1. Time for some upgrades in server hardware and software to enhance security and take AK to the next level. Please contribute what you can to sales@audiokarma.org at PayPal.com - Thanks from the AK Team
    Dismiss Notice

X-202B heaven

Discussion in 'Fisher' started by dcgillespie, Oct 25, 2013.

  1. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,768
    Location:
    Ball Ground, GA
    This unit likely represents Fisher's finest effort in all of their integrated offerings, being offered at the peak of Fisher's tube product years, and representing the best of their top of the line 202 amplifier series. It basically only gives up the power capability of their most ambitious integrated, the EL34 based X-1000. The B retains all of that unit's most important and usable features, only dropping the dubious "kitchen sink" features of its bigger brother -- features that could only be categorized as marketing (rather than use) oriented tools. When you compare the features, power, and overall performance this unit offers, it is also likely represents Fisher's most practical top of the line model as well.

    Thanks to Audiodon, I now have my own superb example of this wonderful piece of Fisher history. And, it came complete with all of the cleaning details applied to it that he has developed over the years -- making this truly one fine example of the breed indeed.

    Bone stock, but amazingly, after giving it a thorough going over, the only thing I can see that needs addressing is the two bias pots which appear to have succumbed to heat over the years -- which are also parts that currently occupy my junk box IIRC. It came with no less than 6 tan based Westinghouse 7591s -- four of which were well used Fisher labeled, and two others which tested nearly new by power output. To the latter two tubes, I was able to source two new coin based Sylvania tubes, of very similar tooling to that of the Westinghouse tubes. Together with the two strong Westinghouse tubes, it should make for a very nice quad of output tubes. This unit will remain bone stock, with the exception of adding individual output tube cathode resistors.

    The weekend is already tight, but I hope to get these tubes installed and bias up to get this beauty on line.

    Along with the X-202 rescue unit project, this unit will become part of my group of reference units, representing the best of what was from Fisher's integrated lines (the 202 for the 7189, and the B for 7591). These units will then act as a base line to measure any potential improvements from modifications made to other units against, to see if they're all they're cracked up to be -- be it mine, or anybody elses!

    Really happy to have this piece, and a tip o' the hat to Don for making it available!

    Dave
     

    Attached Files:

    baconbadge and bobva like this.

     

    Please register to disable this ad.

  2. derekva

    derekva This ain't no picnic... Subscriber

    Messages:
    4,848
    Location:
    Woodinville, WA
    Man...AK needs a 'like' button in the worst way.

    Looks very nice, Dave, and I bet it sounds even better than it looks!

    -D
     
  3. audiodon

    audiodon Addicted Member

    Messages:
    9,293
    Location:
    RIP 1957-2018
    I was fortunate enough to stumble across that one a few years ago. I never did anything but play it . . . fortunately.
    The kids will miss it. My youngest used to use it to demonstrate her turntable skills playing a Lady Gaga picture disc for her friends on sleepovers.

    I've grown increasingly fond of the coin based Sylvanias also. A fine tube.
     
  4. sony6060

    sony6060 Super Member

    Messages:
    4,416
    Location:
    New York, NY
    Don certainly owns or in this cased owned some fine Fisher products. I am playing with a Fisher TA-600 at the moment. This is a well built receiver. The power transformer looks to be 500VA- a brute of a transformer.
     
  5. bhamham

    bhamham AK Subscriber Subscriber

    Messages:
    2,030
    Location:
    Austin
    Dave, you do a great service for us Fisher lovers. What a lovely X-202B!


    Hi Sony, just bought a nice, working TA-600 from BarterTown. Won't be able to fool with it for a while but really interested in what mods you do to yours. I hope you start a thread for it. Thanks.
     
  6. sony6060

    sony6060 Super Member

    Messages:
    4,416
    Location:
    New York, NY
    Ok.
     

     

    Please register to disable this ad.

  7. AlTinkster92

    AlTinkster92 AK Subscriber Subscriber

    Messages:
    3,282
    Location:
    NC
    Dave that's one fine X202B, pics of under the hood please?
     
