| Author | Message | ||
     Anonymous |
I installed a Groove Tubes E34LS Duet in my 210-65 (solid state driver). At a driver bias value of 25mVDC, the sound was thin and cold. The amp 'came alive' at a driver bias of 19mVDC. I understand that the 25mVDC value is a base value. I wonder if 25mVDC may not have been meant for the Slovak EL-34's we see today. In what year were the posted bias insructions written? I want to know if 19mVDC "smooth sounding' value I found, basically by ear, is undesirable from an electronic point of view. The tubes aren't 'glowing' improperly. How else would I know if I've exceeded parameters? Once set (hot), is the bias value static, or might it vary up or down - by a mVDC or 2 (idle)? Also, what does the difference, in the voltage drops, across the 3.9 Ohm bias transistor emitter resistors - indicate? Thanks. | ||
| Steve Kennedy |
The original bias instructions were written back when Music Man was in business... prior to 1984. Back then, 6CA7 tubes were still available and EL34 types were not in general use (although Music Man started using 6L6 types because the 6CA7 was going out of production). In order to directly read bias current, you would have to LIFT the 3.9 ohm resistor and insert a current meter in series with it. Rather than do this, you simply measure the voltage drop ACROSS the 3.9 ohm and using Ohm's Law you can calculate the current. V/R=I V=Voltage (in Volts) R=Resistance (in Ohms) I=Current (in Amps) So .025V (25mV) divided by 3.9 ohms = 0.0064A (6.4mA) quiescent current (at rest). Your 19mV yields 4.8mA bias current. The 25mVdc but not to exceed 55mVdc (14mA bias) range for the worst case gives you the maximum difference and defines the top of the range. Anything below 14-15mA should be OK and no cherry-red plates should result. Keep in mind that they were biasing 6CA7 tubes so the EL34 variety you used may be happier at a slightly reduced bias. This isn't always an "exact" science... some tubes are happier at a different bias, that's all! Your amp will actually run at a bit cooler temperature at the 19mV but it will hardly be noticeable from an electronics standpoint. Stay below 55mV on the "hottest" tube and anything that sounds good is the right value for your amp. Steve | ||
| Anonymous |
When setting the bias on my '65 with Solid State driver, I notice that there is a slow 'upward creep' in the 3.9 ohm resistor's voltage drop. (amp warmed for 10 min's prior.) Is this normal? If it is, at what point in the 'upward creep' cycle do I need to make my readings? Thanks. | ||
| Steve Kennedy |
Bias is measured with no input signal and it should be relatively stabilized before you accept the results. If there is a constant "creep" upwards, the amp would sooner or later eat the output tubes for lunch! It has to stop somewhere. It is moving either because the tubes are still warming up (although probably not in your case), you just made an adjustment and the circuit hasn't stabilized, or you have a wandering bias condition. This last condition (wandering) can be due to component aging and worn out output tubes. Just keep in mind that adjusting the bias isn't instantaneous with a tube. If you change the bias current flowing through the tube, the tube reacts by getting hotter or cooler (depending on the direction of change) in response, which then causes the circuit to draw a little more or less bias current in response to the tubes temperature change... back and forth until the circuit reaches a point of equilibrium and it stays put. Where it stabilizes is the true bias setting. Steve | ||
| thefishingline |
At the risk of sounding stupid...which I ma on these matters...what is "bias" Also is there a way for these boards to possibly remember who we are so we do not have to log in or give our names everytime we make a post? I have a fihsing biards at www.thefishingline.org and once you register, you are alsways recognized once you log in each session | ||
| Steve Kennedy |
Yeah, I know it is a pain to post multiple messages on this system. I am running the "light" (free) version of the "Discus" (discussion board) software. In order to have registered users I would have to mange all of the passwords and assign levels of access manually and I do not have time to that! I barely have time to answer messages as it is! The "Professional" version of this package has automatic user registration and a host of other cool features, but it costs $150. I already provide all the web hosting, registration, software and source material I have to buy to set this system up. I don't feel that I can justify spending another $150 for the "pro" version right now. If the users of this system want to take up a collection and spring for it, I'll get it installed but otherwise the economy will have to improve and I'll have to get a raise before the "discretionary" funds for such an upgrade would be available. "Bias" is basically the current that flows through the amp's output tubes while at rest. It is a "head start" towards passing the signal you are trying to amplify. Too little bias and the tubes run colder, but they sound more "brittle" and lifeless. Too much bias current and the tubes sound wonderful but will burn-up quickly because of excess temperature (the tube's plate will glow red). You can look at bias as the tube's "idle speed" (like in a car engine). If the idle is too high, the car's engine gets hot and wastes fuel at idle. If the idle is set too low the engine may die and you will have to start the engine first before you can accelerate. It is better to have a smooth idling engine that is ready to go at the touch of the gas pedal. Theoretically, you could "over-bias" an amp and it would sound great as long as you had some sort of active & effective cooling solution in place (fans, etc.). However, since tubes are made of glass and have a tendency to be sensitive to vibration (microphonics), this dictates that the common methods of cooling (fans and liquid heat transfer) can have a somewhat limited effectiveness in this application. I can highly suggest fans in "hot" amps (like the HD-130)though. A 240VAC fan running at half-speed (running on 110VAC) is much quieter and moves enough air to make it worth the hassle of installing it. The fan doesn't need to blow air directly on the tubes... it just needs to promote the movement of air in the confines of the amplifier cabinet. Cooled tubes will last longer and will prevent internal components from becoming "cooked" and dried out as soon as they might otherwise. | ||
| TFL |
I just had my Musica Man HD 210 130 given a clean bill of health by Jess Oliver of Ampeg and Oliver amps legend. He lives 20 minutes from me here on Long Island. His eyes lit up when I brought the amp in. He says this is one of thebest they ever made and the "Phasing" output tube?? is a tube and not the solid state version that he says..."put them out of business." This old timer is the coolest and knows his stuff big time!! |