New X-101-B owner

Are you measureing at the end of both 1/2's of the power cord??? Turn it off, remove the fuse and measure from the power cord both ends. Then install the fuse and measure again, the same points with the POWER OFF. Then with mini clips on the ends, same places, turn on the unit and measure the voltage. List them. I'll get dave on here.
 
I measured the outlet at the hot and neutral plugs. The other measurement was positive lead where the power cord terminates at the fuse and the negative lead on the chassis.

I'll check your measurements either tomorrow am or after work.
 
Gentlemen -- If I may: The reason that V5 is giving a negative voltage reading across the 10Ω resistor is because the white wire connected to pin 5 of V5 must be disconnected from that terminal, and connected to the other side of the 10Ω resistor currently connected there. This move would have the white wire connected then to the cathode buss (brown wire) lead.

The reason that the configuration as shown produced a negative voltage across the 10Ω resistor for V5 is because while V5 was trying to draw its quiescent current through the 10Ω resistor and returning this current to the B+ "returns" appearing at the screen and plate terminals, the white wire -- connected to the heaters, is trying to draw power through the 10Ω resistor and returning this current to ground. Since the current to ground through the heaters is far greater than the quiescent current of the tube, it caused the voltage at pin 5 of this tube to actually appear negative with respect to the common meter connection point, which is the cathode buss. Correcting the connection of the white lead then will correct the issue with the indication for V5.

As for the AC voltage reading discrepancy, the meter shots do show a mystery, but solving it starts with a wider shot (and closeup) showing the terminals your meter's test leads are actually connected to inside the amplifier. Then, we can trouble shoot from there.

Dave
 
Just saw your other post on where you placed your meter leads inside the amplifier to check the AC voltage at: The meter leads were not connected across the AC line, but across one lead of the AC line and chassis ground. Such a configuration can place phase shifted leakage voltage from the power transformer (that appears on the chassis) in series with the 120 vac power line, making it appear like there is more voltage applied to the set than appearing at the wall plug. The easiest way to correct this is to install a TWO prong polarized power cord set (or, designate and mark one side of the Fisher power plug to be the neutral side, and always insert that side into the neutral (wide) slot of the wall plug). Inside the set, the neutral side should connect to the lead going to the AC power switch. Also, the neutral side should connect to one end of a .01 uF X-Y rated cap, with the other end of this cap grounded to the chassis. This modification will safely bleed any power transformer leakage on the chassis back to the low side of the AC line, without creating any potential ground loops in the process. It will also promote the lowest possible noise from the amplifier as well.

I hope this helps!

Dave
 
Thanks, Dave. My problem wasn't negative voltage (although I wasn't paying 100% attention to whether there was a "-" sign in front of my reading). I was getting a lot higher voltage reading on V5 than the other 3. Either way, I should move that white wire to after the resistor? So, in my photo, I can just move it and install it anywhere between the red lines I made?

IMG_20180317_184753.jpg

I did check the polarity of the plug and made sure to have the hot wire going to the fuse. I marked the plug so that I would remember which way to plug it in. What voltage rating do I want for the X-Y cap? Would I install it like the cathode resistors? Cut out a section of the insulation and solder on lead to the wire like in the yellow circle?
 
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You can move the white wire to anywhere on the brown buss lead that all the 10Ω resistors connect to and that will be just fine.

An X-Y cap rated for operation on a 120 volt line will be fine -- alternatively, I would think a traditional rating of 600V or more would be fine.

Vintage equipment was not designed with the AC power wiring so clearly defined as we understand it today. Back in the day, the plugs were non-polarized so that the side of the line going to the fuse could be either be the hot or the neutral. Often a cap was used to reference one side of the AC line to the chassis to produce a virtual ground to achieve the lowest noise. Depending on which way the plug was inserted, you might then feel a little tingle on the chassis due to this cap, or from general power transformer leakage. With out clear definition then, your Fisher was wired with one side of the power cord, going first to the power switch, then to the fuse, and then to one lead of the power transformer primary winding. If you connect or mark your line cord so that the hot side of the AC line connects to the switch and the fuse, this is not the lowest noise configuration since it places the hot side of the line in close quarters with the sensitive volume control wiring at the AC switch. You have two ways to go here then:

1. As built, the side of the AC line that connects first to the power switch and then the fuse should be the NEUTRAL side of the line, AND is also the side of the line that the new X-Y cap is directly connected to. However, this approach violates modern design practice where the hot lead first goes directly to the fuse.

2. If you want to correct this, then you will need to re-wire the AC power wiring in the Fisher to represent the following:

A. The hot AC power lead goes straight to the rear terminal of the fuse post, and one side of both auxiliary AC power outlets.

