Fisher 400 Bias question

Discussion in 'Fisher' started by jvstratman, Apr 21, 2016.

  1. jvstratman

    jvstratman New Member

    Messages:
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    I was setting/checking the bias in my receiver and noticed that the red pots (Bias setting?) doesn't change the neg. voltage much until you get near the end of the rotation going more negative. From the lowest -19.2 to -19.8 until you get near the end of adjustment and then goes to -23 rather quickly. Is this normal? Both pots had anti-tamper hardened liquid and I could only get one of them to rotate, as I thought I would deform the red plastic if I tried harder.

    I'm running EH 7868 MQ and mine is a low serial number with the lower impedance OT primaries. All electrolytics and rectifier have been upgraded.

    Years ago I swapped the two .047uf 400v ero fol II with something else but, found some .047uf 630v ero fol II pulls from a working 400 and installed them so now all four are the 630 volt version. Sounds amazing, no hum, -19.6 at each tube and .026 ma on three tubes and .028 ma on the other tube. I'll swap tubes to see if the higher quiescent current moves with it. Yes, I have the 10 ohm 1/4 watt fusistors installed. My plates are all 420 volts or so and I'm running theses @ approx 11 watts and it would appear that I'm stuck there without making some mods like IBAM sp?

    I have new ceramic "B9D base 7868 sockets coming from China to go with a (recently acquired) matched quad of 1964 RCA blackplates. Does changing back to that era tube bring the quiescent current closer to 14 watts?

    Do you think I should change the bias pots or go IBAM? I don't want to do any mods that would alter the sonic characteristics of the amp. It sings!!! Original Klipsch Forte's with Ti diaphragms and upgraded XO's btw.

    Best Regards,
    Jerry
     
  2. larryderouin

    larryderouin You can be sure if it's Westinghouse??????? Subscriber

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    The "RED POTS" up next to the 12ax7 Phase inverter tubes? If so those are the Phase inverter adjustment, NOT BIAS Adjustment. The Stock 400 did not have any adjustment capabilities. Center those pots for now, and then read the "QUICK Phase inverter adjustment for the 400 in the sticky threads. http://audiokarma.org/forums/index.php?threads/quick-and-easy-400-phase-inverter-adjust.559415/

    Any bias adjustment boards you build and install WILL NOT have a sonic effect on the unit, other than making the tubes operate in their Safe Operating Area. (SOA).

    Changing the sockets and installing Old Stock tubes will not bring the quiesent current up or down. changing the bias voltage to change the tube draw at the 10ohm resistor is the only way. An IBAM or Dave's IBBA board will do this and match the tube draw of all the tubes to an equal point.

    You should seriously consider changing the 330K grid return resistors to 200K to 220K and at the same time change the coupling caps to .1uf. The grid resistors are way over the operating parameters of the tube. I copied a blurb from a previous thread on the common changes and why they are done on the FISHER's 400-500-800 units running fixed bias and 7591's or 7868's by Dave Gillespie. All of these go hand in hand. They will NOT affect or degrade sonic quality of the 400. If anything they will enhance it some.

    START QUOTE"

    400-500C-800C MODS for BIAS and POWER SUPPLY
    1.) replace the 4 coupling caps with .1uf/630V caps (OP Choice of cap)
    2.) replace 330k grid resistors with 200K R 1/2Watt.
    3.) install 10oHm 1/4W resistors to ground @ pin 5 on each output tube.
    4.) install 100oHm 1/4W screen stabilization resistors to jumper @ pin 3&8 on each tube.


    Explanation by Dave Gillespie;
    If I may, here is some detail behind the recommendations for these modifications:

    1. "Reducing the grid resistors to 220K": The published maximum DC resistance for grid #1 of 7591 class tubes is 300K (by RCA, Sylvania, and others). This is the maximum value allowable when the tube is operated under conditions of maximum dissipation -- which many designs nearly did -- so yes, using 330K is using a value that is right on the ragged edge.

