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Fisher x100c weaker volume level in right channel

Discussion in 'Fisher' started by mp53, Nov 9, 2017.

  1. mp53

    mp53 Member

    Hello, I got this rebuild and modifying project going on my Fisher . Right now I have a very low volume from the right channel that I have narrowed down to V4. I can jumber into the tag right before the volume pot I get normal volume using my phone but jumper at the tube very low volume as comparred to the left preamp tube. My deduction then is there is a problem with the last half of C4. The tube socket is new, replaced the tube twice and the coupling capacitor twice. The resistors are new and checked them for the right value. The voltage checks out close. The caps are .047 ero fol types which I had in service in another amp. What the hell could be wrong? I'm missing something here.


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  2. Tim D

    Tim D AK Subscriber Subscriber

    Melbourne, FL
    Have you checked the plate voltage? I had an output transformer with one side of the primary open that caused almost zero volts on the plate of one of the two output tubes for one channel. The channel still worked, but I had to crank the balance to 3 o'clock to get it to be somewhat balanced. I have a x-100-B and the plate is pin 9 of the 7868 tubes. I haven't seen a diagram for the X-100-C.

    Not sure this is your problem, but it might be something to check.
  3. rufleruf

    rufleruf AK Subscriber Subscriber

    I only ask because I'm guilty of asking for help before covering the basics... are all controls cleaned and working as expected? RCA jacks are seating correctly and not grounding? Tube pins are tight?

    You had a few threads about this thing - you're running 7199's instead of 12AX7's in some positions? Do you have a diagram or anything about how you implemented this? 7199's are not symmetrical like a 12AX7 so I'm not sure how you would use them.

    I have a stock X100C in my studio doing daily service and it sounds great. What were you trying to accomplish with the modifications?
  4. mp53

    mp53 Member

    I'm using the 7199 tubes in the amp section to deal with the inherant Miller effect in the 12ax7 . The other threads are dealing with hum problems which I have since mostly solved. I still have some hum now so I may end up abandon the 7199 set up.
    But the unbalance problem now is not in the amp section because I made a jumper rig with alligator clips so to clip into any point in the whole amplifier from the headphone jack of my phone. So at the input of the volume pot I can play my phone and I get balanced sound from my speakers.But if I jumber into pin 7 of V3 & V 4 I get unbalanced sound. So the problem is in the second half of V4. But it has a new socket, new resistors, put other tubes there as well changing out the coupling capacitors twice.I've ohmed out the pins of the tube from from one side to the other, I balled the socked to see if there's any solder shorting it out. Unsoldered the input wire to rule out the tone PC. Jumpered the ground of the cathode resistor, checked the value of the cathode and anode resistors. I thought maybe its the heater circuit since V4 is the second tube in the heater string but my voltages at both V3 & V4 anodes are the same.
    I'll put up a diagram of the circuit I'm using to support the 7199 tube
    There are some differences between whats on diagram and whats on the amp. The bias resistors are not 220k but are330k, coupling capacitors are not .1uf but are .047uf , I'm using IBAM also. The feed back is open loop too.

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    Last edited: Nov 10, 2017

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