B. One power transformer primary lead goes to the side terminal of the fuse post.

C. The neutral side of the AC line goes to the power switch and the X-Y cap.

D. As before, the two outputs of the AC power switch go to the other power transformer primary lead, and the other terminal of the two auxiliary AC power outlets.

This more approximates modern design approach, but still leaves the hot AC power lead appearing on one side of the auxiliary AC receptacle outlets at all times when the unit is plugged in. If this is of concern to you, then the best recommendation is to simply disconnect all wiring to the outlets, since they are likely not being used anyway. The resulting configuration then has the hot lead going straight to the fuse so address modern practice, but still has the amplifier power being switched in the neutral side of the line for lowest noise. It also has the chassis always referencing the neutral side of the line via the X-y cap for lowest noise as well.

It is best to install the X-Y cap directly at the terminal where the neutral AC power cord wire connects to where it enters the chassis. Likewise the other side of the cap should be grounded to the chassis at any good nearby chassis ground terminal in the power supply area.

Dave
 
I believe I have it set up the way you describe at least in A and B. I'll just need to add the X-Y cap at some point. You're correct - I have no plan to use the 2 aux outlets. So the cap could just have one lead go to the terminal strip where the A/C line connects - pointed out with yellow arrow and the other lead to ground.

IMG_20180227_205653_2.jpg
 
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Thank you for your assistance, Dave. I moved the white wire and all is well. Here's my bias voltages and current.

V4 - 42.90V & 49.6mA
V5 - 42.84V & 42.3mA
V9 - 42.80V & 43.5mA
V10 - 42.88V & 45.2mA

So I checked A/C voltage coming into the unit the proper way, and with the CL-80, I was at 119-120VAC. Looking at the schematic, it looks like my voltages are still really high in some aspects. Like on the output tubes, pin 3 is supposed to have 424V and I have in the 440s. Pins 4&8 are supposed to have 379V and I'm getting in the 390s. How is this corrected? Technically, voltage is within Fisher's spec which is 105-120. I assume a variac or bucking transformer is my next purchase?

IMG_20180319_184519.jpg
 
Tom; Even tho FISHER spec'ed all the schematic voltages at 117VAC input voltage, the unit is rated to 120VAC. Voltages on the schematic are an ideal @ 117VAC and they spec'ed a 15% variance up or down due to individual component tolerance and variances. Your voltages are pretty much between 3 and 5% high, which is well within tolerances and very close to what I'm seeing on my X-101-B with Tung-Sol ReIssue (russian tubes). I run mine direct off the wall at 122VAC, with no problems.
 
Understood. I'll run it off of wall voltage for now. My readings of cathode current are ok, right? According to a tube datasheet for the 7591, it looks like max current is 85mA.
 
Max cathode current is way different from Max Plate dissipation. Max plate dissipation is used to determine safe operating condition. Max CAthode is a dynamic that can be reached for short peaks and nothing else. Most units run 7591's in the 30 to 40 ma range. Lets use your plate voltage of 440VDC. You have a cathode current of 49.6ma in V4. So we'll use that for the example.

V4 Example 49.6ma is .0496a. Multiply 440 by .0496 and you get 21.824W This is over the Maximum plate dissipation by almost 3 watts. Lets look at the other 3 also.
V5 440.x .0423=18.612W Better as it's under 19W but it needs to come down a bit into the 70-80% Range.
V9 440.x .0435=19.14W Just over the MAX Plate dissipation. Needs to come down.
V10 440. x .0452= 19.888w Again Just over.

NOW you also have screen dissipation of approx 3ma that we can subtract from the cathode reading. This would be the effective actual reading of each tube.
V4 440. x .0466= 20.504W
V5 440 x .0393= 17.292W
V9 440 x .0405= 17.820W
v10 440. x .0422= 18.568W

1.) I would swap V5 and V10 to get two closer matched pairs.
2.) Recheck your grid bias voltage again. It may be higher than 44 specified. Readjust to 43V, and recheck your numbers for V4 thru V10 again. You may be able to go down to 42, but the heaters on the 12ax7 filaments may not heat up enough to operate. Or have you waiting for a Great Grandchild to be born 1st before these tubes heat up enough to conduct, but with a good amount of distortion. I'd bet Dave would have an IBAM setup that could be used for the X-101-B. Maybe even similar to the one in the X-101-C or but I don't know. But all of those voltages have to come down. A bucking transformer might do it, or a variac.
 