    The reason for the maximum resistance specification is to limit the value of reverse grid current voltage that can build up under conditions of maximum dissipation (read that, maximum heat). But there are other concerns that can aggravate this issue as well. Higher line voltages push the heater voltages higher, which means that today, even more heat is generated. The basic concern is that excessive heat (however it's generated) can cause cathode material to boil off and deposit itself onto the grid. This means the grid has now become a partial emitter, with any current flow it provides developing a voltage across the grid resistor that is opposite in polarity to the negative voltage applied to the bottom of the grid resistor. This negative grid current voltage then works against the negative bias voltage, causing the tube to conduct more current, generating even more heat, boiling off more cathode material, and ultimately causing thermal runaway. The production of gas comes into play with these events as well to further aggravate the issue. Therefore, to help control this condition, a maximum DC resistance is specified for the control grid to limit the amount of ill effects that can be caused by reverse grid current.

    As a result, with the high temps that these units typically run at inside their cases, and importantly, because of today's higher line voltages, and because the tubes are biased "on" pretty hard to begin with, it makes very good sense to pull the value of the grid resistors back a little to help minimize the chances of developing any significant reverse grid current voltage, and the possible thermal runaway that can produce -- and this is even before the quality of modern production tubes is considered!

    2. "Double the value of the coupling caps": This has nothing to do with loading on the driver stage, in fact, lowering the value of the grid resistors actually increases the load on the driver stage. The reason the cap values are increased is to keep nearly the same time constant in the RC coupling circuit between the driver stage and the output stage with the new lower value grid resistors installed.

    3. "Install 10 Ohm cathode resistors": This makes for the easiest way to monitor output tube current draw, and therefore allows for the easiest way to set the bias and balance of the output stage (if such controls exist), or check to see that matched output tubes are in fact matched. These resistors are typically used for balancing currents under quiescent conditions, but can also be used to check for balance under dynamic conditions if the proper equipment is available. Balancing output tube quiescent current maximizes OPT low frequency power handling ability (by minimizing core saturation), while balancing dynamic currents minimizes overall distortion.

    4. "Installing a 10K bias pot": Modern tubes are manufactured with far less precision than the best tubes from yesteryear were, and so, fixed bias amplifiers operating with tubes of modern manufacture on higher line voltages will usually need some adjustment from the original fixed setting a manufacturer provided. Installing some type of adjustable control is therefore highly desirable, and adding a DC balancing feature to that is even better. Installing these controls so that they cannot be inadvertently altered from their setting goes hand in hand with the installation of any such controls. (NOTE: This is a global adjustment and does not account for the variances between the tubes. An IBAM or IBBA board is a better way to go as you get individual tube adjustments)

    5. "Installing 100 ohm Screen Stability Resistors": So many folks have posted on various forums -- from all degrees of experience -- how everything is as it should be, but the output tubes will arc for no reason that they can determine. How expensive does that get when output tubes can be ruined in an instant?

    Long ago, I determined that these arcs were in fact occurring from the screen grid. Many pieces of old hifi gear were designed with very little resistance in the screen circuit, which usually didn't present too many problems as the power supply caps of the day had much higher ESR values, which tended to help keep things stable. Even at that however, some designs (Eico HF-89 for example) were simply notorious for arcing output tubes because of the larger overall power supply capacitance values they employed.

    Today it is all too common, with the installation of new power supply caps that have much lower ESR values, and which are so often increased in size as well (all installed on new drop in power supply boards), to have all manner of output tube arcing events occur, which leaves folks puzzled because it just never used to do that before. New manufacture tubes got the lion's share of the blame -- until even NOS tubes were arcing in these refurbished pieces of equipment as well.

