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Running power amp with tube head.

Discussion in 'Amps and Cabs [BG]' started by creis2, Mar 8, 2013.


  1. creis2

    creis2

    Nov 11, 2011
    Cambridge, MA
    I tried to do some research, but I haven't found anything that helped.

    I'm looking at a 100W tube head. It should cover 99% of gigs, but the idea of possibly running the preamp into a high power power amp for trying different setups interests me.

    I know a tube amp has to be plugged into a load at all times.

    Is there a simple dummy load solution, then use the preamp out into a poweramp?

    Or is this just a bad idea all around?

    Layman's terms please :D.
     
  2. Primakurtz

    Primakurtz Registered Nihilist Supporting Member

    Nov 23, 2011
    Denver, Colorado
  3. creis2

    creis2

    Nov 11, 2011
    Cambridge, MA
    Ouch, kind of expensive.

    I'm better off getting the VT Bass DI when it comes out.
     
  4. mystic38

    mystic38

    Dec 4, 2012
    Mystic CT
    well... not to divert you from a full tube amp.. but the GB Streamliner has a 6 stage, three tube preamp with a phenomenal quality DI ..and a free 600 or 900W amp :)
     
  5. creis2

    creis2

    Nov 11, 2011
    Cambridge, MA
    I have the 900, love it.


    Not really necessary for me to do this with the 100w tube head, I didn't even buy it yet. I just threw out a hypothetical, just to understand how tube amps work and what their limits are.
     
  6. And the G-K Fusion 550 or MB Fusion have the same with a 500 watt amp and over 800 watt burst power (550 tested at 836). ;)
     
  7. mystic38

    mystic38

    Dec 4, 2012
    Mystic CT
    agreed..hadnt realised the Fusion 550 was an all tube pre..looks nice so yup call that an option too!.. how is the DI on those things?.. so far the STL has the best DI i have ever (not) heard!

     
  8. If the tube amp has a master volume, you can just turn that all the way down and run the preamp out to an external power amp if you're careful to make sure it doesn't get turned up inadvertently.

    If the amp has a post-EQ effects send, you can use that to drive the power amp as plugging into it automatically breaks the connection between the preamp and power amp. Once again, you'd have to be careful that it doesn't get unplugged while playing.
     
  9. Passinwind

    Passinwind I am Passinwind and some of you are not. Supporting Member Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics
    If (and only if) your amp has a power amp in jack that cuts preamp signal to the power section:

    • Prepare and insert a shorting plug into the power amp input jack.
    • Insert another shorting plug into the power amp output, or better yet use a dummy load of any wattage you have handy. A 25 watt resistor will only run you a couple of bucks. I have actually just relied on the shorting jack at the output of my power amp section many times, but not all amps have one, and they tend to get dirty and intermittent over the years. You could also just leave a speaker plugged in.
    • Could something still go wrong? Yes, but I have used this method successfully hundreds of times and so have a few of my former customers. As Mark says, always verify that your connections are good.
     
  10. capcom

    capcom

    Mar 23, 2005
    You can also remove all power tubes if that is a technical possibility for that kind of amp. If it is possible, then you won't damage the amp while an actual speaker load is not connected.

    I have 1967 made all tube 50 Watt Fender Bassman head that it also have a line out modification made 15 years ago in order to get the pure preamp sound for either recording or for amplifying with another amp. In this mode I used my amp this way several times without connecting it to a speaker and never had a problem.

    50 watt classic Bassman Heads has 2 power tubes and you can also use it with only 1 power tube while connected to a speaker load.
    100 Watt Bassman Heads have 4 power tubes and I believe all can be taken out if only its preamp will be used.

    However some tube amps may not permit removing all or some of power tubes like Bassman. I guess it was mentioned again before in this forum that For example 300 Watt Ampeg SVT all-tube heads shouldn't be used by removing their power tubes. Need more details about this subject from more knowledgable people.
     
  11. The DI is excellent, pre or post your choice, adjustable level with ground lift.
     
  12. Mr. Foxen

    Mr. Foxen Commercial User

    Jul 24, 2009
    Bristol, UK
    Amp tinkerer at Ampstack
    Cab loud enough to work with 100w is safer option. Or get a valve preamp and a selection of power amps, I used to do that, until I got the loud cab, various valve slave amps to go with a V-type pre.
     
  13. I wondered if this would show up. When you pull the output tubes there is a real possibility of the high voltage going way out of spec to the preamp tubes. Don't do this without an experienced tech checking the B+ to the preamp valves.
     
  14. Passinwind

    Passinwind I am Passinwind and some of you are not. Supporting Member Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics
    +1

    If you're only going to use the preamp section of an all-tube amp, just cut to the chase...;) The OP did say he's just looking for a 1% of the time solution though.
     
  15. taurus1

    taurus1

    Sep 13, 2006
    Vancouver B.C.
    I have a Ho's Electronics Attenuator, as well as a Ho's Electronics dummy load.
    the dummy load has a built in variac as well as a variable line out.
    I use a 1970 Bassman 50 into the dummy load into an eq and then to the power amp. works really well.
    Ho built if for me for $125.00, this was years ago but they probably wouldn't cost a lot more now.
    the attenuator can work as a dummy load as well but costs more.
     
  16. Tuned

    Tuned

    Dec 6, 2007
    Even at idle a tube power amp needs a load or it's bad for the tubes. The simple solution is to pull the power tubes and use the preamp out. Perfectly safe.
     
  17. ThisBass

    ThisBass

    Aug 29, 2012
    Germany
    I don't know your amp, but most of times it is possible to fix such problems with a shortened plug into power amp in (or plug with same treatment).
     
  18. ThisBass

    ThisBass

    Aug 29, 2012
    Germany
    In general the OT (the output plug) is shortened if nothing is plugged into the output plug.
     
  19. Nah. No signal, no worries. The failure mode of running no load on a transformer coupled amp is related to inductive flyback. No signal no flyback. :)
     

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