When I posted my original thread on the 8600 series, I had absolutely no idea of the interest it would generate in the AK tube community. Then when AKer Kidmoe asked me to go through a 9300 series model he'd donate for the cause, the undying love for Magnavox console amplifiers was again confirmed and reinforced. The listening pleasure these amplifiers bring is absolutely undeniable. Well, the Kid has been at it again, this time donating up a totally stock, unmolested 8800 series amplifier for analysis, dissection, pondering, and general "what if" thinking, original tubes and all. It arrived here back in November of last year, and as before, I told him I didn't know when I'd get to it, but that I would in fact get to it. That effort has move along in the slow lane as the holidays and life events all took stage front and center, but it has moved along, and now to the point that I can get a thread started. Along the way, a lot of information has been gleaned.
Initial Observations -- stock unit
Others will certainly know more than I, but it would appear that the 175/185 6V6 amplifiers were Magnavox's first stereo vacuum tube push-pull amplifiers, and the 9300 series (6BQ5) was the last, with the 8800 6V6 series sandwiched in between these two models, almost certainly acting as a transitional piece while manufacturing converted from the larger octal tubes, to the smaller 9-pin miniature tubes. As such then, the 8800 is most certainly the bottom feeder runt of the Magnavox vacuum tube stereo litter. Consider:
1. Power Supplies: Both the 175 and 9300 units have power supplies of higher voltage and current capability, employing heavy duty rectifier tubes and a choke for B+ filtering. The 8800 series utilizes a smaller medium capability rectifier tube that is operated near maximum current limits with R/C filtering only. The 8800 also uses the same filter cap as used in the 8600 Single Ended amplifier series. The end result is that B+ capacity and filtering is quite limited in the 8800 series, versus that of its more capable siblings.
2. Operating Conditions: Magnavox is well known for designing their equipment for extended tube life. In the 8800 series, this was pushed to the max by operating the output tubes at a mere 20 mA of quiescent current each. With each output tube operating with a plate dissipation of less than just 4.5 watts then, the unit operates with high output stage distortion (on the edge of cross-over distortion), which is readily visible on a 1 kHz sine wave scope display at anywhere near approaching maximum power output (6.76 watts RMS per channel into 4Ω, both channels driven). This operating condition was no doubt driven by the limited power supply capability these units had. But by using an inherent characteristic of the 6V6 and a common cathode resistor for all four output tubes, the unit could be kept safely away from producing gross distortion, and so was "good enough" for its application. This will be discussed in more detail later on.
By comparison however, the 175/185 series used an identical biasing scheme, but operated the tubes at a much more appropriate operating point to minimize output stage distortion. And, with a higher B+ voltage, it also produced this lower distortion with greater power output as well. In the 9300 series, the same biasing scheme was also used. And while a reasonable operating point was used in that unit as well, resistor based cathode bias in general is hardly an optimal way to bias the 6BQ5 family of tubes when operating in Class AB, and therefore formed one of the legs upon which improved performance could be produced from that unit. However, with greater power output available in the 9300 over the 8800, the biasing scheme worked well enough at the typical power output levels that console duty would dictate. So with the 8800's operating conditions as well, it is again at a notable disadvantage compared to the other two series of amplifiers.
3. Design Effort: On top of the previous comments, the design itself was almost surely an exercise in achieving a minimal part count. While this would generally be true of any design built to a price point, it seems to have been taken (again) to the max in this unit. While this unit uses different OPTs than that of the 175/185 series, they are no doubt electrically very similar with regards to response characteristics. In the 175/185 however, there was an effort made to stabilize the HF resonances of the OPTs to produce good transient and frequency response. The sensitivity of the design was maximized as well. In the 9300, the choice of tubes helped to maximize sensitivity, and reasonable HF stability was achieved on a case by case basis based on the differing installation scenarios. In the 8800 however, sensitivity was sacrificed, and HF transient and frequency response was basically allowed to run uncontrolled as will be seen in the performance data gathered. The one sole component used to act as a limiting agent for the HF response characteristics is shown as optional on the Sams schematic, and was not installed in my unit here. Once again then, the 8800 is the step child of the family.
