STA-25MkIII Power Amp

This page contains the story of a vintage tube valve ampifier named Radford STA-25 Mk III that I - believe it or not - found some time ago, together with other valuable components in a container with building debris, and the steps undertaken to make it functional again.

I was able - thank google - to dig out a schematics of it instantly at www.drtube.com , but only after a long search I found a picture of it on the net. Unfortunately I forgot to Ctrl-D it and later on could not locate it again. AFAIR it was from someone in France who acquired it on a flea market and paid near-nothing for it - looks pretty good!

Also described here are some more personal emotions and biographic details of mine which you may skip if interested in the technical details only.

To me these sentiments are part of my deep love with music and the desire for an as much as possible authentic reproduction of it. Since I found myself beeing not talented enough to become a musician I´m now and forever stuck to that (aka the high end HiFi) approach.
Please feel free to click right through to the more interesting part... I inserted a link below for you extra for that purpose. Don´t miss it!
If you don´t shortcut and later on become boared or offended by either my writing or my horrible english - please remember I told you.
If you like my story or are amused a little - let me know! My email address is hidden somewhere in here.

Text and images were quickly hacked into Netscape Composer and fine tuned using CosmoCreate on a 1996 SGI Indigo 2 and then placed somewhere on one of my web sites after I got unexpectedly rich response from a posting in rec.audio.tubes. Many thanks to all responders! The advice and information you gave were most valuable.

The page is deliberately designed without the use of any "content rich" features like frames, flash, java and javascript - pure html, though. No bells and whistles.
Like an old saying about what it is that makes a good sportscar... it simply has nothing on it that does not make it faster.
Maybe some time in the future I will make this a complete web site of its own, together with a real site structure, link list etc. pp., but for now it is one of my personal projects pages. The very first, actually, but I´m amused by the idea and there are others soon to follow.

(You may click on some of the images to view an enlarged version if desired)

(some personal excurse - you may proceed to the pictures of the Radford STA-25 below if in a hurry)

One needs to know that at the time when I grew up and got more or less socialized - that were the late sixties - the most important thing for a wanna-be man to own besides his motorbike (which unfortunately was limited to 50cc until the age of 18 back then) was his stereo. In order to deeply impress friends, esp. the female ones, one had to possess huge speaker cabinets and weird looking amplifiers, even if the girls were not a bit interested in who the musicians were that made the sound coming from it.

We guys were aware, of course. A very big part of the identity my generation has built up was pretty much related to the contemporary music.
That´s what author Nick Hornby already narrated in his successful and readworthy novel High Fidelity- but IMO AFAIR he missed to tell us a big chunk of the story: the technical aspect.

All that dating would have be no fun at all if you were to take a bus home and there find your stereo equipment hum, crackle and tweet during some sensitive... say, tea ceremony.

The concept was to be ultra cool to the edge of excentricity in order to attract a girls attention, then carry her home at breathtaking speed on a motorbike and lay her while her favourite music played from that impressive stereo with near-live crystal clear quality ;-) ... it never quite worked for me, though (of course), and may not work at all that way.
But at a certain age kids are easy prey for advertisement that suggests the like of that concept...
the proper stereo would get you a girlfriend all by itself.

To be honest: I don´t like neither the slim blonde ready to untie nor the strange speakers that obviously attracted her. Besides their ugly appearance back in 1974 it was far too early for a discrete subwoofer/satellite design. We had been to rock concerts already and knew for sure that huge walls of marshals did the trick.

When I got my first amp at early age 13, after a long time of evaluating all the possible alternatives - not too many, though - I finally decided to get me one from a manufacturer located nearby. I did not know that most likely all of these were made in the far east back then.

Anyway, it were to be a solid state Teleton 2x6W with a neat wooden housing named SAQ-203 (Not quite impressive nowadays), which I could afford and which resembled a little a much more expensive Marantz amp :-).

I  carried it home from the "factory" on my bicycle and then proudly sat in front of it for a while without touching all the knobs on the front panel, reading the manual over and over.

Some friend of mine had donated me a turntable, and I had spent money for a pickup system and a headphone already. The amplifier was the missing link... finally ending the times when I had to use the equipment in the living room of my parents when I wanted to listen to my music.

Do I have to tell that I still own that amp (and it never failed in 35 years)? I also possess - at least the housing of - the two 35 litre Rank Arena two-way speakers I got me about a year later - after I had earned enough money during my holidays. But first I was stuck to Toshiba HR-50 2-way headphones for a about a year, those ones with the "crossover" circuitry for a more natural (not binaural) stereophonic appearance.

