Lots of data has now been collected on the design of the original power amps in the TA-800. When looked at collectively with the tone control amplifier stages, there are a number of design elements that stand out between this receiver, and that of the 500C/800C series units:
1. As I had surmised earlier, the tone controls in the TA series (both models) have much more loss built into them, which ultimately allows for more boost to be had when the tone controls are advanced from the flat setting. Whereas the TA can provide about 20 DB of boost or cut, the later receivers had toned this down (pun intended) to about 14 db. The effect of this is that the TA's have twice the boost/cut capability in their tone controls than the later receivers have -- and it's definitely noticeable in use when switching between the TA and later receivers. This is in keeping in general with a trend in Fisher design, where less tone control range is available in their products throughout the 60s, versus those of the 50s. I have often commented that the amount of bass boost produced in a 400C preamp by the two-step loudness circuit of that unit is just gross, even on the first step -- to the point of being unlistenable/unusable. This was not an engineering defect, but a result of the sources available in that day. 78 RPM records and early microgroove recordings had little LF information to offer, so Fisher's tone controls of the day had more boost built into them. As source material improved, less range was needed in the tone altering circuits, so they were adjusted accordingly in later years. The TAs were part of the pivoting process, and so did not yet represent the end goal of the pivot.
All of this plays into this project because Fisher had the option to account for the greater signal loss of the tone control networks via the greater gain of the pentode section of the 7199 driver tube in the power amplifier section -- and they did take
some advantage of that fact as will be seen later. All else being equal then, if the unit is modified to use a lower gain driver tube, that loss of gain will need to be made up somewhere else in the circuit if the ultimate high level (Aux input) sensitivity specification (250 mV measured) is to remain unchanged. The tone control amplifiers is the only available place to go and find it.
As designed, with the tone and balance controls centered, the overall gain of the complete 12AX7 tone amplifier stages at 1 kHz in the TA-800 (from Aux input to the top of the volume control) clocks in at a measly X1.60, and is derived as follows: With each 12AX7 section in the tone amplifier stages having a raw gain of about 40, this becomes a total raw gain of 1600 when they are cascaded from one section into the other. This total raw gain gets reduced by a factor of 10 due to the 20 db loss in the tone controls, and then by a factor of 10 at
each 12AX7 section due the 20 db of NFB that each stage employs. That results in the overall gain of the tone control section reaching a factor of just 1.60. When Fisher converted the early version 600 to the later version of that unit, they had to deal with the loss of gain in the power amplifier section as well. They did that by slightly reducing the NFB applied around the first tone amplifier stage, which is ultimately what will be done with this project as well. But first, the sensitivity of the new power amplifier design will have to be accurately established to know how much the NFB will need to be reduced in the first tone stage to maintain the original high level input specification. So moving on........
2. The power amplifier section in the TA-800 is rather conventional, with two three notable points regarding its design:
A. The output tube screen grid dropping resistor is nearly 185% larger in the TA-800, versus that of later receivers. This causes the screen grid voltage to drop 75 volts (which is significant) when both channels are driven to full power output, which in turn causes 1 kHz THD to come in at 1.75% under those conditions, which is not a very attractive result at this frequency. Even when a single channel is driven, 1 kHz THD at full power output is still 0.8%. Fisher lowered the value of the screen dropping resistor considerably in later units, and while the screen grid voltage still drops in those units, it doesn't do so as much, allowing (in part) those units to develop more power, at less distortion. The addition of EFB will make a notable impact on the performance of the TA-800.
B. With the greater gain of the 7199 available in the power amplifier section, that could allow for more NFB to be used to reduce distortion -- especially to counter that produced by the large drop in screen grid voltage in the output stage as power output increases. Except, Fisher did not opt to do this, with the power amplifier sections operating with 15 db of NFB -- the same amount that the later receivers use as well. OK, then at least the extra gain could be used to make the power amplifier section more sensitive than with a lower gain drive tube -- and it does, but only increasing it by an estimated factor of about 2.5. The stock 7199 based power amplifier in the TA-800 has an input sensitivity of .40 vac to develop full power output. In the later receivers with a 12AX7 driver tube, that figure is more typically around 1.0 volt for the 2.5X reduction in sensitivity as noted. Yet, the pentode section of a 7199 tube has a gain of nearly 8X that of a single 12AX7 section, so where did all the extra gain go? In this design, Fisher chose to introduce NFB into the screen grid circuit of the 7199, to help stabilize the gain of the pentode section of that tube from one example to the next. High gain is a virtue of pentode stages, but it isn't always consistent from one example to the next. By introducing the screen grid feedback as they have, the gain is stabilized, which helps to stabilize the performance of the design.
C. Since Fisher didn't really need to use all the gain advantages the 7199 offered over that of a more conventional dual triode driver tube, then why was it used? One advantage it does have is its being immune to the effects of Miller, where HF response can be reduced in an otherwise triode stage due to the internal plate to grid feedback that takes place in those tubes. For those with really super sharp ears and a capable source and speakers, the early TA receivers have the capacity then to sound incrementally brighter than later receivers that use a dual triode tube in the driver position. But the effect is almost entirely pushed to very highest extremities of the audio spectrum where so very little audio exists, so the difference will be miniscule at best. Fisher clearly didn't have an issue converting to the use of a dual triode driver stage, so wanting to develop this modification in the spirit of "How would Fisher have done this?", that's the path this modification will take as well.
The best answer as to why Fisher used the 7199 in these units likely falls into the realm of traditions in the industry. When a new tube came out, it was traditionally offered to manufacturers at rock bottom prices to entice them into designing them into their sets. That gives the manufacturers a break, gets the tube "out there", and consumers get the latest technology, so everybody wins -- and often, that process did work great. Just look at the 7591!! But the 7199 wasn't filling a gap like the 7591 was, and with performance that was really little improved from the previous tri-pent tubes already out there, it just represented another inventory line, and as history has shown, didn't really solve any of the well known tri-pent tube construction problems. Fisher did their part to help out RCA and gave it a go, but in my opinion, made the smart move
back to using dual triodes for their driver tubes.
One last specification of the 7199 design: HF response of the power amplifier section was down .70 db at 20 kHz, and down 3.0 db at 50 kHz. A 10 kHz square wave from Channel A is shown for future reference. As I customarily do, the development work for the new driver stage will all be done on one channel (Channel A in this case), leaving the other completely stock. This allows for real time side by side comparisons at any time throughout the project. Once the development is finalized and complete, then the other channel will be modified to match.
With that, work on a new 7247 based driver stage can now get under way.
Dave
View attachment 951545
Havin' fun now!
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7199s aren't able to easily service this decade -- let alone a century!
