1965 Astrosonic 50

[Motorola minion]

Based on the catalog, few models were built with the R208 tuner/preamp and A531 amplifier chassis. The recapping of this one was much simpler than some of the lesser Astrosonic models. It really sounds great at high volume, close to the power of the 9300 series tube amp. The main power supply cap (brown paper cover) was partly open, which I rarely see on a SS model.

This was brought to me by a customer who came from downtown PHL just to buy my 59 Zenith phono-only console. Getting a very decent price, I promised a free servicing of this Magnavox and thought I'd be sorry, not so. Even the Micromatic record player was in good shape, requiring no parts (whew!!!!) Most of the problem was a gummed up motor and very scratchy (stepped) bass control, which can stop audio from getting through.

Lesson here: I have restored many Magnavox one-chassis receivers (R204, 211, 214...) of this variety and I do not recommend a total recap if they ALREADY sound bad. I fooled myself a few times and wasted time on cheaper, early solid state. Stay away!~

[electra225]

Dave, I have never gotten into solid state stuff. That may be my next frontier. I tinkered with a Silvertone solid state console stereo chassis, but ended up tossing it. It had rocker switches for AM AFC, stereo/mono and one for external speakers. Those rocker switches evaded any effort to clean them. Being dirty, they would open one channel or the other. What have you found with some of the "lesser" solid state chassis that would support your recommendation to "stay away?"

[William]

Interesting that it has a separate amp, I thought only the Magnavox 100 series had that.

Bill

[Motorola minion]

Dave, I have never gotten into solid state stuff. That may be my next frontier. I tinkered with a Silvertone solid state console stereo chassis, but ended up tossing it. It had rocker switches for AM AFC, stereo/mono and one for external speakers. Those rocker switches evaded any effort to clean them. Being dirty, they would open one channel or the other. What have you found with some of the "lesser" solid state chassis that would support your recommendation to "stay away?"

Without a doubt, heavy usage of the early solid state amplifiers, especially the lower end models can only sound good again by replacing those once-cheap and plentiful Germanium (Ge) transistors and capacitors. The '66 HH Scott stereomaster 344b I have performs far better than the Fisher SS receiver's I have condemned due to aforementioned solid state Ge deterioration, just like old tubes!!!!

Fisher and others who did not "wait for silicon" developed way too much around Ge, before they could re-design with silicon it was too late. The Japanese receivers then became the go-to unless you could afford boutique audio like Macintosh.

Anyone who has serviced imported solid state prior to the Japanese taking over versus say, HH Scott and older US Harmon Kardon would agree these are not worth repairing*. Even when new, these were a step-down. Few tube amps were truly a "step down" in fidelity like those store-brand solid state models which are to be avoided. Sorry but Penncrest, Sears, Delmonico, Electrophony, Arline and who knows what else are squarely on this list.

*My friend in Florida likes gutting those if the cabinet is "mcm" enough to get a couple grand with new components (and weight ) added.

[William]

Thanks, Dave, that is all good info to know. So, with your statement, what about the stereo in my Avatar which was my grandparents RCA home theater and I believe you have one just like it. It is pushing 60 years old and still sounds really good.

Bill

[electra225]

Your RCA Stereo Theater is not a low-line instrument. It was at or near the top of the line. Dave was talking about cheap equipment being not worth fixing after all these years.

[Motorola minion]

Thanks, Dave, that is all good info to know. So, with your statement, what about the stereo in my Avatar which was my grandparents RCA home theater and I believe you have one just like it. It is pushing 60 years old and still sounds really good.

Bill

Bill, The RCA solid state equipment was some of the best because RCA used their own transistors, even in the most common budget equipment.
Not surprisingly, I find that most Zenith and Magnavox consoles used RCA-sourced transistors.

[hermitcrab]

I guess I don't get the post... is Dave saying low buck SS units like the magnavox pictured are not worth recapping?

[electra225]

My understanding of Dave's post is that RCA germaniums are generally reliable, and that Magnavox, Zenith and others who used RCA transistors can be repaired successfully. Other, cheaper equipment not blessed with RCA germaniums may not be worth the effort, since replacement germaniums of any description are not plentiful.

[wparks]

This posting on the 1965 Astrosonic 50 console with the A531 amplifier is one of the posts that brought me to this forum. I had an A531 that I purchased as a stand alone amplifier from ebay for $25, and is my current obsession/project. I would like to do a more detailed write up and post service information and photographs.

