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Fisher 440-T tube amp stand alone

Discussion in 'Fisher' started by vintage cltr, Jun 22, 2015.

  1. vintage cltr

    vintage cltr Active Member

    Messages:
    206
    Location:
    milwaukee
    This is my second Fisher tube unit, and I remember on my 460 there was a modification made to operate it as a stand alone.

    Does this need to be done on the new 440? And if so, what mod?

    It's either run this or my Mag 175 console amp.... and I really want to hear this one to make a fair comparison.

    thanks
     
  2. fred soop

    fred soop Super Member

    Messages:
    1,703
    Do you mean 440 and not 440-T ? The 440-T is a transistor unit from around 1965 and was the smaller version of the 600-T.
     
  3. vintage cltr

    vintage cltr Active Member

    Messages:
    206
    Location:
    milwaukee
    It's the 440 with the (4) EL84's output tubes
     
  4. larryderouin

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

    Messages:
    19,081
    Location:
    Glen Burnie Md.
    61 Custom electra is the console it came out of. You'll need to do a mod similar to the 460-A to make it standalone. I'm not familiar with the mod (what parts and such) so I'll leave it there. However, the correct designation for your amp is 440A. As Fred Soop already pointed out, 440-T is a 1st generation mid 60's receiver that is the smaller brother to the 600-T. [​IMG]
     
    Last edited: Jun 23, 2015
  5. stewdan

    stewdan New Member

    Messages:
    13
    Hi --- I also picked up a Fisher 440A Amplifier from a Custom Electra V e44 and am looking into getting it working as a Standalone Amplifier.

    Did you succeed in getting yours to work Standalone and would you be willing to let me know your standalone Mods for the 440A?

    Please let me know.
    Thanks,
    Stew
     
  6. FlaCharlie

    FlaCharlie Active Member

    Messages:
    253
    Location:
    Gainesville, FL
    Hi Stewdan . . . I rebuilt one last year and posted about it on this (Fisher) board and described my mods.

    Here are the links where I discussed it: http://www.audiokarma.org/forums/in...4/&q=440A&o=date&c[node]=172&c[user][0]=88202

    I won't have time to comment on them for a while but my description should get you started in the right direction. Please note that some of the pics are "before" and some are "after". Also, I removed the wiring which allowed me to check the bias from the top of the chassis because it was creating noise (the yellow wires) but I left the 10 ohm resistors in place on the sockets to I could check from underneath.

    I'd also suggest that you start a new thread on this, the Fisher, board about your amp since the title of this thread refers to a different model, albeit erroneously, and the original posts are a couple of years old.
     
  7. stewdan

    stewdan New Member

    Messages:
    13

    Hi --- Have a few questions after reading your 440A Mods Thread and looking at your pictures:

    (1) Is what you are calling the Cathode Bypass Cap actually C93A, the 100MF/100V cap?
    And is it the BLUE ELECTROLYTIC CAP in your After Mod Picture?

    (2) How did you arrive at 270 Ohms 20W for the Cathode Resistor?
    I ask because a 12AX7 is like 84 Ohms and there are Four 12ax7's in the Tuner tied to the Pin in the Tuner Connector.
    So, 4 * 84 = ~ 350 Ohms. (I saw something in an Audiokarma Post from another thread, that called out using a 500 Ohm or 550 Ohm resistor which also dropped the voltage on the tubes in the amp.)

    (3) Where did you find the 20W power resistors? I can find the cement variety but usually only in 10W size
    .
    (4) From the After Mod Picture, it looks like you replaced the "Kill Capacitor C106" with 2 Safety Caps in series. C106 was a molded cap 0.01 MF @ 600v, what values did you use for your series safety caps. I can't seem to read what the safety cap says.

    (5) How did you connect the CL-80? From the After Mod Picture, it looks like it is across the fuse???? I can't really tell from the picture.

    (6) You said you removed the Yellow Wires from the EL84's back to unused Pin Connections on the Tuner Connector because of noise problems.
    Were you connected to Pin #2 of the EL84's or Pin #3? I don't think you would have needed resistors for Pin #3 since were would be reading Cathode Voltage and not a drop.

    Any answers you could supply would be greatly appreciated.
    Thanks
    Stew
     
  8. FlaCharlie

    FlaCharlie Active Member

    Messages:
    253
    Location:
    Gainesville, FL
    Hi Stew - Again, I'd suggest you start a new thread on here rather than continue with this one. It will probably get more attention by the fact that it's new and will have a correct title.

    I'm out of town so I don't have access to the schematic or the amp itself but I'll try to answer some of your questions based on my recollection and the pics I posted.

    (1) I think that is the blue cap - I'll confirm when I get back home. No schematic so I don't know about the C93A designation but I think the stock cap was one of the sections of a can cap so it makes sense. OTOH, it looks a bit large for a 100v cap so I'll have to check when I get home.