My new amp had cost me 149,- and the phones 98,- Deutsche Mark. Remember? I´m nostalgically talking about the late sixties here.
And I of course needed some money to afford the most essential vinyls of that time, which I handled only with extreme care and most of which are the precious jewels of my vinyls collection now. But I already owned a few and now it was about time to process them on my very own stereo...

Never again - besides, perhaps, with women I dated later on - have I felt that sweet sensation in expectance of some overwhelming joy to come soon. I already knew what a really good stereo could sound like - I had much of opportunity to listen to a Braun CSV-13 which my grandfather owned, but he always refused to let me listen to my own vinyls on his PCS-5 turntable (that would have made him want to vomit I believe). These were the high end components of that time!

(No enlargement of these images here since I have downloaded them from some recent eBay auction)

Do you remember the V15 -IV Shure pickup with the little brush on it? I kept watching it gliding through the grooves of symphonic orchestra vinyls for hours, both feeling the impression of a beethoven concerto and the pain of the thought that I could never afford even that pickup system myself eventually. Sigh.

My hope was my own little stereo wouldn´t be too bad against that - no sinfonies, of course. Today I know that I wouldn´t have heard much of a difference without. Listening capabilities evolve over the time, only once one has become aquainted to good sound it is possible to tell from it the better. Mine were still to evolve a lot.
Hearing subtle differences works only, besides, in direct comparison eventually - which hardly happens outside a HiFi equipment seller´s studio.

But I finally got me the desired bike (a Honda ST-50... LOL ;-)), the stereo... now I was ready to date! As said already the concept most often badly failed - I later gave up on it and kept having much fun with the bike and enjoyed music alone, mere for itself, which I learned is always the better way to do.

Later my first serious girlfriend happened to be quite a lookalike of the girl depicted on the back of the toshiba leaf I had carried home from a teenage music fair. The Toshiba girl at least looked pretty innocent, though - my real one was most likely not, only handed over to me from a very good friend of mine (being a jazz musician of world fame today).

Isn´t she cute? Sure she´s going to be a grandma nowadays...

Needless to say that my bikes, the girls they carried home and my listening equipment of course evolved both in performance and cost over the very long time since.

I almost forgot about all the different stereo apparatuses I acquired one after the other, pushing fidelity high and higher step by step... until most recently I urgently needed a headphone, for my daughters drumkit lessions. I remembered the Toshiba and went searching my archives for it - and literally stumbled over the Braun CSV-13.

Out of a spontaneous impulse of curiousity, I carried it upstairs and connected it (replacing the solid state Rotel amp that is in charge momentarily) - and it instantly sounded much better, despite the fact it does not deliver a quarter of the power the Rotel does ! So at last there is something with the old saying about tube valve amplifiers! Soon the Rotel went into the vault and ever since the Braun has taken it´s place. And since there´s no phone jack on the front of th Braun my daughter got my ancient Teleton ;-).

It seems the circle had finally closed - and I had matured to the serious listeners heaven - the world of tube valve amplifiers.

Now - so much for (yes, it´s been more than enough) my socialization, tube valve stereo equipment, girls and motorcycles. I spare more for my still-to-write biography.
The lengthy excurse was meant to explain why that thing among the debris I spotted all of a sudden caught my thoughts like magnetic:
Another vintage tube valve amp! Though I didn´t have any clue at all what a jewel that one was... but let´s have the story in chronological order now...

</some personal excurse - I told you earlier that you might have proceeded to here as well!>

One day some weeks ago I accompanied my little daughter to school as I use to do each morning.

The distance is only about 800 meters, and I always see people in the neighbourhood hurry their kids into the car to drive them to school and into adipositas...
since it gives me the opportunity to take a walk in the early morning´s fresh air and talk a little to my daughter - she hardly does the rest of the day - I enjoy being a pedestrian.

But this morning was going to be different.

Only around the corner and over the next intersection I spotted something in a dumpster that caught my interest. Looked like an ancient stereo - and not a cheap one!
Hadn´t I been walking I would have carried away my find immediately (or never noticed at all eventually)!

I hurried to get back there as soon as possible - were my findings gone in the meantime - taken by someone else who easily recognized its value?
Most luckily they were not.

