Not sure what your skill level, so I'll presume you have at least a rudimentary understanding tube electronics in particular. If your skill level is higher you can gloss over most of this. But getting any information and or Re-enforcing it is always a good thing. We'll start with the Manual then Wall voltage.
Service Manual is on hifiengine.com
https://www.hifiengine.com/manual_library/fisher/x-101-b.shtml
Need to Register at https://www hifiengine.com It's free. Hopefully it'll be on http://www.fisherconsoles.com soon.
At 125V
Definitely needs to be dropped to at least 120 which is the max voltage the unit is rated for (105v-120v). All voltages in the Service manual are based on 117Vac. You can make a "BUCK TRANSFORMER" and drop the input voltage 6v to 12v. A 6Volt Buck transformer will drop that 125 to 121v to a range of 119 to 115V. They work good but you are still dependent on wall voltage staying somewhat stable.
A variac (basic adjustable transformer) will do the same thing as a buck transformer, but you have more lee-way in that you can set it for pretty much any voltage you want. I'd set a variac to 115v or whatever voltage it's set to and still gives you heater filament voltages as shown below and let it ride. A CL-80 Thermistor installed on the HOT SIDE of the power cord will drop the voltage a couple volts and "soft start" the unit in that it ramps up the voltage over a few seconds and lessens arcing in the power switch which helps extend it's life. .
Double check the 12ax7 heaters on V1, V2, V6, V7 for 12.6VDC. V1, V2, V6, V7 are the tubes connected to the cathode bias circuit. The heater's on these will probably be around 11V or so, as the cathode bias uses the heaters for resistance in the circuit. Due to tube internal differences and wear, the voltages might be spread out some from 10 to 12 or a little more or less but the sum of all 4 will be 44v or so. Test from Pin 4 to 5. If the heaters are ok, then the rest of the voltages usually fall right in to line. At 121vac my plate voltage is 430v (vs. 424 factory value) and screeen is 385v (vs. 379 factory value). This is due mainly to the higher input voltage. I did however install a CL-80 Thermistor (Current limiter) inline at the power cord. Will reduce the incidence of switch arcing and cap over(surge) voltage on startup. Highly recommended to save the switch and increase it's life.
V3, & V8 are the other 2 12AX7's. They are NOT connected to the bias circuit. But the heaters will test at 6vac (20Vac on meter or AUTO) as they are wired in parallel the output tube heater circuit). So test the two filaments separately. Pins 4 to 9 and then Pins 5 to 9. Both sides should give you approx 6.0VAC to 6.3VAC. Adjust your input voltage until these tubes read 6.0VAC to 6.3VAC (No more than 5% higher tho or 6.615Vac)
The Bias is Cathode biased, with a set voltage target of +44 on pin 5 on all of the output tubes. Mine is set for 43V. Most all of the 7591/7868 cathode biased FISHER integrated amps run btwn 40v and 44V on the cathodes.
Cathode biased units are highly dependent on tightly matched (cathode draw) tubes, it's best to have output tubes that are very tightly matched(less than 5%) for current (cathode) draw. I would check all of them and if they are off by more than 5% (2.2 volts.) you will have increasing amounts of distortion from the imbalance and the tubes that draw more will be working harder eventually leading to runaway tubes down the road. Take the highest reading tube and make that your bogey tube for this purpose. DO NOT exceed the cathode voltage rating on this tube so the rest are in the ballpark. You'll end up burning out
the higher testing tubes. The rest of the tubes should be within 2.2 volts(5%) of the target voltage for closely matched tubes.
HOWEVER!!!! To determine actual current draw on each of the tubes,(and this is a more accurate way to determine health and performance of the tubes) Install 1/4w 10 OHM resistors on each tube on pin 5. Remove the Cathode wire from Pin5. Solder and heatshrink a resistor to the wire, leaving a small bare area of the resistor lead for the meter probe, then solder the remaining end of the resistor to PIN 5.
Now you probe both ends of the resistor in volts(20V setting or AUTO DC) and then convert to ma. for example .32v x 10ohms= .320a or 320ma. Note the voltage on paper for each tube along with the plate and screen voltages. Then convert to ma. Which reminds me, something you can do to limit or negate any tube arcing (output tubes) is to install screen stability resistors. These are 100ohm 1/4w resistors. The jumper between pins 4 & 8 on the tube socket stay, but if there is a jumper between the tubes cut it out. Solder in a 100ohm 1/4w resistor (metal oxide) from pins 1 to 4. Pin 1 is unused in the 7591 and is now used as a tie point. Take the screen voltage lead, and solder to pin 1 on each of the tubes. INSULATE BOTH METER PROBES with HEATSHRINK TUBING all the way down to the end(leave enough to probe with), so you don't slip and have an AW SHIT MOMENT
(almost all of us have BTDT) and short the probe on something else and end up replacing more parts and possibly the meter and probes. You're done, and the tubes are protected from arcing from the screen in the tube. It's very cheap insurance and will keep it running a long time.