    (2) I don't recall how I arrived at the value. I did see the post about 84 ohms per 12AX7 but I don't recall how many of them there were in the tuner/pre section. I did increase the value of the PS resistors in order to compensate for the higher wall voltage and the missing preamp. The end result was voltages, including cathode voltage, that are pretty close to those listed on the schematic. As I noted, however, the output tubes are dissipating slightly more than the 12 watt data sheet spec - which the stock unit also does. When I get back I'll see if I have notes on the exact voltages and resistor values. It probably wouldn't hurt to lower the dissipation some regardless of how hard Fisher ran the tubes.

    (3) The 20 watt resistors were purchased on eBay a while back. I think I also got some 25 watt too. I imagine the larger distributors, like Digi-Key, would also stock them and higher wattage too. I have a thread on here somewhere about how to get free shipping from Digi-Key. Do a search.

    (4) The safety caps (X1/Y2) are .022, although I have read that using a lower value is actually safer. They are not in series. There is one connected between each side of the AC line and the chassis. The one on the right is mounted just like the molded green cap it replaces - between the fuse holder and a ground tab on the rivet that mounts the Aux AC socket. The one on the left goes from where the other side of the AC line connects to an Aux AC socket and then to the same ground tab.

    (5) The CL80 is mounted between the incoming AC line and the end of the fuse holder.

    (6) The 10 ohm resistors were mounted on the cathode (pin 3) of each output tube. The idea was to be able to check the current draw of each tube for matching purposes. I left the resistors in place, I think, so measurements can still be taken from underneath. I did something similar on a Sherwood S-5000 that I rebuilt so I thought I'd try it here since the socket was empty but it didn't work out so I just removed the wires.

    . . . Charlie
     
  9. FlaCharlie

    FlaCharlie Active Member

    Messages:
    253
    Location:
    Gainesville, FL
    Stewdan - not sure if you're still following this thread or if you might have already finished your amp, but to clarify . . . yes the blue cap is the replacement for C93A. The original was one section of a can. The blue one is a bit large because it's a 200v cap, not a 100v as the original. I was out of the stock value and used what I had on hand.

    The changes made to the power supply: I replaced R126 (2.2k 7w) with a 3.3k 10w, as seen in the pics. I also replaced R123 (2.7k 1w) with a 6.8k 1w.

    As for PS voltages: With modern wall voltage (122v) I got 120.2 at the fuse (slightly lower than the wall due to the CL80). The B+ is 397v (vs 385v on the schematic with 117v wall), G2/Screen voltage on pin 9 is 327v (vs 320 on the schematic) and voltage at C98A (supply voltage to the 7247s) is 293.8v (vs 290 on the schematic).

    On the output tubes: I measured around 389v on the plates (vs 381 on the schematic at 117v wall). That's only about 2% over the schematic values, well within the accepted +/- 15% that is considered to be "in spec". Cathode voltage (pin 3) was 46.2v using the 270 ohm 20w resistor (vs 44v on the schematic) so thats only 5% higher.

    The outputs ARE being run over their 12w dissipation rating, which was also true in stock form. Fisher ran these puppies hard! I'll eventually make some changes to bring them into spec but I'm using NOS American tubes so I'm not too worried. If I was using modern production tubes I think I would be a bit nervous running them that hard, although guitar amps do it all the time. I haven't been using the amp much - it's currently residing on my bench where I was breadboarding a preamp design a while back - but it's an excellent amp. Well worth the effort.

    Let us know how your project is going.

    . . . Charlie
     
    Last edited: Jun 19, 2017 at 2:05 PM
  10. stewdan

    stewdan New Member

    Messages:
    13
    Hi Charlie ... thanks for the reply. I am waiting on some parts but the standalone conversion is moving along slowly, since I am undecided about replacing the cans with separate caps. All though, that is probably the cheapest way to go!!

    Your numbers look real good!! I saw a response from Dave Gillespie somewhere on Audiokarma where he says 10% over the specs on the schematic is still within the Fisher Design Limits for EL84 type amps.

    Thanks again for the response.
    Stew
     
  11. FlaCharlie

    FlaCharlie Active Member

    Messages:
    253
    Location:
    Gainesville, FL
    Stew - Not only are separate caps mounted under the chassis cheaper but it also allows you to find higher quality caps that are rated for longer life and higher temperatures than most can types. I just leave the original cans in place so as not to change the appearance. With an amp like this there is plenty of room to mount them too. If you look at the pics you can see that I mounted the new doubler caps (220uf 250v) on an existing terminal strip after removing a couple of preamp-related resistors that were no longer needed. I usually use either Nichicon or Panasonic: https://www.digikey.com/products/en...=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=25

    Here's how to get free shipping from Digi-Key: http://www.audiokarma.org/forums/in...hipping-on-parts-orders.739513/#post-10018562

    . . . Charlie
     

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