More Fun With Magnavox: The 9300 Series

dcgillespie

Fisher SA-100 Clone
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As my thread on the 8600 series of Magnavox amplifiers was winding up, I was contacted by fellow AKer Kidmoe, who was willing to donate a completely stock 9300 series unit for the purpose analyzing, dissecting, and otherwise going over every aspect of the design in this series from Magnavox. The Kid came through too as not long after we talked, a package showed up containing a well packed 9300 series amplifier, complete with (by all appearances) the tubes as he originally received it. I told him I didn't know when I'd get to it, but that I would make good on the deal, and so the saga starts now. However, this one will be handled a little differently.

Unlike the previous effort that was presented as virtually a completed project from the outset, this one will move along in real time -- which is not to say real slow, but move along on a piece by piece basis, running along in tandem with various other projects, including the holidays that are virtually upon us. With this first entry then, a base line of stock performance will be presented, along with a highly recommended (but simple) modification to this amplifier, if not already performed. As the thread unfolds then, the various sections will be addressed until an optimized design emerges. Going forward, the amplifier will serve as a test cell for future ideas for these amplifiers.

As before, to offer the most to the most, all modifications will stay within the confines of the existing chassis, and the basic topology of the design (i.e. a single small signal tube and push-pull output stage per channel with common power supply) will remain intact. Also, the original power transformer will remain in place as well. In other words, the thing has still got to resemble and represent the essence of what the original 9300 series was designed to be. Beyond that however, pretty much everything else is fair game. Those of you who want to go outside of those constraints have at it, but all who follow are encouraged to participate.

With that then, what arrived was a well preserved 9302-00 unit, that required little effort to get up and running again. The can cap reformed itself within a matter of a minute or two, and the tubes -- most of which were still original -- all tested quite strong -- a testament if anything, to Magnavox's desire to ensure long tube life in their products.

Before powering up, the OPTs were measured and in fact found to be wound for a 4 Ohm load, as is in keeping with most Magnavox products. The primary impedance of both transformers was measured at 7628 Ohms when the secondary is loaded with 4 Ohms. Components were also tested, with all the important ones within their rated tolerance to permit testing for base line development.

Power was first applied with the rectifier tube removed, so as to take a reading of the heater voltage applied to the tubes. At 7.0 vac, that is excessive, and is certainly enough to reduce tube life, being a direct result of today's higher AC line voltage levels. Fortunately, as with the 8600 series project, this amplifier included the Molex plug to power a separate tuner/preamp chassis, which meant an extra heater winding is available for buck service to tame the high line voltages. For those with these amplifiers who have not utilized this winding to buck the AC line voltage, I highly encourage it for the health of your tubes, and the power transformer as well.

The modification is easy to make:

1. Disconnect the brown and brown w/tracer transformer leads from the Molex plug.

2. Disconnect (or isolate) the black transformer primary lead.

3. Connect the black and brown w/tracer transformer leads together. There should be enough terminals on the nearby T-strip to dedicate one for this purpose.

4. The brown transformer lead then becomes the new transformer lead to apply one side of the AC power to (the red/black being the other).

This modification produces a heater voltage of 6.40 vac, and a B+ at the OPT CT connection of 318 vdc -- all very much in line with the original design center operating voltages with the unit operating from a 121 vac line at my locale.

Of course, it is assumed that you have already configured the unit so it will power on without the need for the external tuner to do so. If not, that information is readily available from many places, and simple to do. In converting it to stand alone operation, an on/off switch is usually added, and a 2 Amp Fast Blow 125 volt fuse should be added as well -- things I have yet to do to the development unit.

For the purposes of all the performance tests run then, they were performed with the transformer buck connection in place, and all the wiring removed from the Molex plug. Otherwise, it was absolutely bone stock. This model also includes the balance control onboard, so it was adjusted to provide an equal resistance from each outside terminal to ground, while the hum balance control was centered.

