With both tuners, the power supply, and now the complete audio control sections operating properly, I could finally turn my attention to testing the power amplifier sections. The restoration work in both the driver and output stage areas had already been done, but up until this time, the unit has been operated without any of the power amplifier section tubes installed -- this to keep the heat down, and prevent needless wear on the tubes while the other sections of the receiver were serviced. During that time, an external amp and monitor speakers let me hear all I needed to know. But now, the new pair of TS output tubes along with the new pair Rob had recently purchased, as well as the last two 7199s that Jim McShane had were installed, so that the set could finally operate under its own steam for the first time since work on the unit began.
The output tubes were initially biased to a current draw of 32 mA each, with the new pair showing a decent match to Rob's original pair (thanks to Jim's efforts) -- although this is much less important now that individual bias controls have been added. However, in terms of the bigger goal being to have the two channels of any stereo equipment perform as identically as possible, having the output tubes of both channels represent a good match to each other certainly aids towards that end. With this, some basic checks were made, with problems right off the bat.
A. In this unit, Channel B seems to be the stepchild of the unit, since it seems to have had most of the problems to date in terms of working to achieve identical performance from both channels. In this case however, it was the luck of the draw, as the problem turned out to be one of the brand new JAN 7199 tubes.
The 7199 was the ultimate tube developed to implement Laurent's driver configuration (of Dynaco/Hafler fame), wherein a high performance pentode/triode tube is employed with the high gain pentode direct coupled into the triode section, which acts as the phase inverter. When all is right, it is a very high performance configuration indeed. Fisher, with being first an ever present goal, gave the tubes a whirl in both of the TAs. But the tubes that are appropriate for this application are prone to various problems that can make them unacceptable to use in this application. When supply is plentiful however, that's not particularly a problem. But with availability of good NOS examples all but gone now, we're scraping the bottom of the barrel even with the best tubes to be found. The problems these tubes can present include:
1. They are prone to developing heater cathode leaks in the pentode section, leading to the introduction of hum into the audio signal from an AC powered heater.
2. With two high performance sections in one small envelope, they tend to run very hot, making their life span notably shorter than that of a conventional double triode tube.
3. The high heat the tubes operate at tends to cause migration of cathode material to the control grid, causing grid current issues in the pentode section. When this happens, and a tube so afflicted is installed into a circuit with the topology that Fisher used, the immediate effect is that the volume control becomes "noisy" with rotation. No amount of cleaning the control will solve the issue, because the noise is caused by grid current from the tube passing through the volume control, causing the noise to be heard whenever the wiper is moved.
4. The plate voltage of the pentode section can be highly variable if the precise manufacturing tolerances required for the screen grid varies at all. If that happens, the pentode's plate voltage can vary widely from one example to the next in a given application. Because of the direct connection between between the pentode and triode sections, any significant variance in the pentode's characteristics then can (and will) upset the bias in the triode section, where upon the whole house of cards collapses, with performance going right out the window.
In the case at hand, gain was notably reduced in the Channel B power amplifier, distortion was well over double that of Channel A at all power levels, and upon inspection, it was found that the pentode plate voltage in the 7199 installed in Channel B was just 29.5 volts -- less than a third of the 100 volts specified to appear there. Since all the parts had been checked/cleared/replaced as needed previously in this section, the problem had to be the tube, and of course it was. Neither of the original Fisher tubes installed suffered from this problem as outlined in #4 above, but both suffered greatly from that of #1, 2, & 3, besides testing poorly.
In my experience with these types of designs using the 7199, as well as all such tubes like it, while once were a very viable design option, they are in more and more danger today of not being able to maintain proper operation because the bottom of the barrel with these tubes is in sight. Scott widely used this type of design in nearly all of their products, but wisely did not place a shield over these tubes, in an effort to have them operate as coolly as possible. That would be a very good suggestion for those with a TA to adopt, which will help to minimize the problems noted in 1, 2, & 3 above. But the problem with #4 above is becoming more and more prevalent, simply because the tubes originally culled as having the tightest screen grid tolerance are all but gone now, leaving the crumbs that display wide variation. Since the 7199 was one of the last tubes developed, it of course would be one of the first to run out due to lower overall manufacturing numbers, and less demand from application versus that of other similar tube types.
