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Tube Experts I need Your Advice

Discussion in 'Amps and Cabs [BG]' started by bassboysam, Dec 29, 2011.


  1. bassboysam

    bassboysam Supporting Member

    Apr 26, 2001
    Ottawa, Ontario
    I am in the process of possibly having a custom amp designed loosely based on an Ampeg B25. The designer has suggested using 4 x 7591 tubes in the power section. The amp will have a half power switch to run at either 50 or 25 watts. I am not very familiar with this tube and there isn't a lot of information on it. What are its sonic characteristics? It seems to be mostly used in hi-fi amps. Are there any other tubes you would suggest?

    The amp will be tube rectified and have a tone stack compromised of the ampeg Baxendall circuit combined with a Fender style rotary graphic EQ to control the mids. Looking to use it mostly for home and studio.

    Thanks
     
  2. Is this to be a guitar, bass or general audio use amp?
     
  3. bassboysam

    bassboysam Supporting Member

    Apr 26, 2001
    Ottawa, Ontario
    Sorry, low powered bass amp.
     
  4. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    I don't know much about them, but I don't know that I'd want a tube that's fairly rare. JJ and EH make them, but there aren't a whole lot of them floating around the NOS world.
     
  5. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Don't bother, that's a totally unnecessary feature. It has some merit in a guitar amp, but only if you could choose between 5 and 50 watts.
     
  6. bassboysam

    bassboysam Supporting Member

    Apr 26, 2001
    Ottawa, Ontario
    So the amp won't distort significantly sooner at 25 watts compared to 50? I wonder if it would be possible to drop it to 5 watts?
     
  7. 7591 were/are a very nice sounding Hi-Fi tube, new production for these though??? Don't know if I ever heard these pushed into live bass guitar duty. KT66 or 6L6 family would be my choice. Tube rectifier for a bass amp when SS diodes perform so much better is not a good plan IMO (guitar amp yes). The 6SL7, 6SN7's should be used as well as 7199's in preamp and tone stack (depending on vintage).
     
  8. The difference between 25 and 50 watts would be noticeable but not greatly. 5 watts from a bass amp? Not worth the power to light the filaments. ;)
     
  9. bassboysam

    bassboysam Supporting Member

    Apr 26, 2001
    Ottawa, Ontario
    He is suggesting a 6u8a in place of the 7199 and a 5ar4 rectifier.

    Can you explain why a tube rec is not recommended?
     
  10. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    Because B-string 2 doesn't like tubes.

    :D

    A SS rec will make the amp a little tighter sounding and will eke out a little more clean power. The SVT has a SS rec and it still sounds great, but my B-15 has a tube rec, and I've flirted with trying a SS rec but I never do because the amp sounds great to me as is.
     
  11. bassboysam

    bassboysam Supporting Member

    Apr 26, 2001
    Ottawa, Ontario
    I'm looking for that sweaty overdriven sound so that's why I'm thinking tube rec. he said we can compensate a little to keep the low end under control with an over spec'd output transformer.
     
  12. bassboysam

    bassboysam Supporting Member

    Apr 26, 2001
    Ottawa, Ontario
    So i guess the question is do you lose any magic by going with 7027/6L6/KT66 tubes instead in the power section?

    Any other suggestions for power tubes?
     
  13. Tube rectifiers are inefficient from the start and suffer voltage sag under high demand. This voltage sag get worse as the tube ages. You can't build a lot of reserve with filter caps as rectifier tubes and electrolytic caps don't play well together. The voltage sag causes loss of low end definition and power (low end gets "flabby"). Loss of high voltage also increases distortion. If you are looking for weak, distorted, hollow bass then tube rectifier is you ticket. It was difficult to design power supplies using tube rectifiers that could hold up well enough for bass guitar use (usually required dual rectifier tubes).
     
  14. 1958Bassman

    1958Bassman

    Oct 20, 2007
    Think of power supply voltage as if it's a clothesline. The more demand, the more it sags. A tube will sag, two in parallel will sag a lot less and diodes won't sag much, if at all. You can also do a hybrid rectifier and if the tube fails, the diodes will still be there.

    If you use an Electro-Harmonics 5AR4, it doesn't drop the B+ the way the old rectifier tubes did, so you could count on that to keep the voltage more consistent unless you're digging in for a long time.

    If you want dirt, the 7591 may be a bit too clean. Look at the tubes used in the amps you really like and use those. A 50W Fender is a nice amp if you want that kind of sound and the 6L6 is very easy to find, old or new.
     
  15. LOL you a funny boy :p

    Careful if you try a SS rectifier in the B15 as the B+ can go up as much as 20%. Some plug and play (built into a tube base take that into consideration and drop through a resistor).
     
  16. ampegfuzz

    ampegfuzz Supporting Member

    Feb 13, 2007
    Davenport, IA
    Jimmy might know because I don't, but did the b25 have a SS rec?
     
  17. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    Right, that's why the B-15 sounds so crappy when you crank it.

    :D
     
  18. 1958Bassman

    1958Bassman

    Oct 20, 2007
    6L6 and KT66 are very different animals. Same for EL34/6CA7 vs 6L6.

    Maybe a master volume control would be something to consider.
     
  19. ampegfuzz

    ampegfuzz Supporting Member

    Feb 13, 2007
    Davenport, IA
    I still wish I got a later v4 with master instead of my v4b.
     
  20. bassboysam

    bassboysam Supporting Member

    Apr 26, 2001
    Ottawa, Ontario
    The amp would operate in 2 modes, master and non-master. Personally I like my pre to be relatively clean and get the dirt from the power section. So turning up the gain to get preamp tube distortion in totally not the point of this endeavor.
     

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