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Can I use 20 watt tube amp as preamp?

Discussion in 'Amps and Cabs [BG]' started by mebrown20, Dec 31, 2012.

  1. mebrown20


    Jul 22, 2005
    I need some advice, and I know this is the place to find it. I recently traded some old equipment for a boutique, handmade 20 watt tube guitar amp. An idea struck me to plug my bass in, disconnect the internal speaker, and run a speaker out to my QSC power amp, and then the power amp as usual into my cab. Are there any risks in using the guitar amp like this, and if so, what are they?:confused:
  2. scotch

    scotch It's not rocket science! Supporting Member

    Nov 12, 2006
    Albany, NY USA
    Please see Profile for Endorsement disclosures
    If you disconnect the speaker from the tube guitar amp & run it without a load (either a speaker or appropriate rated soak), you will damage the output transformer.
  3. BassmanPaul

    BassmanPaul Gold Supporting Member

    Aug 25, 2007
    Toronto Ontario Canada
    the signal level of a 20W speaker out is way, way too much for the inputs of a power amp. You will have to provide a load for the tube amp. It can be done using a dummy load for the amp with a resistance divider across it that will reduce the signal strength by at least ten to twenty times.
  4. scotch

    scotch It's not rocket science! Supporting Member

    Nov 12, 2006
    Albany, NY USA
    Please see Profile for Endorsement disclosures
    Oh, I missed that! Yeah, using the 'speaker out' into a ppwer amp is a no-no! I read 'preamp out' in my mind. Whoops!
  5. Jeff Scott

    Jeff Scott Rickenbacker guru.......... Supporting Member

    Go for it, it is your gear, after all, not ours. :smug:

    I strongly suggest you try it out shortly before midnight, tonight, it will help add to the festive celebrations! :D
  6. Alex1984


    Jan 16, 2010
    Don't, just don't.
  7. Steve Dallman

    Steve Dallman Supporting Member

    Between the output tubes and the speaker is an output transformer. This transformer changes the higher impedance of the tube outputs to match the low impedance of the speaker. (It also provides the B+ voltage to the tubes, and mixes the "out-of-phase" signals from a push/pull amp back into phase.)

    A tube amp must operate into the impedance speaker load it is designed for. If you go into the input of a power amp, which is dozens, hundreds, or even higher times higher than a speaker, you will damage and/or destroy the amp. The amp needs to be terminated with the same impedance/resistance load of the intended speaker, and must be able to handle the power. A "load box" may be as simple as a higher wattage, 8 ohm resistor. Whatever resistor is used, it should be at least double the peak of the amp, and it WILL get hot in use.

    Once you terminate the amp properly into a dummy load, you can then pad it down (a couple resistors will do it) and run that into your power amp. Speaker to line out simple circuits are available all over.
  8. Primakurtz

    Primakurtz Registered Nihilist Supporting Member

    Nov 23, 2011
    Denver, Colorado
  9. Ukiah Bass

    Ukiah Bass Supporting Member

    May 10, 2006
    Not a problem. I'm doing exactly that with a 20 watt all tube Model One bass amp by Old School Amps. The Model One is a terrific preamp for recording, PA, or as a front end to your own more powerful amp. I'm pairing the ~16lb Model One with a ~2lb EA Micro. This 18-19 pound configuration boosts the all tube signal at 350 watts/8ohms or 550 watts/4ohms.

    How it's done: I feed the Model One's line out to the Micro's Effects Return jack. The Model One's line out is dimed; so is the Micro's effects loop volume. I control volume by setting the desired combination of Model One preamp / master volume (because the line out's signal is Post EQ), and the Micro's master volume. The cab is connected to Micro; the Model One's cab jack is connected to a Dummy Load. Photos are below.
  10. Steve Dallman

    Steve Dallman Supporting Member

    I used a blackface Bassman as a preamp in the early 80's. I built a dummy load (which I also used on the bench) of available parts. I series/paralleled 8 ohm/20 watt resistors into a 160 watt load. The resistors were mounted in the air inside a cage made of expanded metal. I wired a line out onto the box. I still use that supply on the bench. Occasionally I do use a small fan on it, but it was fine without a fan with the 50 watt Bassman. I didn't drive the Bassman to distortion.
  11. mebrown20


    Jul 22, 2005
    Wow, thank you all for the quick responses! You all have given me such helpful information on what I need to do if I want to try this arrangement! Best of all, everyone has prevented me from a catastrophic amp failure!! So thanks again everyone...:hyper: