Modifying the Fisher 400 with EFB

Discussion in 'Fisher' started by dcgillespie, Mar 16, 2013.

  1. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    It seemed like such a simple job. By now, I've installed three of the four basic EFB(tm) configurations into three different 6BQ5/7189 designs, and all with superb outcomes: output stage distortion was significantly reduced (by at least 50%), and output tube quiescent dissipation was lowered to typically 75% of Design Center ratings, or 65% of Design Maximum ratings for the tubes -- as opposed to so many designs that operate these tubes at 90% or more of their Design Maximum ratings. Most importantly, these reduced dissipation operating levels now represent the lowest distortion operating point for the tubes, producing significantly less distortion than the stock design did at much higher dissipation levels. Couple these performance improvements with the significant effect this has on tube and component life, and its easy to see why those who have installed the EFB modification in their equipment to date have heartily endorsed it.

    I've always subscribed to the notion that a job worth doing is worth doing well. I wanted to install EFB in a 7591/7868 application, because soooo many pieces of equipment use those tubes, and many of those designs operate these tubes very hot as well. Therefore, a design with these tubes became the next logical target for EFB, and my own Fisher 400 became the unit of choice to take aim at. Except, I've come to find that the 400 is kind of like the proverbial thread that just keeps coming when you pull on it.

    First up, was identifying what I had. My unit is a latter unit with an extra FM limiter stage installed, and a revised bias circuit to cool the output stage down. Except, that when I was making my stock performance measurements to develop a base line for comparison, I found the stock unit could not meet the original published specifications for the 400. What gives? Some quick checks showed that with the turns ratio in the OPTs installed in my unit (original Fisher devices), it could not possibly produce the power output claimed by Fisher -- even on a sunny day with matched NOS tubes and a high line voltage. Checking the Service Manuals showed different part numbers for OPTs in the two basic versions, and searching further turned up yet more clues pointing to an OPT change as well. The point, and bottom line of this information, is that the base information and the EFB operating points I've developed in this exercise potentially only relate to the latter unit. I wish it could relate to all of the versions, but based on all available information, those wascally Fisher engineers apparently decided to make a change in the output stage -- and one that would appear to be big enough so as to require different EFB operating points for proper operation in each version. Make no mistake, both versions can benefit greatly from EFB, but the operating parameters would be different in each application.

    So what have I got with my "version II" of the 400? Basically, a unit that produces 25 watts RMS per channel maximum (or 22.5 watts RMS both channels driven), from 30 Hz to 20 kHz. This is not as powerful as the specifications state, or as possibly the original version of the 400 is, but is still a very nice power niche to be in with efficient speakers. Distortion is pretty typical with single channel performance at 25 watts returning 3.75% THD @30 Hz, .5% THD @1 kHz, and 1.90% THD @20kHz. With both channels driven at 22.5 watts, distortion at 1kHz doubled to just over 1%, while distortion at the frequency extremes more than doubled. This is all as driven through the Aux/Tape input with the tone and balance controls accurately centered.

    Actually, I'm ahead of myself however, because to deliver even these performance results, I had to deal with the rather unique phase inverter circuit, so there is a lot to tell with this tale. As a result, this effort really amounts to a redo of the power amplifier circuits in the 400 version II, but as a teaser, some trial results in developing EFB for the modified power amplifiers returned a 1 kHz THD of just .22% with both channels driven, and a 20 kHz THD of only .80%. This is nearly an 80% drop in distortion at mid frequencies and over a 70% drop at high frequencies, while at 30 Hz, distortion fell to about 2.5% with both channels driven at 22.5 watts output each. The transformers themselves are the main factor at this frequency in terms of delivering a low distortion output.

    This was also produced from a very good and well matched (+/- 1 ma) quad of NOS 7868 tubes from Sylvania, of late 70s origin. I should also say that with the unit operating at 117.0 VAC, I found tubes that would idle as low as 26 ma, and some as high as 40 ma in this unit, with the vast majority of tubes idling at about 34 ma, which I believe was the target quiescent current draw by Fisher for this version. This is the current level each tube in my NOS quad drew in the stock design, and the current draw the base measurements were made at. This results in a stock dissipation of just under 14 watts per tube, which is hardly a bad place to be for plate dissipation in these tubes -- except for how closely they are physically spaced together on the 400's chassis, and particularly so when a cabinet is used. Overall however, in the Version II design, plate dissipation is not nearly the concern with these tubes that another factor is.

