Dutch, July 2000 explains to Konrad, how a dummy load works === If you're willing to put some time into tweaking equipment and some $ into some extra gear, you can get a good reduction of that "solid state" hiss by running a small tube power amp head (an ancient tube hi-fi monoblock can work well--use the "auxiliary" or "tape" input) on a dummy load, fed by your preamp, in your T-Fex's FX loop. A small (15W or so) tube amp with 6V6es or EL84s will work well, as its volume doesn't matter when used with a dummy load--you'll get no sound out of the little amp, just some primo signal-conditioning. Use a reactive dummy load. You can build one with speaker crossover parts like I did, or spend a chunk of $$ on a THD or similar. If you build your own load, use the lineout circuit from the 50W JCM800 Marshall. It's a good, simple line-out circuit that tackles the "flyback tone boost" effect and gives you a nice, balanced output signal to feed to your FX return. Most commercially available loads have a similar filter built in. That's what I do with my TubeFex, and it kills! You'll get a bit of natural tube-amp compression (a lot if you use a cathode-biased amp), and if you jack up the FX send all the way, it gets you some real tube overdrive to add to whatever the TB is doing. === > What is "dummy load" ? Dummy load is the name for it. It's just not everyday "conversational English", so most books don't include the term. You typically won't catch idle chat about them at the coffeehouse over a mocha latte and a muffin, unless you hang out near a major guitar tech's shop and get lucky.... :^) It's not your shortcoming or the book's, it's just an "electronics tech" term. Are you ready for some "tech talk"? :^) A simple dummy load is a resistor that is rated at the speaker load your amp expects to be hooked to (often 4, 8 or 16 ohms) and is capable of absorbing the total output of that amp. It substitutes for a speaker in much the same way a crash-test dummy substitutes for a real human when you test a car's reaction to a crash. A "reactive" dummy load is more than just a big, fat resistor, but includes inductors and capacitors to mimic the impedance curve of a real speaker. A real speaker's impedance isn't 8 ohms (or whichever rating it has) for all of the audio frequency range. There is inevitably a peak at the mechanical resonance (it's usually around 80Hz or so, depends on the speaker) of the cone, voice coil, spider, surround, etc. (the moving parts). Also, the speaker, being an inductive device, has the characteristic that its impedance rises as frequency rises. There is a trough in most speakers' impedance curve at around 400 Hz or so, and it is the impedance at this trough that the nominal impedance is named from. The purpose of a dummy load is to give the tube amp a load at its output without hooking a speaker to it (and therefore getting sound out). A tube amp can't run without a speaker load for long before it self-destructs. The dummy load is usually used for the purpose of testing on a test-bench, but was found to be useful for the purpose of using that tube power amp section as a signal processor. The earliest examples I can think of off the top of my head of cranking a dummy-loaded amp for its overdrive characteristics is Terry Kath of Chicago Transit Authority (later just "Chicago") using an old tube Bogen PA amp on a load into his Fender Showman to get controllable distortion at whatever volume he wanted. Just give a listen to the CTA album and dig his SG/Bogen/Showman overdrive tone. For 1968, it kills! The biggest way that the dummy-loaded amp reduces hiss in your preamp is by the compression (and mild distortion if you crank it for leads) you get when it's running at a high volume. This compression gets you the sustain you want at a lower preamp Pre-gain level, and thus, lowers the hiss. You also get some of the higher frequencies (which are the most objectionable) rolled off at the tube amp's output transformer due to its frequency response at high power output being narrower than its frequency response at low power output. So, you'll gain a bit of that touch-sensitivity that tube amps are famous for..... :^) > what is "fed by your pre amp", in your T-Fex's FX loop. Does it means signal > from FX LOOPOUT -> Tube AMP IN, Yes. An FX return on the amp would be the best place to go in from the T-Fex, if the amp has one. If not, the regular guitar input (a "low-gain" input would be better if it has one) will work if you set the tone controls flat and send a fairly small level (in this case, start with the Send level at about 25 and adjust from there) from the T-Fex's FX Send. > Tube amp (speaker?) OUT to FX LOOP IN ? Well, sort-of. Your line-out circuit would tap some signal from the tube amp's speaker out and make it smaller (and tailor its frequency response) so that it can feed the signal back into the T-Fex via the FX Loop Return jack. > > A small (15W or so) tube amp with 6V6es or EL84s will work well, > 15 W tube amp is VERY LOUD. What's is with impedance and voltage IN/OUT > adaption? Does is not hurt the FX LOOP IN input circuit ? You take the tube amp's speaker signal and reduce it to a lower level in the line-out circuit to interface with the FX Return. The dummy load takes care of matching the tube amp's output and absorbing the power the tube amp makes. The tiny bit of extra load that the line-out makes won't upset the tube amp's output one bit, and then after the signal level is cut down and tailored to fit, the signal level won't harm your FX Loop Return circuitry one bit. I've been running mine like that for about 3 years now, and for several years with just my old Quadraverb before I got the T-Fex and the Q2.... > > or EL84s will work well, > As far I know, one EL84 has 25 W !? EL34 has 25W plate dissipation. A push-pull pair of EL84s will typically put out about 17W. > On my polish guitar group someone want to trade a hand made tube amp > (2XEL84, 3xECC83), maybe I could use it ? Sounds like a winner. Does it have an effects loop? > > volume doesn't matter when used with a dummy load--you'll get no sound out > > of the little amp, just some primo signal-conditioning. Use a reactive > > dummy load. > ??? See above. The dummy load takes the place of a speaker, and allows you to crank the little amp without making any sound. A reactive dummy load's impedance varies with frequency in the same way that a real speaker's impedance varies with frequency. > You can build one with speaker crossover parts like I did, or > > spend a chunk of $$ on a THD or similar. > Whatta hell is THD ? THD is a manufacturer that makes amps (Pitbull, etc.) and a dummy load for tube amps (the "Hot Plate"). > > If you build your own load, use > > the lineout circuit from the 50W JCM800 Marshall. > Where should I take it from ? The input of the load. If you build it into the load, you can change amps and find the one that sounds best with your rig if the little EL84 jobbie doesn't "do it" for ya.... I'm a 6V6 nut, and just groove on the sweet overdrive of those tubes. > Can You send me any sound samples of Your tube amp adaptation ? I have a song which I recorded on my computer that uses this setup, direct-injected into the computer's sound card, using the T-Fex speaker simulator (SS) effect to take the place of amp, speaker, and microphone. I could also set up some audio samples that compare with/without the dummy-loaded amp, if you have a little time to wait for it.... > Any schematics, parts ...? Sure. I'll see if I can't whip up a drawing of the dummy load (and line-out) circuit for ya this week at work when it's slow..... Parts can probably be obtained from a local hi-fi speaker builder, or over the web from a speaker builder. > heeeelp > > K O \/\ I :^) Koni, I know sometimes, to the average Joe, I sound like I'm speaking Martian! If you want to read a little more about how a speaker works, and how it "loads" a tube amp, try www.webervst.com, and look around there in the "Speaker Terms", and the "Let's Talk About Speakers Q&A" areas. I originally got the circuit for my dummy load from Ted Weber, the founder of WeberVST (I've known him for years), and tweeked its design to suit me. You could maybe even stop by the bulletin boards at www.firebottle.com/ampage/ to chat with some other "tech" folks if you want to, also...... Lots of good folks out there! C ya, Dutch