The Fisher Phase Inverter Noose Revisited (for 500C/800C receivers)

Possible they backed down on that rating over time if there was evidence that running at those values reduced life enough to cause problems under warranty. That seems to be the usual reason specs are made more conservative.
 
EPILOGUE...
For this thread, the phase inverter modification offered herein for the 500C/800C receivers represents a significant improvement over that of the stock design, enhancing both the performance and dependability of the tubes used in the driver stages of these receivers, while at the same time, allowing for a very wide range of tubes to be used, without forever wondering if they are operating at optimum performance. By converting the inverter stage to operate with pure cathode bias and standing it on an R/C tail, the intentions of the original Fisher design are maintained, but optimum performance is assured with virtually all tubes, allowing them to easily perform the necessary functions of the driver stage under worst case conditions, and you to receive maximum return on your tube dollar spent...
I have installed the phase inverter modification in my Fisher 500-C with good results. I was curious about the variability of 12AX7 tubes with this circuit, so, I built a stand alone test fixture to test the performance of various 12AX7 tubes that I had on hand. Testing the necessary peak to peak voltage required to drive the particular 7591 tubes in my 500-C, I measured 40Vp-p. Of the 72 tubes that I took data on, any of them could easily make that drive requirement. The average tube could provide 70Vp-p. The very worst tube could still do 50Vp-p.

test circuit schematic:
Phase Splitter test ckt.jpg


Data from 72 12AX7 tubes:
Untitled.jpg

Output Vp-p:
APS Tester 04 (800x635) (2).jpg

test fixture:
APS Tester 09 (800x600).jpg
 
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so two questions,
what is the "std" line referring to? Looks like zero plate volts? I don't understand what that would accomplish

why the diode on the inverter side?
 
so two questions,
what is the "std" line referring to? Looks like zero plate volts? I don't understand what that would accomplish

why the diode on the inverter side?
Good catch. The std should have read STDEV for standard deviation. In the A column the B+ voltage for all tests is 370V so there is no deviation.

The diode is there to prevent high (>370V) grid to cathode voltage at turn on. I cannot find specifications on maximum grid to cathode voltage in my tube manuals. I just included the diode as a precaution. Maybe it is not needed. In any case, it does not seem to hurt.
 
TC -- Thank-you so much for publishing the results for your test of my 500C/800C Phase Inverter modification! You tested about double the number of tubes I did, and had almost the same range of AF Amplifier stage plate voltage for the tubes you tested, as I did for the ones I tested, although my tests were conducted at the schematic specified B+ of 350 vdc. Any additional B+ voltage over that value would only aid the phase inverter in providing even more un-distorted reserve drive capability. Besides the good tubes tested, my group also included a sub-group of weak tubes whose condition was: weak in sect 1 but good in sect 2, good in sect 1 but weak in sect 2, and weak in both sections (as determined by my 539B). All would have driven your output tubes to full power output with low distortion, although my tests were conducted using the stock coupling cap and following stage grid return resistor values. Additionally, I also tested for available un-distorted drive capability at 20 Hz, to ensure adequate drive was available at that frequency as well since there would be a signal loss through the coupling components at low frequencies. All in all, the results of your tests align well with those of mine, providing even greater assurance of the modification's ability to allow for proper operation of the driver tube in Fisher's largest receivers, when using a wide variety of tubes and tube conditions.

Thanks again!

Dave
 
TC -- Thank-you so much for publishing the results for your test of my 500C/800C Phase Inverter modification! You tested about double the number of tubes I did, and had almost the same range of AF Amplifier stage plate voltage for the tubes you tested, as I did for the ones I tested, although my tests were conducted at the schematic specified B+ of 350 vdc. Any additional B+ voltage over that value would only aid the phase inverter in providing even more un-distorted reserve drive capability. Besides the good tubes tested, my group also included a sub-group of weak tubes whose condition was: weak in sect 1 but good in sect 2, good in sect 1 but weak in sect 2, and weak in both sections (as determined by my 539B). All would have driven your output tubes to full power output with low distortion, although my tests were conducted using the stock coupling cap and following stage grid return resistor values. Additionally, I also tested for available un-distorted drive capability at 20 Hz, to ensure adequate drive was available at that frequency as well since there would be a signal loss through the coupling components at low frequencies. All in all, the results of your tests align well with those of mine, providing even greater assurance of the modification's ability to allow for proper operation of the driver tube in Fisher's largest receivers, when using a wide variety of tubes and tube conditions.

Thanks again!

