So much has been written about this brand and these fabled little amplifiers that it could pose a serious threat to the amount written about Dynaco products! And just like Dynaco products, the modifications available are never ending for virtually any model you chose to discuss. And so it is with the Magnavox 8600 series as well. However, unlike the Dynaco products, the little 8600s rarely get any serious engineering attention to get the most out of them.
A search returned that the vast majority of circuit modifications are basically limited to removing the voicing components in the NFB loop (not always done correctly), or adjusting voltages to match the published schematics (also done with varying degrees of effectiveness). The rest of the efforts typically center around which tubes and components sound best, and dressing the little suckers up to disguise their humble beginnings.
I mean nothing against these efforts in the least. But after getting one of these cuties for myself (for a variety of reasons), the potential to get so much more out of it was immediately obvious. Ultimately, because these amplifiers represent so many different things to so many different folks (myself included), I decided to tear into mine and see how far a complete makeover could take it. Within the world of flea power and what the unit represents in stock in stock form, the results have been nothing short of spectacular.
Consider the wide attraction this amplifier has (in no particular order) to:
1. Those longing for the holy grail of single ended midrange sweetness.
2. Those using one in a biamped flea system for the mids or highs.
3. Those who are simply enamored with flea power.
4. Those who want to see what tube sound is all about in the cheap.
5. Those newbies to tube audio who want to take the plunge and get hands on service experience, but want to start out small and simple (very wise).
6. Those who were paid to haul away an old console; found one installed in it and wanted to see if it would still work (hey, we had one of those growing up!).
7. Those like myself, who have a young grandson that seems to have every attraction to vacuum tubes and music that I did at his age. That makes this a pilot/development project to set up the real one down the road in a couple of years.......
However or why ever you came into the possession of one of these units then, and regardless of what you use it for, with a working example, a moderate budget, and a few fun evenings, you can turn that (basically) stock unit into one that will cause folks to take notice -- of the sound.
Now first off, your ticket into this flea show starts with good, efficient speakers. Without those, you simply have to take your toys and go play elsewhere. In my case, I've owned Cornwalls from before it was popular to own Cornwalls, so at 101 db, my ticket is punched for entrance. So how is the stock amplifier with these speakers?
I got my unit (an 8604-00) without tubes from a fellow AKer battradio, in stock unmolested condition. A quick once over determined that all the parts that count were in good working order -- enough so that an accurate base line of performance could be established. A fresh set of tubes were installed, and the amp was then tested in the listening room. I gotta tell you, I was frankly underwhelmed. The unit displayed classic ground loop hum all by itself (I haaate hum), and just seemed asthmatic: It just seemed to run out of steam more than I am accustomed to. I know this is flea power. But I am accustomed to what good flea power can do, and to me, this wasn't it. Beyond that, the sound was not engaging, and didn't hold my attention. But it was in bone stock condition, so next up was putting it through the paces on the test bench to try and correlate what it was doing, to what I was hearing. Oh boy......
For perspective, I doubt there was ever any published specifications for these amplifiers. Be that as it may however, they are generally regarded as having anywhere from 3 to 5 watts per channel depending on who is offering the information, and that's about it. Beyond that and the tube complement, about the only other information known is that each channel is designed to operate into a 4 ohm load.
A quick glance at the tube manual shows that tubes of the 6BQ5 class are capable of developing 5.7 watts of power when operated in single ended mode. Of course that power level is never realized due to losses in the output transformer. Still, when that is accounted for, about 4.6 watts of power can be realized. Looking at the schematic for these amplifiers, they operate with similar or near similar voltage levels as those shown in the tube manual, so the claim of 3-5 watts would certainly seem reasonable. Except, that isn't what's happening..........
Using a a proper 4 ohm load and a mid band frequency of 1 kHz, each channel could develop just 2.3 watts of power before (unequal) clipping commenced. The power bandwidth (-3db or half power points) extended from 100 Hz to 20 kHz. But that's at 4 ohms. When an 8 ohm load is used -- as most speakers are today -- the available power is then cut in half. Now the unit is capable of just 1.2 watts per channel before clipping commences, with little change in the power bandwidth. That's not a very good head of steam, and correlates exactly with what I heard.