  8. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,768
    Location:
    Ball Ground, GA
    Should be able to put some up tomorrow.

    Dave
     
  9. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,768
    Location:
    Ball Ground, GA
    Underside Shots

    As promised, here are some underside shots of the B.

    Compared to the 202, parts quality has improved, as has the build quality. That's not to say that these things were necessarily bad in the 202, but that both are notably better in this unit -- and indeed in all the Fisher equipment produced when this B was.

    Besides the change to bigger output tubes for increased power output and a SS power supply, there were other notable changes as well.

    Output transformers are bigger of course (not wider or deeper, just taller), but the power transformer is notably smaller. No longer needing a 5 volt rectifier tube heater winding, only a 1/4 of the HV winding by turns (but with larger diameter wire), and with slight adjustments to the 6.3 volt and low voltage DC winding, a more efficient transformer can be made that uses only 2/3 of the same size laminations as used in the 202's transformer, yet draws the same amount of power from the grid, while allowing the amplifier to produce more power output. Keeping the weight basically the same then yet producing more power output, is it any wonder that manufacturers changed over to the no tube voltage doubler supply virtually overnight? I don't think so! Cheaper costs, greater efficiency, and cooler operation are dollar signs to any manufacturer!

    Fisher also wisely changed from 12AT7 tubes to 12AX7 tubes, as it is as hard to find a noisy 12AX7, as it is to find a quiet 12AT7.

    Minor features also changed: With the B, I can finally have that wonderful hifi reverb capability I've always longed for........OK, so maybe that wasn't such a big deal after all. But the filter switches got properly designated, more low level inputs are provided as are more appropriate EQ settings as well. But some over achiever was also able to sneak in that goofy Tape Monitor/Aux bleed circuit too, so you can't win them all.

    But, the B still retains the wonderful close tracking double loudness tap volume control, the active feedback type tone controls, and the excellent double balance control design, that actually raises the volume of the targeted channel when the control is rotated in that channel's direction, to compensate for the loss of the channel being attenuated. How neat is that?

    Also important, the interior surface of the chassis in the B has a very nice milled finish to it, that not only improves appearances, but also virtually guarantees a solid ground connection over time by all the various T-strip ground terminals, tube socket ground terminals, and the like. As a result, whereas in the 202 there are numerous ground wires snaking their way through out the entire chassis, in the B, these are virtually absent. Yet another good change to help reduce labor construction costs, and improve long term durability.

    Finally, the B reverts back to using an ECC83 for phase inverter duty, which is of no real consequence since the 7591 output tubes are so easy to drive. The 7247 tubes of the former unit are still retained however, now serving in the 2nd line stage/cathode follower output function previously performed by a 12AT7. This results in a very quiet design, with only a fraction of the rejects that the 12AT7s would have produced due to noise. Overall however, the design of the B version retains the same basic format of the former 202 model.

    Dave
     

    Attached Files:

  10. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,768
    Location:
    Ball Ground, GA
    DC Balance Pot Repair

    As I had originally commented, the only obvious thing I saw about this amp was the melted DC Balance Pots. Don't know what it is about Fisher Red adjustment pots, versus Fisher Blue pots as used on the X-202, but the red ones tend to deform with heat.

    Luckily, I had a couple non-deformed red pots in the junk box I could steal the wiper assemblies out of, making for an easy repair. I didn't even need to unsolder the pots from their leads. I just snapped them out, took them apart, cleaned them up, and put them back together again with the wiper assemblies from the junk box pots. With a decent set of NOS output tubes installed, I thought it best to fix the pots first before even applying power.

    Pics:

    1. The obvious deformity can be seen. One leans right, while the other leans left.

    2. Carbon track of the second control. The spot on the track is not a crack or worn spot, but where the wiper had obviously rested for a very long time.

    3. First pot all done and reinstalled.

    4. Job completed.

    5. They look and rotate smooth and normal again!

    Dave
     

    Attached Files:

  11. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,768
    Location:
    Ball Ground, GA
    Modifications

    I've really had to work hard to hold off doing some of the most logical, common sense modifications to this unit. And for the most part, I have.