    With some form of transient trigger (music, an open ground, whatever), the lower ESR value caps can set up incredibly strong oscillations in what is effectively a tank circuit that's formed with the screen grid, and can cause an arc at any time. Installation of 100 ohm Screen Stability resistors (for 7591 class tubes) stops this problem virtually every time. I've never had anyone tell me that installation of these resistors did not stop the problem. Personally, I've never had an output tube arc event in nearly twenty years now, where before I had boxes of blown tubes until I could determine what the cause was, and develop an appropriate cure. In equipment such as this Fisher represents, it is very cheap insurance against such events, and produces no audible or measurable difference in performance.

    Hopefully, this will help with an understanding of why these modifications can make such worthwhile improvements in so many of these wonderful instruments. With some components being virtually unobtainable now, minimizing any damaging events that might occur helps all of us to enjoy this equipment even more.

    Good luck with your receiver!

    Dave

    END QUOTE
     
  3. bhamham

    bhamham Super Member

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  4. jvstratman

    jvstratman New Member

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    Larry, Thank you so much for all your advice and link. I thought I read somewhere on this forum that the EH 7868's needed the 220K's but period correct tubes handled the 330K's. I'll read more. I'm really hesitant to mess with these .047 ero fol's. They kick serious butt!!

    bhamham - Here's the sockets - http://www.ebay.com/itm/251115481334?_trksid=p2057872.m2749.l2649&ssPageName=STRK:MEBIDX:IT

    I seriously doubt if I could re-tension the existing sockets to accept old stock but, I may have to try if I can't find the right size.

    Best Regards,
    Jerry
     
  5. bhamham

    bhamham Super Member

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    Those ebay sockets are for big-pin 7868s like the EH. You can use small jeweler's screwdrivers to bend the contacts in the sockets. Not the best solution but it can work.
     
  6. larryderouin

    larryderouin You can be sure if it's Westinghouse??????? Subscriber

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    While it is true that the old stock handle the 330K grid return resistance better than the new stock EH and Tungsols, all would be better off BELOW the TUBE MANUFACTURERS RATINGS which is 300K. I've been running FISHER 400 and 800c's for going on 8 years with 200K and 220K resistors(which is the now recommended value range) with absolutely no problems with regard to tube operation, sound quality, etc. These units get approx 40-80 hours a week usage on average.

    You CAN, get Ero-fol's in .1uf. They sound just as good as the .047's.
    They are out there, but you'll have to look on e-prey for them. And be prepared with deep pockets.

    Just so you know, Dave Gillespie is one of the TOP TECH GURU's for tube units (especially FISHER
    's) on Audiokarma. This guy breathes and dreams tube circuits as long or longer than I've been out of High School (43 years now) and believe me when Dave says jump, you don't ask HOW HIGH? A very high portion of the guys here on the Fisher Forum would be lost (or at least highly confused) without Dave's generous help and guidance. He's one of the guys you listen too. If you are old enough, you may remember the E.F. HUTTON commercials in the late 80's. ("When E.F. Hutton talks, people listen.") Well, that's Dave on A.K. When Dave talks, people read the WHOLE THREAD! Sometimes twice or three times.

    Larry
     
  7. jvstratman

    jvstratman New Member

    Messages:
    18
    Last evening I re-tensioned the stretched sockets with surprisingly good results utilizing a dental pick. The 64 RCA 7868 black plates are in and one pair are running @ 21 ma & 22 ma while the other pair are both @ 18 ma. Bias at all tubes is -19.5 volts. The plates are all @ approx. 422 volts. These tubes are amazing. I'm hearing nuances in Fagen's Nightfly CD that I've never noticed with the EH counterparts.

    So here's some questions from my observations.
    1) My listening space is 12'x16' driving the Klipsch Forte's and I'm quite comfortable giving up power for running cooler tubes. Can I still achieve high sonic quality running these tubes so cold or does the sonic quality increase simply by running them hotter?
    2) I guess this question is to early w/o knowing the answer to (1) but, in your opinions, will increasing the 18 ma tubes to match the higher pair sound better than lowering the 22's down to 18 ma?

