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6550 power tubes in an Ampeg V-4B?

Discussion in 'Amps and Cabs [BG]' started by MotorTed, Oct 24, 2006.


  1. MotorTed

    MotorTed

    Apr 16, 2006
    CT, USA
    Has anyone here tried this? What was your outcome? I thought it might provide more clean volume out of this amp.
     
  2. I'm keen to know the out come of this. "King...." did putting in the different tubes have ANY adverse effects to the amp? Di U have to do any mods for the amp? Anything else U can 'enlighten' me with would be helpful.
    Rod.
     
  3. KingOfAmps

    KingOfAmps Inactive

    I'm blown out tonite but I'll chime in tomorrow.
     
  4. Cheers, Thanx.
    Rod.
     
  5. JGR

    JGR The "G" is for Gustav Commercial User

    Jun 29, 2006
    Maryland
    President, CEO, CFO, CIO, Chief Engineer, Technician, Janitor - Reiner Amplification
    el34's and 6550's 's are interchangeable (with a re-bias of course) as the pin-out (heater, plate, etc. ) is the same, but the pin layout for a 6L6 is NOT the same as the other two. if the amp comes stock with 6L6's, you will need to swap a couple of wires on each of the power tube sockets in addition to re-biasing the amp. there also may not be enough range with the current bias set up to accomodate 6550's with out changing a resistor in the bias circuit as well - not hard to due but should be left to a qualified technician.

    all that said, i think it would sound great. not a fan of 6L6's at all, for guitar or bass...

    JR
     
  6. KingOfAmps

    KingOfAmps Inactive


    When I got my V4B it was stock but two of the four power tubes (the 7027As) were physically broken in transit. They were the original Magnavoxes. My amp guy, who I take all my amps to when I first get them whether or not they have broken tubes, said that because only NOS 7027s were available (this was around three years ago before New Sensor or whoever re-issued the 7027) that I could use a 6L6 or mod it for 6550. Because I had previously been made aware of such a mod and was possibly interested I decided to go for it.
    The biggest part of the job was obtaining the “clearance” to accommodate the larger tube. This required changing the tube sockets from top mount to bottom mount (or vice-versa) on the chassis. That sounds kinda confusing because the chassis hangs upside down in the head box but keep in mind the tubes occupy the same “spot” on the chassis.
    Compared to unmodded V4Bs it sounds some different. Some bassier and more guttural but still can get very mid-rangey and trebly. I like them both yet knowing what I now know about the modded V4B tone I can’t say that I would have to get mine modded if it had 7027s in it. Changing gears slightly, a lot of folks disagree but when I discovered the V4B is not the little brother to the SVT (totally different tone IMO) I figured it probably would be better in the long run to have the V4B sound as different as possible from the SVT. So what I’m saying is that I may de-mod in the future.
    About the clean headroom. I can tell you that mine puts out 110 watts at 4 ohms and sounds very nice thru an 810. I feel like there’s more clean headroom versus the others (regular V4) I’ve played with but that’s not scientific or anything.
    More later about my tube snafu…
     
  7. MotorTed

    MotorTed

    Apr 16, 2006
    CT, USA
    Excellent info. I didn't take into consideration the physical dimensions!
     
  8. mthoople

    mthoople

    Nov 1, 2001
    Denver, CO
    6550's and 6L6's have the same pinout, and both are beam tetrodes. IME, their tone is similar, with fat low end and glassy highs. The 6L6 tends to be "looser" in the low end, and has more tendency to "fart" (subjective, of course). I like 6550s, and tend away from 6L6s.

    The EL34 is a power pentode. Its pinout is the same EXCEPT that the grid is wired to PIN 1. The grid of the 6550 and 6L6 is wired internally to PIN 8 (ground), so amps designed for these tubes usually have nothing wired to PIN 1, or (worst case) use PIN 1 as a tie point. An EL34 amp will usually have PIN 1 and PIN 8 tied together to ground.

    EL34 amps are more likely to be able to be converted to 6550s than 6L6 amps due to the differences in the current draw for the heaters. An EL34 draws 1.5A per tube, and a 6550 1.6A per tube, while a 6L6 only draws 0.9A per tube. Consequently, a power supply/transformer designed for 6L6s could be damaged by the current draw of EL34s or 6550s, while one designed for EL34s should have enough factor of safety for 6550s.

    I've done 7027 to 6550 conversions for multiple Ampegs (using the small base Sovtek 6550WD), and they usually only require a rebias. The change in power output is minimal. The primary reason for it was lack of availability of 7027s (at the time), and the change in tone (tighter). My personal preference is for the 6550s.
     

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