Next, some basic response and stability tests were run to finish out the base line testing. Now remember that the unit still includes the voicing components of the stock design. Still, the only time I've ever seen such contorted square waves is when something was drastically wrong -- except that nothing was in this case. Needless to say, the response was anything but flat......
I didn't get any pics of my unit in stock form before I got started, but needless to say, it was in very good physical shape, and pics of these units -- in all manner of physical form -- abound all over the net, as does the schematic. Search engines will bring it up on the first attempt. Still, a few pics will get the ball rolling:
1. Here channel 1 trying to develop 1.35 watts into an 8 ohm load at 1 kHz. Clipping is obvious, as is the fact that it is unequal as well. Channel 2 was no different.
2. This is a 2 kHz square wave applied to channel 1. This frequency is used because to accurately reproduce a square wave at this frequency requires a flat response within the audio bandwidth to 20 kHz. From this display, it is hard to even tell it's a square wave, with strong peaking and notable ringing across the top of the wave. The peaking causes early clipping within a narrow band for the power that's available, and the ringing can always be shown to affect clarity and detail. In short, it's a mess.....
3. With a 10 kHz square wave, you might as well take your toys and go home. It isn't square, it isn't.... heck I don't know what you'd call it...... But knowing what's going on in the supersonic realm of the amplifier is important to amplifier stability -- as will soon be shown..
This is all pretty bleak -- and likely why many single ended advocates simply avoid performance specifications and only talk about the "sound". The battle cry is that you don't listen to specifications. Uuuuhhh. I'm sorry, but you do -- and I heard these specifications in my listening room. However, it can all be dealt with quite effectively, with simple changes to turn Cinderella into a princess:
1. Frequency response can extend +0/-.4 db from 40 Hz to 40 kHz.
2. Power bandwidth can extend from 90 Hz to 25 kHz.
3. Mid-band power output into 8 ohms can be increased by over 300%.
4. High frequency transient response (square wave display) can take on an ideal wave shape, and
5. Stability can be maintained to a very good degree into capacitive only loading.
We'll start getting into all of this, next time.
Dave
A search returned that the vast majority of circuit modifications are basically limited to removing the voicing components in the NFB loop (not always done correctly), or adjusting voltages to match the published schematics (also done with varying degrees of effectiveness). The rest of the efforts typically center around which tubes and components sound best, and dressing the little suckers up to disguise their humble beginnings.
I mean nothing against these efforts in the least. But after getting one of these cuties for myself (for a variety of reasons), the potential to get so much more out of it was immediately obvious. Ultimately, because these amplifiers represent so many different things to so many different folks (myself included), I decided to tear into mine and see how far a complete makeover could take it. Within the world of flea power and what the unit represents in stock in stock form, the results have been nothing short of spectacular.
Consider the wide attraction this amplifier has (in no particular order) to:
1. Those longing for the holy grail of single ended midrange sweetness.
2. Those using one in a biamped flea system for the mids or highs.
3. Those who are simply enamored with flea power.
4. Those who want to see what tube sound is all about in the cheap.
5. Those newbies to tube audio who want to take the plunge and get hands on service experience, but want to start out small and simple (very wise).
6. Those who were paid to haul away an old console; found one installed in it and wanted to see if it would still work (hey, we had one of those growing up!).
7. Those like myself, who have a young grandson that seems to have every attraction to vacuum tubes and music that I did at his age. That makes this a pilot/development project to set up the real one down the road in a couple of years.......
However or why ever you came into the possession of one of these units then, and regardless of what you use it for, with a working example, a moderate budget, and a few fun evenings, you can turn that (basically) stock unit into one that will cause folks to take notice -- of the sound.
Now first off, your ticket into this flea show starts with good, efficient speakers. Without those, you simply have to take your toys and go play elsewhere. In my case, I've owned Cornwalls from before it was popular to own Cornwalls, so at 101 db, my ticket is punched for entrance. So how is the stock amplifier with these speakers?
