Clean Fisher 400 need advice

Discussion in 'Fisher' started by grindfix, May 31, 2017.

  1. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

    Messages:
    19,509
    Location:
    Glen Burnie Md.
    Dmitry; There are schematics, and parts listed on them in the STICKY "Most Common Fisher PARTS needed". Page 2, POST #39.
    http://audiokarma.org/forums/index....needed-for-fishers.327561/page-2#post-6364459
    All resistors are 1/4w. and the caps for these are no more than 50V.
    On the IBAM you'll see two demarcations "existing parts" . The parts between the two demarcations are the parts you need.

    On the IBBA board (Yellow paper) jpg. Dave has written down all values. Pots for both are bourns 3386 types. Link here...
    http://www.mouser.com/ProductDetail/Bourns/3386P-DF6-103LF/?qs=sGAEpiMZZMukHu%2bjC5l7YewRvTbBNT1xiERk5N2DVYQ=
     
  2. audmod01

    audmod01 Well-Known Member

    Messages:
    754
    Location:
    Tioga, TX
    Dave;

    OK, thanks and sorry about the error on the front end tubes - I should have looked at a schematic or layout diagram. I was unsure of the serial number sequence at which the change to 4 IFs was done. I knew it showed up in the 40001 to 49999 sequence, but since I had no access to the 30001 to 39999 sequence I realized it could have been done in that range too. Mainly I was trying to nail down which sequence the extra IF was implemented in. If I were looking for a model 400 to restore that last SN sequence would be my choice.

    Joe
     
  3. grindfix

    grindfix Active Member

    Messages:
    155
    No reply from Jim McShane so far. Emailed him yesterday around 10 am. 7868 supply must still be an issue?
     
  4. ncwalz

    ncwalz AK Subscriber Subscriber

    Messages:
    72
    Location:
    Alberta, Canada
    I use Gmail and every timeI have emailed Jim this year, his replies have ended in my junk mail for some reason. I would check that first. I think he mentions this at the top of his tube webpage.
     
  5. grindfix

    grindfix Active Member

    Messages:
    155
    I saw that on his tube page. Have been checking junk folder. Hope he replies soon. Feeling more confidence buying tubes from him then from Depot even while planing to install individual bias board
     
  6. grindfix

    grindfix Active Member

    Messages:
    155
    Larry,
    How much have your bias has drifted since you built bias board? Is it even necessary to make it externally accessible? Seems like perfect spot freed-up behind new cardboard cap.
     
  7. mrphilco

    mrphilco AK Subscriber Subscriber

    Messages:
    375
    Location:
    Ferdinand, IN
    Hi Dmitriy,

    Earlier this week I placed my very first tube order from Jim. I emailed my inquiry on Tuesday night, heard from him late Wednesday morning, immediately paid his invoice, the tubes arrived today. I will definitely order from him again. :thumbsup:

    Perhaps he has not yet answered because it's Friday? Hopefully you will hear from him soon.
     
  8. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

    Messages:
    19,509
    Location:
    Glen Burnie Md.
    With either board, and I've got 1 400 running now, 2 800c's,a TA-600 and a Sansui 1000A with one or the other boards, the bias has not changed on the tubes as long as they stay in the same unit. granted, I swapped EH7591's from the Sansui into one of the 800'c's and had to reset, but after that I put them on the meter and they stay right at what I set them for. No drift +/- 1ma. The board's keep it really tight. Same with the EH7868's and Tung-Sol 7591's. The oldest board is now in the Sansui, about 7-8 years now.
     
  9. grindfix

    grindfix Active Member

    Messages:
    155
    That makes it easy to decide to place the board inside. The way it's going, it looks like I'll make bias board before new tubes get here. Would it make cense to try it with originals to see how much gain can be achieved? Just for stats? Or not good idea for OPT health?
     
  10. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

    Messages:
    19,509
    Location:
    Glen Burnie Md.
    Go ahead and try it. The most you'll probably get for bias voltage is close to -12 which IMO is getting too close to ZERO. The originals probably won't comes up much more, but you can see what you can get out of them as long as you have a meter on the bias voltage at the tube.
     
  11. grindfix

    grindfix Active Member

    Messages:
    155
    Off the subject, I'm curious how Sansui 1000 compares to Fisher receivers? Build quality, repairability and sound, of course.
     
  12. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

    Messages:
    19,509
    Location:
    Glen Burnie Md.
    Build quality is fair to good considering Sansui had only just started making receivers a few years prior. I will note that my 1000A is an early 1966 model nd it was built from 1964 (late) thru the end of 1971. Build quality went up withthe later models, but it wasn't grossly different. The 1000A has a very shallow chassis and parts are stacked in there very tightly. Also the 1000A was the last of the Tube Receivers that Sansui made. Reception in both AM and FM even on a mis aligned unit is quite good.

    Repairability is what you'd expect from a 60's Japanese tube receiver. Sometimes it can be a bloody nightmare. My 1000A was the 1st receiver I rebuilt after a Radio craftsman 550A amp. It took me almost a month to get it done, but it was a precursor to doing FISHER's. The only saving grace was the fact that the replacement parts, save the output coupling caps, were smaller in size, so you weren't competing with a double jointed midget to get some parts in.