4. Output Transformer Impedance: I'm not sure what process Sams used to determine the OPT primary impedance of the transformers used on the 8800, but it could not be more wrong. It's not even close, or anywhere near the ballpark for that matter. The part #320295-1 OPT used on the 8800 chassis has a 50:1 turns ratio, indicating that with a 4Ω secondary load, the reflected primary impedance is 10K Ohms, which would be entirely correct for this application. The 6K Ohm specification that Sams indicates becomes yet another of their classic schematic errors.
5. Sams Schematic C4: Shown as a permanent installation in Channel 1 and optional in Channel 2 on the Sams schematic, it's installation no doubt had to do with the cabling and characteristics of the preamp used in the various console application scenarios. It's installation does act to control somewhat the otherwise run amuck HF response, and the HF transient response (somewhat) as well -- but even still, with its installation, both could hardly be considered as yet properly controlled. Do note this however: If you operate the stock amplifier into 8Ω speakers, then the installation of C4 in both channels is mandatory to prevent the formation of parasitic oscillations on medium power (and higher) frequencies below 100 Hz.
4. Performance (operating from 115 vac, which produces normal operating voltages):
A. Power Output @ 1 kHz:
At 4Ω: Individually: 8.27 Watts RMS. Both channels driven: 6.76 Watts RMS.
At 8Ω: Individually: 5.28 Watts RMS. Both channels driven: 5.12 Watts RMS.
B. Full Power Bandwidth (one half power) based on individual channel performance:
At 4Ω: 37 Hz - 11 kHz.
At 8Ω: 41 Hz - 15 kHz.
C. Frequency Response (Ref: 1 kHz) without C4 installed:
At 4Ω: @ 20Hz: -.65 DB, @ 30Hz: +1.0 DB, @ 20kHz: +1.2 DB, @ 47kHz: +6.5 DB, @ 62kHz: -1.0 DB.
At 8Ω: @ 20Hz: -.70 DB, @ 27Hz: +2.4 Hz, @ 20kHz: +.80 DB, @ 55kHz: +9.2 DB, @ 72kHz: -1.0 DB.
With C4 installed:
At 4Ω: @ 20kHz: +1.0 DB, @ 30kHz: +2.8 DB, @ 36kHz: +3.4 DB, @ 49kHz, -1.0 DB.
At 8Ω: @ 20kHz: +1.4 DB, @ 30kHz: +3.7 DB, @ 42kHz: +7.3 DB, @ 57kHz: -1.0 DB.
D. Sensitivity:
At 4Ω: 1.20 vac rms for maximum power output.
At 8Ω: 1.10 vac rms for maximum power output.
E. Total Harmonic Distortion at 1 kHz at:
At 4Ω: At 1 db below 6.76 watts rms: 1.4% (2.4% at the onset of clipping).
At 8Ω: At 1 db below 5.12 watts rms: .8% (unchanged at the onset of clipping).
F. Stability: Unloaded to .05 uF.
G. Negative Feedback:
At 4Ω: 11.9 DB
At 8Ω: 16.3 DB
H. Internal Impedance:
At 4Ω: 1.26Ω
At 8Ω: 1.30Ω
A few pics:
Below: In progress: Basic components checks, setting the unit up for proper AC power (connecting the bucking winding), and removing the needless wiring to the Tuner Power Plug.
Below: The unit operating for initial testing.
Below: Oscillation into 8Ω at 90 Hz (without C4):
Below: 10 kHz Square wave into 4Ω without C4:
Below: 10 kHz Square wave into 4Ω with C4:
So there you have it. 5 or 6 watts of power depending on what impedance speakers you're using, potential parasitic oscillation, HF transient stability that's anything but damped, and 1 kHz distortion that leaves you wondering if it's that high at 1 kHz, how much higher must it be at more demanding frequencies? At any rate, sandwiched between the 175/185 and 9300 series, the 8800 series -- by comparison -- is hardly Magnavox's best effort with these types of amplifiers, leading me to believe it was considered to be a transitional piece.
