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Tube amp quesions

Discussion in 'Amps and Cabs [BG]' started by tomtrb, Aug 10, 2001.


  1. tomtrb

    tomtrb

    Apr 17, 2001
    Ive heard that it is damaging to have an amp turned on with no load on it. Im pretty sure this is correct when it comes to amps with power tubes, but Im not sure if this applies to SS amps. My amp is a Trace Elliot 250 with SS power amp and two preamps - one SS and one tube(12AX7) which I can mix between. If I use my amp with no cab attached, only using the effects send, am I screwing up?

    Also, from what Ive read on TB, biasing is not neccessary on preamp tubes. Is this for real? What if I change brands - do I have to bias then? And BTW, what does it cost to have a single tube biased?

    One more thing. A few weeks ago I had an electrician friend of mine replace my fan and I just told him to put any old cheap fan in there and he did exactly what I said. Now, I regret it. I could just ask him to put a better fan in there, but I would like to do it myself (for educational purposes). Can I just buy a good cooling fan like for a computer? Also, how do I know if it is quiet enough? I am looking for the quietest fan I can get so I can use my head in the studio.

    Boy, that seems like a long post
    Thanks alot,
    Tomtrb
     
  2. Nightbass

    Nightbass

    May 1, 2001
    Seattle, WA
    That's right, it is bad practice to run a tube amp without a load, but it is very safe to run a solid-state amp without a load.

    Preamp tubes don't need to be rebiased, and changing brands wouldn't affect this, either.

    As for fans, you might try one of the quiet fans made for PCs. Another option is to install a better and quieter fan, and hook up a fan defeat switch like SWR puts on the Bass 350. Older SWR's meant for studio didn't even have fans. A third option is to put some resistance in series with the fan to lower the voltage and slow it down when you use it in the studio. Some of those PC fans even have high-low speeds.

    When I had the Bass 350, I left the fan off all the time, and the amp didn't get that hot. YMMV.

    Nightbass
     
  3. tomtrb

    tomtrb

    Apr 17, 2001
    Thanks for the reply Nightbass,

    Thats pretty much all good news. Ive started to just use my head at smaller gigs and use the effects send or DI to go to the board, just using monitors and no bass cab.
    The search is on now for a new 12AX7 - wish me luck in finding a high gain, low noise, clean, warm, indestructable tube.
    I think Ive found the fan I want. Operating noise level of 20dbA whatever that is, but its the lowest I could find. I think Ill just put an on/off switch in there for studio use.
    Thanks
    Tomtrb
     
  4. Phat Ham

    Phat Ham

    Feb 13, 2000
    DC
    If you are not using the power amp section of your head you probably won't need a fan at all, since that's the part that generates the most heat. I second the idea of using an on/off switch so you can just turn the fan off in the studio.
     
  5. engineer7

    engineer7

    May 11, 2001
    Tomtrb,

    Seems my other post didn't go through. Whether your amp is solid state or tube, you need to have aload connected to it. Whether it is your speaker or a "dummy" resistor. The things you mentioned are the preamp circuits which have nothing to do with output circuit. You can use a 4 or 8 ohm resistor to act as a load for your amp. See what your amp is rated for. So if you are using your amp with no cabinet, yes you are screwing up. Give a little time and the repair man will be happy to see you! :) (of course after a customer picks up an amp from my shop, I have to teach them how they broke it and how to avoid it)

    For biasing, you don't have to bias a preamp tube but it sure helps. If your not happy with what you have now and get a new one, you won't be getting its full potential (about 60%). The other 40% is where extra gain, headroom and sweetness come from. So, spend 30 minutes or so and bias that tube to perfection :)

    Fans, ah the most commonly asked question on amps. Heat is the enemy of electronic components. The cooler they operate, the longer the life. A Computer cooling fan, lets think about that.....computer..... low voltage lower heat, less demand. Not good for an amp. Why a computer fan for goodness sakes?!?!? Go and get an origianl equipment fan (SWR, GK and Trace operate best with original equip). Then get your electrician friend to help you put it in. Let him be your "co-pilot". Fans can be by high voltage capacitors which can knock you flat on your rear end!!

    Hope this helps, now I'm really late for the shop, hope this post goes through this time!

    Jason
     
  6. engineer7

    engineer7

    May 11, 2001
    forgot one more thing. The voltage for the computer fan is not the same as for your amp, so the computer fan will fail after a short time. 20dbA, you can get quieter then that man! Also, one of my guys here has a Line 6 Bass Pod Pro. He really likes it and it makes it easy for him to practice at home. The only complaint he had was it was a hassle to have to reach over and change effects or bypass them. So, spend $100 more and get the little foot pedal and save some hassle and inconvience. Well, that's my advice and his. Hope it helps.

