EPILOGUE
The TA finished a 24 hour burn this morning without incident, acting and performing at the end of the period the same way it did at the beginning. At the end of the test, the power transformer temperature was stabilized at 135F -- certainly quite warm to the touch, but quite cool by transformer standards. This was with no forced air to aid its cooling, in an ambient condition of 77F. There is no doubt that the transformer's temperature would raise significantly from this point if installed in a cabinet, so again, forced air cooling of the unit when enclosed is the name of the game.
I was able to complete the final performance testing of the unit this evening, which returned results that were beyond expectations. Rob might have a little trouble getting this one back!
In the case of the TA-800, comparison of before and after performance is easy enough to do, but any traditional comparison to published specifications is all but impossible since to my knowledge, no owner's manual has surfaced to date to know what any published specifications are. The (hardly exhaustive) search I did on line turned up that a reviewer reported that the TA-800 was advertised as a 32 watt per channel receiver in his Allied catalog. Scant little to go on.
Of course, as has been well established with so many of Fisher's other pieces, that rating is an "IHFM" power rating based on one channel being driven at a time. When these two elements are combined, it results in some rather optimistic (ahem......over inflated) power levels to be sure. Institute of High Fidelity Manufacturers (IHFM) power ratings represent at least a 15% inflation of RMS power levels, and often as much as a 20% increase. This fact alone brings the single channel power output capability of the TA down to an RMS power level of about 27 watts. When you factor in the use of Russian power tubes -- which have improved by leaps and bounds over the years but still none the less only produce about 90% of the power that the original American pieces can -- you end up with an individually driven power output capability of just a little over 24 watts RMS per channel -- which is exactly what I measured under those conditions. When both channels are driven however, power supply voltages sag, so that the single channel power capability of the unit sags as well. In the case of Rob's TA, when both channels of the stock design were driven, the measured power output in each channel fell to 20 watts RMS each. For the record, this was the power produced from the unit with it fully restored to the stock design, as measured during the session when I was gathering my initial information to develop the new driver circuit.
On line, there is no information (that I could find) regarding any testing of a TA-800 for comparison purposes to the data I gathered-- other than one small piece on information gleaned from the AK thread on RS Steve's TA-800 restoration. In that thread, his tech commented that he measured Steve's unit as producing 24 watts in each channel. It is an extremely safe bet that this was measured with only one channel being driven at a time. And while I hardly know the details of that restoration, it is exactly the same power I measured from Rob's unit as well, when each channel is driven individually. Ultimately then, in terms of real world use, the stock TA-800 is really a 20 watt RMS per channel receiver (40 watts RMS total), based on both channels being driven. For lack of any information otherwise then, and because I only measure power in terms of RMS watts with both channels driven, this is the rating I will use to judge the modified results against. Measuring power output in terms of RMS watts with both channels driven is the most honest and accurate way of defining the real work that each channel of a stereo amplifier can produce, and is the rating system ultimately adopted as the standard for use by the FTC in the 1970s.
With an honest power rating of the TA established, there finally becomes something to measure the modified TA against. The other significant point of comparison then is the distortion produced on the power levels measured in the stock design. For reference, recall that I indicated earlier that at 1 kHz, the restored stock unit generated .80% THD at 24 watts RMS when a single channel was driven, and rose to 1.75% at 20 watts RMS when both channels were driven. History has shown however that Fisher customarily specified a distortion level of .5% THD at this frequency for the vast majority of its models, when a single channel is driven to full power output. While Rob's unit would appear to be outside of this spec, the distortion readings given at the time were made with the output tubes idling at a customary 35 mA of quiescent current each -- customary for its bigger 500/800C brothers that is. However, with the generally lower B+ levels that the TA-800 operates at (in comparison to the bigger units), the output tubes in the TA-800 must be set to a quiescent current of 40 mA each, which results in a distortion level of .45% when a single channel is driven to full power output -- in line with Fisher's traditional goal line for distortion performance. At a 40 mA quiescent level, this results in a plate dissipation of 15.75 watts per tube in the TA, which is 82.9% of the 7591's plate dissipation rating. While Fisher never specified output tube quiescent current levels, a 40 mA quiescent current for the TA's output tubes is also supported by the voltages that Fisher specifies these tubes to operate at on their schematic for this model. Unfortunately, even with the output tubes operating at this quiescent current level, when both channels are driven to full power output, distortion still rises to 1.60% in both channels, which certainly won't win any blue ribbons for performance.
