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Converting A Fisher 600A to 6L6 tubes !

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

  1. triode17

    triode17 Super Member

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    Yes,in the Bohemian region, that's how they can use leaded crystal envelopes. BTW, i have been using a quad of JJ 7591S in my LK-72 for hundreds of hours without incident. Plus, I am using JJ KT77s in my ST-70 also without incident.
     

     

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  2. triode17

    triode17 Super Member

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    Gary - It sounds like your tone control pots. have drifted over the years. (Here's yet another reason to have a square wave available.) If you input a 400Hz, Sq. Wave, then set the Bass and Treble controls to obtain the squarest wave possible, you will have found the tone control centers. Mark the panel at these positions. Then listen to see if the Treble is still too high. You may have to repalce the tone controls to get proper action.
    Also, replacing the tone control caps and resistors with modern parts ( I like Vishay "Orange Drops", polypropylene film/foil caps.) should widen your tone control and improve the sound. Carbon film resistors will work well.
     
  3. Gary Mattis

    Gary Mattis Active Member

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    You are correct the tubes were running too hot , the 9.09 k resistor I added to the cathode bias resistor after removimg the 4k resistor seemed to solve that problem without messing up the pre amp enough to hear any real differance and like you said their is always the easy way out by just putting it back to stock again ! .The tubes still run hot but they are not as hot as they were running before. On 6L6 usually at least a 10 K resistor is on the bias pot , the 4 k resistor was just not enough. I knew from the beginning there would need to be some changes made and that they would be gives and takes not total perfections ! . My goal was just a reasonable sound quality that I have already achieved pretty well . This is a good way for me to learn more about tube amp design and testing because it is not a normal amplifier build to start with but one with a kind of strange design. Certainly looking at the sku for awhile should help me quite a bit !. I need more test equipment at this point so I am limited some. Thanks again for the help.
     
  4. Gary Mattis

    Gary Mattis Active Member

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    How about this for a solution ? At R 126 the 1.5k 3w resistor where I put the 500 ohm resistor, how about if I replace the 1.5 k and run a separate 550 or 600 ohm resistor from source to G2 instead of changing the 1.5k ? Would that solve the problems ?. The bias pot resistor change brought the voltages to 43,454,446. I would like the screen to drop lower at 10% of Or less than plate around 400 v. I just got the sku and I have only looked at the output section at this point.
     
    Last edited: Jun 13, 2018
  5. gadget73

    gadget73 junk junkie Subscriber

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    It should still run a 1.5K at R126 otherwise G2 voltage will be high. I'd probably put the power supply back to the stock setup and see what that does with the tuner connected.
     
    Gary Mattis likes this.
  6. Gary Mattis

    Gary Mattis Active Member

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    457/423/41 now, If I added a separate 2k resistor from supply to G2 that would lower the screen more maybe closer to 400 volts, is 423 too high or can the tubes handle that ok ? Or just add 500 ohms between R126 and G2 making it 2k from source to G2 only and it would stay 1.5k to everything.
     
    Last edited: Jun 13, 2018

     

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  7. gadget73

    gadget73 junk junkie Subscriber

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    The reason I wouldn't run just a drop resistor to G2 is that the voltage regulation gets poor. 6L6 tubes will really swing the screen current, and that makes the voltages dip a lot at higher output. When all that shifts it makes for distortion. If you have a smaller resistor it won't swing as much. Using a smaller resistor with the current load of the driver and the tuner keeps the voltage drop where you need it without the big resistor that causes large voltage swings.

    Also keep in mind any voltage is less the cathode voltage, so if the plate is at 457 and the cathode is at 41, the tube only "sees" 416.
     
    Gary Mattis likes this.
  8. The Fuxtor

    The Fuxtor AK Subscriber Subscriber

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    Maybe too far gone for this thread, but I did notice that you can buy 7591 to 6L6 tube converters ? Not exactly sure if they work decently, but ......??
     
