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Converting A Fisher 600A to 6L6 tubes !

Discussion in 'Tube Audio' started by Gary Mattis, Jun 5, 2018.

  1. Gary Mattis

    Gary Mattis Active Member

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    Location:
    TN.
    Greetings Earthlings ! , Top of the day to everyone out there ! . I started working on the Fisher 600 A amplifier two nights ago and I replaced a missing output transformer and I rewired the output sockets last night in it for 6L6 tubes. I am using the Fisher 470-T tuner with it also. I ordered new caps and I tested the power supply , I think it was around 480 volts. I found a broken 3.3k resistor that went to the negative feedback on the 16 ohm tap of one of the output transformers, I have another OT like the one I replaced if it is bad, for now I just cut the feedback wire sense I am only using the 8 ohm tap. It has 1k grid resistors and a 1.5 k - 3 watt common grid #2 resistor , I guess my questions would be ; can I just put a 3/5 watt 470 ohm common resistor in place of the 1.5k common resistor on The wired togather Screen Grid's #2 of the 6L6 tubes and use the stock fisher bias adjustment ? Or do I need to have separate 470 ohm resistors on each 6L6 tube ? . My other question would be if leaving the 1k resistors on G1 is ok ?. I have not done this tube conversion before from 7591 to 6L6 and I don't want to do it wrong , I thought that maybe someone here has done it before or knows the right way . I do not want to change the amps design if I can help it other than changing the output tubes. Thanks !
     

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  2. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    This set up is out of a '62 Ambassador Console. The Amp is actually a 660A. Why go to a 6L6GC when it's optimized for 7591's?
     
  3. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Your project is very doable, but doing it correctly depends on how much re-engineering capability you have -- not only in the understanding of theory, but test equipment capability as well. I say the word "correctly" based on either making the amplifier simply "work" (i.e., operate reasonably well) dependably and safely with the alternate tubes, or having it produce all the performance of the original design that Fisher intended for it with the alternate tubes installed. That is two very different things. In either case, basic issues to address include:

    1. Mismatched OPTs. The original Fisher OPT is no doubt significantly superior to the replacement piece. It would be best to get an original replacement transformer. Otherwise, understand that there will be performance given up by using the replacement unit installed.

    2. 6L6 class tubes have about half the Gm of 7591 tubes, therefore, the 6L6 tubes will require about double the signal drive voltage and double the required grid bias voltage. But these amplifiers do not include a bias supply but rather, used the heater elements in the audio tubes on the adjoining tuner preamp to provide cathode bias to the output stage. This will clearly need to be adjusted to suit. The increased bias voltage required will represent a loss of B+ voltage, so power output will be reduced with such a move. NFB will be reduced as well due to the lower output stage gain, so that if it is restored to its original value, the input sensitivity of the unit will be cut in half -- again, all owing to the fact that the Gm of 6L6 tubes is much lower than that of the 7591.

    3. To promote a proper impedance match to (at least) the original OPT, the screen voltage will need to be dropped to about 350 vdc. But doing this by simply upping the value of the screen grid dropping resistor will produce poor results as then the regulation of the screen grid voltage will be very poor.

    All these things would need to be addressed to achieve the performance of the original design. If you just want to get it working reasonably well, I would consider installing a separate negative bias supply and an EFB™ screen grid regulator. This will allow the tubes to be biased properly and develop a similar amount of power output (if not more) than that of the original tubes. But the NFB, sensitivity, and stability issues would all have to be sorted out from there as desired. Is there any reason you don't want to use new manufactured tubes as used in the original design in the unit?

    Either way, good luck with your project!

    Dave
     
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  4. kward

    kward AK Subscriber Subscriber

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    If this is your ultimate goal, I would suggest you wire the sockets back up for the 7591 and call it a day. That way you don't need to make any other serious modifications to support the 6L6 tube family in the areas of biasing, screen voltage, feedback circuit, etc. You can get new production 7591's from Tung Sol that are true 7591 replacements.

    Now on the other hand, converting over to the 6L6 family I think would be an awesome challenge, and one I'd be up to do myself, perhaps on a test chassis first, just to see how it would work and where the gotcha's are. There are more varieties of new production 6L6's available than there are new production 7591s, thus many more tube rolling choices. That by itself would be a good reason in my view to consider this mod, if you don't mind moving away from the stock design. Even though tube rolling choices would be expanded if moving to 6L6, I'm sure a modded unit would not fetch as much money on the open market. Tradeoffs to be considered.
     