Establishing accurate performance data for the stock design relative specifically to power output, distortion, and output tube quiescent current and plate dissipation levels is very important, because it is precisely these parameters that are expected to be positively impacted by the application of Enhanced Fixed Bias (EFB) to the design. And while the new driver circuit has virtually no impact on output tube quiescent current and dissipation levels, an incorrectly designed driver circuit can limit power output, and send distortion levels to the moon. With this backdrop then, just how well does the EFB/7247 driver modified TA-800 perform? In a word, spectacularly! Consider:
1.
Power Output: Single channel driven power output rose to 31.64 watts RMS. And while this is a nice increase in power output (131.8%), it pales to the fact that with both channels driven, each channel now produces a very healthy 30.25 watts RMS -- for an increase of 151.25%. Effectively then, EFB transformed the TA-800 from a 40 watt RMS receiver, into a 60 watt RMS receiver. But there's more.
2.
Distortion: With a single channel driven to a power output of 31.64 watts, THD at 1 kHz is .35%, for a 22.2% reduction in distortion. But as with power output, it is with both channels driven where the benefits of EFB really shine. With both channels driven to 30.25 watts each,
distortion remains at .35%, for a phenomenal 78.1% reduction in distortion. And remember, this is at a power level that is 151.25% greater than the level at which the stock design is already producing 1.60% distortion! The fact that the reduced distortion level does not change whether a single channel or both channels are driven, and occurs at a higher power output level, is a hallmark benefit of EFB action, whereby the EFB circuits automatically correct for sagging power supply conditions, and maintain the optimum output stage operating point at all times, under all conditions.
3.
Output Tube Quiescent Current: Optimum output tube quiescent current was found to be 32.0 mA under EFB control. While this may not appear to be much of a reduction from that which its bigger brothers operate these tubes at, it is a 20% reduction in current flow through the tubes based on that which the stock TA circuit operates them at. This then relates directly to tube life, since the cathode (the element that normally wears out in a tube) emits electrons to support a given current flow. Less current flow means less electrons are drawn from the cathode, meaning that the cathode effectively will have a longer life.
4.
Output Tube Plate Dissipation: With the lower quiescent current flow, plate dissipation under quiescent conditions falls accordingly to 12.39 watts, or just 65.2% of the 7591s plate dissipation rating. This is a 21.3% drop in energy being dissipated by the plate of each output tube, resulting in collectively nearly 13.5 watts less heat generated in the output tube area, and the same reduction in wattage drawn through the power transformer and power supply components as well. This hardly turns the TA into the cool operation of a SS receiver, but any reduction in heat generated returns exponential benefits in terms of component life and dependability.
Together then, the addition of EFB to the TA-800, and re-designing the power amplifier driver stages to operate with 7247 tubes completely transforms the problems of the stock TA, into a quiet, dependable, powerhouse. Gone is the constant background noise of knowing it's "on" and hoping it will remain decently quite, to a unit that is dead quiet on highly efficient speakers in a small, quiet listening room. It simply stands quietly ready now for whatever you wish to throw at it.
In the listening room, it's new muscle easily shows itself in how it handles demanding bass passages -- as in sustained 32 ft pipe organ notes, live performances, and high energy digitally recorded jazz/pop music. The overall tonal presentation remains largely unchanged from that of the original design, since that quality is largely established in the tone control section of the unit. What did change significantly is the focus of the sound stage, since now there a very close match in frequency response and amplitude between the two channels at any given volume setting. By comparison, the sound stage of the original design was smeared and uneven.
The unit has a very dynamic sound now, giving you the feeling that it always has more to offer than you demand of it. The sound is crystal clear, sharp, and detailed, but not fatiguing. It is balanced and powerful sounding, with a range to its tone controls that its bigger brothers could only hope to match. Within the vintage Fisher vacuum tube stereo receiver lineup,
this TA now takes a backseat to nobody!
Dave