KT88 v 6550?

Discussion in 'Audio Hardware' started by ogdens_sliced, Oct 17, 2020 at 2:26 AM.

  1. ogdens_sliced

    ogdens_sliced Walnut Plug Thread Starter

    Location:
    Albion
    Sorry if this is explained somewhere but I searched and could not find. Most of what I've read elsewhere is too detailed for my simple mind.

    Can one of the valve people please tell me in simple terms what the fundamental difference between a KT88 and a 6550 is, specifically:

    1) KT88 beam tetrode v 6550 beam output pentode?
    2) can both of these be run in triode / ultralinear?
    3) typical differences in sound signature / power output?

    4) bonus question - does anybody know who makes the current production Brimar labelled valves?

    Feel free to treat me like I'm stupid :edthumbs:

    Cheers.
     
  2. TarnishedEars

    TarnishedEars Forum Resident

    Location:
    Seattle, WA
    1) Both tubes are technically beam power Tetrodes.
    2) Yes, and both may be run in so-called "Pentode mode" as well.
    3a) Power output is about the same as these were designed to be essentially electrically equivalent.
    3b) This is much harder to answer because different makes of 6550 don't sound alike, nor do different makes of KT88s. Plus the bet sounding tube in one amp isn't always the best sounding tube in another amp.

    4) I didn't realize that somebody had now purchased the Brimar brand name as well. Is nothing sacred anymore? But it should be possible to determine if it is Russian, Slovakian, or Chinese by comparing it against other current production tubes.
     
  3. ogdens_sliced

    ogdens_sliced Walnut Plug Thread Starter

    Location:
    Albion
    Thanks for prompt and understandable reply.

    Brimar: Brimar Tube Range - Brimar Thermionic Products - Official Site
     
  4. KT88

    KT88 Forum Resident

    They are very similar, the KT88 being of UK origin, and the 6550 of US origin. They can be used as drop-in replacements for each other without mods. Both can be run in either triode or pentode wired circuits as well as UL. The best choice these days is the KT120, which is the most robust current version and increases the output and thus headroom / bandwidth within distortion spec, and it also drops right in. It draws slightly more current so that might be an issue with unmodified vintage amps which were designed to run the transformers on the bleeding edge of their capabilities. Most amps designs have more margin for error built-in though, so it's not usually a problem. One thing that distinguishes these tubes is physical size, so if an amp was designed to be as compact as possible, substitutions could be more problematic, The KT88 is a bit wider than the 6550, so it's possible that a few amps designed for the 6550 might make the spacing of the KT88 replacements a bit tight so that they don't cool as effectively. The KT120 is about the same diameter as the KT88 but is taller than either, so amps with covers may need to be modified or left uncovered to accommodate them. Lastly, the bases of the KT88 and KT120 are just a bit wider than those of 6550 tubes, so amps with cut outs in their chassis when the tube base must pass through to reach the tube sockets (usually on PCB designs), the cut outs need to be larger to allow the larger diameter tube bases to pass through.
    -Bill
     
  5. The new production Brimars are made in Slovakia by JJ.
     
    Rolltide and ogdens_sliced like this.
  6. ogdens_sliced

    ogdens_sliced Walnut Plug Thread Starter

    Location:
    Albion
    Thanks, I'd like to roll the power tubes in a Primaluna evo 400 integrated.
    The KT88, 6550 and KT120 can all be used.
    Currently have the stock EL34, I enjoy them but looking for a bit more bass extension.
    Which tube type would dig a bit more bass out without sacrificing midrange - any advice?
     
  7. ogdens_sliced

    ogdens_sliced Walnut Plug Thread Starter

    Location:
    Albion
    OK, what's the verdict on JJ for Hifi use - reliable?
    Recall reading a few posts in here saying that they can fail early.
     
  8. There's no way, I have a PL Dialogue Integrated myself and I've rolled KT88, that I like, and KT120 that I don't, but in both cases, midrange is lost in favor of the bass extension and highs.

    If you're just interested in bass extension and you're not interested in boosting the top end, you can try EH6CA7, but I still think a nice set of EL34 is the best for PL amplifiers.