I got my unit (an 8604-00) without tubes from a fellow AKer battradio, in stock unmolested condition. A quick once over determined that all the parts that count were in good working order -- enough so that an accurate base line of performance could be established. A fresh set of tubes were installed, and the amp was then tested in the listening room. I gotta tell you, I was frankly underwhelmed. The unit displayed classic ground loop hum all by itself (I haaate hum), and just seemed asthmatic: It just seemed to run out of steam more than I am accustomed to. I know this is flea power. But I am accustomed to what good flea power can do, and to me, this wasn't it. Beyond that, the sound was not engaging, and didn't hold my attention. But it was in bone stock condition, so next up was putting it through the paces on the test bench to try and correlate what it was doing, to what I was hearing. Oh boy......
For perspective, I doubt there was ever any published specifications for these amplifiers. Be that as it may however, they are generally regarded as having anywhere from 3 to 5 watts per channel depending on who is offering the information, and that's about it. Beyond that and the tube complement, about the only other information known is that each channel is designed to operate into a 4 ohm load.
A quick glance at the tube manual shows that tubes of the 6BQ5 class are capable of developing 5.7 watts of power when operated in single ended mode. Of course that power level is never realized due to losses in the output transformer. Still, when that is accounted for, about 4.6 watts of power can be realized. Looking at the schematic for these amplifiers, they operate with similar or near similar voltage levels as those shown in the tube manual, so the claim of 3-5 watts would certainly seem reasonable. Except, that isn't what's happening..........
Using a a proper 4 ohm load and a mid band frequency of 1 kHz, each channel could develop just 2.3 watts of power before (unequal) clipping commenced. The power bandwidth (-3db or half power points) extended from 100 Hz to 20 kHz. But that's at 4 ohms. When an 8 ohm load is used -- as most speakers are today -- the available power is then cut in half. Now the unit is capable of just 1.2 watts per channel before clipping commences, with little change in the power bandwidth. That's not a very good head of steam, and correlates exactly with what I heard.
Next, some basic response and stability tests were run to finish out the base line testing. Now remember that the unit still includes the voicing components of the stock design. Still, the only time I've ever seen such contorted square waves is when something was drastically wrong -- except that nothing was in this case. Needless to say, the response was anything but flat......
I didn't get any pics of my unit in stock form before I got started, but needless to say, it was in very good physical shape, and pics of these units -- in all manner of physical form -- abound all over the net, as does the schematic. Search engines will bring it up on the first attempt. Still, a few pics will get the ball rolling:
1. Here channel 1 trying to develop 1.35 watts into an 8 ohm load at 1 kHz. Clipping is obvious, as is the fact that it is unequal as well. Channel 2 was no different.
2. This is a 2 kHz square wave applied to channel 1. This frequency is used because to accurately reproduce a square wave at this frequency requires a flat response within the audio bandwidth to 20 kHz. From this display, it is hard to even tell it's a square wave, with strong peaking and notable ringing across the top of the wave. The peaking causes early clipping within a narrow band for the power that's available, and the ringing can always be shown to affect clarity and detail. In short, it's a mess.....
3. With a 10 kHz square wave, you might as well take your toys and go home. It isn't square, it isn't.... heck I don't know what you'd call it...... But knowing what's going on in the supersonic realm of the amplifier is important to amplifier stability -- as will soon be shown..
This is all pretty bleak -- and likely why many single ended advocates simply avoid performance specifications and only talk about the "sound". The battle cry is that you don't listen to specifications. Uuuuhhh. I'm sorry, but you do -- and I heard these specifications in my listening room. However, it can all be dealt with quite effectively, with simple changes to turn Cinderella into a princess:
1. Frequency response can extend +0/-.4 db from 40 Hz to 40 kHz.
2. Power bandwidth can extend from 90 Hz to 25 kHz.
3. Mid-band power output into 8 ohms can be increased by over 300%.
4. High frequency transient response (square wave display) can take on an ideal wave shape, and
5. Stability can be maintained to a very good degree into capacitive only loading.
We'll start getting into all of this, next time.
Dave