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More Fun With Magnavox: The 9300 Series

dcgillespie said:
Leland -- The 5U4GB is a quick heating (employing no cathode) type of rectifier tube, that begins to conduct literally in a matter of 3 seconds or so. Since this is well before the audio tubes heat up and begin to draw current, the initial B+ at turn on rises very high -- to over 400 vdc, which is why an amplifier that was designed to originally operate at 300 vdc required 450 volt filter caps -- to handle the turn on surge. But also however, the 5U4 produces a rather high voltage drop across it, which only grows when power output is increased. So in effect, when you don't want the voltage, it's too high, and when you do want it, it's too low.

Against that backdrop, the 5AR4 is a much improved rectifier tube. It is a slow heating cathode type rectifier tube, so the turn on surge is completely eliminated. As the tube warms, the audio tubes are also nearly warmed, so there is no voltage spike. But also, the voltage drop produced by the 5AR4 is greatly reduced over that of the 5U4. This tends to make general voltages higher, but neither will they drop as much when extra current is needed by the amplifiers, either. This allows the amplifiers to put out more power with less distortion, since the regulation of the power supply is better. It also requires less current to power its heater as well, so it is simply a better tube all the way around for the application. At the time the console was produced, the 5AR4 was a relatively expensive tube, compared to the dirt cheap 5U4. The bean counters will always win that game.......
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I've read the whole thread, but I just want to clarify/verify: If I have a restored (but not significantly modified) 9300 amp, can I drop a 5AR4 (say, a new Sovtek) in place of the 5U4 and expect satisfactory performance? I understand that the advantages are no turn-on "spike" and less sag under load? Any concerns?

Edit: Yes, the tuner winding is wired as a bucking winding, with new filament voltage around 6.3v with my relatively high line voltage.

Thanks!
 
dcgillespie said:
4. Your transformer however is frankly all over the map, leading me to question the ultimate quality of the transformer. Edcor is one of those brands that everybody always "says" is great -- but who has any hard data to back this up? There is absolutely none that I know of. In any event, with 8 ohms being the mainstay of most replacement transformer designs, I think it's fair to say that your transformer is intended to be a 10K primary -- particularly if you are actually using the 8 ohm tap -- and that it is the reason for your power loss.
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Just curious as to what you would recommend from current manufacturers as good / great transformers, Dave (especially in the 'beer budget' range).

Thanks!

-D
 
To be fair, I haven't gotten around to re-measuring that transformer since I got a voltmeter that wasn't full of lies. Its very possible that the Edcor does actually present itself OK with an accurate means of measurement.
 
I agree with MP -- Beer budget could be anything from the original transformers, to garden variety Hammond. I have been thinking about Transcendar from the get-go for this unit. I want to contact them first regarding some of their specifications, but on the whole, I think their product looks great.

Dave
 
I may give this a whirl with my 9304-20. It's already "rebuilt" with upgraded caps, resistors, IEC power plug, fuse, RCA and speaker jacks,etc. But, if I can get a little more out of it, that's always a good thing. I'll use either my Stromberg Carlson ASR-120 or Carver M-1.0t MkII in the interim. I'm not as knowledgeable as any of you in the tube forum on rebuilding, testing, etc., of these amps but I c an reasonably follow a diagram and pictures and have soldered for 30+ years between building crossovers, recapping amplifiers, fixing computer server boards, etc. Hopefully I can figure this out. I'm going to replace the rest of the sockets as I've done a few on this one that were having issues.

Is it worth it to go ahead and rewire the sockets or just stay with the same wire? Also, are there better caps to use than the orange drop caps I have in it? Or will changing make that much of a difference?

After swapping out the Magnavox for the S/C ASR-120, I may have to expedite the project some. In comparison between them having both been equally updated on parts, the S/C sounds better still. More low end, stronger midrange, etc. Hmmm.... Maggie sounds good, though.

Dave, thanks for sharing your knowledge of tube amplifiers with everyone so freely and the help you offer as they go. You're an asset to the community!
 
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How does the "enhanced fixed bias" circuit compare with Lincoln Walsh's "Automatic Bias Control' . They seem to have very similar design goals and results. I must say using a 3 pin regulator to achieve the regulation is an excellent idea!
 