    Gary Mattis likes this.
  9. Gary Mattis

    Gary Mattis Active Member

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    I was not considering the lower voltage the tube actually sees , at 420 and 400 the tubes should be ok unless they are weak to begin with. I'm gonna try it with the extra 500 ohms to G2 just to see what happens , I was not thinking about the large voltage swings at high output either ! , Just trying to get the tubes to run a little bit cooler than they were.
     
  10. tekuhn

    tekuhn Active Member

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    Here's my novice attempt at helping:

    1) The 1.5K resistor value is important because changing it will also effect the voltage drops downstream to the AF/PI plates and the tuner voltages as well.

    2) Dave recommended a screen voltage of about 350v to be a better match to the OT primaries. You know that everything downstream of the 434v B+ is drawing 30ma because of the 45 volt drop across the 1.5K resistor R126. 45/1500 = 30ma. You want to move the screen tap further down in the 3700Ω (2.2K+1.5K) network. To get the 84v drop you need for 350v on the screens, you need a 2.8K resistor to replace R126 and then a 900Ω to replace R123 which will keep the total at 3700Ω as before. This should drop the screen voltage to about 350v and still maintain the original design voltages downstream from that. For simplicity you could probably use common 2.7K for R126 and a 1K for R123. R126 will be dissipating 2.5watts and R123 .9watts, so (over)size accordingly. Keep in mind that the experts have said that using more resistance to drop the screen voltage will result in poor screen voltage regulation which means increased distortion, but at least you might avoid burning up more tubes.

    3) To get your idle current in the ballpark, I believe you need to be more negative on the grids, so adjust resistors R104 and R125 to drop the voltage, but again keep their sums the same to avoid changing other parameters. Do you have a way of measuring your idle current on the 6L6's? I know it's common to use an adapter on guitar amps that sandwiches between the tube and socket which allows you to insert an ammeter on pin 8 to measure cathode current. Someone else will have to chime in on where his idle current should be.

    Someone please speak up if I'm giving bad advice here. I honestly would not be going this way, but would be looking into building an EFB circuit and fixed bias supply as was Dave's minimum recommendation. With that, you can know that your 6L6's are operating exactly in the best possible configuration, producing the most power output, and will live a long happy life :). From there you can choose to address the other issues previously mentioned if you feel like the sound can be improved upon to your ears.

    BTW, you mentioned changing to tube rectification. I see this is using a voltage doubler circuit which means you cannot replace the two silicon diodes with a standard full wave rectifier tube since the tube plates share a common cathode. You would have to use 2 separate tubes to make the conversion - either some uncommon tube single diodes or a pair of standard full wave tubes using both halves in parallel.
     
    Last edited: Jun 13, 2018
    derekva likes this.
  11. Gary Mattis

    Gary Mattis Active Member

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    I
    Thanks, I put a 500 ohm resistor from R126 to G2 and it sounds very good at half volume now and the tubes seem cooler but the voltage on G2 stayed about the same, but let me start things out now by checking the ma's on the tubes, I looked and I Cannot find the really nice bias tester I made , I have rooms full of parts and it is buried down in there somewhere , I will just make another one real quick and then I will be back with the results.
     

     

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  12. tekuhn

    tekuhn Active Member

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    I’m pretty sure the voltage didn’t drop because there is very little current draw. If you do it the way I suggested, you are assured of voltage drop due to current being drawn by other components.
     
    Gary Mattis likes this.
  13. Gary Mattis

    Gary Mattis Active Member

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    Tube 1 = 48ma. , Tube 2 = 43 ma. , Tube 3 = 38 ma. , Tube 4 = 37 ma. Seems ok on one side and too hot on the other , 40 is Avg. At 450 volts. 47 hot and 33 cold. I adjusted the AC balance pot and I got it to 43 ma on both tubes, the tubes should be ok now ! 37/38 and 43/43. One side is a little hotter but both pairs are now in the acceptable mid range.
     
