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to play a large harp. After a few minutes of live music passed, I asked my friend what he thought of the sound system. He became animated.
"So that's it. That's why you wanted to come to this restaurant: let me guess these new age hippies are too cheap to buy a new stereo and they are running some old Dyna gear. And they probably are running the original tubes," he said triumphantly to me.
He then cocked his ear and said, "Yes, definitely tube gear, notice the flabby bottom end and the rolled off highs."
"What about the mids," I asked.
"Sure it sounds smooth, but too smooth, artificially smooth, no bite...just too romantic don't you think?" he replied.
"You're right, it sounds just like tube gear to me," was my answer.
When we then got up to leave and my friend turned around to see the harpist still playing and gasped. A truly great moment.
Subject: ParaFeed Single-Ended OTL
Please excuse my English. I admire your journal. It is the densest concentration of tube circuit information I have found anywhere. So please do not stop publishing.
I have a question for JRB. Couldn't an OTL amplifier be designed along the design principles of a para-feed single-ended amplifier...minus the output transformer? The speaker would connect to the junction between the output tube's plates and the choke. Of course, a large valued coupling capacitor would be called for (.1F for example).
Since single-ended amplifiers are half as efficient as the push-pull amplifiers, I am guessing that four EL-509's would deliver about 40 to 50 watts.
What do you think? If such an amplifier is possible, many of us who need a more powerful amplifier would be very happy.
Paulo S.
? South America ?
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Do not worry too much about the journal, as I believe the sale of the article related software will save the day. Of course, I assume you will be among the first to buy a program so as to ensure the continuation of the journal ;)
Yes, certainly an OTL para-feed amplifier is possible, but it maybe not as desirable as you would hope. In a nutshell, the amplifier must run in strict Class-A mode, not the advertising copy Class-A mode. Now 40 watts of RMS power would demand at the very least 80 watts of dissipation, as the maximum theoretical efficiency of an inductively loaded Class-A amplifier is 50%. That is 50% with perfect output devices but vacuum tubes are not perfect output devices. For example, the plate curves for the EL509 on the previous page show that if we want to prevent any positive grid voltages, the 0v grid-line splits the area of useful operation in half. And even this halving assumes that the load equals the rp of the EL509; but as the effective rp of four EL509s is about 30 ohms, we can expect a further quartering of the potential efficiency. What we are left with is about 10% efficiency. Thus 40 watts of output power requires 400 watts of output stage dissipation, which far exceeds the limit of the four EL509s. So at least a doubling of the number of output tubes is needed.
The point that need clearing up in in your e-mail is the part about the relative efficiencies of output stage topologies. The efficiency difference between a single-ended and a push-pull Class-A amplifier is zero, both are 50% efficient if inductively loaded. (If constant current source load, both are 25% efficient; resistive loaded, 12.5%.) In other words, 8 output tubes in a totem-pole push-pull Class-A amplifier equal 8 output tubes in a single-ended Class-A amplifier. In the push-pull version, the output tubes must see an idle current equal to half the peak output current; for example, if the peak output current swing is 2 amperes, then the idle current must equal 1 ampere and as totem-pole amplifier has four output tubes in parallel per bank, each tube must draw 250 mA at idle.
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