Since many use this amplifier with the original OPTs, but with 8 Ohm speakers, information is given for both loading conditions. The results are as follows:

1. POWER OUTPUT in RMS watts, both channels driven, average both channels (presented in a 4 Ohm/8 Ohm format):

40 Hz -- 5.29/4.65

100 Hz -- 7.84/8.2

500 Hz -- 8.70/7.80

1 kHz (ref) -- 10.24/9.25

5 kHz -- 8.12/7.51

10 kHz -- 3.40/5.28

Comment: From this you can see that power is concentrated in the mid-band as would be expected, with the amplifier honestly rated as a 20 watt stereo amplifier, 10 watts per channel, with a power bandwidth of basically 45 Hz to 10 kHz (half power points).

2. FREQUENCY RESPONSE: (average both channels)

20 Hz -- -7.0 db

45 Hz -- -1.0 db

1 kHz (ref) -- 0.0 db

10 kHz -- +2.0 db

20 kHz -- +3.5 db

30 kHz -- +3.8 db

45 kHz -- -4.5 db

Comment: Coupling caps are often increased in value in this design, which will certainly help on the low end of things, but will do nothing for the rising response on the high end.

3. HUM AND NOISE (input shorted) -86 db below 10 watts each channel, hum adjustment set for lowest noise. The addition of a temporary bottom plate improved this figure by 2 db.

4. NFB: (measured) 12 db (average both channels)

5. PHASE INVERTER BALANCE: (average of both channels)

Open loop (NFB disconnected) 82.9%

Closed loop (NFB connected) 87.1%

With this large of an imbalance 1 kHz THD was well over 1% at 1 db below 10 watts (7.8 watts), so no further distortion tests were made, as they would all rise from this point. Clearly, phase inverter balance is a point of opportunity in this design.

6. SQUARE WAVE RESPONSE: With a 10 kHz waveform, rise time is very slow, with significant overshoot, and ringing across the wave top.

7. STABILITY: Any practical value of capacitance would not cause instability when loaded into a 4 Ohm resistive load. Under conditions of no load however, less than .01 uF connected across the output terminals would cause sustained oscillation. This is not inconsistent with other power amplifiers of more prominence, but for maximum stability, the ringing should be equal on both the top and bottom of the waveform.

Pics include:

SAM_1574.JPG
A closeup of the transformer connections to provide buck action.

SAM_1577.JPG
The complete underside before testing. The only modifications are the removal of the Molex wiring and creating the buck connection.

SAM_1579.JPG
Ready to start testing.

SAM_1573.JPG
A 10 kHz square wave into a 4 Ohm resistive load.

SAM_1582.JPG
The left channel has only a .005 uF cap connected to it, with any more capacitance causing it to spill over an oscillate.

So there you have it. Other things can certainly be tested such as internal output impedance, channel separation, and the like. But this will do for kicking things off. We'll start getting into it next time.

Dave
 
I always enjoy reading about your projects Dave. Looking forward to how this one works out.

Scott
 
Yes! Haven't been able to find an 8600, maybe I can find one of these and follow along.
 
Just out of curiosity, what value are the feedback resistors in this? There are typically 2 different values, 2.2k or 4.7k I think. It would be interesting to see what switching between them actually does. It sounds arguably better to me with less feedback, but I've never done any measurements on the thing to have a guess as to what it does.
 
This unit has the ubiquitous 750 Ohm balance control, with the wiper grounded. Off of each of the outside terminal of this control is a 2.2K resistor to the cathode of the input AF amplifier stage, and a 3.9K FB resistor to the 4 Ohm output connection. The resistors can be seen at the bottom of the pic showing the complete underside . When the balance control is centered, the NFB is identical in both channels. At either extreme, the NFB is completely eliminated -- not a good thing!

