6550 / KT88 with lowest heater draw?

Discussion in 'Amps [BG]' started by christw, Jan 22, 2013.


  1. christw

    christw Get low!

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    As some of you know, I like to mod my amps almost incessantly. Usually it's for well thought out reasons like "MOAR POWER" or making it go to a figurative eleven. Occasionally, I try to take a more tactful route than sticking 6550's in everything I see. :smug:

    My question is in regard to minimizing heater draw off of a pair or quad of 6550 or KT88. Are there any brands with lower draw? How about new production versus old stock? Does it matter at all?

    I know of guys who run 6550 in V4's without issue but I've also read that they're running near the limits of the PT's capabilities. I'll be picking up a VT-22 later today and like most things, I want to put 6550's in it down the road. I just want to know if I can minimize extra wear through my choice of tubes or if they're all close enough it won't matter.

    Thanks guys!
  2. BassmanPaul

    BassmanPaul Gold Supporting Member

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    The best way is to add a supplementary heater transformer. Changing from one tube type to another will not garner any significant increase in power.
  3. dunwichamps

    dunwichamps

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    In order to do this you would need to get large batches of tubes then find ones with the minimum draw through a process of testing. 88s and 6550s all vary in their heater draw but a certain amount and only though this kind of weeding of large batches could you find one with the smallest draw.

    The Ampeg PTs are known for being quite robust and overspeccing the heater lines. the 7027s only draw on average 0.9A of heater current so your asking 0.7 or so more per power power tube in there. I would be more worried about having such close spacing between tubes. Its already below spec for even 7027s and adding KT88s needs even more space to avoid mutual heating between the tubes. Couple that with the fact that all those tubes are upside is kind of a ****** proposition IMO

    finally you most likely need to adjust the bias circuit as 6550s and 88s need a large negative grid bias to properly be biased.
  4. christw

    christw Get low!

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    The amp will get a precautionary tech visit soon. The bias will no doubt be checked and/or set while it's out (esp. if I want to put 6550 in). My concern lies with the fact that I'm looking at this amp to be one of my main workhorses. I don't want to stress the PT in the long term just to get a little extra fatness out of it.

    If I were to look at a separate heater supply, could someone recommend me an appropriate transformer? (Perhaps I should just ask one of my techs...) If the power tube proximity becomes and issue I could always just add a fan, no?
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  6. dunwichamps

    dunwichamps

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    Yes but then ur hacking the amp up. Not usually my preferred approach. Its not the worst, majority of commercial amps are already underspec so its nothing new.

    How many tubes are you changing 2 or 4?
  7. B-string

    B-string Supporting Member

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  8. christw

    christw Get low!

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    I was looking at running a quad in there. I only mentioned the use of two because of a separate discussion in the Portaflex threads about trying 6550 in a B-15 with a Weber copper can rectifier. That's another discussion I'm not worried about now...

    I don't mind deviating from stock if it helps an amplifier serve my purposes. In this case I'm buying it at a good price to use as a workhorse, not a collectible. I'm going to leave it on for five and a half hours as the backline to a jam and still cart it around to practices and shows I want it to be as robust as possible. That said, I don't plan to do much of anything that a reputable tech couldn't undo later on. I really like the voicing of the older Ampegs, especially when loaded up with 6550. :D

    Also, talked to whathisname at Chrome Dome and he gave me a ballpark price that I consider to be really reasonable.
  9. dunwichamps

    dunwichamps

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    that one is a bit over sized IMO. If your just going to do the power tubes then all it needs is 7A if you need to power 4 88s or 4A if you need to power 2 88s.
  10. dunwichamps

    dunwichamps

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    For 4 88s something like 8A is kool. Technically 7A will get you through fine 2. So long as your only going to use it for the power tubes.
  11. BassmanPaul

    BassmanPaul Gold Supporting Member

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    Forgive me Chris, but I still don't see what you are hoping to gain from all this. If you intend to leave the amp on for long periods of time you should not be over stressing the PT in any way.
  12. beans-on-toast

    beans-on-toast Supporting Member

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    The heater current specs of the 6550A is 6.3V +/- 10%. The nominal current draw is 1.6A. If the wall voltage is above 120, you heater voltage will be a little higher and the current draw a little lower, and vice versa.

    The heater specs for NOS and new production 6550A's are the same. GE, Tung Sol, RCA, SED/winged C, JJ, Svetlana, Electro Harmonix all have the same spec.
  13. christw

    christw Get low!

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    It's probably all a product of me worrying too much about asking too much of the PT running 6550's for hours of hard use. I should probably try that first before I worry about modifying it though. :p
  14. BassmanPaul

    BassmanPaul Gold Supporting Member

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    Why not just run it as designed?? These amps got their reputation by being themselves. My curiosity is killing me!! :)
  15. Tuned

    Tuned

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    My friend that did this to his V4B had it done by a tech who wouldn't have added the extra filament power transformer if it weren't necessary. The PT and OT can handle the 6550's 30-40% extra power output, but the filaments more than double the power draw.

    You should also mod the input bias caps and coupling caps to V4B specs or the deep low end won't be there. If the caps need replacing you should kick them up a notch too.
  16. B-string

    B-string Supporting Member

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    Choices from AES were 6 amp and 10 amp.
  17. dunwichamps

    dunwichamps

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    yea, not a ton to choose from. I would probably contact Heyboer for a custom one.

    I dont think you will see much ouput power increase if you swap in 6550s. The plate voltage is not high enough, and the screens are a bit to close to the plates IMO.

    Those amps are damn loud as is. Of course might not be enough on bass if you gotta keep up with some 100W full stacks on either side of your. Unless you plan on playing bass like guitar
  18. B-string

    B-string Supporting Member

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    Screen resistors need to be changed for 6550's. I gained about 20 Watts (in a V4B). Seemed to increase clean headroom a tiny bit. Worth the effort, eh undecided. 7027A's were hard to find back then or high plate voltage 6L6 family. The Heater draw spec on Winged C's went up to 1.9 amps with the "C" rev.
  19. dunwichamps

    dunwichamps

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    You can get some 7027s from JJ nowadays but they look like 6l6s IMO and run overspec in the V4 family of amps. Esp on the screens.
  20. B-string

    B-string Supporting Member

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    7027A's had higher design voltage ratings but were/are essentially a 6L6GC with an external suppressor grid connection.
  21. christw

    christw Get low!

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    Well, I got the VT-22 home. I'll try it out at rehearsal tomorrow to get a feel for headroom. I'm going to eat some dinner before I start noodling around with it.

    I'm a habitual tinkerer. The main reason I was looking to put 6550 in the VT-22 was because with I've been very pleased with substituting 6550 for 7027A in the B-12XTC. It doesn't get significantly louder. However, it does have a fluffier/punchier low end while otherwise retaining the same volume levels and character. The biggest difference I find is a (perceived?) delay of the onset of distortion on the low E string, most notably finger style. That line is what separates gig rig from bedroom toy with me. For the jams, I'm sure this amp will be enough. I just don't know that it'll keep up with my rock band though.

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