Target -- Both EFB(tm) and the Lincoln Walsh concept are similar, in that they are both a servo circuit of sorts, and both provide control of output stage bias voltage. But beyond that however, there is nothing similar about them at all in what they are attempting to achieve.

The Lincoln Walsh circuit would appear to be the first effort (to my knowledge) of applying a servo circuit to maintain a constant quiescent current through the output stage, eliminating the need to adjust the bias every so often as the output tubes age. With the ease that modern SS components allow for such circuits to be devised, a number of modern vacuum tube amplifiers use this very same basic servo concept to control quiescent output tube bias, often using a series of LED lamps to show that the bias is either in or out of range.

With EFB however, the concept is quite different. Here, servos are used as well -- for the control of both the control, and screen grids when applicable -- but in the case of EFB, the servos are not working to control grid bias based on current flow through the tubes (which of course is basically self defeating), but working to hold the control and screen grid potentials in relative lockstep with that of the plate supply voltage. In this way, even though the plate supply voltage is not regulated, the optimum operating point -- as established under quiescent conditions -- is maintained under all possible quiescent and dynamic conditions. Therefore, like cathode bias, the tubes are automatically compensated as the AC line voltage changes, but very much unlike cathode bias or the Lincoln Walsh concept, EFB does not limit current flow through the tubes with increasing power output demands. Therefore, with EFB, both power output and distortion are optimized, and in most cases, quiescent current flow is also reduced, resulting in cooler operation, and longer tube life.

The installation of EFB in this project closely follows my original launch of EFB as described in my white paper on the subject, and as implemented in the small Dynaco products (SCA-35 and ST-35), where by both EFB action and conversion to fixed bias operation is achieved through the use of a simple 3 terminal regulator. EFB can also be implemented into designs already employing fixed bias operation, and as such, has been depicted in a number of Fisher products over on the Fisher forum.

Thanks for the interest!

Dave
 
Looks like the L-W circuit only samples the cathode current draw and corrects for it rather than tracking the bias voltage as a function of screen voltage.

That is an interesting circuit, and one I'd not seen before though. Probably works a treat on triodes where there is no screen voltage to be concerned about. Looking at the original patent drawings, that looks like 2A3 or similar tube types, back in the 30s with interstage transformers for the inverter function.

**edit** ok, filed 1941. Close enough.
 
Ok thanks Dave,
I think I see the difference now. Bias as a function of B+ vs Bias as a function of quiescent current in the output tubes. Basically with EFB, if i'm following correctly, at all times the ratio of Vp to Vs to Vc is maintained and this is done by regulating Vc in ratio to Vp instead of regulating Vp directly. I suppose a nice stiff B+ would have a similar effect. IE the better the power supply of the amplifier the less impact full this will be. In the case of the lowly 9300 i'm sure it benefits a great deal. It would be interesting to see how it would compare if you pulled the 5u4 and replaced it with a regulated b+. With a well bypassed cathode resistor the effects should be the same ... no?
 
Dave - awesome thread. This is great stuff to read about.

I'm sending you a PM - I'd love some deets on your dummy load setup. Looks rather slick.

Mike
 
Target -- You are correct in that the stiffer that the main B+ supply becomes, the less EFB action takes place -- to the point that if the main B+ is perfectly regulated, then the EFB regulators would also perfectly regulate Vc and Vs, so that no potential change would occur for any tube element.

However, please consider that while a well bypassed cathode bias resistor will help -- and is certainly required -- for any cathode biased Class AB1 amplifier to minimize the changes in bias produced under dynamic conditions, it can do nothing to address the fact that as average power output increases, so does the average current draw across the cathode resistor. Increased average current draw equates to a general increase in bias voltage at elevated power output levels, in the case of this project, leading to the output stages producing notable cross-over distortion before clipping is even reached. No amount of cathode bypass capacitance will solve this problem.

The installation of EFB completely solves this problem, by operating the output stages with true fixed bias, and then maintaining the optimum operating condition, even when the power supply droops under elevated dynamic conditions.