    Sound quality is quite good, comparable to the 800c. Lows are a bit on the thin side, but mids and high's can be very brash if you crank it up with the tone controls past center. With the Lows kind of thin it's better to run it with the presence and loudness taps on to get some semblance of normal sound in the lower volume settings although by 10:00 on the volume control you can turn the loudness off.
     
    Last edited: Jul 1, 2017
  13. grindfix

    grindfix Active Member

    Messages:
    155
    Thanks for transformer clarification. I'll monitor my wall voltage and see how much it fluctuates.
    I ran across Dave's EFB board post and was wondering what is your opinion on it? Do you have it installed in your 400? As I understand it can be used with IBAM board, Cathode resistors and Screen Stability resistors installed, correct?
     
  14. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

    Messages:
    19,509
    Location:
    Glen Burnie Md.
    I've never put one in. Solid State I can't wrap my head around, just like math with letters instead of numbers. But what I've heard of it on the forum it's definately something to look at, and possibly install.
     
  15. grindfix

    grindfix Active Member

    Messages:
    155
    Larry,
    Jim finally replied and I got matched quad of 7868EH tubes with higher current range then originals. According to him they will work with adjustable bias current. Does it mean that draw needs to be set higher then 0.350v (0.035a) for those tubes? Or bias needs to be set higher to achieve 0.035a draw?
    So I built and installed IBAM board. 8 ohm load resistors in. Old original power tubes. Powered-up through Variac several times between steps and before removing eliminated resistors and capacitor. Allowed to warm-up on wall voltage and adjusted draw across cathode resistor to 0.346v. I stopped at that because I see tendency of the draw to slowly increase with heat. (unit is upside down)
    Pin 7 = 389v
    Pin 9 = 428v
    What is the formula to calculate individual power output?

    Thanks


    IMG_1242.JPG IMG_1243.JPG IMG_1244.JPG IMG_1252.JPG IMG_1255.JPG IMG_1256.JPG
     

    Attached Files:

  16. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

    Messages:
    19,509
    Location:
    Glen Burnie Md.
    Higher current range means for any set voltage the current on the cathode will be higher, so you'll need to go larger on the bias voltage (away from ZERO(0)). This is normal the the EH 7868's.

    plate voltage x bias current in ma(ma = MV / 10) in other words take the MV and move the decimal 1 place left. 428 x .0346=14.80W per tube. Right in the middle of the 70%-80% mark. But when you subtract the screen dissipation of 3ma you get 428v x .0316ma = 13.52W
    19w x .70=13.3w So you are still in the 70% - 80 % Range. Bump it upto 35mv if you want,and it'll end up a little higher inthe range but you'll still be in.
    428 x .035=14.98 428 x .032= 13.696W
     
  17. grindfix

    grindfix Active Member

    Messages:
    155
    So, my original Fisher 7868 tubes are still good?
    Does it matter what actual bias voltage it takes to get to 0.035A draw? I forgot to measure it yesterday. How far away from -17v (either way?) is considered acceptable?

    Thanks
     
  18. grindfix

    grindfix Active Member

    Messages:
    155
    Another thing I was thinking is to by-pass Hi Filter switch and use it to switch between 2 different sets of coupling caps. (original value .043uf vs. recommended 0.1uf) I have original value polypropylene caps there now. The reason behind is that while Advents (stacked 4 or 16 ohm) are great sounding speakers with SS, tube output may give me options with the cap switch. Any thoughts to that?
    Thanks
     
  19. grindfix

    grindfix Active Member

    Messages:
    155
    Here are today's measurements after about 20 minutes idling with 8 ohm load.
    Wall = 120.5v
    Adjusted bias to 0.348v
    Actual bias supply settings:
    V14 -15.80v
    V15 -16.35v
    V16 -16.87v
    V17 -16.95v
    Pin 7 = 373v
    Pin 9 = 411v

    Looking at values I suspect that heat is still the factor as receiver is on it's side now with power transformer down. (V14 is lowest and V17 is affected with the heat of all 4 tubes)
    My question is whether it is a good idea to switch to EH tubes since there is no way back with pins being larger on EH tubes.
    How much of the power increase is realistic expectation?

    Thanks
     
  20. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

    Messages:
    19,509
    Location:
    Glen Burnie Md.
    You're overthinking the bias issue with regard to heat. I've had 400's upside down for extended periods of time (also on their sides) and Not had any problems. I would only worry about the BIAS voltage if it dips below -10V (toward ZERO). The reading you are looking for is the CURRENT FLOW THRU THE RESISTOR to be equal, REGARDLESS of what the voltage is (within a reasonable factor)(I use +/-5v total high to low) BUT NO CLOSER to ZERO VOLTS THAN -10VDC . Your EH's are tighter than a ticks ass in a fart contest.

    411 x .0348 = 14.3028W per tube. Subtract 3ma for screen dissipation and you have 13.0698 Watts per tube. Bump them up to 36ma. That will give you 13.56watts per tube after screen dissipation which is just over 70% of Max dissipation. Your tubes will last a good long time.

    You won't hear any difference. And if you DID do this, you'd need to use shielded cable to keep RF/IF interference out. Not a good idea. Find some .068's in there. They are the closest to .072uf IDEAL for 200-220K. And they are a stock size.
     

Share This Page