Magnavox Flea Power: Getting More Out Of The 8600 Series - A Lot More!

I'm thinking Magnavox 9300 series amps.

And I think the 9300s had good enough iron on them, when modded up right. You have to be realistic about how far down you need to go, and if it's still a Magnavox when you get done, or really a home brew on a Maggy chassis. IMHO of course.
 
I'm more just curious about the merits of various mods and what they are legitimately capable of. I don't quite have the knowledge or the test gear, or the amplifier itself to determine that information.
 
And I think the 9300s had good enough iron on them, when modded up right. You have to be realistic about how far down you need to go, and if it's still a Magnavox when you get done, or really a home brew on a Maggy chassis. IMHO of course.

Jay,

I don't think anyone really wants the compromise that is typically a "Stock" Magnavox Console pull amp.

I have really thrashed in my tiny cat brain the conundrum that is either living with the stock 4 Ohm OPT's or what is the point of actually swapping them for better OPT's with 8/16 Ohm taps. What is the cost/benefit actually when you in effect redo the amplifier so far away that its better to scratch build something in the end. (Please don't debate the merits here as I do think this could be a different thread and not detract from this excellent thread)

Dave,

This is a fantastic build you have here and actually makes the most effective cost/benefit reasoning of any maggie build I have seen to date.

You cut through the fog like no one else. I truly enjoy reading your posts.

Bravo,

Frannie
 
Thanks so much for the kind words Frannie!

Kid -- 250 volts is fine.

Derek -- Dynaco choke is fine.

Schematic should be up by sometime this evening.

Dave

Dave
 
Schematic and Final Comments

As promised, the schematic is posted with this thread.

For a project that started out as a simple development effort to ultimately build for real with a grandson in a few years, it quickly took on a life of its own, and as I have said, surprised me with the quality of reproduction it has returned. It is easy to see why these amplifiers have such a following, as they come off as the ultimate little engine that could.

Within the bounds of it's simplicity and non-high fidelity beginnings, it really is quite an impressive performer, being as impressive in the specs it produces, as what it delivers in the listening room. For those accustomed to the stock (or basically) stock performance of these amplifiers, the modified amplifier will show that we really do listen to specifications. It's just that with high caliber equipment, the differences are usually so small as to make discerning the difference more difficult. On this level however, the difference (for me anyway) between the stock and modified units represents a large quantum leap improvement. The physical appearance of the unit simply belies the sound it produces through capable speakers.

There will certainly be those who will want to experiment with the design, which is fine. But as experimentation relates to component values, every component value used was chosen for a specific reason to address a particular aspect of the design. Therefore, if your favorite grid stopper resistor for a 12AX7/6EU7 (for example) is a 240 ohm resistor, that's fine, but it will change the stability characteristics of the unit. Or maybe you think the output stage is running too hot. That's fine too, but power and distortion will suffer significantly. Maybe your favorite small signal audio tube is the 12AT7. Fine and dandy. But such a change will significantly lower open loop gain, and reduce circuit performance. Experimentation is great, as we all have our own preferences. But I would encourage those who wish to copy the design to listen to it first as offered, and let that sink in. Then try experimentation if you want.

For those who champion "voicing", that's fine too. But there was no effort of that kind involved with the development of the modification. This was a classic engineering exercise through and through, with appropriate components of today's typical good quality used to fill the various locations -- as surely as any manufacturer did with the components available to them back in the day for this type of exercise.

The resulting sound of the modification is high quality in nature, lacking any console "bloom", mid-bass kick, or rolled off high end. The unit presents a very uniform presentation across the audio bandwidth, with surprising bass capabilities, very smooth midrange, and an overall detail and is not hard to miss on material that is so recorded. Therefore, if you are not familiar with a truly flat presentation (that is, being more familiar with a colored sound presentation), you may want to use a preamp with controls to achieve the settings you favor. As for the unit itself, it is very uncolored, with a wonderfully balanced presentation throughout.

Finally, some may want to add a volume control which is fine, too. However, as with virtually all tube power amplifiers that use a triode input stage, the unit will perform best when driven from a constant low impedance source, which only an active preamplifier can provide (or if directly driven from a SS source).

Well, that's about that. This has been quite a departure from my usual projects. But I learn on each and every one of them, and this one was no different. I hope you have as much enjoyment out of this design as I am having!

Dave
 

Attachments

  • scan0001.jpg
    scan0001.jpg
    74.9 KB · Views: 2,493
Dave - as always, you are the glue that holds together Audiokarma. Thank you for all your hard work!

-D
 
Yes, thanks to Dave for another great tutorial and taking the time to set me straight on a few things. But I am sure there are many that are not going to shell out $30 for the replacement OPTs, in which case, the second best thing to do is follow Dave's instruction to re-bias the 6BQ5's and live with the reduced performance.

I also want to circle back to the earlier point on the "nominal impedance". Dave has already corrected my use of the word "nominal"... What I meant to say was that even though many speakers are sold as "8 Ohm" speakers, their actual impedance are often lower, so the output impedance mismatch may not be as bad and you should still be able to get some usable power out of the amplifier.
 