    I will probably do some bias balancing via Dave G. mods.

    Those RCA's sound so good, I caught myself squeezing the chair's arms a few times. Very intense, captivating sonic experience. I have doubts that it can get any better than this and my main concern is obviously tube life at this point.

    Also, I built a variac back in my guitar tube amp tinkering days. I may drag it out and run down to 117 vac and re-check voltages and listen for effects.

    Dave, please chime in with your thoughts on power and how it relates to sonics.

    Best Regards,
    Jerry
     
  8. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Jerry -- Operating your output tubes down around 20 ma (on average) will significantly reduce the performance of your Fisher, by introducing significant cross-over distortion. This level of operation has the tube plates only dissipating about 7 watts each, which is far too low to produce rated power or distortion levels. Tube life is always a concern, so if you want to back off from the redhot factory setting, that's fine. But in any event, I wouldn't let any of the tubes operate at less than about 27 ma cathode current, as distortion rises dramatically below that level.

    Dave
     
  9. jvstratman

    jvstratman New Member

    Messages:
    18
    Thanks for the response Dave.

    Well then, today I will lose the 330K and go with some combination of other resistors to try to get close to the more ideal 27 ma.
    So, I plan on doing the bias controls eventually but, I have loads of parts and would like to experiment now.

    Currently: 1964 RCA's read
    V13: 18 ma - 330K so, what value resistor to get to 27 ma = K
    V14: 18 ma - 330K " " = K
    V15: 21 ma - 330K " " = K
    V16: 22 ma - 330K " " = K

    I have 200K's , 220K's, 270K's - Is carbon comp 1/2 watt OK? What did fisher use? What sounds best?

    For this test's purposes, I'm leaving the (4) .047 erfol II's and adding some NOS Cornell Dubilier PM 6S47 .047 600 volt in parallel. Are these green weenies any good for coupling? Remember, this will be temporary as, I don't want to leave 4 extra caps in there.

    Dave, what are your favorite coupling caps in the Fisher 400?

    bhamham: I received those ceramic Chinese sockets today and they fit nice and snug with the old RCA's. There's no way that they would accept the EH's w/o major force and I'm not going to try.
    $9 (shipping included) for 4 ceramics. They seem to be very good quality.

    Does anyone build/sell the IBAM/IBAA/EFB boards? Life's busy right now...


    Best Regards
    Jerry
     
  10. larryderouin

    larryderouin You can be sure if it's Westinghouse??????? Subscriber

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    Jerry;

    Please read my 1st post AGAIN, ref: the role of the 330K resistors. apparently you misread something in that post or have misunderstood their role subsequently. They do NOT adjust bias. The drop in value to 200k to 220k is to get them below the maximum recommended tube manufacturers fixed bias resistance value and is the accepted value range with .1uf coupling cap value to maintain R/C timing value. see tube book pages below for 7868. This also applies to 7591's as the 7868 is a 7591 in a different pkg.

    Item 4 will give you a basic GLOBAL ADJUSTMENT for BIAS Voltage. For 400's below serial 48001 you will REMOVE C74- R94-R102. Above Serial 48001 remove C42-R34-R35. Replace all with a 10K pot. Connect pin 1 and 2 together and connect that to the bias cap side of the circuit. Pin 3 goes to the output tube side. Set initially for least resistance (bias voltage at caps or about -22v to -24v to keep from overloading the tubes on startup). connect your meter across one of the tube 10ohm resistors (preferably the known hottest tube) and start up. when the voltages stabilize, adjust to approx .32v (320mv or 32ma across 10 ohms). Then check the rest of the tubes. 32ma with 422 on the plates gives you just over 13.5.watts per tube. Which is just over 70% of max dissipation. If the lower tubes stay proportionately lower as they are now, the lowest tube will be about 28ma. or 11.81w. This does not account for screen dissipation, so it will be slightly lower. This should be good for good sound quality and just out of the cross distortion territory. Taking the highest tube to 34ma with the lowest tube at 30ma (provided the tubes come up proportionately), will be more in the center to high side of the SOA of the tubes for output without going above 80%.


