Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jordan86 on January 14, 2021, 01:57:34 pm
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I apologize if this is overly basic. I have an output power question in relation to my single ended amp that I recently posted on. It's a well known build from the AX84 community. It's the SEL. High gain, Single Ended Lead. The stock power section is a KT88 but it will accept most of the common 8 pin power tubes. Post schematic again fwiw.
Schem calls for a 15W Hammond 125ESE. Mine was built with a custom Heyboer Doberman 20W SE OT by the ax84 community guys. Seen here. https://reverb.com/item/37912853-ax84-heyboer-20w-semi-universal-single-ended-tube-output-transformer-el84-el34-6l6-6v6-kt88-etc
My question is less about the model of the OT and more so about biasing and plate dissipation. It's class A, cathode biased so I bias my KT88 for 42w. Roughly 110ma at 380v. Does that translate to 42W hitting the 20W OT though? My suspicion is that the answer is no, and that the amp is fine because it was spec'd that way. I just don't have an electrical or mathematical reason for it. It anyone has good links that I could read up on as well, I'd love to see those.
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Just found this on another audiophile sort of forum:
"I think of it in terns of incandescent light bulbs.
A bulb is rated at 100 watts. But it's output is in lumens.
A tube's plate dissipation is one thing, it's output is another. They both just happen to be measured and rated in watts."
I guess my thinking is off. Ie. that plate dissipation is not the same as output power.
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Just the process of turning steady DC into the Sine wave customarily used for testing "audio power" is a factor of 2. So 42W DC input can not be more than 21W sine output.
The "rating" of an audio transformer is very situational. The most common first limit is bass THD. If this tranny was razor-designed for 5% THD at 20W at 82Hz, then 21W may be 5.25% THD. Very high current may worsen bass response. Very high voltage may punch-through insulation. I'd be sure Heyborer gave ample margin on these troubles.
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most of my SE builds had 2X "watts" flowing at idle, my last kt88 is about where yours is. I use Edcor OT and after an hour wailing on the amp, the ot just yawns as far as residual heat.
maybe convert to coulombs so you don't get confused :icon_biggrin: