The output tubes are designed to run hot. In a typical operation, I measured the turkish proxy output of the 7591 and EL84 types running north of 300 degrees Fahrenheit on a glass jacket, even if it seems a bit daunting compared to the world of solid-state components. In AB1 two-cycle mode, even under no-load conditions (no signal), at 34 mA cathode current, your 7591 tube plates dissipate about 13 watts per piece, and some standard Fisher devices have heated lamps much more than that. That's like having a 60-watt bulb under the hood, not even considering the heat from the shields and other tubes and components such as power resistors.
Power tube operating temperature a good proxy for power output???
Background:
Recently got a lovely Linear Tube Audio ZOTL40 Reference tube amp. It runs 4 pcs EL34 power tubes, autobiased, 2 driving each channel. Being auto-biased, the user does not see the actual biasing condition at each tube (or pair of tubes). Amp sounds wonderful, works perfectly. One of the reasons I like this Berning design amp so much is that it runs the power tubes very easily, thus allows for very extended tube life -- especially attractive to me as I still have several quads of UK made EL34's and KT77's collected in the 80's and 90's, not the current Russian reissues... the real stuff. I can run these with long tube life.
Just for fun, I used a laser thermometer on hand (from track driving, checking tire temps) to check the operating temperatures of each tube... found there is a decent variation in temperatures (160-185-185-178 degrees F) at the exposed top domes of the 4 tubes in the amp...
Question:
So this raises a question in my mind... would such a 10-15% variation in individual tube operating temps be a fair indication or proxy for output level of each tube? The 4 tubes do sit in a row, so the two end tubes running cooler makes sense as they see cool air on one side, while the middle tubes don't... and of course, even identically constructed tubes have meaningful variances... we can also test GM in a tube tester -- but those test parameters is not close to real operating voltages and currents...
Input welcome... thanks in advance. No problem here -- just curious to know, from those very experienced with tube amps and tube amp design, if you have an informed point of view on this.
Recently got a lovely Linear Tube Audio ZOTL40 Reference tube amp. It runs 4 pcs EL34 power tubes, autobiased, 2 driving each channel. Being auto-biased, the user does not see the actual biasing condition at each tube (or pair of tubes). Amp sounds wonderful, works perfectly. One of the reasons I like this Berning design amp so much is that it runs the power tubes very easily, thus allows for very extended tube life -- especially attractive to me as I still have several quads of UK made EL34's and KT77's collected in the 80's and 90's, not the current Russian reissues... the real stuff. I can run these with long tube life.
Just for fun, I used a laser thermometer on hand (from track driving, checking tire temps) to check the operating temperatures of each tube... found there is a decent variation in temperatures (160-185-185-178 degrees F) at the exposed top domes of the 4 tubes in the amp...
Question:
So this raises a question in my mind... would such a 10-15% variation in individual tube operating temps be a fair indication or proxy for output level of each tube? The 4 tubes do sit in a row, so the two end tubes running cooler makes sense as they see cool air on one side, while the middle tubes don't... and of course, even identically constructed tubes have meaningful variances... we can also test GM in a tube tester -- but those test parameters is not close to real operating voltages and currents...
Input welcome... thanks in advance. No problem here -- just curious to know, from those very experienced with tube amps and tube amp design, if you have an informed point of view on this.