    In the Version II design, because of an apparent change in OPTs, the screens of the output tubes get particularly pummeled at elevated power output levels -- much more so than than they already do with more traditional 7591/7868 operating parameters. Indeed, at full power output, they show themselves to glow quite well, and with even the slightest overload, they illuminate the inside of the bulb. Fisher was obviously aware of this, which resulted in the goofy phase inverter design that they used in many of their 7591/7868 designs. In this unit however, the screens are of particular concern, so the measures taken with the phase inverter are particularly severe. Fortunately however, since this is a pentode design, the screen control portion of the EFB bias supply regulator can easily deal with the screen issue in a proper way, allowing the operating parameters of the phase splitter to be altered to a linear portion of its operating curve. For all of the performance figures quoted here, this change had already been made. All of this will be dealt with over the course of this thread, and will take some time to lay out, but I hope those of you interested in the 400, or EFB in general, will follow along.

    Oh yeah, initial results from applying EFB to the Verson II 400 yielded the exact same amount of power output, but with the significantly lower distortion levels mentioned above, and no screen heating of any kind -- even with severe overload. And the quiescent plate dissipation? Just 8.5 watts per tube, with each tube only drawing 22 ma total cathode current each. That's just under 45% of the Design Maximum rating. The tubes gotta love that! When finished, this should prove to be a very worthwhile modification in deed.

    Some pics:

    1. The candidate.

    2. The output stage as Avery wired it. I got started and thought I better snap a pic before I got the first connection completely disconnected.

    3. New 10 ohm cathode resistors are installed to know just what the heck is going on.

    4. Developing the basic EFB parameters for the Version II. At this point, the unit is developing 25 watts RMS as a single driven channel with under .20% THD.

    5. The resulting quiescent current draw at the EFB low distortion point.

    Thanks for following along!

    Dave
     

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    Last edited: Mar 29, 2013

     

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  2. bhamham

    bhamham Super Member

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    The Master at work :thmbsp:

    This should be a giant leap for Fisher-kind. Many thanks Dave for all you do to help guys like me.
     
  3. audiodon

    audiodon AK Subscriber Subscriber

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    Student #2 subscribing to this thread.
     
  4. sony6060

    sony6060 Super Member

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    I am interested. I have serial number 55256. What I noticed is this Fisher 400 sounds excellent. Using 98dB efficient speakers, I am only using a few watts keeping distortion low in the power output range.
     
  5. notdigital

    notdigital AK Subscriber Subscriber

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    I'm going to foxhole in the back of the class with the hope that Dave doesn't call on me .......
     
  6. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    I'm in! Although I'll be waiting for the 500-C/800-C version and cataloging the 400 version should I get another.

    Larry
     

     

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  7. audiodon

    audiodon AK Subscriber Subscriber

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    This might be the only one.
    The principles should be the same on any fixed bias Fisher, since the outputs are coupled similarly, bias is similar with a 4-diode bridge, and the OPTs have center taps but not ultralinear taps. The bias circuits with two diodes are different, I think.
     
  8. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Don; What do you mean 2 diode versions? All I've ever seen in the 400 to 800-C is a bridge for bias and tube heaters, with 2 diodes separate on the HV circuit.
     
  9. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Don; You mean integrated with a 1/2 wave rectifier (2 diode)? OK I've seen them.

    Larry
     
  10. Mid-Fi

    Mid-Fi AK Subscriber Subscriber

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    I'm in.
     
  11. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Turning Facts into Stated Goals

    OK. Now that the little excursion down the OPT road has currently played itself out, a plan to proceed can be developed. The excursion wasn't wasted time however, since the intention is to have the installation of EFB be as easy and appropriate for all possible versions of the 400. Examining the OPT issue was necessary to see just how big an issue it was.