    For example, I would love to replace the Reverb In/Out jacks with Preamp Output/Power Amp Input jacks, and make the Common of the speaker outputs operate at ground level. But.......I have beaten myself back from the edge.

    However, I could not except the potential damage that could be done by not installing individual cathode resistors and test points, and not installing Screen Stability resistors, as the design of this amp is very close to the edge of the danger zone, where circuit conditions actually encourage output tube arc events.

    I studied the amp for a number of days to determine the best way to install these safe guards. And, in keeping with my goal for this unit, I wanted to install them in such a way as to have them not draw attention to themselves once installed, and not do any physical modifications to the unit in the process of installing them. That way, the unit could be returned to stock conditions if need be.

    The pics tell the tale of how these mods were installed, which I completed before ever applying power to the unit. I'm glad I did, as the Westinghouse tubes are mismatched enough as to not fully allow for equalization of the quiescent currents between the tubes within the range of the DC Balance control provided. Without the test points, I would not have known how imbalanced these tubes were without doing matching tests before hand.

    Pics include:

    1. The chosen test point format and location. A standard four post barrier strip that is held down firmly with industrial two sided tape. On the back set of screws are connectors for the internal leads to connect to, while on the front set of screws are connectors to either clip to, or insert a meter probe into for taking measurements. The posts align with output tube location, making it easy to know which tube(s) you are measuring. If the back cover ever needs to come off, the back set of screws can be removed, disconnecting the connection leads to the strip, to allow for the cover's easy removal.

    2. The back side of the mounted strip, showing the twisted leads for each channel's test point leads, disappearing down through two of the cooling holes in the chassis.

    3. The finished output stage wiring. Close inspection will show that the original screen wiring was removed, and reinstalled to pin #1 on each output tube, with these pins acting as a buss connection point to the screen grid B+ supply. Quarter watt 100 ohm resistors then run between pins 1 & 4 on each output tube to effect their installation.

    Also, all of the original cathode wiring was removed, although the original cathode resistors were left in place to effect the least amount of invasive surgery. The twisted leads from the barrier strip were routed with existing wiring over to the appropriate cathode terminals (pin #5), with quarter watt 10 ohm resistors added from each of these pins to ground.

    4. An overall shot of the bottom side after all the work was done, to show how much the modifications blend in with the existing wiring.

    5. An overall shot of the rear panel after installation. It does change the look of the rear somewhat. But I consider it acceptable considering no physical modifications were made, and the output tube protection these modifications provide.

    The unit is all biased up properly now, so with the modifications all done and the DC balance pots repaired, tomorrow I will get to hear a 202B for the first time, and not have to worry about how well the output tubes are being treated!

    Dave
     

    Attached Files:

     

    Please register to disable this ad.

  12. bhamham

    bhamham AK Subscriber Subscriber

    Messages:
    2,030
    Location:
    Austin
    Nicely done, Dave :thmbsp:

    I really like the 4-post barrier strip for the output test points - very descrete and not imposing.
     
  13. audiodon

    audiodon Addicted Member

    Messages:
    9,293
    Location:
    RIP 1957-2018
    Oh those pesky output tubes.
    I spend a lot of time moving tubes around and selecting between tubes to set up matched pairs. It's not that easy a task.
    In both 7591s and 7868s, I have more than a dozen used tubes that I cycle through to come up with matched pairs. I reserve them for integrateds now and go straight to EHs for any receiver that will allow the EHs to fit in the case.
    This is all in-circuit matching. My Hickok 800A tester seems no better than an emissions tester at sorting out power tubes.
     
    Last edited: Nov 1, 2013
  14. AlTinkster92

    AlTinkster92 AK Subscriber Subscriber

    Messages:
    3,282
    Location:
    NC
    Dave I see you retained the original ERO FOL II's, do you know the values on them? I think I would like to replace the orange drops in mine with the originals if I can find them. The Orange drops sound ok, wonder if its worth it..
     
