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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    Thanks again, Your right ! , at first it was not that great but now it sounds pretty good actually , after cleaning the controls up and replacing two RCA pre amp tubes with some Telefunkens the sound is pretty good now , could it be better ? Certainly it could, a little too much treble is all that I really see wrong with it now and that can be turned down but when half way up on the control it is a little too much treble, that seems more like a pre amp issue more than output issue to me , I have not messed any with the pre amp/tuner other than changing tubes and cleaning it. I noticed that the speakers Fisher used on this model were pretty non effeicient sponges - if I had to put it into words, those speakers would certainly tame the highs down quite a bit sense they do not have any horns. I'm just using two 12 inch 8 ohm test speakers right now, I have not bought any speakers for it yet. I usually like 8 ohm 3 ways with dual mids and horns up top ! . I don't know if I should try to lower the highs some now or just leave it alone until I get speakers for it. I do not have a sku for it either to sit and examine their pre amp design. I did notice high notes were extending themselves higher and lower once in a while on the scope patterns. It is so close to being right that it is hard to describe what I am hearing, just an extra millisecond of high sustain is the best way I can describe it or a slight schrill on certain highs. If I lower the treble it goes away but so does the shine on the vocals. I have a marker generator (pic) can that work ?
     

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  2. Gary Mattis

    Gary Mattis Active Member

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    I do not know what it was before ? How could I compare ? It was not working when I got it.
     
  3. 62caddy

    62caddy Trust but verify Subscriber

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    That's unfortunate as it would have made for an interesting experiment. Next best thing would be to obtain test data for the same Fisher model in stock form (restored to original spec).
     
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  4. Gary Mattis

    Gary Mattis Active Member

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    I just remembered that I disconnected the negative feedback on both channels , that could be what is messing with the highs ? , I cannot hook it back up until I get another Fisher OT because it hooks up to the 16 ohm tap on the Fisher OT, the replacement OT has no 16 ohm tap on it. I should replace the OT and re install the NFB before I do anything else I think. The complete circuit is not hooked up at this point so I do not really know if there is a problem or not until I fix that.
     
  5. tekuhn

    tekuhn Active Member

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    Absolutely! Why not connect it back on the channel with the correct OT and then you can listen to it and compare the left and right channels directly and hear the difference? Bear in mind what Dave was explaining previously that the neg feedback is not optimized for your new 6L6 since it was designed for the circuit using 7591.
     
    Last edited: Jun 12, 2018
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  6. Gary Mattis

    Gary Mattis Active Member

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    Yes I understand it was made for the other tubes but it simply uses the 16 ohm tap on the OT or double the 8 ohm output from ground on the OT windings. I am sure it must affect the EMF of the OT and it's sonics.
     

     

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  7. tekuhn

    tekuhn Active Member

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    You could also take the same approach you are using with the output tube and just re-attach the NFB to the replacement OT to a different tap and see if it works. You might like the way it sounds.
     
  8. tekuhn

    tekuhn Active Member

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    NFB does not alter the performance of the OT. It is using the output of the OT and feeding it back in to the input to alter the signal for increased accuracy.

    By including the OT in the feedback loop, it ensures that any distortion caused by it is also corrected.
     
    Last edited: Jun 13, 2018
  9. Gary Mattis

    Gary Mattis Active Member

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    I reconnected the NFB to both channels and added a 10 ohm resistor on the 4 ohm side to equal out the ohms on both channels. The Fisher OT secondary was not exactly 8 ohms it was less around 6.7 ohms . It Cleared up the too much treble problem and light schrill with the high notes.
     
    Last edited: Jun 13, 2018
  10. Gary Mattis

    Gary Mattis Active Member

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    A 6L6 Tube went bad after 2 days. Replaced it , seems ok now.
     
    Last edited: Jun 13, 2018
  11. Gary Mattis

    Gary Mattis Active Member

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    I am thinking Add More cathode resistor at the 1.5k bias pot resistor to ground ? Plus it has a 4 K resistor and 4 K on bias pot . Amp has 330 K on G1. 1 K on plates. I do not see any negative voltage on G1. It must be there somewhere or the tubes would all be dead by now at 450 volts !. From my past experience I decided to just add 10k to the bias pot resistor and to leave it at that.
     
    Last edited: Jun 13, 2018

     

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  12. KentTeffeteller

    KentTeffeteller Gimpus Stereophilus!

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    Simply put, get an amplifier which used 6L6 unless you know how to properly redesign this to use 6L6 and perform to Fisher standards.
     
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  13. tekuhn

    tekuhn Active Member

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    Gary, I would suggest you re-read post #3 from Dave Gillespie. I know you highly prize your 6L6's and I hate to see them failing, probably due to them being operated improperly. I personally do not posses the knowledge he mentions being needed to properly re-design this circuit to work optimally using 6L6's. If I did, I would attempt to help you. Unfortunately, I suspect you risk losing the support of those who are capable of helping you with your dismissal of their advice as if it was nonsense. This forum is filled with people who offer advice and knowledge freely to those who are willing to learn.

    To get an idea of what's involved in optimizing a circuit, I would suggest reading this thread in it's entirety.

    http://audiokarma.org/forums/index.php?threads/improving-the-fisher-400.511867/

    If you get nothing else out of it, you should understand the methodical approach Dave takes by setting a performance goal, designing the solution, and then proving the results through testing. His depth of knowledge is invaluable and his willingness to assist people is amazing. And there are many others as well.

    At a minimum to get your 6L6 tubes to live in that amp, do some research and look at schematics of proven 6L6 designs to find the operating voltages used - anode, screen and grid - and make changes to get them operating at those points.
     