The listing: https://www.ebay.com/itm/145755373859

I have been doing a dive into germanium lately, really started with the garage sale purchase of two 70's automotive 8-track decks for $5, both of which had 12V class AB germanium amplifiers of Japanese design and manufacture with Sanyo transistors. After hand drawing out the schematics for the pre and power amplifier stages, researching other similar schematics, and understanding thoroughly the biasing design and thermal concerns, I rebuilt them into stand-alone cabinets and have been enjoying listening to them. I have found them to have surprisingly good fidelity and a wonderful tonality, once divorced from the awful 8-track source and given decent speakers to drive. Only about 1Wpc, they are great for desktop near field listening.

I wanted something germanium that was bigger to fill the room with my larger vintage 3-way speakers, so was looking for something in the 25W per channel range, and found this A531 on ebay for a song. I'm cleaning it up and reworking it to be a stand alone amplifier component. The transformers have date codes from 1962. The PNP Germanium output transistors, (38PIC or 38P1C) have a source code of 274 indicating RCA manufacture. I have figured out that Magnavox seems to have been big enough to get manufacturers to print Magnavox proprietary numbers on their transistors instead of the common part numbers to confound repair techs working without the Magnavox literature, or to give Magnavox licensed techs more business. (Does anyone here have more information on that?) Anyway, SAMS typically list a table of equivalent replacements, which might give clues as to what actual part numbers these should have been printed with. I would love to learn more about this.

The amplifier seems to have come in two versions using the same chassis. The larger, the A551 is listed as 25 Watts per channel, has a larger power transformer, and different transistors (47PIC or 47P1C) than the A531. Other minor differences include a main supply of -44V vs. -39V, half-again bigger input coupling, power filter and output caps, and some slightly different bias and gain values. I have found much less information on the A531 version except for one of the schematics I will attach here that shows the A531 component values with the values for the A551 next to them in parenthesis. I am not sure what the rated power output of the A531 is, but as Motorola minion posted it's probably comparable to the Magnavox 9300 series 6BQ5 push-pull tube amp (which I also will do a write-up on) that is around 15 watts per channel. I have not purchased the SAMS (669 folder 8) for the A531/A551 yet, but will and post it in the downloads section soon.

Attached are views of a good clean condition A551, My dirty A531 in a support frame, 38PIC output transistors, and two schematics.

And now, some gratuitous key words, purely food for the search engine to bring in more intrepid future members:
Magnavox Astro Sonic AstroSonic solid state Germanium Class AB push-pull console 25W 50W 50WPC power amplifier amp
A551-01-00 A551-02-00 A551-03-00 A551-04-00 A5510100 A5510200 A5510300 A5510400 A55101 A55102 A55103 A55104
A531-01-00 A531-02-00 A531-03-00 A531-04-00 A5310100 A5310200 A5310300 A5310400 A53101 A53102 A53103 A53104
38P1C 38PIC 47PI 47P1 47PIC 47P1C 274 637343-2 320108-1 300209-1

[TC Chris]

Interesting that the output transistors are transformer-coupled to their drivers. My Heathkit AA-22 has the same scheme. What was it about early transistor amplifier design that required interstage transformers?

Chris Campbell

[wparks]

Interesting that the output transistors are transformer-coupled to their drivers. My Heathkit AA-22 has the same scheme. What was it about early transistor amplifier design that required interstage transformers?

Chris Campbell

Hi Chris-

I have noticed that just about every germanium output transistor amplifier design I have looked at uses interstage transformers. I'll take my best crack at why this is, and welcome more educated input on this because I am a newbie at analog design and don't have much historical knowledge in early transistors.

Most typical class AB output designs using silicon transistors will use complementary drivers- a PNP and an NPN that are well matched. Because they are opposite polarity devices (a rising base signal turns the NPN on, and PNP off, and vice versa), both bases are driven with the same phase input signal (with different DC bias points) so the predriver output is either directly or capacitively coupled. From what I have read, it was more difficult and expensive to manufacture NPN germanium transistors. With PNP devices being prevalent, the output stage was most typically totem pole (two same polarity devices) so the predriver output had to be phase inverted for the two bases. Tube AB output stages had the same problem (there is no PNP analog in tubes) so a wide variety of phase inverter circuits were designed to present opposite phase input signals to the two totem drivers. Tube designs typically did not use interstage transformers, so why do germanium designs?