  15. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,768
    Location:
    Ball Ground, GA
    Hi Al -- Yeah, they're the original couplers -- ERO .047s. Since this is a reference amp for me (a reference for Fisher sound), I am intentionally trying to keep the unit as stock as possible, replacing or modifying only that which is absolutely necessary. These caps are all good in my amp.

    Also of note, starting with the third (and last) generation of the X-202, Fisher started choking off the drive capability of the phase inverter tube. Then, starting with the X-202B , the famous all-out choke hold resistor on the phase inverter tube appeared.

    This was a design noose that Fisher gradually tightened as time went on. By the time the 400 receiver was produced, this noose was all but choking the poor thing off. This is a topic I covered in some detail with my rework of the Fisher 400 receiver.

    I hope to get some good listening in on my 202B this weekend, but in comparison, my 1st Gen X-202 does not include such throttling measures. In my 202B, the noose is in place, but is comparatively loose compared to later designs. In any event, I will be interested to see if this produces any sonic impact that I can detect between the two units.

    Dave
     
  16. AlTinkster92

    AlTinkster92 AK Subscriber Subscriber

    Messages:
    3,282
    Location:
    NC
    Thanks Dave, glad to see you retained the originals, just wondered if its worth the time and effort to obtain and install the originals over Orange drops is all.. Yours look great!
     

     

    Please register to disable this ad.

  17. nj pheonix

    nj pheonix AK Subscriber Subscriber

    Messages:
    10,041
    Location:
    New Jersey
    Dave,
    On the verge of picking up a 202b, I went to read your thread (this one) I was somewhat surprised it ended (so to speak) in mid form. Where you left off, you were going to use it as a reference against other pieces and do some comparisons. So, with the not so choked off phase inverter, is there much of a sonic difference to the completely choked off 400?
    As other pieces have come through and upgrade (improvements, EFB for instance) have been made, sorry if I'm putting you on the spot here. Would you say sonic benefit, measurable benefit, and /or just reliability, stability improvements were noted.
    Did you ever work through the EFB for the 202B?
     
  18. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,768
    Location:
    Ball Ground, GA
    Now those are great questions! As for my X-202 and X-202B, those units -- other than needed repairs and/or safety upgrades made to the output stages (individual cathode and Screen Stability resistors added) -- have remained completely stock, for them to remain as reference pieces. It was hard to resist the temptation to make some common sense changes to them, but I have succeeded in leaving them stock, which I'm happy I did. If I started modifying the audio circuits, then they could no longer be considered as reference pieces. In fact, it is precisely because I have left them stock that I can make the comparisons I have.

    A comparison with those units to my 400 is somewhat unfair, as both integrated amplifiers have superior line/tone stage sections to that of the 400, so while a comparison can be made, that fact must be considered. Also of course, the 400 can't be compared to itself, as without a second stock receiver, no direct stereo comparison can be made. However, a channel to channel comparison was made as that project went along, since I used one channel for development, and one left as stock for comparison until the very end of the project.

    A number of points of comparison then:

    1. From a purely sonic standpoint, the modifications that made the biggest audible difference in my 400 was the installation of the small buffer stage, and the development of the new power amplifier section: Specifically, the development of the new NFB and HF stability circuits in the power amp section. Both of these efforts worked to add significantly to the detail presentation (the buffer circuit), while maintaining the smoothness of presentation (the new NFB and HF stability circuits). Fisher achieved smoothness as well, but did so through a rolled of HF response to cover the rather ragged response produced by the power amplifier NFB and HF stability circuits. The result was a loss of detail. The effect of the modifications was to maintain the same non-fatiguing sound, but now with all the detail that was subdued before.

    Other modifications more addressed matters of practicality (tone control defeat, Preamp Output/Power Amp Input jacks, etc), but also, EFB(tm) was added to this receiver.