    Last edited: Jun 14, 2018
  14. Gary Mattis

    Gary Mattis Active Member

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    I was wondering if EL 34 tubes would work better in this amplifier ? Pin #1 is blank so woulden't EL 34 tubes work in it also ? I was not planning on changing it right away to EL 34 but after the 6L6 wear out I thought why not try some EL 34's in it.
     
    Last edited: Jun 14, 2018
  15. GordonW

    GordonW Speakerfixer Subscriber

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    EL34 tubes do have somewhat more transconductance than 6L6s- so it might help just a little. It's still not going to have the same "effective gain" as the 7591

    Regards,
    Gordon.
     
    Gary Mattis likes this.
  16. kvflyer

    kvflyer AK Subscriber Subscriber

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    At the risk of upsetting the apple cart, the 7591 is hard to beat. It was the zenith of vacuum tube design and manufacturing (think Westinghouse). That is why so many audio manufacturers used them (and they were inexpensive, costing $1.61 in 1969 dollars). Very high gain, high output, small envelope; it doesn't get much better than that.

    Then, the 7868 came out. Essentially a 7591 in a novar envelope. I guess I don't understand reinventing the wheel...
     
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  17. Gary Mattis

    Gary Mattis Active Member

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    I'm seeing the guy this weekend who I got the amp from and hopefully he will have the other OT for it. I rolled a few tubes in the PI today and the short plate Amperex sounded better than long plate Telefunkens somehow ? The RCA long blackplates and Telefunken longplates both were noisey at idle in the PI and the Amperex were silent.
    Gain is not everything ! , The 6SN7 tubes proove that ! , Sometimes less gain has a much sweeter tone to it. No tube number to me sounds any better than The Blackburn Mullard EL 34 tubes.
     
    Last edited: Jun 14, 2018
  18. Gary Mattis

    Gary Mattis Active Member

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    I am sure the guy I got it from would give me the 7591 tubes if I told him that I wanted them, I just do not want them - no matter how great you all say they are ! , I would rather use RCA 6l6 holy grail or Mullard EL 34 tubes. My whole purpose was to get away from 7591 tubes forever !. This amplifier sounds pretty good now at any volume, it is as good sounding as any 6L6 amp I have heard before ! . I have very good tubes in it and the more the tubes and caps break in the sweeter the tone gets ! , This thing is really starting to sound good now. 6 Amperex and 4 Telefunkens !
     
    Last edited: Jun 14, 2018
  19. GordonW

    GordonW Speakerfixer Subscriber

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    The problem with the gain mismatch, is that it upsets the feedback loop, and probably won't have enough gain to be properly driven by the preamp/tone stage.

    If it's OPTIMIZED everywhere (redesigned to have a proper gain structure throughout, so it doesn't run out of headroom, and the feedback is properly re-set)- then yes, sometimes low-gain stages can sound better. But, ONLY if everything is set up to work with those stages.

    Regards,
    Gordon.
     
    Bill Ferris likes this.
  20. Gary Mattis

    Gary Mattis Active Member

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    Maybe Gain isn't everything ? The Mullard EL 34 Blackburns sure do sound good with less gain !. I am not a fan of 7868 tubes either ! Nothing but bad connections in Fisher 400's when they get hot , work ok in Ami jukeboxes usually. Not bad sounding in 400's but not the best sounding tubes by any means to my ears !, Maybe I do not like a real sterile sound and just perfer tubes with more color to them. Some people think all distortion sounds bad and others like guitar players would disagree with that statement. To me where it is at and how much would be more important than having it or not having it. Little 6V6 tubes are pretty good sounding tubes but they are not real loud. I may try making (two) 4 or 6 output tube 6V6 mono blocks with tube rectifiers next ! Just a thought I have had for years about hi powered 6V6 !
     
    Last edited: Jun 14, 2018

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