Clean Fisher 400 need advice

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

  1. grindfix

    grindfix Active Member

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    Larry,
    All posted readings are with original Fisher tubes, not EH's that I got from Jim. V14 & V15 are 110 emissions and V16 & V17 tested at 120. Should I swap 14 & 16?
    These tubes were drawing around 20ma with stock bias supply. Does it mean that with proper bias they are good enough that it wouldn't make sense to switch them over to EH's?
     
  2. larryderouin

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

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    Leave them as is. As with any electrical voltage and current equation. When you change one, the other will change also. Now with the tubes all biased for "CURRENT" not VOLTAGE anymore, your measurements are more accurately reflecting what the tube is doing. The voltage setting that FISHER advocated 52 years ago was for when output tubes were a buck or 3 each. And they wore out in a year or so. Today, we want them to last longer so we bias them down using the CURRENT FLOW of the cathode at idle. If all the TUBES CURRENT FLOW is identical, (and I'm presuming it is on pin 3) then by virtue of small differences in tube construction the voltages are different. And they are. This is NOT a problem as we're measuring CURRENT (amps or milliamps) rather than Voltage. It's MORE ACCURATELY reflecting the work that the tube is actually doing rather than guestimating what the tube is outputting at a known voltage. Don't worry about the bias voltage in any of the equations unless it hit's about -10v to get your ma's. That is a bit low for comfort.

    Your TUBES are FINE. Run the piss outta them. Bump them up to 35ma so you are within 70% of Max dissipation. Your distortion will be slightly lower in the 70-80% range than below 70%.

    Run those tubes until they won't bias up to 27ma with the pots at full lock anymore. But keep the EH's in reserve just in case one of the Fisher's decides to commit ritual seppuku!
     
  3. grindfix

    grindfix Active Member

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    Larry,
    Can you recommend replacement for STF10NK50Z for EFB board?
    Thanks
     
  4. larryderouin

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

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    Sorry No. PM Dave as this is his baby and Solid state is definitely not my forte.
     
  5. grindfix

    grindfix Active Member

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    171
    Larry,

    I'm setting bias using 8 ohm 50W resistors as a load with jumpers set to 8 ohm.
    Speakers I'm using are stacked Advents, so it will be either 4 or 8 ohm (approximately)
    Does tube current change with different loads? Does bias need adjustment for every different type of speaker?
    Load jumpers will be set accordingly, of course.
    Thanks,
     
  6. larryderouin

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

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    In a word, NO for all. The Transformers do all the load calculations (secondary windings). The amp will work harder at a lower impedance than a higher impedance (output loads). I would set them so you have a 16 ohm load.

    Depending on whether the stacked Advents are parallel or in series, the load will be either 4ohms(par) or 16 ohms (series).
     
  7. grindfix

    grindfix Active Member

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    171
    In my understanding Advents are not efficient speakers at 89db. Having them stacked improves it to about 93db level regardless of the connection type parallel or series. This is not proven number and just one engineer's opinion. I'm not worried about overloading the amp in parallel 4 ohm connection as my listening room is small enough and I do not listen at very high levels. It is difficult to compare sound quality between 4 and 16 ohm connections but I think (I'd like someone's opinion on that) 4 ohm parallel setup improves speaker sensitivity and frequency response.
    If that is true, wouldn't it make sense to adjust bias with speakers connected to 4 ohm tap?
     
  8. larryderouin

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

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    Not really. Bias is not affected by the secondary windings of the output transformers no matter what impedance is. NFB (Negative Feed Back) can be, which affects gain. Agreed that the OLA and NLA speakers are not really efficient @89db. In parallel I don't see that the efficiency of the speakers would go up, however because you have double the drivers, it increases the total area driven (no it's not doubled), which could give you the impression that the efficiency of the speakers had gone up.
     
  9. grindfix

    grindfix Active Member

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    171
    The reason I ask all these questions is that after installing IBAM board there is a noticeable distortion I will run it with Advents in 16 ohm / series and monitor bias current as well as pin 7 and pin 9 voltages.
     
  10. fred soop

    fred soop Super Member

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    Connecting identical speakers in series or parallel does nothing to change efficiency.

    If in parallel with a constant voltage source, the current is double. If in series, the voltage is doubled for the same current. Either is a 3 dB increase in output but also double the power from the amplifier.

    There are also rumors floating around for many years that series speakers would result in a major drop in the damping factor. Ken Kantor set this one straight with a couple of links filled with a lot of math, but the short answer is that damping will not be affected. Yes, there is twice the voice coil resistance, but there is also twice the back emf.
     
  11. grindfix

    grindfix Active Member

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    171
    So math is that there is no sonic difference between series or parallel connections? Volume drop but less load in series and higher volume and higher load in parallel, right?
     
  12. fred soop

    fred soop Super Member

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    In series, the volume would remain the same because the current would drop by a factor of 2 for the same output voltage. This would be a 3 dB reduction from each speaker but there are now 2 speakers.

    In parallel, volume would increase by 3 dB because the second speaker would behave the same as the first.

    Both scenarios assume that the volume is not changed and the amplifier is a constant voltage source. A tube output would not necessarily be constant voltage because the transformer tap would be changed when adding either the series or parallel speaker.

