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Fisher 500B .....add bypass capacitors?

Discussion in 'Fisher' started by 1rebmem, Nov 5, 2018.

  1. 1rebmem

    1rebmem AK Subscriber Subscriber

    Messages:
    20
    Location:
    Wilmington, NC
    On my 500B, the AF amplifier stage immediately before the PI does not have bypass caps on the cathode resistors. The 500C has 50uF caps but the cathode and feedback circuitry is a bit different.

    500B Service Manual with schematic:
    http://akdatabase.com/AKview/albums/userpics/10004/Fisher 500-B Service.pdf

    And Dave G wrote this in one of his threads where he added 22uF caps to his 400 receiver:

    "First, with the output stage now operating under the control of EFB, the grid bias voltage for this stage has been reduced from typically -18 vdc or more, down to ~ -15 volts. Less bias voltage equals less drive voltage for the same power output, meaning that the output stage has more gain. Then, the rest of the lost gain can be recovered by bypassing the 1200 ohm cathode resistor of the AF Amplifier stage with a 22 uF cap. These two measures then allow the revised circuit to display the same amount of open loop gain (OLG) as the original design did, allowing the same level of NFB and overall sensitivity to be achieved as well. Except that now, with the revised circuit, there is plenty of low distortion drive available for the output stage, that the original design was simply not capable of producing."

    On my 500B I have added EFB and IBAM along with all the other standard safety improvements.
    I also removed the PI noose including feeding the AF stage with a lower B+ (node F).

    So...... I believe adding bypass caps would be appropriate and if true what size?
     

     

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  2. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,703
    Location:
    Ball Ground, GA
    I would not recommend adding the bypass caps as that would likely invite LF instability due to the excessive NFB that would produce.

    The open loop gain of the phase inverter circuit between the 500B and 500C is virtually the same, with the degeneration of the AF Amplifier's cathode bias resistor handled differently in each design. Left un-bypassed, the cathode resistor of such a stage inherently produces degeneration due to the negative current FB the resistor introduces. In the 500C, the degeneration is handled in the classic manner, by bypassing the resistor with a relatively large cap. In the 500B however, no such cap is used, but what is used in positive voltage feedback. In the 500B, the phase inverter's cathode resistor is not returned to ground like is done in the 500C, but returned to the global NFB insertion point. Because the voltage inserted at this point from the phase inverter section is out of phase with the current feedback developed across the AF stage's cathode resistor, it acts to counter act the negative current feedback developed there, much the same way that bypassing the cathode resistor would. This is why the OLG of both phase inverter/driver circuits is virtually identical. Adding a bypass cap to the 500B then would increase the OLG even further, producing the instability -- particularly in light of the positive feedback being used.

    Both approaches have their advantages and disadvantages. In the 500C, the use of a bypass cap creates predictable results with a wide variety of tubes, but introduces an additional time constant in the circuit that must be accounted for when stabilizing the LF performance of the design. The positive feedback approach does not introduce any new time constants, but is less tolerant between various tube examples in producing consistent performance results.

    I hope this helps!

    Dave
     
  3. 1rebmem

    1rebmem AK Subscriber Subscriber

    Messages:
    20
    Location:
    Wilmington, NC
    Dave...thank you for the opine on my post.
    I will read and reread a few times in an attempt to fully understand.
    And of course, I won't add the cap's.
    Jef
     
  4. 1rebmem

    1rebmem AK Subscriber Subscriber

    Messages:
    20
    Location:
    Wilmington, NC
    So Dave, are we talking 12AX7 tube examples or for example a 12AT7?
    If a 12AX7, what parameters are desirable?
     
  5. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,703
    Location:
    Ball Ground, GA
    Not particularly either. It was a general statement about the results of using that approach with various examples of tubes of a particular tube type.