One niggling problem I had was a crackling noise
on startup which I tracked to the Speaker phase switch. If it's in the FWD (ON) position it crackled like crazy and I'd get intermittent speaker cutout's for a couple minutes. The problem with the switch is that it's right up against the 5AR4
and gets overheated and breaks down. I ended up replacing the switch with a new switch which cured it.
Make sure the speakers are in phase with the switch in the off position.
The X-101-b is a very good amp, but can be a bit finicky at times especially with the Balance and Stereo Dimension pots. Leave the Stereo Dimension all the way clockwise. Yearly DeOxit Treatment withDeOxit D5 and then Faderlube F-5(don't forget this part or you'll be looking for pots as the lubricant is washed out with the cleaner and the carbon tracks can be wiped off due to high friction)will help it a lot. Mine runs very well
now that I built a shielded box around the phono inputs.
I'm running Tung-Sol ReIssue (Russian) 7591s in it at 43V on 121V wall voltage. The 4 cathode bias 12ax7's(V1,V2,V6,V7) are running at 10.4vdc to 10.6vdc,which is a bit cool, but the tubes take a little longer to come up to full voltage. It won't hurt them. . This is normal for Cathode bias The 12ax7's are a mix of Tung-Sol Russian 12ax7's in V7, V2, Sylvania's in V8, V3 and Sovtek 12ax7LPS in the Phono(V6,V1) Section. The Voltage Amp/Phase inverters(V3, V8) heaters are running at 6.4vac. Heaters on the 7591's are at 6.4vac. It's Nice and Quiet. Plate and Screen voltages are within 5% of nominal with the Tung-Sols. The Rectumfrier is a Mullard IEC Branded 5ar4 made in the 70's. It keeps the voltages where they are suppossed to be a lot better than the SOVTEK I bought. The Sovtek sags about 18volts more than the Mullard.
Try and get output tubes from someone like Jim McShane who has a custom setup and runs all tubes at or slightly above operating voltages and tests for current draw at the cathodes, then matches quads to that number at full voltages. Most stores just test them on tube testers and check for gm and matches for gm. I have no affiliation with Jim McShane but am a highly satisfied repeat customer for 10 years now.
If the original Can caps (Main Filter Caps) are in it, I would suggest they be checked for puking
(look at the bottom and there is a vent hole in the middle of the bottom plate). If you see the rubber seal poking it's head out of the hole, or you see what looks like a off white caking at the center hole, replace all of the can's. (There's 2 Can's a 3 section and a 2 section). These cans do not have exact value per section replacements.
You can either gut and stuff with individual caps(105*C at least), or if you're lazy
like me, get custom replacements.
www.HayseedHamfest.com makes Custom Can caps. Cost of these is comparable with the C-E caps that Antique Electronics Supply (AES)in Az sells (
www.tubesandmore.com) but, the AES Can Caps..........
1.) these are more suited to Guitar amps or custom assembly's, No exact value replacements are available.
2.) Temp rating is usually 55*C with a spattering of 85*C caps.
No affiliation with HAYSEEDHAMFEST and a repeat customer. C-19 is the only other Electrolytic in the unit. It's a 20uf 350V cap. Suggest a 22uf 400v Nichicon UPW series cap. or if you prefer an axial cap the TVX Series. Both listed at the end of the post with Mouser part #'s.
You can replace the Mylar caps with Illinois MKP axial's(Polystyrene) or AES yellow house brand caps (polyester). Cornell Dublier DME series caps or Panasonic ECQ for Mylar replacements (especially the Dark Brown drop caps(DOG TURDS)). If the coupling caps are EROFOIL caps, test them. >90% are still good. I just leave them in now. CERAMICS leave them in unless broken.
Suggest keeping the tube shields on the 12ax7's as RF from a tuner close by and transformer electromagnetic fields can raise the noise floor. The shields will prevent this as they are grounded to the chassis.
Everything I've listed here will allow you to have an amp that will run a good bit warm (temps under 140*F) but well within operating spec. Another piece of gear you might want to consider is a Laser or Infrared Stand off thermometer. Transformers run hot, it's a fact of life just like death and taxes. At nominal input voltage of 121VAC mine runs about 140*F on the POWER Transformer after a couple hours, checking with a Harbor Freight Laser Thermometer. The side facing the 5AR4 runs about 5*F higher due to radiant heat from the 5AR4. HANDS are a LOUSY
indicator of how hot something is. They can't determine temps other than it's HOT!
. A hand held IR or Laser Thermometer is a good idea.
Here's a few pics of mine. It's hooked up to a KM-60 Tuner, and a Dual 1249 with a Shure V-15-III . For the guys wondering
that white thing is on the KM-60, its a white tube box. In the Dark it glows a diffused light and isn't obnoxiously bright.
View attachment 1111634
View attachment 1111632
View attachment 1111633
View attachment 1111635
C-19 replacement.
Radial UPW...22uf 400v Mouser #647-UPW2G220MHD
https://www.mouser.com/ProductDetail/Nichicon/UPW2G220MHD?qs=sGAEpiMZZMtZ1n0r9vR22RxOeB5kHP9BZ1hlfnE2oR0=
Axial TVX 22uf 400v Mouser #647-
