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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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In Reply to: RE: Why do my NOS power tubes take so much to bias compared w/ new production? posted by blazter on April 29, 2007 at 15:16:19:
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.
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Follow Ups
- Re: RE: Why do my NOS power tubes take so much to bias compared w/ new production? - shermanr@prw.net 14:12:46 04/30/07 (0)
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