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I am building a PSE amp using 300B.I would like to find out the distortion of the parallel 300B stage(only on power tube stage). It is because the distortion level of 300B seems to have relation on its Grid bias, plate voltage and plate load impedence.
Suppoing a voltgae gain and driving stage have THD of 1.5 percent when producing max volt swing of 65V rms. At the OPT output THD is measured at 6% when producing 12V rms.
My question is: Is that the distortion of 300B stage + OPT as simple as 6% - 1.5% = 4.5 % ?
Does anyone in this forum have suggestion on those parameter combination(grid bias, plate v, load impedance) which can minimize the distortion(primarily on odd harmonic). My current custom-made Power Transformer has 455V plate V and I am planning to use self-bais on 300B.
Follow Ups:
"My question is: Is that the distortion of 300B stage + OPT as simple as 6% - 1.5% = 4.5 % ?"
No it isn't. Could be additive, subtractive etc.
Hi Eric,Check the 300B Western electric datasheets at the WE site. They have nice distortion curves and distortion specified for different operating points.
The transformer distortion is somewhat more difficult. More airgap is good but you'll also need the inductance to match the reflected impedance or else the loadline will become elliptical resulting in more distortion. For pretty straight loadlines I'd recommend a reactance eaqual to the reflected impedance at the desired -3db frequency. So for a 3.5K opt -3db at 20Hz: 3.5k/(20*2pi)=28H. With enough core area to keep the flux levels in the linear area; Basically just get an opt designed for the job...
But this is not the whole story as a reactive speakerload will present a reactive load on the tube anyway. So you also need to consider your amp/speaker combination. And a reasonable amount of H2 in your amp can be used to cancel the H2 of your speakers suggesting the choise of a lower reflected impedance to achieve lower total thd.
Do you have the site address of WE ?
Thanks for your help. I've found the website pretty useful.
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