    Jason
     
  7. tomtrb

    tomtrb

    Apr 17, 2001
    So it seems there is a difference of opinion on the load issue. I would be running my Trace for its preamp and eq only, then going out the post eq DI (or the effects send if I dont have an XLR cable), right to the board. Hence, there is nothing connected to my speaker outputs(8 ohm), but Im not using my power amp section. Im not sure what is meant by a "a signal being passed" - passed thru where? My preamp or power amp? If your talking about just turning my amp on with nothing at all going in or out, well, that may be safe but it isnt very useful to me. Im not sure what you mean.
    About the resistor, is that just something I plug in to my speaker jack and it acts like a dummy cab? If that is what I need, where can I get one?

    About biasing preamp tubes, most of the people I have talked to have said that it is not neccessary to bias them. One person however, said that I should try different tubes to see what I liked, then after Ive got the one I want, bias it to really max it out. I have never heard about self-biasing tubes like Psycho is talking about. Since I know someone who can bias it for free, why not, right?

    Fans - Long story short - I have a factory Trace fan and a 20dba computer case fan. I need my amp to be quiet, and possibly have a fan switch for studio use. I do not push my power amp hard at all (.5 - 1.5 is LOUD), so I guess my amp doesnt get that hot. I have not seen any fan rated lower than 20dba. I guess Ill try the Trace fan and go from there. The 20dba fan is 12v and some tiny amount of amps. My old fan was 12v with a larger amp rating, so the case fan would fail because of the difference in amps?
    Yes, I have thought about the Pod Pro, still thinking - its expensive you know, and the footswitch is more like $250
     
  8. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    I've seen some tube guitar amps that have a resistor of maybe about 100 ohms or so across the output so there's always a load even if the speaker's accidentally disconnected. But I wouldn't rely too much on it.

    Any solid state amp can be run without a load, because it's just producing voltage.

    -Bob
     
  9. Nightbass

    Nightbass

    May 1, 2001
    Seattle, WA
    No difference of opinions; we're right and he's not. :)

    Seriously, in the Class AB/B complimentary-symmetry output topology that nearly every transistor bass amp has, the output transistors are fully capable of handling the rail-to-rail voltage swing, and since there's no load, the current is almost zero. Can't hurt a thing, and you don't need to use a dummy load. The transistor itself *is* the load.

    Psycho meant that if you could somehow keep the output from the tube amp's preamp section from getting to the tube amp's power amp section, you could theoretically just leave the power amp hanging without a load, and it won't kill it. If you were to hook a cab to the amp at that point and no sound came out, you'd be safe. Meanwhile, you would be using the preamp section for recording, DI, or feeding some other amp. Some tube amps have that provision in the form of two 1/4" jacks in the back, jumpered with a short cable.

    However, I still wouldn't run a tube amp without a load, for the sake of the output transformer.

    Self-biasing or auto-biasing describes the bias circuit, not the tubes. It's the opposite of fixed bias, and is more relevant for output tubes, though. When you plug a new tube into a auto-biasing circuit, the tube finds and maintains it's own bias current. When you plug a new tube into a fixed bias circuit, the tube gets a bias current that was designed into the circuit.

    A good analogy is the idle circuit on your car. Let's say that idle was set to 650 RPM at the factory. When your car goes in for a tuneup, the tech might readjust the idle to compensate for aging of the engine. And if you installed a new carb or FI system, if the idle wasn't spot on, you'd have to reset the idle back to factory specs. Same thing happens with an amp - the idle current is set at the factory. It may drift with age. You might replace a tube resulting in a new idle current value. So you have to adjust it back to normal.

    Now if you try preamp tubes and find the one you like, and then change the bias, the tube won't sound like what you liked anymore! Making the bias hotter changes the tone, the headroom and compression, and the point where the tube begins to distort, and this doesn't make sense - unless you are after pure distortion. It's not right to make a blanket statement that you should run a hot bias, any more than it's right to say your car should idle at 1200 RPM.

    Nightbass
     
  10. engineer7

    engineer7

    May 11, 2001
    I agree with them on the part about the input signal. if there is a way you can have the input signal not go to your power amp section, then great! whether its a switch installed on your amp or a modification you make to the input of your output circuit. But from my 15 years of experience of repairing these dad gum things. Please just hook up your cabinet or some type of load. In your case a 8 or 4 ohm, 50W resistor would work well with the output level down.
    Remember this is a power amp, hook it to your speaker cabinet like it was designed to do. If not, then go get a preamp! :)
     
  11. Sorry, this is a misnomer...there can be alot of heat generated by the switching power supplies in today's high-end computers. The fan on those not only cools the power supply (which, as systems get more powerful, need to keep up) but circulates the air within the case (obviously not enough to cool the CPU but that's not what they're designed for).

    The current rating of the fan is what the DC motor in the fan will draw...so as long as it's equal to or less than the original, you'll be fine.
     
  12. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Again, that applies only to tube amps.

    -Bob
     
  13. Again, Bob is right.

    No current = no power = no heat in the output section, plus, no inductive output transformer = no inductive voltage spike when driving into no load = no problems when running solid state output stage with no load.

    I don't doubt that you know a lot about all this electronic stuff and that you've repaired a lot of gear, but on this issue, Bob is right.

    Chris