Well, that's about that. This has been quite a departure from my usual projects. But I learn on each and every one of them, and this one was no different. I hope you have as much enjoyment out of this design as I am having!

Hi Dave, per usual, and entertaining and informative piece. Now, that you have tasted the fruit from the single ended tree will there be more to come? I hope so.
I really like your schematic. Clear and easily read. I just have a couple of questions regarding that. The first is that on your output transformer you grounded the green wire and used the black as the speaker tap. I always thought black should be ground. Second; I also noticed you introduced some heater bias by taking the cathode voltage from one 6bq5 and injecting it into the filament circuit via 100 ohm resistors. Is there a reason you chose to add some filament bias to the amp? I have always wondered if one could take cathode voltage for bias from just one tube and not upset the bias voltage vs the tube that was not tapped. tc&br, primo
 
Last edited:
I really like your schematic. Clear and easily read. I just have a couple of questions regarding that. The first is that on your output transformer you grounded the green wire and used the black as the speaker tap. I always thought black should be ground.

Depends on the transformer wiring diagram - also, may be adhering to the romex standard of black = hot, white = neutral, green = ground.

-D
 
The color of the wire really does not matter, the use of negative feedback dictates which wire on the seconadry should be used and which one should be grounded - the correct polarity must be used for the NFB to work. For heater elevation, please see this article (middle of the page).
 
Primo -- The article that Jaz provided and his response regarding the secondary wiring hits the nail directly on the head regarding your questions about the modification. Specifically however, the heater reference circuit draws zero current from the output tube cathode it is connected to, so the output tube doesn't even know it's there......

As for anything else of a SE nature in the future? I would say likely so. I always try to approach sound quality using sound engineering doctrine. I believe it can be achieved with the right approach from a variety of topologies. But as I have stated before, the need to have copious amounts of power to achieve it (i.e. the Tim Taylor syndrome) has waned in me somewhat over the years -- although I do keep plenty of it close at hand to keep that itch scratched. However, when this trend in power reduction is coupled with the fact that my downstairs audio facilities have no A/C (they really don't need any), it really makes SE designs even more attractive: Even though no A/C is needed, I am always mindful of the amount of heat my equipment is throwing off versus the practical need I really have for reserve audio power. In the summertime, heat dissipation can build up rather quickly down here without any active means to remove it, so in that light, SE operation makes even more sense.

The biggest thing that stands between these small Magnavox amplifiers and an all out SE design effort of course is the output transformer. Therefore, a new scratch build with this topology would certainly start with a high quality transformer, and go from there. The amount of clean sound level that this project here returned is quite amazing (it can get loud!), so 6-7 watts RMS per channel across the whole audio spectrum should be about right. That will require an output tube to idle at about 20 watts of quiescent plate dissipation, after which the SE topology starts becoming real impractical from an energy efficiency standpoint: For nearly the same level of heat dissipation, the Heathkit AA-30 project I recently did can produce twice the high quality audio power that a high quality 6-7 watt SE design can, so this level would appear to be the practical limits for a SE effort.

And yet, the sonic results of this Magnavox project continues to amaze. I hope that all who decide to implement these modifications will report back with their results!

Dave
 
Most of my 8's read @6.5 for DCR but I'd venture most 4's would read 3 -3.5 which would match the original OPT's @3.2 ohms. But the DCR is really only good for trying to figure out what you have if the speaker is unmarked.

Without changing the OPT's it'd still be best to put a scope on it with the 8 ohm load and tweak the FB line with the right cap, then get the input impedance set for a modern source, and then do the biasing if you intend to drive it hard.
Agreed, spend a bit more time on the bench and save some $ is always a good thing in my book.:D
 
"As for anything else of a SE nature in the future? I would say likely so. I always try to approach sound quality using sound engineering doctrine. I believe it can be achieved with the right approach from a variety of topologies. But as I have stated before, the need to have copious amounts of power to achieve it (i.e. the Tim Taylor syndrome) has waned in me somewhat over the years -- although I do keep plenty of it close at hand to keep that itch scratched. However, when this trend in power reduction is coupled with the fact that my downstairs audio facilities have no A/C (they really don't need any), it really makes SE designs even more attractive: Even though no A/C is needed, I am always mindful of the amount of heat my equipment is throwing off versus the practical need I really have for reserve audio power. In the summertime, heat dissipation can build up rather quickly down here without any active means to remove it, so in that light, SE operation makes even more sense."

It is much greener embracing a low power SE amp. Parts count is minimal, power consumption much less than push pull amps. And you are recycling what used to be an item for the wheelie bin. Oh, saving on the AC bills too. Plus great sound quality making all that eco awareness much less of a chore.

But, I wanted to ask if you would be willing to implement replacing the cathode bias resistor with a LM317t voltage regulator to set the bias. It seems to be used quite often in this type of an amp. I have tried it in a few amps and it seems to have a positive effect but I have no means of quantifying that. I can send you some along with some resistors if you want to try those on your amp. cheers, primo
 
Back
Top Bottom