Fisher 400 Service Bulletin

dcgillespie

Fisher SA-100 Clone
Subscriber
Many of you might be aware of the comments I made in my thread "Improving the Fisher 400" regarding the performance of the phase inverter circuit in this receiver. In particular, Fisher employed what I have come to call a "noose" on this stage, which effectively limits the amount of peak output the stage can produce. I have speculated that this noose was employed as a limited measure of protection for the output tubes, as it was a circuit "feature" that Fisher widely employed in many of its receiver, integrated, and dedicated console designs.

This noose was applied in varying degrees of strangulation across the various Fisher models it was used in. But none have it applied so tightly as that employed in any version of the Fisher 400 receiver. In this model, the phase inverter stage is gasping for what precious little current draw it is allowed to have (the lifeblood of any driver stage), causing it to operate right on the ragged edge of being able to produce any kind of clean, low distortion drive to the output stage, let alone drive it to its full power capabilities with such a signal. The term noose then is appropriately applied, as the stage is ultimately choked down to being able to produce no more drive than the output stage theoretically requires to produce full power.

The design goal is worthy enough I suppose, and the circuit does generally work "OK", but depends heavily on very consistent characteristics for the tubes being used -- both in the phase inverter, and even in the output stage as well. Since Fisher could control those characteristics by way of the tolerance specifications it provided to its tube manufacturers, the units employing these nooses all generally ran pretty well, with the noose always being close by, but largely remaining invisible -- as long as the original tubes were installed that is.

Enter the recent 400 receiver I just encountered. All the component values were well within specification, if not very well matched and close in tolerance to the specified value between the channels of the phase inverter stages in this unit. In the listening room, the unit sounded nice enough for generally listening. But play some more demanding material and kick it a little (who doesn't do that from time to time?), and the unit just seemed to be asthmatic. Heavy bass tracks suffered the most of course. This was not the sound I was accustomed to from a properly operating 400 receiver.

Relevant to the concern at hand, this receiver had just had all the goodies installed to provide some real protection for the output stages (Screen Stability and cathode sampling resistors), individual bias controls, and completely rebuilt power supplies. It also sported a quad of truly NOS GE bottom exhausted 7868 output tubes, which while not matched, all excelled in my power output tube tester. With power supply voltages on the mark, the output stage was clearly not the source of the audible woes.

On the scope, the displays clearly showed what was happening: The phase inverter noose was kicking in as a result of two basic issues:

1. The output tubes employed were all set to 35 ma of quiescent current, generally requiring at least -19 volts (with some over -20) to achieve that level of quiescent current draw. Against the stock unit's bias voltage of -17 volts and much higher quiescent current draw, this means that the phase inverter now had to generate more drive to cover the increased bias voltage the output stage was now operating with -- drive it already struggles to provide with the stock quiescent current levels. In essence, reducing the quiescent current to more sane levels in the output stage then only aggravates an already highly compromised condition.

2. This unit had been purchased sans any tubes, so all tubes for it had to be procured before I received it. As received then, all the 12AX7 positions sported brand new modern production Tung Sol 12AX7 tubes -- which is the real meat of this bulletin.

With these tubes installed -- that is ANY of the 9 versions of this tube that the unit employs installed in the phase inverter positions -- they all exhibited early -- and slow/lazy recovery clipping, this because the tubes simply do not draw enough current in the circuit as designed. However, when good American produced 12AX7s were installed, the effect was largely eliminated.

Upon testing the Gm and AC gain of these Russian tubes against the precise conditions published by RCA under which these parameters of a 12AX7 tube were developed, the Russian tubes get an A+, with the average Gm coming in almost exactly at 1600 micromhos, and the Mu displaying a factor of nearly 100. So what the heck is going on then? American tubes seem to work satisfactorily in the phase inverter position of the 400, but the Russian tubes don't.