    Well, they don't have good reputation in the reliability department and some people consider them dangerous, but that applies to power tubes. I haven't tried the 12AU7 versions they make, because I've tried a few new production tubes and none of them sounded better than old production tubes, so, for the small signal tubes I stick to those, cheaper, and they are supposed to last longer.
     
    ogdens_sliced likes this.
  9. ogdens_sliced

    ogdens_sliced Walnut Plug Thread Starter

    Location:
    Albion
    OK, that's very interesting to hear from a PL owner. And that's why I asked in here :righton:

    What would you say a nice set of EL34 is for the PL? For listening to multiple genres.

    Only tubes I've rolled in my amp is the 2 gain tubes with a couple of NOS Brimar. Added a touch of cream to the sounds.

    Tried current production gold lion before that but they sounded a bit brittle to be honest.
     
  10. About EL34, I'm currently running a set of Winged C´s, but since they are not made anymore, they are considered Old Stock, so, if you want to take this out of the equation even if they are not that expensive yet, other two that I liked are Mullard and JJ´s, but like I said, JJ´s are considered dangerous (PrimaLuna amps are very safe anyway with the bad tube indicator). Still think their plain EL34 (because JJ make a few EL34 models, like EL34II, EL34L) is a very good sounding tube as well.
    In any case I think any EL34 made in russia that doesn't add any letter to the model is fine and will retain the mids, and improve the sound compared to the stock tubes. Some people tend to like Tung Sol EL34b, but I think they loose mids, even if they sound nice.

    I agree with Gold Lions, and those were the best new production I tried, all the other were even worse.

    What speakers are you running with your PL? The reason I ask is because I used to run mine with a pair of Monitor Audio RX6, and I also missed some bass. That's when I tried bigger tubes and realized I was losing mids, so I replaced my speakers with a pair of Focal Aria 948, and now everything is in it's place, and with EL34´s.
    I could have kept the RX6 by adding a subwoofer, but I saw no point in adding a SS subwoofer, as I prefer full tube sound.
     
    ogdens_sliced likes this.
  11. ogdens_sliced

    ogdens_sliced Walnut Plug Thread Starter

    Location:
    Albion
    OK, have seen them for sale in UK at a reputable tube shop.

    Re speakers - I'm currently driving a pair of Harbeth Compact 7. So they aren't exactly a "bass monster" speaker. I do get nice bass in my room, can follow bass lines nicely. Occasionally though it can get a little muddy / sofen up when there's too much else going on. Wondered if a tube roll would assist with some headroom / control as adding add a sub doesn't really appeal to me.

    Prior to the Harbeth I had a pair of PMC twenty5.23: the bass out of the transmission lines on those was too much for my liking as it was too front and centre all the time.

    Ive driven my Harbeth with a 100 wpc Mcintosh 5200, but that just makes the Harbeth sound lame compared to the Evo 400, even when it's triode at 38 wpc.

    The Compact 7 are 6 ohm and I have them on the 4 ohm taps on the primaluna. Has a bit more punch / definition than the 8 ohm tap.
     
  12. I think replacing EL34 with Winged Cs will help, also it will help if you replace the other four driver tubes.
    Then, your speakers are 6 ohms 86db, not sure if they are tube friendly, even for an EVO 400, but I really don't know, I never tried a pair of those...
     
    ogdens_sliced likes this.
  13. ogdens_sliced

    ogdens_sliced Walnut Plug Thread Starter

    Location:
    Albion
    Quite the debate over on HUG ref Harbeth with tubes.
    I like it, 98% happy - just the slight bass niggle now and then.
    Thanks for your input today, much appreciated, have a great weekend :righton:
     
    Stonegarden71 likes this.
  14. KT88

    KT88 Forum Resident

    So adding a sub is really the way to go to get proper bass. You are essentially biamping that way, using the subs amp for the lowest frequencies. You can run your main speakers full range so that they are all tube driven, and then introduce the sub where they roll off. That gets you extended bass response, and it's solid state driven so it'll be tight. Then you have the advantage of tuning just the bass to the room via the subs level control and phase control. That's a big advantage in itself and it won't get overblown if you set its output carefully.