One additional point to remember in determining actual plate dissipation when it comes to the designs that power the small signal tube heaters from output stage quiescent current: First, subtract out the cathode voltage.

Larry gave an excellent run through on determining real plate dissipation, by also accounting for the screen current that flows through a pentode tube as well. But as a basic point of theory, plate dissipation -- being the product of plate voltage X plate current -- is more specifically the voltage that exists between the plate and cathode X plate current.

Applying this to the worst case example that Larry laid out in your unit then (V4), and using your approximate cathode voltage of 43 vdc, the new exercise becomes (440 - 43) X .0466 or 397 X .0466 = 18.50 watts.

This shows that this particular tube is actually operating within its designated limits -- if only barely. Plate dissipation will be reduced somewhat when the quiescent current is reduced -- but remember, as quiescent current is reduced, plate voltage tends to increase, causing the two actions to somewhat counteract each other. Plate dissipation does decrease, but not as fast as a simple reduction in current alone would imply.

These particular designs of Fisher were really threading a needle so to speak, in that the output tubes need to pass enough quiescent current so as to be able to light the heaters of the preamps tubes effectively, but not pass so much current as to cause the plate dissipation rating of the output tubes to be exceeded. The unit also had to be able to compete in the market place, so maximum practical B+ voltage was also required to produce a competitive amount of power output from the scheme as well. All of these requirements then work to make the window of acceptable quiescent current rather small in these amplifiers. A little too much current and the output tubes cook. A little less, and you can pack a lunch waiting on the small signal tubes to warm up. So all of these designs operate the output tubes at the high end of what would normally be considered as safe operation. But there's one other thing as well.

Because the tubes operate so much closer to their limits, the importance of using a well matched quad of tubes in these types of designs cannot be overstated. Using the best case example (V5) in the OP's unit, actual plate dissipation becomes 15.60 watts which is quite reasonable, but represents over a 15% difference in dissipation that V4 is incurring over V5. In other words, V4 carrying more than its share of the work load (lighting the heaters), while V5 is comparatively loafing. Besides the degradation in amplifier performance this causes, it also causes the tubes to wear unevenly, which only is accentuated with use as time goes on.

My intent is hardly to trash the design, as for the price point of these amplifiers, the scheme produces the huge benefit of operating the small signal tube heaters off of DC voltage, which is one reason why Fisher equipment was considered to have such a low noise floor back in the day (and still does), where so many other manufacturer's pieces did not. However, in today's vacuum tube audio environment of higher AC line voltages and modern manufactured tubes, these particular types of designs require close monitoring and the types of modifications as already done here to ensure proper operation, and the long term health of all the components involved. There are modifications that can be done to completely circumvent these problems, which also produce a significant improvement in amplifier performance as well -- but they are extensive, and only for the advanced hobbiest to consider. An example of one such effort can be found here, offered primarily to reinforce the level of modification required, but also to show what the performance potential of the amplifier truly is:

http://www.audiokarma.org/forums/index.php?threads/improving-the-fisher-x-101c.582379/

Good luck with your X-101B!

Dave
 
Something else learned from the Master. Subtract the difference between the plate and screen from the plate voltage to get true plate voltage. And still subtract out the 3ma for screen dissipation. Neat. :bowdown::bowdown::bowdown::bowdown::bowdown:
 
Playing the unit some more and the tube in V4, current seems to get higher each time I turn it on. It's up to ~55mA now. I think I'll be looking into a matched quad of 7591s at this point for stability.
 
GT -- You might continue to use that tube a little bit more. If it is a vintage tube that has not been operated (for any length of time) for a very long time, then it may just be gassy. If that's the case, the condition will correct itself with normal use and ultimately become stable again at it's normal characteristic. It should not take long for this process to play out (a few hours at most), after which the tube will be fine. Of course, there are other things that can cause this behavior too, but letting this process play out to see if it is just gassy will at least prevent you from discarding what might otherwise be a perfectly good tube.

Dave
 
Thanks for the info. I was really bummed about the thought of junking these original tubes. I'll run it for a while and see how the tube responds.
 
You could always give it a high colonic! Guaranteed to get rid of gassssssssssssssssssssssssssssssssssssssssssssssss.
 
I emailed Jim McShane and got the spiel about changing resistors and coupling caps with modern 7591s, so I just wanted to make sure that with the cathode bias setup, I don't need to do that. Larry, I think you said that you run Tung Sol reissues in your X-101-B? How do you like them?
 
I know you asked Larry, but I'm using Tung-Sol 7591A reissues in my 500-C and I like them. I had to dial down the bias settings though as they are a lot hotter than the Sylvania's I replaced.
 
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