    Moving forward from here then begins with accurately summarizing what is known. From that, and knowing the capabilities of EFB, realistic goals can then be established. Finally, a plan can then be developed to implement it .

    Considering on all the information gathered to date, we know that:

    1. Based on my own data, and that supplied by Larry Derouin, when operated from a 117 VAC power source (which produces the correct heater voltage for the AC powered heaters), the 400 could accurately be rated as producing 20 watts RMS per channel, based on both channels being driven. Both my unit, and an early version independently tested support this rating, with neither being able to produce the RMS power output level that Fisher stated for either single channel, or a both channels driven conditions. While it was somewhat long and draw out sussing out this information, it was important none the less to determine if there was an initial problem with my unit, or if the various versions differed significantly in power output capability. Bottom line: The early and late production units produce substantially the same power output, with neither meeting original published specifications. The installation of EFB will not change the power output capabilities of the 400 receiver.

    2. The output transformer saga determined that at least different venders were used, that some have a different part number assigned to them, and that circuit changes are associated with the units employing different transformers by part number. Since appearances suggest that the core size of all transformers used is the same, and the ultimate power output, power bandwidth, and distortion performance is (or can be) basically the same for all practical purposes between at least one early and late version unit, then it must be assumed that whatever change is represented by the two different transformers used (by part number) is basically insignificant, until any new evidence can suggest otherwise. EFB will provide worthwhile benefits regardless of what changes might have been made in these transformers.

    3. Rising distortion in each channel when both channels are driven is a classic characteristic of nearly all the vintage vacuum tube stereo designs. Regardless of what the published data from the independent lab suggests that Larry provided, the 400 is not immune to this characteristic either. With power being developed in both channels, power supply voltages fall to the output stage, upsetting the operating point established under quiescent conditions. In my 400, the best I could accomplish was a rise from .23% THD from a single channel, to just under 1% THD from each channel when both channels are driven, for a four fold increase in distortion. The installation of EFB compensates for this, maintaining the operating conditions of the lowest distortion operating point, whether each channel is operating individually, or both together, from zero, to maximum power output.

    Initial tests with EFB in my 400 -- with garden variety matched output tubes -- resulted in each channel producing lower distortion at maximum output with both channels driven, than one channel of the original design -- with carefully matched output tubes -- could produce when driven by itself. This was produced with an EFB quiescent current setting of 22 ma per tube, that results in a plate dissipation of just 8.5 watts per tube, for over a 30% decrease in plate dissipation over the lowest distortion operating point (32 ma per tube) of the original design.

    4. The operating conditions used for the output stage in the 400 receiver are curious to say the least. The output transformers in at least my unit reflect a primary impedance of 10.2K ohms, which is quite high for tubes of the 7591/7868 class when operated with fixed bias and the voltage levels they are in the 400. The benefits of this type of operation are that the required dissipation levels for low distortion in the output stage are reduced, and it eases the peak current demands on the power supply with elevated power output as well. Indeed, the lowest distortion operating point of the stock design produces a plate dissipation of some 12.6 watts per tube, which is already significantly lower than many designs operate these tubes at. It all equates to lower under hood temps as well -- so important in receivers where heat becomes such a concern.

    The down side of this design, is that it can be extremely hard on the screen grids of the output tubes. Fisher recognized this concern, and seriously choked off the drive capability of the phase inverter to prevent any possibilities of immanent meltdown from significant overdrive, protecting the screens in that manner. The problem is, that approach brings sonic penalties with it as well. Back in the day, these were likely logical compromises to make. But with tubes so valuable today and more technology available at hand, a correction which can achieve the best operating performance under all conditions of use, protect the output tubes, and help achieve the best possible sonic presentation would seem mandatory.

    This can be accomplished part and parcel with the installation of EFB, specifically by using the EFB modification to lower the basic quiescent screen voltage for the output tubes to about 300 vdc, placing these grids as well as the control grids under EFB control, and then restoring the drive capability of the phase inverter. These moves then produce a superior match between the output tubes and output transformer -- which lowers distortion and eliminates the stress on the screen grids, while leaving the power output capability unchanged. Removing the the choke hold (improving current flow) in the phase inverter then produces its own sonic benefits as well.

