6550 vs EL34 Feedback 8 ohm vs 4 ohm

Discussion in 'The Workbench' started by stickyfinger, May 17, 2019.

  1. stickyfinger

    stickyfinger Active Member

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    Ive had JMPs with 6550 converted to EL34 amps in the past but what was the reason Marshall with 6550 used the 8ohm tap 100k and the El34 used 4 ohm tap 100k? Was this thought to be equivalent for the specs of each tube?
     
  2. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    It was the other way round. EL34 used the 8 ohm tap and 6550 used the 4 ohm tap.
    6550 has harder vacuum and less crunch if you will. So it got less feedback by using the 4 ohm tap.
    That was their attempt to level the playing fields for both tube types.
     
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  3. GIBSON67

    GIBSON67 Well-Known Member

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    Thanks Mickey, I did not know that...I am going to have to look into my 1976 2203 with 6550's, now. I've been thinking about converting...
     
  4. stickyfinger

    stickyfinger Active Member

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    Hmm looking at references on amparchives this doesn't seem to be the case.
    6550 were consistently using the 8 ohm tap and 100k resistor starting in 73.
     
  5. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Those could have been changed or modified.
    I am looking at the schematics.

    Show the links and pictures. Most of the pictures on amparchive are too small to even see and most cannot be zoomed while seeing clearly.
     
  6. stickyfinger

    stickyfinger Active Member

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    You can spend 10 min sarching through the 77-81 era and clearly see the purple wire attached to the yellow 4 ohm tap with EL34. Maybe folks moved them around? But I don't see how the percentage of amps out there would have been moded this way. Most folks don't understand this stuff nor do they care.

    Also when looking at the schematics dont confuse the early non cascaded version (330k plate) with the cascaded master model. You also need to look at the load resistors if they are 220k then the amp was EL34 if 150k then 6550. Also checking to see if the 220k load resistors matches the 220k bias feed resistor in terms of color and shape to rule out customer EL34 amp conversion.

    Edit I'm looking at Unicord schematics right now. El34 = 4 ohm 6550 = 8 ohm.
     
    Last edited: May 17, 2019
  7. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    By 1977-1978 they were all using 4 ohm tap.

    You must have found some uncommon UNICORD schematics. The one 1976 2203 bastard had it pegged to the 8 ohm tap. The 2204 used the 4 ohm tap.

    The 1959 models have feedback setup the same way.

    [​IMG]
    [​IMG]
     
    Last edited: May 17, 2019
  8. stickyfinger

    stickyfinger Active Member

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    Edit-
    Sorry had to much to drink.

    I am talking about 50 waters and should have made that known in the first post. Will have to look in to the SL spes of the decade as I'm now curious
     
    Last edited: May 17, 2019
  9. MarshallDog

    MarshallDog Well-Known Member

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    So regardless of what tap is being used with each type of tube, what are possible tonal implications if a specific tube type is not using the correct ohm tap?? Please Share!
     
  10. stickyfinger

    stickyfinger Active Member

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    lower tap + a bigger resistor = more power amp grind.
    To confuse everyone a 200k resistor on the 16 ohm tap is the same as 100k on the 4 ohm tap or 141k on the 8ohm tap.

    Any ways the 10 JMP Marshall unicord schematics floating around are not good enough to use for historical reference. SO many small component values changes from year to year. You can use them as a guide but you need to use original examples for tech support.
     
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  11. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    There is no correct tap. It is whatever used and expectations per design and usage.

    As @stickyfinger was mentioning, here are some feedback/tap numbers for say a 50 watter:

    04 ohm | 14VAC | 100k | 4.7k | .628VAC to PI
    08 ohm | 20VAC | 150k | 4.7k | .607VAC to PI
    16 ohm | 28VAC | 200k | 4.7k | .642VAC to PI

    That is speaker/output impedance, AC voltage @ secondary/speaker, feedback resistor, PI tail grounding resistor and average feedback voltage to PI. I rounded numbers for calculations and kept things simple. The VAC voltages are based on basic power formulas. This represents average numbers and stock resistor values.
     
    Last edited: May 18, 2019
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  12. stickyfinger

    stickyfinger Active Member

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    I'm thinking the 8ohm tap,100k and 6550 was Marshalls idea to "fix" the EL34 problem with the imports.
    A new tube with more feedback should make less tube warranty issues with Unicord :thumb:.
     
  13. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Why would you think the 6550 needs more feedback? To make warranty better??

    They had issues shipping amplifiers with the EL34 of that time. They choose to use 6550 for the states. Why, I do not know. They could have just as easily used 6CA7 (the better option making more sense).

    I do not see how using the 8 ohm tap is going to make things better for number of warranty issues. If there are shipping problems the feedback choice is not going to help. If so then why are not all state side amplifier connected to the 16 ohm tap?

    I do not trust pictures any more than schematics and I especially do not trust wire colors. They could have used whatever wire was readily available.

    I also pulled up 4 1973 50 watt amplifiers from Amparchives and there were various connection locations for the purple wire from the feedback resistor.

    I do get what you are saying but honestly I do not trust anything.
     
  14. stickyfinger

    stickyfinger Active Member

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    I'm speculating Marshall who probably didn't know any better thought it be easier on the tubes.


    The orientation of the Schurter "oasis" or "window" selector connections did not change so the wire codes were the same.

    The Unicode schematics are some what universal or generic as seen with the 47k and (100k) feedback resistors. The 50 waters evolved with the feedback on the speaker jacks (making them variable) then to the 8 ohm 47k in 1970 and sometimes 100k/8ohm in 71. In 72 it went to 100k 4ohm tap. The 6550 started around this time with 100k 8 ohm and continued for the rest of the decade.

    There'ss only 2 unicord model 1987 schematics floating around I've seen.
    A 1970 and a 1970 converted to MK2 which is approx PCB era 3rd quarter 73.

    Also a 76 pre cascade 2204 and a 81 cascaded 2204 unicord schematics.

    Lets not emphasis on the schematics as they don't tell the whole picture.
    These schematics are not good enough for feedback references and may also be the reason we see so many big box 72+ 1987 models with added V2 .68uf. The 1987 model evolved very often.
     
  15. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    My main point is obviously not to trust any of it and figure out what is there or what the owner has in possession.

    I hope everyone read above, between and below the lines.
     
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  16. ampmadscientist

    ampmadscientist Well-Known Member

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    The feedback corrects the very bottom low frequency boomy-ness. You can disconnect the feedback but it will make the sub sonic lows a little over-done.
    If you want to adjust feedback for different sounding tubes, or you want the amp to sound tighter / more compressed, go ahead and use a different resistor.
    Personally, I don't really want any NFB because it down-grades the sustain of guitar. I am just personally removing all the diapers / take the leash off / let the amp sing type mentality.
    But on the other hand, NFB limits some noise, limits some low frequency, adds some compression, etc...

    "what was the reason Marshall with 6550 used the 8ohm tap 100k..."

    A little more NFB from the 8 ohm tap (more secondary windings at 8 ohms)
    A little less NFB at the 4 ohm tap (less windings)
    They were adjusting according to taste of output tube.
     
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