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In Reply to: RE: ". . . the O/P tubes are worked VERY hard." posted by Eli Duttman on October 13, 2016 at 12:53:54
I was under the impression that specs listed in a tube manual are measured under quiescent conditions. Is that not the case?I can't read the voltages on the schematic you posted. My figure regarding the S-5000 is based on the Sams which lists a voltage at the cathode pin of .5v dropped across a 12 ohm resistor to ground so 41.6 mA for the two tubes or 20.8 mA each. Plate to cathode voltage is 412.5 so dissipation would be 8.59 watts.
So are you saying the Sherwood operates in Class B? I'm not very knowledgeable about operating classes. I would have guessed it would be AB1. I was under the impression that the most common use of Class B was in PA amps, not hi-fi.
Regardless, my point was that I think the physical layout and the resultant concentration of heat plays an important role. The toasty wires around the output tubes should confirm that, don't you think? Exactly how much that affects tube life, I don't know.
Out of curiosity, I just checked the schematic of another popular and well-regarded amp - the Eico HF-81. It lists a cathode voltage of 11.5v dropped over a 165 ohm resistor. That works out to 69.7 mA per channel or 34.8 mA per tube. Plate to cathode voltage is 323.5 which gives us a plate dissipation of 11.27 watts.
So that's 31% higher plate dissipation than the Sherwood. I don't think I've ever heard anyone say the HF-81 is hard on tubes, though.
Edits: 10/13/16 10/13/16 10/13/16Follow Ups:
Work backwards from 20 W. The max. theoretical efficiency for Class "B" is 78.5%. In the real world, the efficiency will be lower. For sake of argument, we'll use 78.5%. 10 W./tube leads to 12.9 W. being dissipated.
Yes, the S5000 works the O/P tubes hard . Factoring near Class "B" (prevents cross over distortion) and real world losses into the calculus, both of which reduce the efficiency, means over 13 W. being dissipated/tube, at full power O/P. Don't think of anything other than EL84Ms in an S5000.
Eli D.
Eli - Thanks for the explanation of how to estimate the actual working dissipation.
I understand that over 13 watts is high and would certainly be considered "hard" on the tube. But, as you said, this is at full output power.
Most users probably never run their amps at full output power. I know I never come close to that. Even with fairly low efficiency speakers (I have KLH Model 6s paired with mine at the moment) I never turn the amp up even halfway on the volume pot. Yes, I know half on the volume pot doesn't necessarily equal half power. But still. Even with fairly inefficient speakers, say 88 db/w/m, 13.2 watts output (66%) would produce over 100 db at one meter.
If the amp is being operated at 66% - 6.6 watts output per tube - and each tube needs to dissipate, let's say, 135% of 6.6 watts to produce that output, that's 8.9 watts plate dissipation. Well under the maximum specs. If plate dissipation is 135% of output, the amp would be putting out 18 wpc (90% of full power output) before the tube exceeds its 12 watt rating.
So wouldn't it be fair to say that, under what most would consider "normal" (not close to maximum) operating conditions, the tubes are not being run all that hard??
My other question is, how does this compare to the other examples I gave - both the Fisher SA-16 (13.34 W) and the Eico HF-81 (11.27 W) dissipate considerably more - at idle - than the Sherwood (8.59 W).
The Eico is supposedly 14 wpc, so less efficient (not near Class B)? I believe I read somewhere that the Fisher is about 20 wpc. Wouldn't this indicate that they are running their tubes even harder, under actual working conditions, than the Sherwood? Yet the Sherwood seems to be frequently mentioned as being hard on tubes yet one rarely hears this about other amps.
Please understand, I'm not just trying to be contentious here. I just don't understand how an amp that has its output tubes idling at what seems like a very reasonable plate dissipation (actually lower than similar amps) could be considered to be hard on tubes. And, for the record, I'm running 7189s in my S-5000.
Am I incorrect in saying that at 66% power output - which most people would consider closer to "normal" operating conditions than full output power - the plate dissipation of each output tube in the Sherwood S-5000 would be about 8.9 watts?Is running a tube rated at 12 watts dissipation at under 9 watts "hard" on the tube?
If my logic is flawed please correct me and explain.
Edits: 10/19/16
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