Dave
Thank you for your comments. It is good to know that we received similar results under somewhat similar test conditions. Regarding my choice of test conditions:

1. I chose 370V B+ because it matches the actual value found in my Fisher 500-C and it is also the value you noted in post #32 of this thread. As you have stated, a lower B+ will reduce the maximum peak-to-peak drive capability.

2. I chose to use the modified values (220kΩ and 0.068µF) for the coupling caps and following stage grid return resistors, again, because they are the values I currently have in my Fisher 500-C.​

Thank you again for publishing your insightful modifications for the Fisher receivers and this driver stage in particular.
 
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Final Pics

BELOW: I finally had a chance to permanently install the phase inverter modification into the right channel of my test bed 500C. It is the simpler version without the heater bias tap, since that feature was executed with the modification installed in the left channel of the unit. Both channels work superbly now without concern that the particular driver tubes installed will operate at anything less than optimum capability. Similar to the installation of the left channel, the right lug of the T-Strip shown below is the mount lug for the strip and soldered to the shield plate, with the plate itself soldered to the chassis ground lug as originally built. Therefore, the right lug is a ground terminal, which cannot readily be seen in the pic presented. In the left channel, it is the center terminal of the T-Strip for that channel that represents ground.
SAM_2707.JPG

BELOW: The full modification installed in both channels. With that, the noose removal modification presented for the C receivers is now considered as fully developed and vetted for optimum performance from virtually any driver tube you would chose to use in these units.
SAM_2708.JPG

Enjoy your tunes!

Dave
 
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Dave - The schematic for this mod is in post 42? Would you mind adding to the schematic (or in text explanation) to where the leads go externally? Thanks, Thorne

PS - I reread this thread. I think the connections are explained on the 1st page. I will digest and ask questions later.
 
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The modification installed in my 500-C (the light blue electrolytic capacitors are the 22µF, 160V bypass caps):

Phase Inverter Mod 07 (534x400).jpg
 
Thorn -- The schematic I posted is in post #13, and shows three partial circuits: At the top is the original design of the phase inverter stage showing the noose resistor. The bottom left is the modification that (ideally) would be used in the right channel, and could also certainly be used in the left channel as well, but the modification is further broken down at the bottom right to show how a slight variation on the modification allows it to also provide heater bias as well. So the installation I have shown shows the version on the bottom left of the schematic installed in the right channel (left side of pic), while the version at the bottom right of the schematic is installed in the left channel (right side of pic). With regards to correcting the bias for the driver tubes, either version of the modification performs that job identically, but the version on the bottom right of the schematic also provides heater bias as well. Using that version is optional, but recommended, and only needs to be used in one (either) channel -- I simply chose to use it in the left channel shown at the right side of the underside pic. The simpler version shown at the bottom left of the schematic is then used in whichever channel the other version is not used in. Installation is not particularly difficult at all. Assuming you will install the simplest version in both channels, you will need: two 47K@0.5W, two 91K@0.5W, and two 22uF@160 vdc electrolytic caps. Based on starting with a perfectly stock receiver then, the procedure in each channel is as follows:

1. Locate pin 3 of the driver tube to find a coupling cap, 68K, and 150K resistor (which you've already removed) located there. Remove the 68K and 150K resistors (the latter of which you've already done).

2. If you installed the unvetted noose removal procedure earlier, then the 470K resistor connected to pin 2 of each driver tube had previously had its other end disconnected from the center terminal of the phase inverter control, and connected to a lower voltage source. That work needs to be removed, so that the 470K resistors are reconnected to the center terminal of their respective phase inverter controls again as before. Any wiring previously installed to apply the alternate power to these resistors then needs to be removed.

3. Determine how you wish to secure the 22 uF 160vdc cap, either with adhesive, or more professionally using the T-strip shown as I did. The negative lead of these caps is grounded to a convenient ground point, while their positive lead will connect to a new connection point (created by the T-strip) that is insulated from the chassis.

4. From the new insulated connection point, besides the positive lead of the 22 uF cap, also connect a 91K resistor to ground. Also, from the new insulated connection point, connect a 47K resistor, with the other end connected to pin 3 of the driver tube. There are now three leads connected to the new insulated connection point.

And that's it. If you install the version in one channel that provides heater bias, you will need to provide two new insulated connection points in the channel it is installed in. That's why you see a 2 terminal T-Strip in one channel of my installation, and a 3 terminal T-Strip in the other. On each of these T-strips, one terminal ground, while the others are insulated from ground to allow the connections made to them to work properly.

I hope this helps!

