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Power tube output questions

Discussion in 'Amps and Cabs [BG]' started by David A. Davis, Mar 7, 2013.


  1. David A. Davis

    David A. Davis Supporting Member

    Apr 5, 2008
    Summerville, SC
    I'm going to base these questions on the output potential of KT88's.
    I'm comparing the Reeves Custom 400 to the new Mesa Strategy (300W). Both amps run KT88's, the Reeve's has 6 and the Mesa has 8. But the Reeves output is 100W higher. I am assuming this is where the output transformer makes the difference. What are the advantages/disadvantages of the different confiigurations? It is a technical preference or does it change the tonal characteristics?
     
  2. David A. Davis

    David A. Davis Supporting Member

    Apr 5, 2008
    Summerville, SC
    Comparing the Reeves Custom 400 (400W) to the new Mesa Strategy (300W). Both amps use KT88's, Reeves (6) Mesa (8). How does the number of output tubes effect the output. How does the output transformer play into this?
     
  3. I think the Strategy runs 12 6L6s not KT88s. In a general sense you can get about 25W per 6L6 and about 50 per 6550/KT88.

    The OT doesn't really come into it as long as it's rated for the power the output section can provide. It only translates the amp's power from the high voltage/low current signal of the tubes to a low voltage/high current signal suitable for speakers. Save for some slight potential loss in the transformer's windings power in = power out. Hope this helps. :bassist:
     
  4. The number of tubes and type in a general output range is a design choice and will vary as to how hard or beyond design values the engineer pushes the valves. Output transformers match the load (speaker's expected AC impedance) to the output tubes. OT's can limit output and/or frequency response. The more output tubes of a given type the lower the impedance can be to match with the load (less windings needed on the OT primary).
     
  5. bb5000

    bb5000

    May 30, 2006
    Sundsvall, Sweden
    In amps with the same output tube type, differences in plate & screen voltages have effect on the output power.
    For example, the Hiwatt DR201 with 6xEL34 produces ~190 W with 700Vp and 400 Vg2 (plate and screen voltage).
    The Sound City 120 with 6xEL34 produces 120W with 375 Vp and 360 Vg2.
    A pair of EL34's produces 50W in most amps, but can theoretically produce 90-100W at Vp=800V , Vg2=400V (in class AB1)
     
  6. David A. Davis

    David A. Davis Supporting Member

    Apr 5, 2008
    Summerville, SC
    Sorry, I double posted. I didn't think my other post went through. Here's the original.

    I'm going to base these questions on the output potential of KT88's.
    I'm comparing the Reeves Custom 400 to the new Mesa Strategy (300W). Both amps run KT88's, the Reeve's has 6 and the Mesa has 8. But the Reeves output is 100W higher. I am assuming this is where the output transformer makes the difference. What are the advantages/disadvantages of the different confiigurations? It is a technical preference or does it change the tonal characteristics?
     
  7. This will also depend on the operating class and bias design of the amplifier. The vast majority of tube bass amps are fixed bias Class AB1 which provides the wattage output by tube type as listed above for a B+ voltage within the design specs of the tube type. Cathode bias and Class A design reduce the power output significantly, while designs like the Fender 400PS use Class AB2 and can, with the correct tubes, produce higher output.

    Output transformers have more of an effect on sound quality as opposed to sound output, i.e. wider bandwidth, and lower distortion. Good designs use transformers that are over-specified as well so that common user errors like impedance mismatch don't immediately cause a failure.
     
  8. David A. Davis

    David A. Davis Supporting Member

    Apr 5, 2008
    Summerville, SC
    Disregard this post, see newer thread.
     
  9. The Strategy specs call for 12 6L6's just like the older Strategy and 400+.
     
  10. David A. Davis

    David A. Davis Supporting Member

    Apr 5, 2008
    Summerville, SC
    From what I've read in that thread, there was a design change to KT88's.
     
  11. Yeah, in generalizing those numbers I probably should have specified typical bass amp applications (i.e. fixed bias AB1). :)

    I think 400W clean watts from 6 KT88s might be a bit of a stretch for the Reeves.
     
  12. The amount of negative feedback also affects how much "clean" power an output section can produce. Excessive negative feedback has many negative effects, care must be used in the design.

    Mesa like to provide a choice of output tubes in many designs EL34-KT77/6L6-KT66/6550-KT88. The 6L6-KT66 being the potential weakest of the three but a preferred sound for some. Some like the other two choices for different reasons.
     
  13. Really? 8 KT88s should be easily able to do 400W in that kind of amp unless they're running them well below spec. Hard to know exactly what's going on without looking under the hood.
     
  14. Hearsay from the Orange forum is that Mesa likes to bias their outputs pretty cold and run lower B+ so that they can maximize the amount of tubes that they can rebrand. They setup their outputs with non-adjustable fixed bias so that customers can order Mesa tubes with the same bias number and never have to worry about adjusting bias for each set.
     
  15. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    As has been mentioned, it all comes down to the design of the amp. There are a lot of factors that complicate things.

    The tube data sheet specifies, for example, how much power a pair of tubes can put out, operating in class AB1, over what frequency range, and with what level of distortion. The designer has to pick a linear operating range and keep the tubes with the specifications for energy dissipation and distortion. It's a balancing act. Sometimes those specs are pushed to get the most power out of the tubes, sometimes the design affords a more conservative operation.

    In terms of sound, more tubes might allow the amp to run in a more conservative way, cleaner, with lower total harmonic distortion. It isn't always about maximal power output. So tube life can come into play which is a big deal when you have to pay for an octet or sextet of matched tubes. Safety can also come into it. If a power tube blows, how much stress does it put on the other tubes. The design can ensure that the other tubes aren't going to be stressed if one goes.
     

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