, with a 100k ohm pot following it.  So I just took this input stage, up to the pot output, and wired it in front of the second two OP-6 stages (starting at the grid of stage #2).  When I did this, the whole system worked fine except when the volume pot was a zero (IE., when the grid of tube #2 is shorted to ground).  This caused weird noise and a little humming.  Not sure if this is due to the negative feedback in that stage, or if this is simply a general characteristic of 1620s when they are run in pentode – but it sounded awful.  The easy solution?  I added a 1M resistor from the grid to ground, and isolated this from the pot with a 1K grid-stopper resistor.  Done and done.  The BA2/OP6 Hybrid is born.




The finished piece is shown above.  I used a 6X5 rectifier tube like the OP-6 uses; in fact, with the exception of using DC filament voltage, I kept the power supply the same as the OP-6.

The audio portion of the circuit is shown above.  What with the three pentode stages and feedback paths, this is extremely complicated for a mic preamp.   A lot of parts.  On the left you can see my usual Jensen 115 input transformer.  Audio caps are a mix of Solen, sprague, and some random Russian ones.  Basically whatever would fit.  Resistors are random as well; now that I have confirmed that the piece works well, I will probably replace the carbon-comp resistors in the B+ chain with some quieter modern resistors.  On the right is the output transformer.  I used an edcor 15K: 600; $10.22!

Even though this edcor is unshielded, there is zero hum following the volume pot.  And as for performance?  I did a frequency sweep through the entire unit; response is absolutely flat from 15hz to 10k hz.  At 10k there is a rise of about 1db up to 22khz, at which point response begins to fall of pretty rapidly.  This is really excellent performance considering the inexpensive transformers.  I did not measure the gain but there is a lot of.  The output level can get extremely hot.

Anyhow.  That’s it for now.  As soon as I have a minute I will provide some audio test examples; I’ll post some A/B examples of this unit versus an API 512, hopefully with both acoustic gtr and drums.

Thanks to DW at EMRR for helpful suggestions regarding this project.

***UPDATE*** Listening test has been done and results are posted here.

A Few Interesting DIY Audio Projects c. 1955

Circuit for a simple bass/treble parametric equalizer that uses no inductors.  Could be  a useful piece.  Not sure if 250k/500k dual pots are still available?  Since I have still not finished building the last tube EQ project that I described, so I’m not likely to craft this thing anytime soon.  If anyone out there picks this up and builds one, LMK…

Going back to AUDIO magazine in the early 50s: a few circuits that caught my eye. As always, if any of y’all out there build any of these, drop us a line and let us know how it went.

A ‘cascode’ preamp which features very high gain and defeatable compensation for phono cartridge.  I am not aware of any available mic preamp that uses this circuit.  a lot of gain available here…

A low-gain preamp that mixes two high-impedance signals to a medium-impedance output.  Add a few transformers (500: 50K inputs, 15k:600 output) and this could be a useful tool for selecting/blending two mic signals to one track of (tape).  IE put a couple of mics on a gtr amp, blend to taste… I generally do this using submasters (busses) on the console but here’s a potential way to do it that uses a much simpler signal path, IE right from the mics into the convertor (or tape machine).

A 50-watt ultralinear power amp using 6550 power tubes

A 12-watt 6V6 power amp with some negative feedback and a pentode input stage.

A simple 8-watt power amp that uses a 6SH7 input tube and a single 6L6 output.  This circuit uses a great deal of negative feedback in order to supposedly get more linear response with cheap output transformers.  I’ve never used the 6SH7 tube before…  curious though.

A simple design for a variable sawtooth wave generator.

A DIY tube mixing console; plans, schematics, methods

Some more interesting bits from AUDIO magazine c 1955:  plans and schematics for a radio station mixing console.   Very little in terms of equalization devices, and no compressors discussed, but a wealth of ideas for low-level and line-line level amplifiers and mixing circuits.  Download the entire piece as a 4-page PDF:

DOWNLOAD: DIY_Console_1955

There is little of real interest here in terms of a complete system, as the system discussed is a mono mixer; but if you are interested in some simple, novel mic preamp and line driver plans, you might find this worthwhile.  Personally I am drawn to the OD-3 regulator tubes; i know zero about voltage regulation tubes; I have always used Zener diodes in the past when i needed regulated voltage; but the very simple way that these tubes are used here leads me to believe that they are pretty easy to work with.. Might wanna try them out.  They are just $5.15 at AES.