The new power amplifier and modified tone control circuits in Channel A played completely uneventfully last night, along side the completely stock Channel B circuits. With the tone and Balance controls centered, the sound was balanced and presented equally from both channels, with a well focused sound stage and balanced timbre. I firmly believe that for anyone familiar with the sound of the 800, asking them to blindly identify which channel was stock and which was modified, would be purely a crap shoot guess. So the amplitude (i.e. sensitivity) and timbre of the modified channel presents for all intents and purposes identically to the stock one. However, there is one notable area where the channels can immediately be identified now as to which is which.
In a block diagram, for a 250 mV signal presented to the high level inputs of the stock TA-800, it can be shown that the tone control amplifiers amplify this signal by a factor of X1.6 which is then presented to the volume control. At its full setting, the volume control presents this (now) .40 vac signal to the power amplifier section, which then develops full power output.
In the modified TA, the same 250 mV signal is now amplified by a factor of X4.0 in the tone control amplifiers, which through a full setting volume control, now presents a 1.0 vac signal to the new power amplifiers, which then develop full power as before. In other words, the new design amplifies the signal 250% more in the tone control amplifiers, to make up for a 250% loss in gain in the power amplifier section. Because the net gain of the unit is identical in both cases however, this is why the two channels can play side by side each other within the unit, with no one knowing the better, even though they are quite different. However, what this also shows is that assuming a full power 8Ω output generates 14 volts at the TA's speaker terminals, it means that the gain of the stock power amplifier section is X35, where as the gain of the modified power amplifier section is now just X14. Because the power amplifiers appear
after the volume control, this change has huge implications on how quiet the set is when turned fully down.
The stock TA -- even with a properly quiet 7199 -- is notably noisier at minimum volume settings than its later brethren is when using sensitive speakers, because of the higher gain the power amplifier section has in stock form. In piling on by adding insult to injury, locating these sensitive power amplifier driver tubes right next to the power supply section invites this higher gain design to produce more general noise, in addition to the extra hiss generated from the higher amplification displayed. With sensitive speakers, this can rather easily be heard in a quiet listening room from a not-so-close listening position. In other words, you can hear that it's on. Nothing to go postal over mind you, but very obvious none the less. However, if you add any kind of extra unusual noises from the 7199s, it gets to be intolerable real quick.
With the new design, the gain in the power amplifier section is notably reduced, while the gain in the tone control stages -- located well away from the power supply area -- is appropriately increased. As a result, now at a minimum volume setting, you don't even know that the unit is turned on. For all intents and purposes then, it is dead quiet now (in Channel A anyway). The total noise from the overall amplification process is the same in both designs. But in the new design, more of it is lost within the un-attenuated signals passing through the tone control amplifiers, making for a notably much quieter minimum volume setting noise level. It simply comes off as more refined.
7199 EPILOG
No doubt some may say, "what's all the fuss about converting the power amplifier section to using 7247 tubes? The 7199s in my TA are quiet." I'm sure they are. I have a precious few that are commendably quiet in my equipment as well. Therefore, if you've got quiet tubes, and they're good, then I have no doubt that all of this may in fact come off as much ado about nothing -- which is perfectly fine.
But that wasn't the case for Rob's unit. Both of the original Fisher 7199s in his unit were very bad, and more specifically to the point, it is the effort to obtain quiet 7199s
today -- where you didn't have any before -- that presents such an unworkable situation -- which will only get worse in the future. All three 7199s that were the result of this effort were hardly blind internet buys. They were all obtained from quality vendors -- two of which were "JAN" tubes from Jim McShane, and one which was a NOS NIB Sylvania from Brendan at Tube World. Let me stress again, that
all of the problems that these tubes displayed are the type that would not be caught by convention tube testing, whether pretested, burned in, you name it. The problems displayed are either due to the way they are used in circuit (direct coupling), or produce significant hum from the slightest H/K leakage. For vendors to cull out these bad examples -- on top of those that fail conventional test procedures -- would require the vendors to additionally plug the tubes into working circuits and monitor them, which either they don't have the time to do, or would send the price even higher. To the credit of both these vendors, in spite of both returned tubes being new and certainly testing as "Good", they immediately took them back, and gave full credit for them. I have dealt with Jim for years who imo is excellent, and it was he who recommended Brendan as a quality vendor when he couldn't replace the one I returned. Even though this was my first experience with Brendan, he took back the Sylvania 7199 no questions asked, for a full refund including shipping, and was then able to supply the two excellent GE 7247s the set will now employ. Cudos to both of these quality/class vendors for standing behind what they sell.
The final nail in the coffin of this story however is the third and last 7199 tube of this saga. It too is now starting to display intermittent noises -- periods of increased hiss and/or static, that can always be relieved for a period of time by tapping on it. It's hard to call it a new tube anymore since it was used for all the testing after the initial restoration was done, and then through out all the developmental work on the new driver circuit. But whatever slimmer of hope I had in finding any of these tubes in truly good, usable condition is now is completely gone. I have zero confidence in
any remaining stock out there, as this project is 0 for 3 with these tubes. By comparison, with the new driver circuit design, the unit turns on with no muss no fuss to inaudibility, and out of the black background comes music -- exactly as Avery would have wanted it to. In
today's audio environment then, unless you've got a stash of known good, quiet, tubes, conversion to the 7247 driver design will keep these particular Fishers performing -- and acting -- like a Fisher should.
Dave
One of the new driver tubes installed, up and running, makes it one down, and one to go......