On second look it turned out that it was a Thorens TD124MkII turntable equipped with an SME 3009 tone arm and a AKG P15ES pickup, together with a Radford SC-2 preamp and some sort of tube valve power amplifier (only "radford" written on it), all of it built into a plywood kitchen plate with plastic adhesive wood imitation on it... ugly! Btw., I did not know the amplifiers at all - never heard of that name before.

What an ignorant I was! I had the short thought of leaving it where it was - but I took it with me because I knew that metal parts among building debris make it much more expensive to get rid of it at the landfill site. In fact I literally ran home and as soon as possible returned with my sidecar (I´m still a motorbike addict) and then carried all the findings away from the debris container. Lucky me!

The Thorens TD124 turntable, the SME 3009 and the AKG pickup were half way good in shape and soon sold at eBay for an astonishingly high price :-).
I then decided to get me some replacement tube valves for the broken ones in the power amp with that money and find out what can be made out of the remains.
There began my research into the - as I now know, thank google - legendary Radford STA-25 tube valve power amplifier.

Some additional remarks about the finding circumstances... I guess the one who dumped the whole stereo had no clue what it was.

Maybe the house was sold after someone died or moved out who left the stereo not knowing about its value and/or not wanting it any more. The most probable scenario is that the first owner died long ago since these components are from about 1960 and were among the most expensive back then (the buyer had to be quite wealthy). Eventually some younger descendant inherited this stereo and made some use of it in a party vault or the like which again became abandoned later. Finally in 2004 someone who was put in charge with a renovation of that house got it out of the way - there it was, partially smashed and half buried under bricks and rubble.

Or it wasn´t from that house at all and some super ignorant carried it to that dumpster by night like smartass people often do to get rid of their junk.
Anyway, it is now safe - besides some speakers that must have gone another way... from here it´s mere guessing... the speakers must have been of adequate quality back when this stereo was first acquired!

Sometimes I wake up at night from a nightmare: the sound of a pair of Quad ESLs being crushed in a giant hydraulic garbage press irrevocably, or processed mercilessly through a shredder, only iron parts finally picked out by a giant magnet in order to re-melt them into steel which will be used to make a cheapo diecast midrange driver chassis eventually.

Here you can see the state the components were in "as found". Note that even the special preamp-to poweramp cabling was included!
 

Power Amplifier - Radford STA-25 MkIII

The very heart of it is the power amplifier, a Radford STA-25 Mk III. Nowhere on it is any hint about what it is, though. It simply says "Radford", that´s all.

The tubes are - as I soon found out - standard types, and I already got me a set of some Philips "new old stock" ones at eBay.

On one side they were all smashed, even the massive cromium plated steel bar could not save them when they crashed into a solid pile of bricks.

Luckily the bakelite knobs on the back were totally unaffected by the impact.

Preamplifier - Radford SC-2

The SC-2 preamp looks pretty good at first, the front seems perfect...

no tubes are missing and there´s no sign of mechanical damage,

besides the sheetmetal is a little bent - not from any impact with ugly things in the dumpster but from some incompetent way of mounting in the wooden plate mentioned above. This should be not too difficult to fix...

on second view, the input selector is damaged. The aux button is stuck and two buttons are broken (sniff). Hope I can fix this with a bit of epoxi and lots of patience,

on the other hand it seems no one before me has been "working" on the electronics up to now - which I estimate to be a time span of about 40 years now.

My hope is the electronics is as solid as it looks and not up in smoke yet.
Pretty lucky find all together, isn´t it?
I´ve been told the preamp isn´t too good later, but I will keep it because it is of no use without the main amp I think.

Both items together are also most valued by collectors and hifi enthusiasts worldwide and were said to be well worth about a thousand dollars... if an owner were willing to let go at all - which is not very likely since these babies are rare nowadays.

First I pulled the tubes that were still in place on one side. I guess the Mullards are from the original manufacturer - Radford, respectively - while the Valvo one was from a later replacement. I´ve learned now that there are even tube valves in existence that are labeled "Valvo" but in fact are Mullard ones.

Then some cleaning of the outer parts of the amp was damn necessary...
It is always amazing what a paint- and a teethbrush, a cloth and some warm water with soap can do!

Once the dirt and encrustings were removed - some water tags left - from the outside I couldn´t stand the tension any more and carefully opened the case.
No bad surprise inside, only much more dirt of course.

It also appeared to have rained a little the night before I rescued this from the rubble, but not too much or too long - luckily there´s no visible corrosion.

Note that the iron parts were marked manually - that must have been done by the original manufacturer or during a previous restauration (which I doubt).
AFAIK felt tip pens were pretty rare back in the late fifties / early sixties.