    Last edited: Jun 13, 2018
  14. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Gary -- One point of explanation might help you with regards to the use of square waves for testing purposes, as opposed to the sine wave testing you're currently doing:

    1. Sine waves are very useful in determining things like maximum RMS power output delivered into a load, the amount of both THD and IM distortion present, tube element dissipation levels under dynamic conditions, etc., etc. The problem is, sine waves aren't a very good replica of what the intended use of the amplifier is for -- amplifying speech and music signals, and in our case, primarily music signals.

    2. Music is highly transient in nature -- not smooth, round, and slow like sine waves. It is fast, sharp, and dynamic. So knowing how an amplifier behaves under transient conditions is very important towards the end of accurately amplifying music signals. An amplifier can amplify a sine wave quite well, but still perform very poorly when passing a square wave. At low frequencies, an amplifier with poor transient stability can steal available power output, cause speaker cone "breathing", and in the worst cases, be quite destructive to the amplifier. At high frequencies, poor transient stability shows up as oscillations or "ringing" that distorts the whole of an overall transient signal by allowing the repercussions of one transient to interfere with the next transition, and so on. With music, this blurs detail information and impacts clarity of sound.

    3. Square waves have an extremely fast "rise time", which therefore allows them to become an excellent transient test signal to see how well the amplifier can deal with a transient, at that it settles down immediately after a transient is presented, so it doesn't interfere with the next, etc. Quite simply, a square wave is the best steadily repeating test signal that is most like a music waveform, and can yield a wealth of information about how capable the amplifier performs under dynamic conditions. As you continue on with your audio projects, I would encourage you to add a square wave generator to your arsenal of test equipment, and study how they can be used to really enhance the outcome of your projects.

    Dave
     
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  15. kward

    kward AK Subscriber Subscriber

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    I tried to find the 600A schematic (only had ~5 min this morning) and I was not able able to locate it. But if there are only two 12AX7 filaments in series connected to the cathodes of the four output tubes, that should generate about, what, 20V to 24V as cathode bias voltage, probably about perfect for 7591 outputs. Then if converting to 6L6 outputs, the output stage will need somewhat more cathode bias voltage than that, so it seems to me (without having seen the actual schematic) your 6L6s could be running quite hot and putting a lot of stress on them.

    You would need probably three 12AX7 filament strings connected in series to be in the ball park for cathode biasing 6L6s (this is what the Harmon Kardon A50K did which used 7355's which bias up similarly to 6L6s). So it looks like to me you are between a rock and hard place here. Either way to do this in a more or less the correct manner if keeping with 6L6 outputs, I think it would be wise to change the topology of the filament circuits/output stage cathode bias circuits. Somehow you gotta either a) figure out how to get three 12AX7 filaments in series to get you into the ball park of the cathode bias voltage you'd need for 6L6 outputs (I wouldn't recommend this), or b) switch over to resistor only cathode biasing or even fixed biasing and power the 12AX7 filaments with a separate quiet DC supply.

    An option you mentioned you wanted to explore, which I don't think will work, is to add extra series resistance into your cathode circuit. The problem here is that's the same circuit that powers the 12AX7 filaments, and the amount of resistance you'd need to add to get the biasing correct for 6L6s will be too constrictive to allow the proper amount of current to flow to power those 12AX7 filaments. Or in other words, you can't meet both of those requirements simultaneously with this approach.

    Of course, I don't think its too late to rewire the sockets back for 7591 operation..but I know you don't want to do that.
     
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  16. tekuhn

    tekuhn Active Member

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  17. tekuhn

    tekuhn Active Member

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    That biasing scheme is sure different to a novice like me. I see the grid is positive 33v, but since the cathode is at 44v, it's still 11v negative in relation. I also see how the cathodes are connected to ground through four 12ax7 (eight 6.3v heaters in series) heaters located remotely in the tuner. I did not realize that the cathode current would be steady enough to provide a constant source like that. Also, at 44 volts, that's only 5.5volts per 6.3v filament which seems too low? Not trying to hijack the discussion, just trying to understand how it works. :).
     
  18. kward

    kward AK Subscriber Subscriber

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    I found the "600" schematic which used 7189's and it used two 12AX7s in series as cathode bias for the output stage. Lacking the correct schematic I assumed this amp would be similar. It sounds like it kinda is similar in that it uses the filaments of some frontend tubes to bias the output stage, but it's apparently a different configuration of that general approach that is used in the 600.
     
  19. tekuhn

    tekuhn Active Member

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    And actually, the tuner filaments are also in parallel to ground with a 1.3K resistor located on the amp. Is it safe to say this is designed so that if you unplug the tuner from the amp, that the proper bias ratio will be maintained between cathode and grid and the amp will continue to operate safely?
     
    Last edited: Jun 13, 2018
  20. gadget73

    gadget73 junk junkie Subscriber

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    Changing the values of R104 and R125 will adjust the bias voltage. Effectively in stock form its cathode voltage - grid voltage, so 44 - 33v = 11v. Spitballing it I'd expect a 6L6 to want closer to 30v.

    Assuming the cathode voltage is about correct at the needed current level, try R125 as 12K and R103 as 3.3K to see if that gets you closer to right. If you don't happen to have them, simply swap the existing resistors and see what that does for it. Either way it ought to be a lot closer to the proper bias level than the stock config.

    Overall the bias scheme isn't that different from the TA-600, except that it has a variable pot in the mix where the TA-600 just has two resistors to form the voltage divider. Both feed some cathode voltage into the grid to make up for the overly large effective value of the cathode resistor.
     

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