    Don't have a direct link to the article because it is a download from a blank web page, but if you Google "Dick Pierce Loudspeaker Damping", it should be the first hit, from DIYSpeakers.net Can't readily find the second article.
     
  13. fred soop

    fred soop Super Member

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    Correction to post #72 .....

    With a constant voltage output (transistor amplifier), the series speaker connection would cut the current in half, but each speaker would also see half the voltage, so the drop for each speaker would be 6 dB but the total drop for the combination would be 3 dB for the same volume setting.

    Parallel would be same as posted above.


    For output transformers on tube units, impedance is proportional to the square of the number of turns on the winding. So, 2 speakers of 8 ohms in series would be connected to the 16 ohm tap. Voltage would increase by a factor of 1.414 but with double the load impedance, total current would decrease by the same. Volume would not change. Total power would be the same but divided between 2 speakers. This would reduce the speaker distortion.

    A parallel connection on a tube amplifier would then go to the 4 ohm tap with a voltage reduction to 0.707 of the 8 ohm tap but with the total reduction in parallel impedance, the current would rise by the same factor, so volume would still not change.

    But, with either connection, the power handling capability of the 2 speakers combined would be doubled, resulting in a 3 dB increase in maximum available volume. This assumes that the amplifier could deliver that power.
     
    Last edited: Jul 9, 2017
  14. grindfix

    grindfix Active Member

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    Strange and unexpected developments. As I posted before there was noticeable distortion after IBAM installation that I attributed to old original output tubes are running on their limit.
    I really wasn't concerned since I have new EH quad from Jim and was planning to replace them after all mods and adjustments were completed.
    This weekend I finally had time to finish EFB board. Installed and connected all power sources but left EFB Bias output disconnected to monitor on power-up. IBAM board is still connected at this point. Powered up through Variac and DBT just incase. All good. Powered up from the wall and was able to adjust bias from EFB to neg 17v without any issues. Connected EFB to IBAM and adjusted individual tubes to .350mv. Connected speakers. Left channel is badly distorted. Right channel seems fine.
    Swapped output tubes Left to Right - no difference. Same distortion through headphones on Left channel only.
    What I've noticed is when receiver first starts playing there is no distortion. Distortion starts within 5-7 seconds and progresses into crackle and almost no volume.
    I guess the only thing that can affect single channel is IBAM but I was using receiver after its installation for several hours while monitoring tube draw. There was distortion but nothing like what I get now. Any thoughts?

    EFB.JPG EFB2.JPG
     
  15. larryderouin

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

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    Swap the driver/PI tubes 1st L to R and R to L. Check ALL FUNCTIONS(FM, AUX, PHONO,ETC) on all tests. If the distortion swaps channels replace the bad tube. If no change, then grab a known good tube with matched sections (Hereafter KGT) and swap it into V11. Test. If no change insert original tube and plug KGT into V10 and Test. If no change swap the Phono preamp tube L to R and R to L. If the distortion doesn't change in any position then it's not a tube. If it changes with a tube swap or channel swap btwn 2 tubes replace that tube.

    V10 is Voltage Amp (one channel per section) and V11 is Tone Control Amp (one channel per section). Phono(V8 & V9) tubes run one channel each. Driver/PI (V12 & V13)run one channel each.

    If all the tubes check out good, tighten all the socket leaves, and check each item you replaced for solder blobs or drips, leads that are touching other parts, poor solder joints, especially joints that are on terminal strips and tube sockets. Solder will run down the terminal and leave a blob on the chassis that can touch the terminal shorting out the part. Double check all connections for proper placement. in the future mark the exact terminal BEFORE you remove the old part. This way you don't or won't get confused as to which terminal is which.
     
  16. grindfix

    grindfix Active Member

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    171
    Swapped v12 & v13 around. Distortion is almost gone but now in both channels. I had EH 12ax7LPS tubes there. Pulled both of them out and put Mullards I had and all is good. No distortion. So I think PI needs adjusted and EH tubes are just too different in current consumption and amplification that current PI setting is way off for them?
    Is there a way to set it without a scope?
    Thanks
     
  17. larryderouin

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

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    Yep. Go in the stickies section and look for a thread called Easy fix for 400 Phase Inverter adjustment. BTW Dave did a thread or a post on domestic vs russian tubes and what happens in the P.I. with Russian tubes. Stick with the mullard's or old stock (RCA, GE , Raytheon, Tele, Mullard, etc.) in the P.I. Your distortion is a prime example of the problem. I'll see if I can find it.
     
  18. larryderouin

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

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  19. grindfix

    grindfix Active Member

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    Found both threads. Will post results soon.
    Thanks!
     
  20. grindfix

    grindfix Active Member

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    Removed 150Kohm noose resistors. Adjusted PI. Re-adjusted bias to about .350v (35ma) Sound is crap.
    Power tube draw fluctuates within couple of minutes about .080v (8ma) on all 4 tubes (load resistors in, without audio source, volume at min).
    Decided to install new EH quad from Jim. Now I can't get draw to any lower then .420v (42ma) on all tubes with EFB and IBAM trimmers maxed out.
    Sound is better though. Far from perfect, but much better. Will the draw go down after some burn-in time or I need to modify EFB (39Kohm resistor?)since it supplies bias to IBAM?
     

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