    Dave
     
  6. thornev

    thornev Well-Known Member

    Messages:
    515
    Location:
    Mid Hudson Valley, NY
    Be careful if you're considering replacing tubes with different types e.g. 12AT7 to replace a 12AX7. Although it appears to work you have to review the tube specs of both tubes. For example if the mu on a tube you want to install is higher than what is recommended for a socket, it can adversely affect your overall system in the long run. If the system runs hotter, it could mean burn out faster. Thorne
     

     

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  7. 1rebmem

    1rebmem AK Subscriber Subscriber

    Messages:
    20
    Location:
    Wilmington, NC
    Thanks Thorne. I wasn't thinking about changing tube type, just trying to fully digest Dave's response.

    How is your 500C sounding?

    Jef
     
  8. thornev

    thornev Well-Known Member

    Messages:
    515
    Location:
    Mid Hudson Valley, NY
    Jef... It sounds absolutely wonderful. I'm running the Tung-Sol 7591A tubes at 35mA and through the AR 2Ax speakers + sub, it's just wow. Lately, though, I've hooked up my Marantz 2265B to a pair of B&W DM601's bi-amp'ed and that configuration is getting a lot of attention with my turntable and well-positioned FM antenna. (just a dipole, but I found that position makes a huge difference). Thorne
     
    Last edited: Nov 10, 2018 at 5:33 PM
  9. 1rebmem

    1rebmem AK Subscriber Subscriber

    Messages:
    20
    Location:
    Wilmington, NC
    Thorne, you have the buffer op-amp installed and ironed out correct?
    Did you hear a nice improvement?

    I have the board built to install the buffer in my 500B, just waiting on the .1uF caps. The board is only 1-1/2 x 2" won't hardly see it in there!
     
  10. thornev

    thornev Well-Known Member

    Messages:
    515
    Location:
    Mid Hudson Valley, NY
    1reb... Not sure what you mean by "buffer op-amp". Is that the Unity Gain mod? If so, I opted not to install it because the one time I tried, I heard no difference and it caused more problems (I forget what). I have installed these mods:

    - 47K ohm piggy-back on R204 to improve FM stereo reception sensitivity.
    - Added CL80 inrush current limiter because voltage here is pretty high, around 125V.
    - Subwoofer circuit board hookup.
    - Cut pin 4 to 8 wire on 7591's, connected all pin 1's together, added 100 ohm resistors to connect pins 1 and 4 (screen stability).
    - IBAM board mounted on top of chassis for easy access to pin 5 for measurements and keeping 7591's outputting the same wattage.
    - Lots of tube rolling to find tubes that sound the best to me. Adjusted all transformers.
    - RIAA mod (I forget what this one does).
    - Removed the noose (I forget this one too).

    Thorne
     
  11. 1rebmem

    1rebmem AK Subscriber Subscriber

    Messages:
    20
    Location:
    Wilmington, NC
    Yes Thorne......the Unity Gain mod. I think I've read where some said it made a huge improvement. I shall see.
    I will post some pics at some point.
    Jef
     

     

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  12. thornev

    thornev Well-Known Member

    Messages:
    515
    Location:
    Mid Hudson Valley, NY
    rebmem... It took me 3 builds to get it right and I'm not even sure I got it right. According to my notes, the 1st time I tried, I got mega distortion and only 1 channel worked. The 3rd time I still got distortion when bass was turned up and stereo separation was diminished so I removed it. Someone suggested that maybe I had too many mods and that their coexistence was the cause of the problem. But later on I found it was a 12AT7 tube I had installed that was causing the diminished stereo separation. Larry suggested trying some sine wave injection and measuring to diagnose the problem, but I never tried. It doesn't seem like that complicated a build, but getting it right seems to elude me. I've tried 3 different designs. Some of the components criss-cross each other which makes it challenging with my newbie board design skills. The board is still sitting in my electronics box so I could try again. There's a thread somewhere where I talked about my 3rd build with pictures. I'll be very interested to hear about your results. Thorne

    Found my thread: http://audiokarma.org/forums/index....-they-be-improved.826401/page-3#post-11757739
     
    Last edited: Nov 10, 2018 at 7:02 PM

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