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6550 in EL34 amp

Discussion in 'Amps and Cabs [BG]' started by jackcregg, Mar 8, 2008.


  1. jackcregg

    jackcregg Guest

    Feb 25, 2007
    Hi

    I recently bought a knackered Sound City 120 amp.

    As some people may know this amp uses 6 EL34 valves to produce 120 watts. Reading about EL34s i found that it distorts very early. So I was wondering if I can use 6550 valves, because of the lower gain.

    Thanks

    PS How much will this affect the voltage?
     
  2. You could use the 6550 but you will need to modify the bias circuitry to suit them. Check the spacing between the output tubes to make sure they will actually fit. If they are too close you can get hot spots on the glass. I would add a small fan to cool them. You will not get more power out of the amplifier afterwards. Personally I would use the '34s as there is really nothing to gain from the mod.

    Paul
     
  3. JGR

    JGR The "G" is for Gustav Supporting Member Commercial User

    Jun 29, 2006
    Maryland
    President, CEO, CFO, CIO, Chief Engineer, Technician, Janitor - Reiner Amplification
    yup, its usually just one resistor you need to change in the bias supply. 6550's should give you a bigger, tighter bottom end and more headroom compared to the EL34's.

    JR
     
  4. Edword

    Edword Supporting Member

    Jun 23, 2005
    Detroitish
    My Traynor amp was designed to use EL34-type tubes, but is now running 6550s. It stays clean until I crank it really loud.
     
  5. jackcregg

    jackcregg Guest

    Feb 25, 2007
    Any clue as to which resistor?
     
  6. Hi.

    If You are positive about the mod, take it to a competent tech IMHO.

    It's not just replacing a resistor with another, there's slightly more involved if You want the result to be what You're hoping for.

    Regards
    Sam
     
  7. jrfrond

    jrfrond

    Jul 11, 2006
    NYC
    Tech Director, dBm Pro Audio Services, New York
    First of all, 6550's don't stay clean because they have "lower gain" as such, they just need more signal to drive them to the same rated output, and this is because of the input impedance. Also, they are a beam-tetrode, whereas the EL34 is a power pentode, which tends to compress more due to higher screen-grid leakage. The sound of these tubes are totally different, which is why many tube bass amps use them. Other beam-tetrodes are 6L6GC, 6CA7, KT66, KT88, 7027A, 6V6GT and 7591A. In fact, the 6CA7 will swap directly for you EL34's, without any bias mod, though the bias should be at least checked after swapping them. In the old days, 6CA7 were also labeled as EL34, because they were a direct swap, but 6CA7 is an American tube number.

    True EL34's bias in the mid -30V range, and 6550's around the mid -50V range. In the case of you Sound City 120, there is a single resistor that grounds the two bias pots whose value must be raised, but to what value would have to be determined through experimentation by an amp tech.

    I recommend the 6CA7 approach. The EH version works really well. You will also use less heater current, the OTHER problem of switching to 6550's.
     
  8. And if you did not change the bias voltage the 6550s will be running hot.

    Paul
     
  9. John, if I may say so, a very nice summation.

    Paul
     
  10. Edword

    Edword Supporting Member

    Jun 23, 2005
    Detroitish

    That was done before I bought it (from an amp tech). I've never heard the amp with the tubes it was originally designed to use.
     
  11. Good, too many people just stick a different tube into an amp and leave it at that!

    If you have never heard your amp with EL34s then you have no frame of reference as to when it starts to overdrive. I have a suspicion that you might not notice all that much difference.

    Paul
     
  12. Edword

    Edword Supporting Member

    Jun 23, 2005
    Detroitish

    I notice quite a lot of difference between the traynor and my Ampeg V-4, which starts to dirty up shortly after half way on the volume control.
     
  13. The actual physical position of a volume control has very little to do with how loud your amp plays. Comparing amp1 at 50% rotation to amp2 at the same position is like comparing apples and oranges. It all depends on the gain structure of the individual amplifier. Breakup also has to do with saturation in the OPT. I assume we are discussing an older Traynor. If so it uses Hammond iron and those things are built like tanks.

    Paul
     
  14. Edword

    Edword Supporting Member

    Jun 23, 2005
    Detroitish

    Yes, I'm aware of all that, as I've been playing through tube amps since 1976. I can also tell when an amp is audibly beginning to distort, as I've owned many of them.
     
  15. So short a time??? ;)

    Paul
     
  16. Edword

    Edword Supporting Member

    Jun 23, 2005
    Detroitish


    I'm not usually this pissy, it's just that I got an unsolicited letter from AARP today - with a membership card inside.
     
  17. jrfrond

    jrfrond

    Jul 11, 2006
    NYC
    Tech Director, dBm Pro Audio Services, New York
    HAAAA!!! Welcome to my world! :D

    BassmanPaul...also a good summation.
     
  18. Edward,

    As a "retired" person I understand. We in canada have a similar organization: Canadian Association of Retired Persons known as CARP. I always thought there was something fishy about them! They are fairly new so I don't think they are out of the water yet!

    Take comfort in the knowledge that your 'Honey Do" list is about to grow exponentially!!! Good luck to you sir.

    Thanks for the kind words John.

    Paul
     

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