Again I patiently wiped off all the dust and carefully brushed all the surfaces, even the inside of the sheetmetal cover.
See the already polished chrome bar in foreground blinking! Also depicted here are the archeologists tools:

a wet paintprush to lift off the dust, a dry toothbrush to remove small encrustings, a nylon brush for the big ones (not shown) and a brass one. I normally use this type of brush to clean the finned cylinder heads of my bike - they do a fine job with tube valve amps as well. The latter is for the metal parts that desire heavy shrubbing, e. g. the zinc plated sheet metal parts.

Finally all of the interiour seemed acceptably clean to me:

(The water tags are clearly visible because of the contrast and sharpness enhancement these images went through) - Now I took a look at the fuse:

it was _not_ blown! Looks like new, doesn´t it? And not like a contemporary replacement - which would not be labeled that good.
But the most exciting part was still to come - the final opening of the king´s chamber:

Astonishingly few dirt inside here... must have been opened before!
The cabling from inside to the transformators seems to be untouched, though (an opportunity for another view from top)

The cabling looks correct, at least for a vintage british product. Japanes standards were unavailable at these early times.

But there has been reworking on the PCBs apparently, one big capacitor was replaced and on the other side near its counterpart a small resistor, too.
Note that its predecessor may have burnt - there´s a little blackening on the PCB visible around it.

Obviously two capacitors at the rectifier PCB were also replaced since these have the same colour as the replacement one mentioned above. I guess it is not very likely that these caps were built into the electronics through production with different colours. I have tried to mark the three suspicious parts red - I hope this is visible.

My first thought was: replace the unmatched parts with new, matched ones... but wait! My inner voice tells me better not to modify anything unless it becomes necessary. Collectors items like these should not be altered just for fun.The green capacitor is 8µF compared to 8.8µF of the orange one. I guess that the latter is from the original outfit - but this is only a guess, it may be vice versa as well (whatever reasons for).

I have learned most recently that capacitors of that kind were of 50% tolerance back in the early days of electronics... so 8µF vs. 8.8µF doesn´t matter at all.

If anybody reading this has some 8.8µF 450V 525V Surge Red Outer Plessey Capacitor NOS please let me know immediately, I´ll then warm my soldering iron.
Yes, with the help of the Internet and some intensive googling I now have some schematics at hand. For the more curious reader here is a close-up of both PCBs first:

None of the electrolytic capacitors looked leaky. Only in one place here was some sticky fluid (near one of the big caps on top) which could not be identified as originated by a leaky cap. Instead there was a trail to an external source of it: on the outer, upper sheetmetal cover there was a piece of this substance, too. Either the amp was mounted overhead so it dripped through a hole or it came from outside and dripped into it. I will know for sure when the thing is beginning to produce smoke during power-up.

And now for the schematics of the STA-25 MkIII (sorry, this one must be big):

To my surprise the 8.8µF electrolytic capacitors are all labeled 8µF here.... OK, this doesn´t matter.

I ordered tubes from various sources now, most of them manufactured by Philips - I hope I needn´t go for the original Mullard ones since these are hard to find and pretty expensive. I have no clue what differences it makes for what types or brands of tubes (I´m ´only´ a mechanic). Before short I learned that Mullards were produced in license of Philips or even a Philips brand like Valvo. Anyway... Philips manufactured tubes (TV screens at last) in the town near which I live. They closed that factory last year and set some thousand mployees "free"... so for nostalgic sentiment I decided for the Philips brand.

One saturday after an near endless wait the last EF86 arrived. I improvised some power supply wiring on the desk of the libroratory (that´s a hybride - a lab in a lib).
A vintage speaker set was also at hand - two Braun LS550 slimlines in chestnut finish which had been sitting on a shelf as mere decoration for nearly 20 years.
Again not the ones depicted here (I can tell white from chestnut, believe me), this is only to give an idea how simple these are. Pretty massive, though.

The whole family gathered around to watch my electrocution, or, if not for that, to see a funny little firecracker performance.

After I had followed all the precautions I had been advised to by friendly rec.audio.tubes participiants, e. g. a 25W lightbulb in series (and an amperemeter) - I was finally courageous enough to switch it on - to all´s disappointment nothing happened at 33V. Then with a 60W bulb I had 185mA current floating - and a hardly audible click came from the speaker. Then a 100W bulb - and there came the music! I finally omitted the lightbulb.

(Watch out for some pictueres of the amp glowing in the dark - soon to come in this section)

Only then I placed the speakers (braun L550, about 1970 - just at hand for testing) into a stereo position, and me in the middle - and off I went listening for hours...

My source: a cheap AIWA portable CD player. OK, I had no direct comparison at this point... but my impression is: that amp is indeed fantastic!
No hum or noise when there is no signal applied (input open, even with a cable). The only audible thing is a silent hum of the transformers, but to hear that one has to put an ear onto the perforated housing sheetmetal.

The tubes glow red and blue in the dark mysteriously, and - funny - the thing emits a sweet aroma which is conciously noticeable only direct above the transformer housing. Once I had located it there I now recognize it whenever I enter the room and the amp is on, like the very personal perfume (not a synthetic product) of a beloved person.

(end of the technically interesting part - only some subjective listening experience to follow)

The recording I prefer to start with whenever I have to find out about the value of an equipment is this (I´m going to explain my strategy for listening sessions soon):

1.) Vladimir Horowitz - "Horowitz in Moscow"

with eyes closed I soon had the impression of sitting backstage near the Steinway (it requires a surround system to have the coughs of the audience from behind - unfortunately impossible with stereo). This does not easily happen with a CD - normally the vinyl version does the trick better... anyway, it sounded quite promising.

2.) Swinger Club - "Monsters of Jazz"

A little up with volume now - and the sound appeared to be not much far from being right in front of one of their live gigs.

I have not been a witness of the impressive horovitz event in Moscow - but I had the Swinger Club playing for a summer party in my backyard a few years ago.

Which leads me to explain my personal test listening strategy...
Most important IMO is to choose material very well known. Acoustic instruments first hand, highly skilled and inspired musicians - preferrably ones one has seen and heard live at least once. If one happens to know some and their instruments personally - perfect!. Luckily this is the case with my chosen example 2 for me - I heard their drummer Martell Beigang first when he was age 12. Now he´s about to approach his fourtieth birthday... a mature jazz drummer and much sought-for high precision studio musician.

Back to the listening experience! The finest timbre of ride and crash cymbals, upstroke-downstroke technique on hi-hat, rim-click accents on the snare... it is of course impossible to fully describe the clarity of the nuances - or is it 3D definition? - this amp easily reproduces, even with some 35 year-old two-way speakers.

Another example:

1.) Pat Metheney - "Watercolors"

I bought this one on vinyl soon after it first came out and enjoyed it very much ever since. Later on I got me a CD of this and it is always amazing to compare it with the vinyl. Again it is the drums and the piano that reveals details unheard before. Btw., the cover art for this album was  made by a friend of mine, photographer Rainer Kiedrowski, so again I have a personal relationship with this example.

Again the reproduction of fine nuances of both drums and piano is brilliant, even if the instruments are under heavy attack. The vibrant timbre of a full size acoustic bass, the tiny scraping noise when the fingers touch the strings... quite indescribable. It´s not easy not to be instantly caught by the music and what this amp does with it. That happened not only to me... my wife and all my children instantly froze when they hit the hotspot within the magic stereo triangle, mouth slightly opened. To relieve them I had to drastically turn down the volume or switch the music off at all. This is not their normal reaction when there´s music playing somewhere!

I can´t say how this happens, even without state-of-the-art high-end speakers. Maybe it is just that: a simple but efficient 35W two-way speaker. The Braun LS550 is not too bad a speaker - I remember that my grandfather had a lot of speakers and a switchbox at home whenever he was going to buy some. Most likely he made his vendor bring all this stuff to his home for testing - then he carefully compared what suited him best and finally gave the rest back.

One thing is for sure now: I will keep this amp, at least for a while until a better one shows up.

My next action will be to compare directly with half a dozen others and a little audience invited.
With some additional speakers, and a switchbox, too... my personal favourite speakers are also simple 2-way ones: a Philips/Audax driver combination with a 6dB/Octave crossover circuit as primitive as possible to avoid any phase shift. Wish I could afford a pair of ESL-63 (won´t fit into the living room either).

I´m very curious... I inherited the Braun CSV-13/CT-18 stereo from my grandfather when he died in 1985 at age 96.
It is - besides a volume knob that was mysteriously missing and a broken power switch I had to replace later - in very good condition :-) .
And I´m even more curious about the performance off the Radford with my vinyls!

Which tube amp is going to sound better? The Braun or the Radford?
Or will a modern Rotel pre/main transistor amp couple do?
My custom made MOSFET power amps?

...to be continued here - sooner or later!