    As far as tube swaps, the EL34 is really a midrange strong tube. It doesn't have the extended highs and lows that a KT88 or KT120 have. So replacing with an EL34 with a KT120 will allow a more clear picture of the recording to comethrough; lower distortion, greater detail. You'll fix the bass bloat, muddy issue, but you'll be adding highs as well. Again, a sub will really help to balance that out. The JJ 12AX7 and 12AU7, etc are pretty darn warm sounding, so they can help to round things out, and they are cheap. The real issue with EL34 and many amps that use them is bass control. Things get muddy down there with many speakers when you are limited to 30wpc or so.
    -Bill
     
    trd, SandAndGlass and ogdens_sliced like this.
  15. ogdens_sliced

    ogdens_sliced Walnut Plug Thread Starter

    Location:
    Albion
    Thats what it's about when all said and done.

    Thanks for the input and advice Bill.
    You've given me a few things to chew on along with other replies to the thread.
    Appreciated :righton:
     
  16. AP1

    AP1 Well-Known Member

    Location:
    TX
    In all amplifiers with 6550 tubes I saw, these tubes ran with higher voltages than any modern KT88 can safely tolerate. That limits options for tube replacement. Some NOS 6550 may not keep vacuum well enough to survive high voltages either.
     
    McLover likes this.
  17. PrimaLuna amplifiers are very easy on tubes anyway, I don't think this will be a problem.
     
    nelamvr6 and ogdens_sliced like this.
  18. KT88

    KT88 Forum Resident

    The KT88 spec is 800v / 600v maximum, whereas the 6550 spec is 600v / 450v maximum. It might be true that most new KT88 designs are really more similar to 6550. But you can now buy the KT120, which has specs of 850v / 600v. You can theoretically even run these triode strapped at 600v. Most amps don't have more than 500v on the plates anyway. It's the grid spec that you have to watch more carefully on the 6550 types.
    -Bill
     
    ogdens_sliced likes this.
  19. AP1

    AP1 Well-Known Member

    Location:
    TX
    800V max means no more than 450V at idle it push-pull and indeed most amplifiers run at 400V.
    6550 based designs I worked on had 550-600V at idle, which means they experience over 1000V peak when pushed close to clipping. KT88 won't work reliably at that level and have high probability to arc inside.
     
  20. unclefred

    unclefred Coastie with the Moastie

    Location:
    Oregon Coast
    in my SE pentode, the KT77 was the best tube for getting better bass response and top end while retained something close to the EL34 mids than the KT88, 6L6, 7081a (I like this tube also though).
     
    SandAndGlass and ogdens_sliced like this.
  21. KT88

    KT88 Forum Resident

    No, 800v max means plate voltage. It's the same spec I listed for all of tubes mentioned. Typical operating voltage (design plate voltage) is another spec which I did not mention as it wasn't as relevant to your post. You were talking about point of failure.
    -Bill
     
  22. McLover

    McLover Forum Resident

    Location:
    East TN
    I have a major distrust on JJ for power tubes. Their small signal tubes are OK, for modern tubes.
     
  23. Rolltide

    Rolltide Forum Resident

    Location:
    Vallejo, CA
    JJ power tubes are so scary and dangerous that Luxman uses them as OEM tubes. I guess they must be swamped in warranty claims, huh?
     
  24. indy mike

    indy mike Forum Pest

    Check out these articles about 6550s and KT88s:

    https://worldradiohistory.com/Archi...-Vacuum-Tube-Valley/Vacuum-Tube-Valley-06.pdf

    https://worldradiohistory.com/Archi...-Vacuum-Tube-Valley/Vacuum-Tube-Valley-19.pdf

    The KT88 / KT66 /6550 Directory

    Keep in mind the Vacuum Tube Valley articles are from the late 90s and and early 2000s - I'm sure that since that time there's been developments in 6550 and KT88 production - the info about tubes produced from the 50s up until the early 90s when GE/MPD closed up shop in Owensboro, Kentucky are still relevant...
     
    ogdens_sliced and McLover like this.
  25. HiFi Guy

    HiFi Guy Forum Resident

    Location:
    Orlando, FL
    I’m running my PL with Gold Lion KT88s and have no desire to try anything else.

    Kevin Deal openly states that they run the tubes at 416 volts at the plates- vey easy as @Stonegarden71 stated.
     

Share This Page