Dave
 
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gREAT dAVE.
Just what I have waited for.
mY 2ND TUBE AMP REFERB WAS TO A 9303 which I still have and love it dearly.
SQ is wonderful on all 8ohm spkrs of hi eff 90db+ I have hooked it to.
It has all new Rs and Cs.
I will be doing any and all mods you come up with for this amp as it deserves it.
Will follow your thread closely.
 

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gREAT dAVE.
Just what I have waited for.
mY 2ND TUBE AMP REFERB WAS TO A 9303 which I still have and love it dearly.
SQ is wonderful on all 8ohm spkrs of hi eff 90db+ I have hooked it to.
It has all new Rs and Cs.
I will be doing any and all mods you come up with for this amp as it deserves it.
Will follow your thread closely.
 

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    9303018.jpg
    122.8 KB · Views: 558
I think the 9302 was the best 6BQ5 console amp Maggy made, that I've seen. Great iron for my old ears and speakers. They sure didn't waste any time on lead dress though, no twisted heater wires or anything.
 
Dave, 10 wpc seems a little low for pp 6BQ5. I wonder if more power will be available with optimization.
 
I think the 9302 was the best 6BQ5 console amp Maggy made, that I've seen. Great iron for my old ears and speakers. They sure didn't waste any time on lead dress though, no twisted heater wires or anything.
And yet its still dead quiet, mine is anyway. :music:
 
They are very quiet as the measured spec shows -- and much credit for that can be given to the choke included in this model, which I'm sure plays into Jay's favoritism of this unit (among other things) -- but it can be even quieter. As to the power output, remember that the measured power I recorded is both real world power, meaning it is with both channels operating, and with intended operating voltages as well. In the regard then, power is on the lowish side, but there is news in that department that is underway even now........

Dave
 
6 -- Now now, calm down and take deep breaths....... Tubes were all very good, arranged for best match (which was pretty good), and components determining symmetry of the push-pull signal all within their tolerance as specified/installed by Magnavox, as were all of the important components. No doubt that close tolerance parts may improve the measured results, but not in any significant way. The performance results achieved is largely due to the design, not the passive components used.

Dave
 
Off of each side of the wiper is a 2.2K resistor to the cathode of the input AF amplifier stage, and a 3.9K FB resistor to the 4 Ohm output connection.

Its the 3.9k I was thinking of. I think the other possible value is 2.2k, but it changes that cathode resistor value as well.
 
Dave, it will be interesting to find out what the optimal resistor ratio for the phase inverter input will be on these.

If you remember- the AMP175 that I brought over to Steve's house- I had installed adjustment pots in the inverters, for balance. That seemed to make a difference, when it was set right, as we got about 11 solid watts out of that thing in the midband- and that was with 6V6s, which should be a little down on power compared to the EL84 amp (the 93xx amps have larger output transformers than the AMP175 series)...

I definitely wonder, that even with stock OPTs, if it can get significantly closer to 15 watts per channel, with proper inverter balance.

I also wonder, if the lower B+ (due to the bucking transformer winding) is dropping available power a bit. It would be interesting to just try dropping the heater voltage with power resistors, and letting the B+ run as high as it can, with the power transformer primary run straight-in. Though, with the power drop-off in the bass already, that may be an indication that the OPTs are already "tapped out", even at the lower B+...

Regards,
Gordon.
 
I also wonder, if the lower B+ (due to the bucking transformer winding) is dropping available power a bit. It would be interesting to just try dropping the heater voltage with power resistors, and letting the B+ run as high as it can, with the power transformer primary run straight-in. Though, with the power drop-off in the bass already, that may be an indication that the OPTs are already "tapped out", even at the lower B+...

Gordon, If the goal was to possibly raise the B+ to change the operating point why not just sub a 5AR4 for the stock 5U4? If the voltage even with buck winding is currently at around 320 the 5AR4 should put the B+ in the 350 range and also keep the heater voltage nice and low without additional circuit changes.
 
Hold onto all of those thoughts gentlemen, much of that is already in the works, and then some.....

Dave
 
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