Fisher (imo) wisely stepped away from this tube rather quickly, never returning to it again in any of their products to my understanding -- and I believe largely due to the potential issues they noted when marrying this tube with direct coupled operation, even during their short stint using it. Besides keeping the covers off, there are circuit modifications that can be done to allow those tubes with wider screen grid tolerances to be used effectively in a direct coupled configuration such as is the case at hand. It amounts to doing much the same thing as is done today to accommodate modern manufactured output tubes whose tolerances are not as tight as those achieved by the original American manufacturers of such tubes. For this project however, in trying to minimize the cost damage to Rob, I decided to have another go at locating a tube that would work properly in the circuit as designed.
Finally, understand that the problem detailed in #4 above is NOT something that will be caught even with the best tube testers, as the problem does not manifest itself as a deviation in Gm in either section -- and particular to this discussion, the pentode section -- as the very nature of a pentode tube causes it's Gm (the parameter that the better tube testers test for) to remain rather constant regardless of the actual plate voltage it operates with. Rather, it is a performance issue that ONLY crops up when the two sections of the tube are tied together with direct coupling -- a condition that NO tube tester emulates. Being ever true to the caliber that Jim McShane represents as a tube vendor however, he credited the cost of the "defective" tube back to me for Rob, and even went so far as to locate and suggest another new example he found for me from a competitive vendor that he recommended. That is simply service without equal, and why I cannot recommend him highly enough.
So, yet another tube is on the way in the hopes that it will fill the bill, with one of my own tubes filling in in the mean time. Understanding that the channel balance was virtually perfect up to the volume control, and with that control turned up full (to eliminate any tracking issues it might contribute), the pic shows just how significant the channel balance is due to the one new 7199 that doesn't work well when direct coupled, but otherwise tests very strong in both sections. The pic shows a 1 kHz signal at 5 watts RMS power output in Channel A, lower in Channel B:
B. Initial data gathered:
1. With a CL-80 installed to protect the power switch, and otherwise operating directly from a 121 vac line, the AC heater voltage measured directly at the output tube socket furthest from the power transformer is 6.35 vac. Therefore, this unit may be operated directly from most residential AC power lines, with external adjustment not required until AC power exceeds 126 vac.
2. Best output stage performance coupled with safe operation is obtained with at least 40 mA quiescent current draw per tube. This unit will display significant cross-over distortion when the tubes are biased to less than 30 mA each.
3. Measured mid-band (1 kHz) power output:
Individually driven: 26.1 watts RMS in each channel at just <1% THD
Both channels driven: 20.48 watts RMS per channel at nearly 2% THD in each channel.
C. Suitability for EFB:
Measurements indicate that the screen grid voltage drops some 75 volts from a quiescent condition (375 vdc) to 300 vdc when full power is produced in both channels. However, the main B+ voltage drops only 20 vdc under the same conditions. Even accounting for the 20 volt drop in the main B+ voltage, this represents a screen grid regulation of >14% which is rather poor, and contributes significantly to reduced power output produced when both channels are driven. Also, with mid-band THD approaching 2% with both channels driven, the impact on distortion is significant as well. This represents a perfect environment for EFB to thrive in, so work will now begin on developing those circuits, with the initial work aimed at determining what the potential gains can/will be.
For now however, a quick pic of the underside as it currently stands. The only work unaddressed at this point is the DC Heater/Bias filter cap installation by a previous tech, which has been left un-touch at this point, as they will be addressed when the EFB board is constructed and installed. In the mean time, while waiting for the new tube to arrive, the unit moves over to the listening room for a little critical listening and burn in time. I'll grab a pic or two once its over there.
Dave