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  5. Gary Mattis

    Gary Mattis Active Member

    Messages:
    336
    Location:
    TN.
    I'm Not interested in using obsolete 7591tubes or newer 7591 tubes , I have Telefunken and Amperex pre amp tubes and Vintage RCA Holy Grail 6L6 GC already and I want to use those tubes . Retail value is not my concern with this project , good sound is !. I just wanted to try to use the origonal bias adjustment if possible ? , I'm not saying I won't change what I need to change for 6L6 tubes , just that I want to do the least amount of change that is needed. I use newer tubes in other peoples equipment if I have to but Only Vintage tubes go in my stuff. I can always put it back to 7591 tubes if it dosen't sound good with 6L6 tubes. I was thinking that I could use one of the old type resistors in the pic for the 470 ohm screen resistor with the stock bias adjustment.
     

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    Last edited: Jun 5, 2018
  6. 62caddy

    62caddy Trust but verify Subscriber

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    If good sound with the use of 6L6GC is the aim, it would be a lot easier to find an amplifier designed for 6L6GCs in the first place. Or simply use the tubes called for in the original design.
     
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  7. Gary Mattis

    Gary Mattis Active Member

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    There are no problems only solutions ! John Lennon. I'm not rich or silver spoon and I'm not really looking to buy another amplifier , anyone could do that ! . My goal was to improve the 7591 mistake that Fisher made !
     
    Last edited: Jun 5, 2018
  8. 62caddy

    62caddy Trust but verify Subscriber

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    There are also solutions to be found in the form of not creating problems in the first place.

    What's the "mistake" about the 7591? It's one of the most highly regarded output tubes ever made and used in any number vintage units held in equal admiration.
     
    Last edited: Jun 5, 2018
  9. s-petersen

    s-petersen Scott Subscriber

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    The 7591 was a great tube, many companies used them, even McIntosh I believe, an 8417 is like a 7591 on steroids, they were some of the last power amp tubes developed.
     
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  10. 62caddy

    62caddy Trust but verify Subscriber

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    7591 used in the MAC1500 FM receiver, MA230 integrated amp and one of the most beloved McIntosh amplifiers of all time - the MC225.



    [​IMG]
     
  11. s-petersen

    s-petersen Scott Subscriber

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    I didn't realize the 225 used 7591's , I thought they were 6L6GC, the other models I kind of knew about.
     

     

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  12. 62caddy

    62caddy Trust but verify Subscriber

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    You might be thinking of the MC240. Also, the MC30 (monoblock) was originally supplied with 1614 metal tubes but nearly everybody runs 6L6GCs in them. As far as I'm aware, they're the only Mc units that used 6L6GC OTs.
     
  13. s-petersen

    s-petersen Scott Subscriber

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    I have a 240, it will run 6L6GC's or 7027's, I also knew about MC-30's, it was just an assumption of mine that the 225's ran 6L6GC's
     
  14. pdm4606

    pdm4606 Super Member

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    I guess that some folks born with a silver spoon in their mouths never see the value of rehabbing something that has possibly worn it's usefulness.
    I for one being in the audio stuff for 60 years find that position a waste of our resources. I think the silver spooners are missing the whole point.
    Every one of my equipment pieces has been rehabbed from useless pieces of steel and transistors and remade to operate as well as my meager electronics education can carry it.
    Show me something all shinny and sparkly and I wouldn't know what to do with it. Give me the next to go into the scrap container all dirty and broken and I get to work.
    This has happened many times during my audio adventure and I expect it to keep on in this manner. My biggest problem was working in a skilled trade that provided nothing in the way of retirement income or benefits.
    This makes it important that I find the lowest common denominator for next to nothing. Same goes for my listening. My stash of early Miles, Count and others that I bought in the late 50's still sound terrific. Played regularly.
    So good luck to the silver spooners. Just think that shinny new amp they just got will be someone's rehab project in 10 years. The joke is on you.
     
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  15. s-petersen

    s-petersen Scott Subscriber

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  16. 62caddy

    62caddy Trust but verify Subscriber

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    I think you have severely misunderstood the advice being advanced. Nobody is suggesting the OP not restore his Fisher unit to its fullest potential. What is being suggested is that it would be a mistake to reconfigure the amplifier to accommodate tubes it was never designed for. And almost certainly it will negatively affect performance in the process.

    I don't know where all this silver spoons nonsense is coming from but it certainly has no place in this discussion.
     

     

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  17. pdm4606

    pdm4606 Super Member

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    Figures......
     
  18. Gary Mattis

    Gary Mattis Active Member

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    336
    Location:
    TN.
    They are obsolete surplus korean war tubes with industrial numbers with one choice new.
     
  19. Gary Mattis

    Gary Mattis Active Member

    Messages:
    336
    Location:
    TN.
    Industrial tubes not audio tubes
     
  20. Gary Mattis

    Gary Mattis Active Member

    Messages:
    336
    Location:
    TN.
    The 6L6 conversion worked perfectly with a 500 ohm 10 watt screen resistor and rewire of tube pinout, no other changes were needed ! RCA Holy Grail 6L6 GC are in there now and sound great ? Next the tuner rebuild, clean everything up and two custom cabinets ! . The guy I got it from says he has the other output transformer for me also !
     

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    Last edited: Jun 11, 2018

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