    7868-modded.jpg
     
    Last edited: Apr 23, 2016
  11. jvstratman

    jvstratman New Member

    Messages:
    18
    Larry,

    I understand now how the 10K pot controls the bias on all 4 tubes together and where to install it. I'll also drop the resistors down to 220K (per the tube manuf. recommendations) and get the capacitance in line with the r/c timing. I'll do the 100 ohm 1/4 watt screen resistors too.

    Does anyone build/sell the IBAM/IBAA/EFB boards for the 400? email: jvstratman@zoominternet.net

    Best Regards,
    Jerry
     
  12. larryderouin

    larryderouin You can be sure if it's Westinghouse??????? Subscriber

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    Nobody makes them commercially.

    As for those P.I. pots. Center them up for now (it's very close). On the 400 (what's your serial #) the Phase Inverter can be adjusted very easily with the use of a Multimeter and a jumper wire. Read up on it in the Easy Phase Inverter adjustment sticky. Also there is a "noose" that can be loosened up that Dave can go into. It involves removing a resistor on the Phase inverter tubes and readjusting. I'm not up on the technical stuff for that one but it does open the unit up some.
     
    Last edited: Apr 23, 2016
  13. jvstratman

    jvstratman New Member

    Messages:
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    Serial number is 19,xxx w/ 6500 ohm impedance on primaries so no noose.

    I'll do the jumper method for the driver pots. Which Deoxit do you use for those? Regular, gold, or I think I have one for carbon or something..

    I really like the sound (PUN) of Dave's EFB & IBBA mods to keep everything consistent. I'll eventually do those, as it's foolish (IMHO) not to.

    Thanks for all your help Larry, I really appreciate it.
     
  14. jvstratman

    jvstratman New Member

    Messages:
    18
    Update: I discovered a 2.7k resistor @ R102 instead of 5.6k. I can't remember why I put that there a few years ago when I upgraded all the filters and rectifier but, if memory serves, it had something to do with modern AC voltages and EH new production tubes but, I digress. I changed it to 5.6k and now have the following numbers.

    I'm running on a variac @ 114 VAC
    Bias supply: -24.5
    Grids: -15.5
    Screens: 356
    V13: 398 x .030 = 11.94 w
    V14: 398 x .030 = 11.94 w
    V15: 396 x .035 = 13.86 w
    V16: 396 x .035 = 13.86 w

    All measurements (Fluke 85) taken while in operation (cd aux input w/ volume @ 1/3rd.

    The outputs are 1964 RCA BP's
    The grids are still 330k w/ erofol II .047's

    A few posts ago I stated that I didn't think it could sound any better. I'm so glad I started this thread. The performance is stunning! I (WE) awoke the beast!!

    Next will be IBAM

    Thanks to all

    Best Regards,
    Jerry
     
    Last edited: Apr 24, 2016
  15. larryderouin

    larryderouin You can be sure if it's Westinghouse??????? Subscriber

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    Jerry: When you measure for quiesent (or idling ) current, you Put it in Aux with no source input, and volume at ZERO and let it idle. Running a source and / or running the volume up skews the results in that it's not idling.

    The numbers look better. you're almost there. Put in the IBBA board and you'll be set after you change the grid returns and the coupling caps.
     
  16. ducati2

    ducati2 AK Subscriber Subscriber

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    same inquiry... anyone building these out there? would love to get my hands on one that would work for 400/500/800 fishers.
     
  17. larryderouin

    larryderouin You can be sure if it's Westinghouse??????? Subscriber

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    All IBAM / IBBA boards are individually made by each owner. There is nobody making them for distribution/sale.
     

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