There is help for these things however, and all without breaking the bank. Of course, to allow the 8800 to really compete against anything in its (potential) power class, a new OPT will be needed. Luckily, there's an answer for that too -- if you are so inclined to go that far.
For now, the unit has undergone my usual approach to these projects: One channel remains completely stock (as much as possible), while the other is modified to investigate all the potential opportunities available. Those will be discussed next time.
Dave
Initial Observations -- stock unit
Others will certainly know more than I, but it would appear that the 175/185 6V6 amplifiers were Magnavox's first stereo vacuum tube push-pull amplifiers, and the 9300 series (6BQ5) was the last, with the 8800 6V6 series sandwiched in between these two models, almost certainly acting as a transitional piece while manufacturing converted from the larger octal tubes, to the smaller 9-pin miniature tubes. As such then, the 8800 is most certainly the bottom feeder runt of the Magnavox vacuum tube stereo litter. Consider:
1. Power Supplies: Both the 175 and 9300 units have power supplies of higher voltage and current capability, employing heavy duty rectifier tubes and a choke for B+ filtering. The 8800 series utilizes a smaller medium capability rectifier tube that is operated near maximum current limits with R/C filtering only. The 8800 also uses the same filter cap as used in the 8600 Single Ended amplifier series. The end result is that B+ capacity and filtering is quite limited in the 8800 series, versus that of its more capable siblings.
2. Operating Conditions: Magnavox is well known for designing their equipment for extended tube life. In the 8800 series, this was pushed to the max by operating the output tubes at a mere 20 mA of quiescent current each. With each output tube operating with a plate dissipation of less than just 4.5 watts then, the unit operates with high output stage distortion (on the edge of cross-over distortion), which is readily visible on a 1 kHz sine wave scope display at anywhere near approaching maximum power output (6.76 watts RMS per channel into 4Ω, both channels driven). This operating condition was no doubt driven by the limited power supply capability these units had. But by using an inherent characteristic of the 6V6 and a common cathode resistor for all four output tubes, the unit could be kept safely away from producing gross distortion, and so was "good enough" for its application. This will be discussed in more detail later on.
By comparison however, the 175/185 series used an identical biasing scheme, but operated the tubes at a much more appropriate operating point to minimize output stage distortion. And, with a higher B+ voltage, it also produced this lower distortion with greater power output as well. In the 9300 series, the same biasing scheme was also used. And while a reasonable operating point was used in that unit as well, resistor based cathode bias in general is hardly an optimal way to bias the 6BQ5 family of tubes when operating in Class AB, and therefore formed one of the legs upon which improved performance could be produced from that unit. However, with greater power output available in the 9300 over the 8800, the biasing scheme worked well enough at the typical power output levels that console duty would dictate. So with the 8800's operating conditions as well, it is again at a notable disadvantage compared to the other two series of amplifiers.
3. Design Effort: On top of the previous comments, the design itself was almost surely an exercise in achieving a minimal part count. While this would generally be true of any design built to a price point, it seems to have been taken (again) to the max in this unit. While this unit uses different OPTs than that of the 175/185 series, they are no doubt electrically very similar with regards to response characteristics. In the 175/185 however, there was an effort made to stabilize the HF resonances of the OPTs to produce good transient and frequency response. The sensitivity of the design was maximized as well. In the 9300, the choice of tubes helped to maximize sensitivity, and reasonable HF stability was achieved on a case by case basis based on the differing installation scenarios. In the 8800 however, sensitivity was sacrificed, and HF transient and frequency response was basically allowed to run uncontrolled as will be seen in the performance data gathered. The one sole component used to act as a limiting agent for the HF response characteristics is shown as optional on the Sams schematic, and was not installed in my unit here. Once again then, the 8800 is the step child of the family.
4. Output Transformer Impedance: I'm not sure what process Sams used to determine the OPT primary impedance of the transformers used on the 8800, but it could not be more wrong. It's not even close, or anywhere near the ballpark for that matter. The part #320295-1 OPT used on the 8800 chassis has a 50:1 turns ratio, indicating that with a 4Ω secondary load, the reflected primary impedance is 10K Ohms, which would be entirely correct for this application. The 6K Ohm specification that Sams indicates becomes yet another of their classic schematic errors.