I believe the best answer is because of the low input impedance of the totem pole germanium output devices. Vacuum tubes have an extremely high input impedance, which can be easily driven by even the relatively high output impedance of a tube predriver circuit. For a germanium power transistor however, the current gain (hfe) was typically quite low, maybe 20-50 or so, so a relatively high base current would need to be driven for any kind of output power. The hfe of the transistors used in the predriver circuits were not much better, so several different stages would be required for voltage amplification, then creation of the inverted signals, then a current buffer (like emitter follower) to provide the necessary drive current. I believe a transformer was a cheaper, more reliable and elegant solution than the 5-6 transistors and supporting components required otherwise. The transformer could generate identical secondary outputs that were perfectly inverted, and the turns ratio between the primary and secondaries chosen to give a good impedance match- the primary was typically 200-400 Ohms (AC impedance, not DC ohms) while the base driving secondaries was around 3-8 ohms. The primary could be driven by a simple high gain common emitter circuit that incorporated the negative feedback signal from the amplifier output, and the secondaries had the low impedance to drive the output bases directly with ease.

In my opinion, the interstage transformer is most of the magic that provides the tonality or color (for better or worse) of the germanium amplifier circuit, just as the output transformers do for vacuum tube amplifiers. I really like this color, and I am working to make a germanium power amplifier part of my audiophile pallet.

Took a glance at the schematic for the AA-22 output stage- interesting that they use a germanium "compensating diode" to establish the bias point of the output drivers- you see that all the time in silicon, with silicon diodes, but in germanium I frequently see "thermisters" used in the bias circuit that can adjust the bias point down as the cabinet warms up and the germanium output transtors start to conduct more to avoid thermal runaway. A germanium diode would be subject to the same effects of heating, so is a nice method to use.

[TC Chris]

Thank you for the extended and interesting reply! My brain has never understood solid-state circuits but gradually I learn.

Early audio circuits used interstage transformers just to couple and not to invert for P-P. Is that from impedance issues with early triodes? And I have a 1936 Zenith with P-P 6L6 outputs that is transformer-coupled for phase inversion. I wonder what drove the choice of the transformer, which was probably pricier than one more tube.

And on to the Heath AA-22. I have the Julian Hirsch review of it. He loved it, praising its RIAA comp. accuracy and its sound. He used the phrase "transistor sound" as a compliment, not the term of derision as later writers used it (meaning cold, harsh). He wrote, paraphrasing here from memory, that it has that transistor sound, a sound most often heard in very expensive tube amplifiers. In other words, we have a medium-power transistor device that sounds like top-end tube equipment. I always think of that when read criticism of Hirsch's work for being specification-obsessed. No, he was listening as well as measuring. Others on this list have read my story about taking the AA-22 to one of the old McIntosh Amplifier Clinics. Dave O'Brien himself measured my amp, and it was one of very few (apart from the Macs and other high-end models) that met its published specs. I've got the graph he drew still.

Chris Campbell

[wparks]

Thank you for the extended and interesting reply! My brain has never understood solid-state circuits but gradually I learn.

Early audio circuits used interstage transformers just to couple and not to invert for P-P. Is that from impedance issues with early triodes? And I have a 1936 Zenith with P-P 6L6 outputs that is transformer-coupled for phase inversion. I wonder what drove the choice of the transformer, which was probably pricier than one more tube.

And on to the Heath AA-22. I have the Julian Hirsch review of it. He loved it, praising its RIAA comp. accuracy and its sound. He used the phrase "transistor sound" as a compliment, not the term of derision as later writers used it (meaning cold, harsh). He wrote, paraphrasing here from memory, that it has that transistor sound, a sound most often heard in very expensive tube amplifiers. In other words, we have a medium-power transistor device that sounds like top-end tube equipment. I always think of that when read criticism of Hirsch's work for being specification-obsessed. No, he was listening as well as measuring. Others on this list have read my story about taking the AA-22 to one of the old McIntosh Amplifier Clinics. Dave O'Brien himself measured my amp, and it was one of very few (apart from the Macs and other high-end models) that met its published specs. I've got the graph he drew still.

Chris Campbell

Hi Chris-

There are a bunch of Youtube videos available that explain a lot of basic electronics if you are interested in learning more- It's never been more easy!

I have seen interstage transformers used for a variety of reasons, primarily due to the ability to easily impedance match between stages, and the generation of identical in-phase or complimentary out of phase outputs is a bonus. I was speaking in generalities. In 1936 octal tubes had just been invented, so multi-section tubes (with two triodes in one package, etc) were not available. With the more simple tubes available, doing a proper phase inversion would have required at least two tubes, which were large, taking a lot more space on the chassis, hot, and power hungry. I'm sure an interstage transformer in that chassis was more attractive.

I enjoy having a selection of different amplifiers to listen to: one tube AB, one silicon AB, several germanium AB, and a very early class D integrated amplifier. I would like to get both a tube and transistor class A at some point. To my ears, I hear what is frequently said- silicon AB (and good class D) has more technical precision, with lowest distortion and a more "accurate" presentation. The clarity, transparency, bandwidth, and sound stage is very pleasing with most music and really encourages dissecting the music to find the details. It's easy to become a purist here and discard all others as inferior for having too much distortion and adding too much color to the sound. Measurements are key when you are talking about frequency bandwidth and flatness, about distortion level versus output power, and about maximum output power level, but distortion measurements are highly misleading and don't tell the whole story. It's more about what flavor of distortion, not just how little.

The way distortion was explained to me really "resonated" with me. Odd harmonic distortion (3rd, 5th, etc) is dissonant, grating, and annoying. Even harmonic distortion however, occurs quite often in nature and is almost expected by our brains. Play a violin note very softly, and you hear almost exclusively the fundamental. Stoke the note harder, and the violin gets louder, but the sound also gets much more complex and interesting- the even harmonics accompany the fundamental in a harmonious and enjoyable way, much like a 12 string guitar sounds compared to a 6 string. Vinyl records and stylus, magnetic tape with tape head, vacuum tube non-linearity, germanium non-linearity, and transformer non-linearity ALL contribute to even harmonic distortion. This is what I love about my germanium amplifiers- they DO sound much like high end tube amplifiers, and it is very enjoyable. I notice a wider sound stage with germanium and tube, and the flavor is different, but very enjoyable, particularly in the gentle handling of midrange, and a more warm bottom end.

All of this is so subjective, in the end you have to set aside the reviews, and just enjoy what you enjoy. It's a good think to have the variety.

[Brettzky99]

Astrosonic was the Greek god of shitty consoles

[wparks]

That's kind of rude. I disagree. To wit:

https://www.diyaudio.com/community/thre ... nt.419757/

Criticize all you want. Opinions are like armpits . . .

[TC Chris]

Yeah, I have learned to listen first and not judge solely on design topology. I am often surprised. That DIY project apparently produced a good-sounding amp. A local guy who was on this list has an extensive collection of elegantly-restored '30s and '40s radios, mostly higher-end, with auxiliary inputs added. On modern source materials they sound much better than I would have expected. Then you have to factor in the fun of working on the devices.

Chris Campbell

[wparks]

Yeah, I have learned to listen first and not judge solely on design topology. I am often surprised. That DIY project apparently produced a good-sounding amp. A local guy who was on this list has an extensive collection of elegantly-restored '30s and '40s radios, mostly higher-end, with auxiliary inputs added. On modern source materials they sound much better than I would have expected. Then you have to factor in the fun of working on the devices.

Chris Campbell

Yeah, that's my amp, I posted that. (Thought my user name would give it away ) And it's an awesome sounding amp, among the best I have heard. Was wondering if he would criticize that too. You know, consoles are a funky thing like some cars or foods- a world unto themselves, you either love them or you hate them. It's easy to be closed minded and jump on the popular bandwagons, I understand the need to do that, but never any need to tear something down that other people enjoy. Unforced error. Too much hatin' in the world.

I really do feel bad that I tore three different Magnavox consoles apart for the three amps that I have rebuilt, but honestly, they were in bad shape- many are. Those that can be saved should be, at least some, and Astro Sonic, love it or hate it, was an notable part of history and should be saved too.

You're right Chris- I spent a good number of years restoring radios, had over a hundred various table top and consoles at one point. The amp is usually far better than the old sources (AM for sure), and speakers really have not changed all that much in the last 90 years, and both can really shine if tuned up a little and given a good source. Great fun to be had there for sure. That's what it's all about.

[TC Chris]

By way of automotive analogy, in the 1960s I was a devoted big V-8 guy. Then in 1970 I needed an inexpensive economical vehicle because of my job circumstances. I bought a 1962 Corvair for $150. It was the antithesis of everything I thought I liked. 6 cylinders, for goodness' sake. And air cooling, wasn't that something that VW had? It was, as they say, cheap and cheerful. I loved it and thrashed it hard and it never gave up (except for the oil is deposited everywhere). As I get older I try to suppress my prejudices, which often get in the way of clear thinking. I just finished reading the latest edition of Stereophile magazine, and here I am defending console-grade audio. But then, the founder of that publication believed that listening was the ultimate test of a component, not specifications.

Chris Campbell

[Brettzky99]

That's kind of rude. I disagree. To wit:

https://www.diyaudio.com/community/thre ... nt.419757/

Criticize all you want. Opinions are like armpits . . .

That is a cool post on the amp, thanks for sharing.

But The tail goes with the hide...now show the big ugly cabinet with the plastic controls that weighs a ton and can't be placed in a corner. You're picking and choosing noteworthy parts, I'm talking about restoring and keeping the entire console.