    Summarizing then, the goals of installing EFB in the Fisher 400 will be to:

    1. Significantly lower distortion in both channels when both channels are driven, to lower levels than the best single channel performance of the original design,

    2. Lower output tube quiescent plate dissipation by 30% from the optimum stock setting,

    3. Protect the output tube screen grids from excessive (destructive) dissipation levels (no glowing grids!),

    4. Allow for improved drive performance from the phase inverter by increasing its current flow,

    5. Extend output tube life, and

    6. Reduce under hood operating temperatures.

    Next up, developing a plan for installation, and documenting that process.

    Dave
     

     

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  12. JonL

    JonL Lunatic Member

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    :lurk:

    It might soon be time to dust off the 400 I've had in storage for 20 years!
     
  13. kevco

    kevco Member

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    Yes I'm in on this thread and SA-100 thread as well. I own both a 400 and a cathode biased SA-16. My curiosity is peaked especially by EFB modding in the 400 however as I believe there's more to be gained there. I've found that the SA-16 with a simple recap has become a real favorite. It's just got what I like to hear. But implementing EFB on a cathode biased amp is also something I want to see implemented. Go Dave!
     
  14. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Progress Update

    All of you solder slingers who attempt to modify the 400 (or its cousins) with IBAMs or power supply boards have my utmost respect. There is just precious little real estate with which to work on these monsters. The mechanical engineering is something in and of itself on all the Fisher stereo receivers.

    In this case however, the installation of EFB(tm) is made even tougher, because I did not want to install it in such a way as to take up the usual spaces used for power supply boards and IBAMs, as then you end up choosing space between restoration of the power supply and no EFB, or EFB and no power supply restoration, which ain't cool. So, that consideration made for a couple of days in just pondering how to best install it. At the very least, I wanted the installation to not require any modification of the Fisher chassis itself, so the modification could be easily reversed if so desired.

    It was determined that to make maximum use of any available space, all of the EFB circuitry would need to be built on a small perf board, which would also emulate a small universal board as might be used in any number of projects.

    Also, because this is a test of my basic universal circuit, it is designed to be minimally invasive into the original Fisher circuitry. But because the 400 is a fixed bias pentode amplifier, it also uses the most involved circuitry of all the possible EFB configurations -- requiring an EFB regulator for the bias supply, and an EFB regulator for the screen supply. Therefore, the 400 was a good pick for this project, challenging all aspects of working with a universal type of installation.

    The board has six leads coming from it as follows:

    1. Ground.

    2-3. AC leads which connect to the original bias supply leads from the power transformer.

    Side Bar: For the EFB bias supply regulator portion of the installation to work most effectively, it requires the original bias supply voltage to basically be doubled in value. This then provides plenty of voltage for the EFB bias supply regulator to operate from, allowing the ultimate bias to the output tubes to then automatically rise or fall under EFB control, as power supply and AC power conditions so dictate, without worry of running out of available operating voltage.

    To accomplish this, a novel universal voltage doubler was devised, allowing half wave bias supplies (as are typically found when a single tap from the power transformer is employed) to have their output effectively doubled in normal half wave fashion, or full wave bias supplies (as found in the 400) to also have their output effectively doubled as well, while still retaining full wave operation. While the universal EFB bias supply modification basically provides a completely new bias supply for the EFB supply regulator to operate from, full wave operation of this new supply uses a portion of the original full wave bias supply in its operation to operate properly. This is of little concern however, since most original full wave bias supplies will need to be left in place anyway to light the small signal tube heaters -- which is the reason a full wave design was used to begin with. Such is the case with the Fisher 400. The universal EFB circuit is such that only the required parts would be populated, depending on the type of supply employed.

    4. EFB Bias Output. This is the bias supply to the output tubes, replacing the output of the original bias supply originally powering this point.

    5. B+ In. This connects to the plate supply B+ point, supplying the EFB screen regulator portion of the overall modification.

    6. EFB Screen B+. This connects to the output tube screen grids, placing them under EFB control, similar to the way bias is controlled by EFB.