Dave
 
I have installed the phase inverter modification in my Fisher 500-C with good results. I was curious about the variability of 12AX7 tubes with this circuit, so, I built a stand alone test fixture to test the performance of various 12AX7 tubes that I had on hand. Testing the necessary peak to peak voltage required to drive the particular 7591 tubes in my 500-C, I measured 40Vp-p. Of the 72 tubes that I took data on, any of them could easily make that drive requirement. The average tube could provide 70Vp-p. The very worst tube could still do 50Vp-p.

test circuit schematic:
View attachment 1435054


Data from 72 12AX7 tubes:
View attachment 1435049

Output Vp-p:
View attachment 1435057

test fixture:
View attachment 1435065

That test jig is cool:thumbsup:
 
Dave - Thank you SO MUCH for taking the time to write up the detailed procedure. It really will help. Thorne
 
Dave; Bringing up a "Dead Horse". For the few of us that have 1963 Executives with the original 10000 series 800c's, what changes in the schematic are necessary to implement the updated phase inverter components, vs. the later 20000 and up series(1964 exec's and standalones.)

If you need the Full Scat for the Exec. let me know and I'll send it to you

Larry


10000 series Output section
10000 series 800c.jpg
30000 Series Output Section
30000 series 800c.jpg
 
Hi Larry -- A superb question to make for a complete look at the big receivers!! Quite interestingly, the very early 800C receivers (19999 and below) -- with the exception of the value of the noose resistor (R26, R28, which Fisher was clearly still playing with at that point) -- is a carbon copy of the earlier 500/800B receiver power amplifier section. Therefore, for that particular serial range of 800C receivers, following the modification presented for the B receivers published in the Revisit thread for the 400/500B/800B receivers is exactly the way to go for your early 800C. For clarity, you will be adding the 22 uF bypass cap to the AF Amplifier section of the driver stage, eliminating the positive feedback connection and noose resistor, and then following the modified inverter circuit shown on the right side of the modification schematic presented in that thread.

Interesting as well -- it is only the early 800C where this quirk exists. I checked the early (10000 series) 500C receiver schematic, and while there are some differences between it and later 500C power amplifier sections, for the purposes of this discussion, the parameters that matter are still consistent throughout the entire model run, so therefore, those units should still follow the phase inverter modification offered in this thread. Apparently the 500C's audio section was the spearhead of changes being made to that section that the 800C then followed later?

Thank-you for bringing this 800C detail to light!

Dave
 
Dave, I know you ran into the same problem with the noose on my 400 restoration a few years back that had new Russian tubes installed, I'm wondering if the fix was just removing the noose or did you do more extensive work to get it operating correctly. Just wondering if I have to apply this new design, or just leave it be.

Edit: Looks like the noose was just removed, after digging up the old thread and doing some reading.
 
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One final question. On my 43355 serial, For the 1uf 350V would a film cap be an ok sub. I've got a ton of them, and hate to see them go to waste, or is this a bad idea??????

OK, TWO Questions. The Heater resistors (R101-R106), If I understand it right, you took one and connected to pin 9 and then to network at the junction of the 16K and 75K and the other to pin 4? Could I instead leave the resistors in place where they are, change the two tap "T" strip to a 3 tap, and move the grounded end to an open tap, and run a wire to the junction of the 16K and 75K??? or would that induce some oscillation/emf????

Here's what I figured out on the 43355 serial.
1.)Remove R26/R-28 (180K) Pin 3
2.)Remove R30/31 (68K) from Pin 3
3.)Install short network on Right Side (47KΩ, 91KΩ, and 22uf 160V) to pin 3
4.)Install long network on LEFT SIDE (47KΩ, 16KΩ, 75KΩ, 1uf/350v, and 22uf/160v) to pin 3
5.)Install 220Ω on pin 4 and pin 9 and to 16kΩ/75kΩ junction on T strip. Or alternately, replace 2-tap T strip on PS with 3 -ap "T" Strip and connect 220Ω (R101-106) grounded end to non grounded tap and run a wire to the junction of the 16KΩ/75KΩ resistors.
6.Adjust for equal resistance.

Correct me if wrong please. (Gotta start these somewhere on my receivers...LOL!)
 
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You know what? A bit off topic, but would someone "in the know" use a post (please) to explain in which situations which kind of resistors (film, metal, carbon, et cetera) are best in which electronic situations? It's a question always on my mind when having to order parts. I've read about the DIFFERENCE between them but have found very little about in which circuits these differences are important. So please? Thanks. Thorne I'll start a new thread if it's lengthy enough of a subject that warrants discussion, but I think not since one post or reference to a web page could do it.
 
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