5. Sams Schematic C4: Shown as a permanent installation in Channel 1 and optional in Channel 2 on the Sams schematic, it's installation no doubt had to do with the cabling and characteristics of the preamp used in the various console application scenarios. It's installation does act to control somewhat the otherwise run amuck HF response, and the HF transient response (somewhat) as well -- but even still, with its installation, both could hardly be considered as yet properly controlled. Do note this however: If you operate the stock amplifier into 8Ω speakers, then the installation of C4 in both channels is mandatory to prevent the formation of parasitic oscillations on medium power (and higher) frequencies below 100 Hz.
4. Performance (operating from 115 vac, which produces normal operating voltages):
A. Power Output @ 1 kHz:
At 4Ω: Individually: 8.27 Watts RMS. Both channels driven: 6.76 Watts RMS.
At 8Ω: Individually: 5.28 Watts RMS. Both channels driven: 5.12 Watts RMS.
B. Full Power Bandwidth (one half power) based on individual channel performance:
At 4Ω: 37 Hz - 11 kHz.
At 8Ω: 41 Hz - 15 kHz.
C. Frequency Response (Ref: 1 kHz) without C4 installed:
At 4Ω: @ 20Hz: -.65 DB, @ 30Hz: +1.0 DB, @ 20kHz: +1.2 DB, @ 47kHz: +6.5 DB, @ 62kHz: -1.0 DB.
At 8Ω: @ 20Hz: -.70 DB, @ 27Hz: +2.4 Hz, @ 20kHz: +.80 DB, @ 55kHz: +9.2 DB, @ 72kHz: -1.0 DB.
With C4 installed:
At 4Ω: @ 20kHz: +1.0 DB, @ 30kHz: +2.8 DB, @ 36kHz: +3.4 DB, @ 49kHz, -1.0 DB.
At 8Ω: @ 20kHz: +1.4 DB, @ 30kHz: +3.7 DB, @ 42kHz: +7.3 DB, @ 57kHz: -1.0 DB.
D. Sensitivity:
At 4Ω: 1.20 vac rms for maximum power output.
At 8Ω: 1.10 vac rms for maximum power output.
E. Total Harmonic Distortion at 1 kHz at:
At 4Ω: At 1 db below 6.76 watts rms: 1.4% (2.4% at the onset of clipping).
At 8Ω: At 1 db below 5.12 watts rms: .8% (unchanged at the onset of clipping).
F. Stability: Unloaded to .05 uF.
G. Negative Feedback:
At 4Ω: 11.9 DB
At 8Ω: 16.3 DB
H. Internal Impedance:
At 4Ω: 1.26Ω
At 8Ω: 1.30Ω
A few pics:
Below: In progress: Basic components checks, setting the unit up for proper AC power (connecting the bucking winding), and removing the needless wiring to the Tuner Power Plug.
Below: The unit operating for initial testing.
Below: Oscillation into 8Ω at 90 Hz (without C4):
Below: 10 kHz Square wave into 4Ω without C4:
Below: 10 kHz Square wave into 4Ω with C4:
So there you have it. 5 or 6 watts of power depending on what impedance speakers you're using, potential parasitic oscillation, HF transient stability that's anything but damped, and 1 kHz distortion that leaves you wondering if it's that high at 1 kHz, how much higher must it be at more demanding frequencies? At any rate, sandwiched between the 175/185 and 9300 series, the 8800 series -- by comparison -- is hardly Magnavox's best effort with these types of amplifiers, leading me to believe it was considered to be a transitional piece.
There is help for these things however, and all without breaking the bank. Of course, to allow the 8800 to really compete against anything in its (potential) power class, a new OPT will be needed. Luckily, there's an answer for that too -- if you are so inclined to go that far.
For now, the unit has undergone my usual approach to these projects: One channel remains completely stock (as much as possible), while the other is modified to investigate all the potential opportunities available. Those will be discussed next time.
Dave
Last edited: