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I've been using a Cary amp for years and have a nice selection of NOS power tubes. I recently picked up some newer monoblocs that use EL-34 tubes & have one of those fancy bias meters built right into the amp. Well, when I stuck my EL-34 Amperex Double-D Getter tubes in there the amp ran out of juice before it would reach the reccommended bias level, I mean I had turned the bias knob all the way and still the Amperex didn't get as high as they should in one of the amps. I stuck the new production SED EL-34's in there and they biased at about HALF the turn required to bias the NOS Amperex tubes? I recall in my Cary amp I really had to crank that bias knob to get these to bias as opposed to my Tung-Sol 6550's but they always went up high enough eventually. All the Amperex test out just fine and I hve about 3 quads of these saved up over the years and all of them are the same as far as how you have to crank the bias knob to get them where they are supposed to be.Any reason why this EL-34 Amperex model takes so much more juice to reach the same numbers on the bias meter as new production El-34 or even my Tung-Sol 6550?
Is there anything I can do to the new amp to get a little more juice out of the Bias knob so I can correctly bias the Amperex tubes?
I love these Double-D Getter tubes and they get "close" to the reccommended bias level but not quite there on one of the amps and just reach the level needed on the other monobloc?
Follow Ups:
as tubes age (and decline in output) they require less negative bias (e.g. more closer to zero) to get to a certain current level. My guess is these are not as NOS as you might think, or at the very least are not biasing up within the spec range. (due to either being out of spec and/or low output, or both). The meter is showing the current draw on them, most likely. Since you can't get the current to the expected range, the bias is off. Changing the resistors may help but it will change the range for more normal tubes, and may also cover up the fact that these tubes may be past their useful life spec, despite what the tester might say (since they rarely measure at real-world operation currents)
thing is these tubes just do not "test" NOS they "look" NOS also. I've seen used El-34 power tubes where the getter flashing is nearly gone even though they still test fine. These arrived here over the years w/ perfect getter flashing and test results. Usually I look around the bottom of where the flashing ends and if it gradually fades away sort of hazy like versus a nice solid line where the flashing ends I know they have had no play time. Still yet, the Cary amp would bias them just fine, although Idid have to crank the bias way past where the Tung-Sol 6550 went or even the Visseaux 6L6 or Mullard El-37. The only tube that has to be cranked more is the 8417 power tube which will nearly wear your wrist out trying turn that darn screw!I understand your point though about changing teh resistor may make it hard to bias current production tubes. However, I've never used any current production power tubes since I went to tube amps anyway so for me it's not an issue as I don't plan on using them ever in the future either but it could be an isssue should I sell the amps later down the road. Think I would just have enough of a change to add a "tad" more to the bias, not a lot? Think I'll let the manufacturer give me the data on that one?
Good NOS Power Tubes can easily out-perform modern Power Tube types in almost all areas, even sonics, in most Vintage amps. Even used NOS Power Tubes can function and sound better than current production. I think that the problem today is Quality Control vs. Price and also the fact that the empirical tests done on these amps used older tubes, not the new breed of mis-aligned screens, loose tube bases and pin diameter issues. Yet this also may have been the case 45 years ago, I was just born then so cannot speak from experience in this case. The one thing that makes current production tubes almost a 'lottery' for Fixed Bias Vintage applications are those amps that do not offer any adjustment whatsoever. In these amps, it's really scary to install so called "matched-quads" of current production Russian, Chinese or JJ Tubes. In most cases, I can see plates turning red and the tube glass making noises as they heat up and burn away. In less than 2 minutes in a dark room I can often decide to keep the amp on or power off the amp and utter some of the more subtle 4 letter words. The only sure way I have gotten around these bias problems in Vintage Cathode bias and Fixed Bias amps is to buy used and tested NOS Power Tubes. One can change the bias resistors or make the bias circuit provide a different negative voltage. But the problem with this method is that one is altering the design of the output stage. Tube amp H.V. supplies are un-regulated. So if you draw more current the plate voltage drops a %, and if you pull back in the current draw the voltage climbs a %. There is really only one sweet bias spot where the THD%, the Power and Bandwidth are optimal, and this is the actual designers sweet spot. Some other bias points may offer different sound and be optimal for that particular amp. One can actually cut back on the tube's current flow to cause some screen and plate voltages to rise and make the amp sound different, but probably not as musical as it can be. In my dozens of old vintage tube amps NOS is better than current production, hands-down. Unless of course one can purchase a matched quad of tubes that will perform as expected. Yet the only practical way to do this is to examine your amps specifications and request tubes that will have a stable operating point at the specified bias voltage, plate voltage, screen voltage and bias current. If you tell'em where your amp 'park's' your power tubes, you just may be able to buy several quads that work well with your amp in standard-normal-un_modified mode. New production amps seem to do the opposite, they tend to favor and work best with current production tubes. I believe that the quiescent points in these newer amps are tailored to what's been available recetly from Russia and China.
Your bias pot puts a negative voltage on the grid of the output tube. This negative voltage acts as a "valve" causing the tube to draw more or less idel current. Some tubes require, say, -25 volts at a given plate voltage to draw a standing current of 50 mA. It is the sensetivity of the "valve" action to negative voltage which determines the current draw of the tube. It has nothing to do with "Juicing" the tube by applying current or plate voltage.So, some new production tubes need more negative voltage for a given current draw than some old production tubes. When you crank up the bias knob with the Amprex, you are reducing the negative voltage. Your amp has a stop, a minimum negative voltage setting that is there to prevent people from blowing up their amp. Your range is probably something like -55 volts to -25 volts and you cannot get below -25 volts. Your Amprex probably needs something like -20 volts to bias properly. There is a one resistor fix per bias pot to extend the range. It is basically a free 10 minute fix if you know what you are doing.
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Long Live Dr.Gizmo
replacing one resister is something I can have done here locally. there is a VERY good old fella that has a repair shop locally and builds his own tube amps for fun. I have had him tweak my gear before by replacing stock caps w/ Jensen oil caps and blackgates in the PS. I think I'll call him and Thor to make certain what value we need and just enjoy the amp for a month or so w/ the new production tubes then have the bais resister changed and see how much better they sound w/ the Amperex.
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