6550s for 7027As in a V-4?

Discussion in 'Amps and Cabs [BG]' started by Rockbobmel, Jan 1, 2005.

  1. Rockbobmel

    Rockbobmel Supporting Member

    I know this has been covered in the PAST but I wanted to hear any new news or experience with swapping out the 7027As in a V-4 for 6550s.... Thanks
  2. Rock, I got my '72 or '73 V4B when it still had the 7027 tubes. However, because it needed some work (and one of the output tubes was cracked), my techie (and PBG) told me about the 6550 mod. I was skeptical from the standpoint of I wanted the V4B to be as unique as possible from my SVT. Now I did have the option (tech's private stash) of NOS GE or RCA 7027s and was told about another substitute (6L6?). The NOS 7027 were pricey and scarce and I decided against them purely on that. As for the 6L6 option I decided that if I was gonna stray from the original complement (something I did not really wanna do) I might as well go 6550. So I gots me four Svet 6550s in thar now. There was a minor snag that I know of. He had to relocate the sockets to top-mount on the chassis (depending on your perspective could be bottom-mount but they're still in the same "spot") to increase the clearance. This was a matter of drilling out the old rivets and re-riveting, that's all. He also added spring tube retainers.

    Because I did not hear the 7027s I can't give you a comparo but the way it is now it sounds great and different still than my SVT.

    Lastly, without a fan, she gets scary hot!
  3. I thought long and hard about changing my '72 V4 over to 6550's. I Googled and Yahoo'd all day long, read Lord Valve's article "Help, Uncle Ned, my amp has 7027's", looked at every piece of tube data I could find, etc..

    There were two main concerns. First, the filament current draw. The 6550's draw a heckuva lot more current than the 7027's. Supposedly the power transformer can handle this, but it did seem like a gamble. Secondly, the issue of space. The 7027's are stuck in a little niche right next to the transformer, with very little clearance. A smaller 6550 would fit, but not a coke-bottle 6550 or a bigger KT88. Like KOA said, that seemed like an awful amount of heat to be sticking next to a power transformer that would now be getting even hotter!

    I considered moving the tubes and adding a separate filament transformer...then stopped and said "Why?? For an extra 30 watts? All that work for a measly 30 watts?"

    So I just bought a quartet of Sovtek 7027's from Parts Express and kept it "stock". But I did recap: every electrolytic plus the coupling caps (with metal films) and the negative feedback cap. Sounds great.

    Of course, some days I still consider the 6550's ....
  4. BassGreaser


    Aug 22, 2002
    Austin, TX
    King if I am correct your V4 is now a 200watt amp now? I have a Bassman 135 with four 6L6's that I might change out for 6550's for more head room, and wattage :p
  5. From the articles I've read, the change to 6550's only brought the V4 up to 130 watts.

    Two pair of 6550's can yield up to 200 watts in an optimized circuit, but the V4 voltages and output transformer do not take advantage of the 6550's power capabilities.
  6. from what i´m told it´s about 130-140w.
    i am wondering if anyone has changed their 6l6 for 6550 on the new V4?
    mine was delivered with 5881 and i have used it with 6l6gc and didn´t notice any difference until the gain and master was at 10(yes, i used to run my amp at max)!
    it did sound fatter then but otherwise no difference.
    like bassgreaser i am also looking for more headroom and am thinking about 6550´s.
    after what i´m told it could possibly be done but i have not seen anyone do it and i don´t want to waste any money on 6550´s just as experimenting
  7. No. Measured at 4 ohms it's putting out 110 watts before clipping. Bear in mind that more wattage was not a goal. I just wanted it to be up-to-snuff and fully functional.

    To achieve more wattage out of the 6550 I'd imagine it would need even more stuff like the bigger power supply that nashvillebill hints at.
  8. It was reassuring to learn that my tech was aware of all of the above potential issues which, when mentioned to me by him, I remembered reading about here in some PBG-related threads. Before giving the green light I conferred with PBG. He agreed with the game plan and so I had two independent thumbs up for the conversion.

    For the record, I have no regrets. It sounds great. It gets played daily thru an 8 ohm B40 ('70s 4x10 w/CTS 32 ohm square mags). I'm gonna incorporate it into an 810 (gotta luv it!) set-up as time goes on. The 8, 4, 2 ohm versatility of the V4 is cool.

    Heat/Reliabilty-wise the work was done in January of '04 so its got a year on it thus far and all OK.

    I'm gonna try to dig up some pics detailing the tube clearance and all.
  9. I just re-visited the work-order. He put in Svet 6550C (so far so good). Here are some notes from the job (Greek to me):

    Ep 518v 21v~ 4 ohms @ clip
    g2 511 =110 watts
    Ip .042A per 6550
    pD 22W per 6550
  10. You're not going to see any benefit from it in the new SLM V4BH and I seriously doubt that its power transformer can handle the extra current draw of 6550's.
  11. thanx, i kind of thought so.