As produced, the Magnavox 9300 was designed with the idea of high power output levels never being required. For the output stages to really operate properly, they either needed to employ fixed bias for the output loading conditions employed, or, raise the plate-to-plate loading conditions for the bias system employed. Of course, the former is more expensive which sends the bean counters into orbit, while the latter reduces power, which sends the marketing department to their anxiety closets. The poor engineers must have just shrugged their shoulders, installed some efficient speakers, and accepted the limitations of the design. For what the product was intended to be, the compromise certainly worked well enough based on the number of units sold. But when laid bare for all to see, the compromise is significant for those trying to seek the most out of these amplifiers.

Dave
 
Thanks Dave.
So when you say "as average power increases" i read that as "as the amp moves into class ab operation" the average or DC component of the cathode moves the bias more negative and in the case of this amp would actually shut the tube off prior to the complimentary tube being able to "take over" the load. I think I follow.

I also think that is class B not AB.

I also see how your EFB design corrects this. Brilliant!

i'm so impressed i may pull down my 9300 and see what i can do with it. More likely however I have a Heathkit A9B that gets quite a bit of use. It never goes into Class B operation but it does get quite a bit of use and I think with your EFB circuit I can cut down on the necessary current draw and make things easier on the outputs. It is a Very Flawed amplifier I've been toying around with :).

Hopefully folks are paying close attention to this thread. I think you've demonstrated very well how limited these Magnavox console amps really are. That is nothing against the engineers nor the company that created them. The were built fully capable to perform the task they were given. Drive some very efficient 4 ohm speakers in a console environment witch included a phonograph. The consoles sound great, the amps on there own are not very good at all. However, thanks to you it seems there may be a very easy and cheap way to bring them up a large notch in performance.

Steve
 
i've been thinking some more about this project. IMHO if new output transformers are in order i think they should be cheap in order to stay in line with concept of taking a cheap console amp (unless you buy one on ebay!) and doing the most you can inexpensively. There seem to be some folks doing interesting things with cheap Line matching transformers and push pull outputs.

By the way. I am completely blown away by the prices some of these amps are bring on ebay. i'm seeing them sell for as much as $400!! What the heck? Perhaps the best way to upgrade this thing is sell it and invest in integrated amp in need of restoration :).

Steve
 
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I'm sort of interested in where the transformer thing ends up as well, but for a different project. There is a 6bq5 console amp sitting around here that has 2.5 ohm outputs on it, and that just isn't useful. Space is limited on it, and it needs to be something budget-minded as well. I suspect if something reasonably smallish and inexpensive works on the Magnavox, it will probably work on this.
 
targeteye said:
...I am completely blown away by the prices some of these amps are bring on ebay. i'm seeing them sell for as much as $400!! What the heck? Perhaps the best way to upgrade this thing is sell it and invest in integrated amp in need of restoration :).
Steve
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It was well over an year that I didn't check these (too busy with work) and you are right, prices o these console amps are skyrocketing (good as I have 4 magnavox amps). My advice for buying for less is to buy a complete console (sometimes they sell for $100-150) and then have only the amp shipped, of course you need to know a bit the console models or see a pic of the internal. I bought two of them very cheaply in this way a few years ago.
 
i'm seeing them sell for as much as $400!!

You may as well build a Dynaco ST-35 clone or similar for that kind of $ as the Output Transformers determine largely the sonics of a tube amp.
 
Well i have a 9302-10 setting on the work bench in front of me. Its a particularly nice one with virtually no corrosion and magnavox branded telefunken output tubes. I keep looking at that open tube socket hole thinking about another dual triode and a better phase inverter. Adding traditional fixed bias and perhaps doing something with the power supply. Or just cloning what Dave did. Then I realize when i'm done. I have no use for it :).

To the right is a Pilot 505 that was bent up by the monkies at one of the shippers (don't remember which one). They lost it, then returned it like this ... nice of them don't you think? It was equally as nice at one time. Interestingly the outputs on both of the amps are physically the same size. The power tranny on the pilot is huge and makes the outputs look smaller than they are.

I keep looking at these things trying to get motivated.

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Magnavox Telefunkens? First I've heard of that, but it seems Magnavox branded anyone and everyone's tubes. Seems like everyone has different tubes in theirs.

Did that Pilot originally pair with a tuner or something? That is a massive power transformer. Seems excessive for just an EL84 power amp.
 
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