    As I have grown older, my need for tremendous power reserves has waned, yet has still allowed me to enjoy very high quality music. At one time, I wanted no less than 75 watt RMS of highly stable, very low distortion vacuum tube amplification over the entire 20 Hz to 20 kHz bandwidth in each channel -- and with careful design and the use of regulated power supplies, I achieved that end. I still have that equipment -- for (again) reference. I came to realize however, that over years of comparison, and with speakers of 100 and 101 db efficiencies, all I was really doing was feeding the power company from the current draw such units pull, on top of the additional A/C such units require for comfort. The actual need for such power capability was rarely, if ever needed. As a result, I have found that an honest 35 watts RMS per channel is all that I could ever want in power reserve capability.

    And yet, moving down into this 35 watt and under power class, it is still quite obvious that even with that reserve, I routinely only use maybe 1 or 2 watts per channel peak during elevated listening sessions. To be honest, under those occasions, the use of EFB will have little sonic impact: There simply isn't enough variation of operating conditions happening for it to play a significant roll. As a basic point then, the smaller the amplifier capability, the more EFB can play a SONIC roll, as smaller amplifiers operate over more of their available power capability range. To that point, the sonic improvements made by the addition of EFB to the small Dynaco amplifiers has been credibly validated literally the world over now. Also, one of the amplifiers I find myself gravitating to most is my EFB clone of the Fisher SA-100. Plennnty of low distortion power when needed over the full audio bandwidth for my speakers, without triggering the A/C. On the other hand, remember the 100 watts RMS/channel Bogen project? EFB also helped significantly to produce hifi like qualities at elevated power levels with that project, but honestly, who uses that much power except for maybe at parties?? Designing for that exception gets crazy -- I know, because I used to do it at one point in time. Frankly, the accomplishment is more of a personal goal in achieving that level of performance, rather than it is any practical accomplishment. As you mature then in your audio experience, you realize that having equipment that is based on the exception rather than the rule is just nuts. Even Tim the Tool Man Taylor started to outgrow his lust for mo' power, mo' power, mo' power! by the end of the show. It's best to use practical everyday equipment, and store the party equipment for when those occasions happen.

    Against this backdrop then, what did specifically EFB do to my 400? At normal listening levels, frankly, very little. On the other hand, the output tubes love it. The output tubes in the modified late version 400 idle at about 8 watts per tube, and produce distortion levels at elevated power levels that the original design could only dream about. Whats more, operation at continuous good time party levels with EFB (which my modified receiver has done!) lets the output tubes produce that power level as little more than just a walk in the park, where as the original design would have converted the output tubes into miniature flash lights due to overloading of the screen grids. So the bottom line is that EFB in my 400 is not so much of a sonic enhancement, as it is a safety device, and a means of adding significant amounts of life to the output tubes. The other performance enhancements it provides however require conditions of high power output to really appreciate.

    Ditto for removing the choke hold on the phase inverter. It was clearly a safety device Fisher included (primarily) for the output stage, but at normal listening levels, you'd never know it's there. It can be demonstrated in the lab however for the choke hold it creates at elevated power levels.

    Ultimately, this is all very similar to the person who has a nice, dependable car to get them where ever they need to go, versus the person who has a highly tuned, high performance vehicle that can run circles around the daily driver at will. It's simply the knowing that the performance is there. But if you're both stuck in traffic on Peachtree Steet, or cruising down the boulevard in traffic at 45 MPH, none of the high performance stuff comes into play. But still, it's just knowing.......

    Good questions that deserve good answers. I hope I've helped to answer some of them!

    Dave
     
  19. AlTinkster92

    AlTinkster92 AK Subscriber Subscriber

    Messages:
    3,282
    Location:
    NC
    Dave as always a great informative read. Best thing I ever did was bring my X202B down to you, as well as the 800C. I learned a ton from you and now have 2 great sounding reliable Fisher pieces, which I listen to daily. Next trip one day, sure would like to hear that SA100 clone. If I recall you said it was around 65 wpc??? (way above norm) :thmbsp:
     
  20. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,768
    Location:
    Ball Ground, GA
    No -- In conventional circuits, 7189s are not capable of that much power in standard push-pull form. It will however produce 25 watts RMS per channel individually, or even 22 watts RMS per channel with both channels driven -- which is definitely above that of the original SA-100, and will give late model Fisher 400s a real run for their money.

    Dave
     

Share This Page