I then tested both the American and Russian tubes at the very low current operating point established by the Fisher 400's design (~.5 ma). It is important to note that 12AX7 designs (including Fisher) routinely operate the tube at this current level and lower in many phono preamp and line level stage designs, which is perfectly fine. But only Fisher does so with the tube in a driver position, and that's the rub. At such low current levels, when the load on the tube increases significantly (is numerically reduced) -- AS IT DOES IN A DRIVER POSITION WHEN THE OUTPUT TUBES APPROACH THE ONSET OF GRID CURRENT -- then the Russian tubes simply fold up shop and go home. Again, the tubes are already running on the ragged edge in this design -- even with American tubes, and in that region, the consistency from tube to tube can go right out the window. In human terms, it's like trying to have all children at the age of precise 2 yrs old learn at exactly the same rate and speed. That simply doesn't happen because at that point, you're still well off of the "linear" portion of human existence. At that age, the learning curve for children is all over the map, and such is generally the case with tubes in these operating conditions as well. The Russian tubes simply do not perform as dependably in this region as the American tubes do, under the fixed bias conditions set up by the noose in the Fisher phase inverter design.

The quick solution of course was simply to remove the noose in both channels. This returns the stage to operate as a cathode bias design, which allows the tubes to settle at the operating point that is best for the individual tube installed. Bingo. This instantly resolves the issue then for ANY good tube installed. Now, the Russian tubes perform in an indistinguishable way from the American tubes. Gone is the early clipping and lazy recovery from it, with drive to the output tubes now remaining strong and balanced, right up to and beyond the onset of clipping. In the listening room, this translates into a strong dynamic presentation when pushed, replacing the anemic presentation heard earlier.

Removing the noose is easy. The resistor creating the noose has different designators on the different 400 schematics, but in all cases, it is comprised of the 150K resistor connected to pin #3 of the phase inverter tubes. Simply remove this resistor, and then reset the phase inverter adjustment using the procedure presented in the sticky information. And that's it.

The bottom line is that for best performance from your 400, remove the noose, or if not, at least use tubes of American manufacture in the phase inverter positions. Then -- enjoy!!

Dave
 
Are you replaceing the resistor with a straight shunt wire or disconnecting the resistor altogether. I've got one on the bench right now waiting on new 7868's and I can do this easily as needed.

R96, R97 on 10001 to 47999 units (EARLY UNITS with T1020-116-A output transformers)
R115,R116 on 48001 and up units (Late Units with T-1020-116-AX Output Transistors)
 
I can vouch that my newly un-noosed 400 might just be the best or close to the best sounding Fisher in my collection. It's hard to turn this thing off! :thmbsp:
 
Nothing goes in place of the resistor -- you simply remove it, and then readjust the phase inverter adjustment.

Sam -- While it would help the other receivers too, it is the 400 that is so strangled, so the greatest impact will be on that unit. Also be aware that if a given unit doesn't have a phase inverter control that can be readjusted, then with the removal of the noose in those units, the value of the plate or cathode resistor (depending on which leg the control is connected in) will have to be adjusted accordingly to bring the phase inverter back into balance.

Dave
 
Unless it's an executive('63) 800c from the 10001-19999 series. then it's 120K.
 
I just did the mod. Easy peasy!!! Checked L&R readings 1st.
L 50.5KoHms on pins 1 & 3
R 49.5KoHms " " " " "

Removed the 150KoHm resistors on pin3 of the P.I.'s. Pin 3 went to 72.3K (L) and 72.6K(R). Both #1 pins rechecks (no change). Brought Pin 3 down to pin 1 readings with no problems. Plenty of pot travel left (at least 1/3 left).

I'll do the 800c's when I have them out for PM checks.

Thanks Dave. Your knowledge and expertise in keeping and improving these treasures is limitless.

Larry
 
WHat exactly is that resistor doing? It looks to be feeding some positive voltage to the cathode. I'd think that would act as a sort of fixed bias rig. By the sounds of it, the bias point that it ends up with is not reasonable and the tube is nearly in cutoff.
 
Exactly Gadget -- hence the "noose" action it provides in limiting drive to the output stage.

Dave
 
The early version 500C that I bought has a 180K ohm resistor in that spot. Looks like it needs removal along with changing the fixed bias circuit to allow for individual tube bias should help this unit to do better in today's real world new tube choices.