    An additional benefit of the universal EFB circuit, is that it is fully capable of powering any of the popular IBAM modifications out there (or factory bias adjustment circuits), so that each output tube can still be individually adjusted for the optimum quiescent current, while still being controlled by the EFB bias supply regulator.

    Finally, the universal EFB bias supply regulator also contains an overall bias level control, allowing all the tubes to be brought up or down in current draw as required to achieve a target set point. While my particular 400 does not have an IBAM circuit installed (the four tubes currently installed are really quite close in quiescent current), the bias control in the EFB bias supply regulator still represents a nice improvement over no bias control at all, as one was not provided for in the original design.

    The installation is not complete yet, but pics are provided to show both sides of the completed perf board build, and how it will appear when installed behind the fly wheel of the tuning assembly. It is mounted via an L bracket to an existing screw. Also note the power Mosfet (part of the EFB screen control circuit) extending from the board that mounts to the corner screw helping to hold the RF tuner chassis in place. This component is specified as a plastic device, so all that is required is a dab of silicon grease, and you're good to go. This device does not even dissipate 2 watts under full sustained power output conditions, so its very long life is assured. Also, the mounting location for the Mosfet is in a rather cool portion of the chassis, so the heat sinking is beyond adequate, even when installed in a cabinet.

    A test point strip is planned on the rear of the unit to measure the current draw of each output tube. With the EFB bias control mounted top side on the EFB board, it means that the bias will ultimately be able to be adjusted without having to turn the unit upside down - a very nice plus as you get older!!

    All that is left is to install the flying leads from the EFB board, and then perform the actual installation of it. I will document that effort in the next entry.

    Dave
     

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  15. eduarsan1

    eduarsan1 Luis Eduardo Sanchez Subscriber

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    Dave.

    Let me tell you something with all respect:

    You're the master!!!:ntwrthy::ntwrthy::ntwrthy:

    Recently I got a fisher 400 in a Ebay's bid (my wife is not happy about this :nono: ), but hope to do the improvements recommended in this forum.(the IBAM, the new PS using a C-L-C configuration and now this EFB(tm))

    For me will be not easy to get some parts due to I live in Colombia, but:

    I'm in!!!

    Once again thank you for share with us you wisdom!


    Luis
     
  16. notdigital

    notdigital AK Subscriber Subscriber

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    Excellent, Excellent, Excellent!!!!!!! :thmbsp: :thmbsp: :D Dave, I was worried about how this thing would fit under the skirt; but the topside mounting is genius! I'm already carving out real estate on my 800-c!!!!! And you've answered my not-yet-asked IBAM compatibility question too!

    Exciting stuff!!!!!!! :banana: :banana:
     

     

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  17. bktheking

    bktheking Gitter Done!

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    Well done Dave, I agree with your sentiments on installing the SDS board and the IBAM, I did both and had to get creative on where to install them, it isn't plug and play so to speak. Thank god for stand offs or it would be padding and zip ties ARGGGG.
     
  18. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Agreed BK!

    The 800-C is tight enough. The Sansui is an absolute nightmare. As much as I like an "uncluttered" topside, it looks like the EFB will go topside on the Sansui. I believe I have a spot in the bilges on the 800-C I can put the EFB and string "flying leads" out thru ventilation holes and mount the remote test points. I'll have to pull it out of the Executive cabinet to check. If I solder a mount to the center ridge plate, then bolt the board to it, that may work, like I did with the IBAM. I'd have to double check tho. I've got Fran's 800C (from the Executive) sitting here. Looks like there should be enough room to mount a small angle plate from light sheet metal, then attach the EFB board to it. But we'll see after Dave finalizes the board dimensions and the parts list.

    Larry
     
  19. gadget73

    gadget73 junk junkie Subscriber

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    This is something I wouldn't mind doing on my Sherwood, but I don't know where it would go there either. Not really any room top or bottom side. If it was an 8000 it wouldn't be so bad but with the AM tuner parts its just impossibly compact.
     
  20. sony6060

    sony6060 Super Member

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    I like this upgrade. Good deal.
     

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