Thanks for making this information available Dave!

Joe
 
Joe -- That's in essence exactly what was done to RS Steve's 400.

His receiver was the perfect test cell for this work, since it included 9 of the brand new Russian tubes I could work with (as well as a few of my own), and his unit also now has individual adjustment capability for the output tubes. In his case, since he elected to stay with the original American 7868 tubes, and finding a truly matched quad of those type tubes anymore is basically an impossibility, the need for individual adjustment for his unit was significant. His unit had beautiful, new GE 7868 tubes in it, but they were not sufficiently matched to use in the stock design, or even with a single bias adjustment.

But even with the output stage operating properly, I am so used to the sound of my own 400 (that had the noose in it removed back when I did the 400 improvement thread), that when I heard his in the listening room, I knew something was just not right rather quickly. It was headed back to the bench anyway for a final checkout, but what I heard told me what to zero in on. Having all the Russian tubes available and everything else basically already eliminated as a possibility really made the task of testing between the American/Russian/Noose/No Noose rather easy. To not have done the tests would have been an opportunity missed. With the (very excellent) Russian tubes in Steve's unit, the difference when removing the noose was really quite pronounced.

Dave
 
Good to know. I'm still not sure why Fisher employed that noose resistor to begin with, maybe to protect the output tubes somehow?
 
I've seen you discuss this noose before but didn't pay it much mind since I don't have a receiver. Do you know Dave, if this approach was used on any of the EL84 amplifiers like the 20A, or 30A. They don't sound like it, I'd say.
 
I didn't see it start showing up until AFTER the X-202 units, or maybe more generally, after the brass faceplate era (think original X-101 for example). When it did appear, it was in everything -- receivers, integrated amps, kits, and console units. Therefore, it was most definitely a design characteristic of Fisher that crossed all model lines. But again, I haven't observed it to be the outright problem that it presented itself to be in RS Steve's 400 receiver -- but his also had the Russian tubes in it. That seems to be the sticking point: The noose AND the Russian tubes. Without the noose however, the circuit works fine with any tube.

Dave
 
For whatever it may be worth, the "noose" does not exist in my late production TA-600. Mine is the 7247 driver stage. I didn't look at the schematics for the 7199 version.
 
Gadget. The TA-600 predates the "Noose" by about 3 years. To my understanding any unit that uses the 12ax7 as a P.I. tube will have it. If it's a 7199 or 7247 driver it won't have it.

Larry
 
The only possible reason for including the noose was to limit the drive to the output tubes, ostensibly to add a degree of protection for them by keeping the drive signal from the phase inverter stage limited to a level that would just produce full power output and no more. Other than this, there is no performance quality or component life that is enhanced by its presence. For its intended purpose then, it was a crude effort at best, but one that Fisher apparently felt the need to build into everything after the gold faceplate era -- receivers, integrateds, consoles, you name it. Therefore, it was clearly a design strategy by the Fisher engineers.

The downside -- if you can call it one then -- is that removing the noose allows greater undistorted drive capability from the phase inverter stage, relative to the needs of the output tubes for the production of full power output. However, for users today who cherish this equipment and know the importance of always keeping a proper load on the amplifier, I just don't see removing the noose as a down side.

If you elect to keep the noose in place, just remember the determination that the noose is at least phase inverter tube sensitive. But now couple on top of that the fact that most folks operate the output tubes cooler than Fisher did, which requires greater phase inverter drive capability. And, if you happen to be using matched output tubes -- by random vendor selection or brand -- that require even greater bias voltage than that normally required to achieve the reduced target quiescent current level, and you've now really created conditions that the restricted phase inverter simply cannot cover for. That's exactly what I heard out of Steve's unit when I called on it to really kick it up a bit. It just sounded tired and worn out. I removed the nooses, reset the phase inverter adjustments for lowest THD, and the thing just lit up in the listening room, with the results plainly visible on the scope. I would strongly suggest removing these in the 400 receiver line, which to my knowledge uses the most restricted phase inverter in all of Fisher's designs.

Dave
 
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