Getting the Best THD Performance Out of Your VTA ST-70: 50mA Vs 40mA Bias Experiments

This video is a follow up to a video that I posted regarding the negative feedback circuit of the VTA ST-70. In this video I go one step further and show that biasing the output tubes at 50mA rather than the recommended 40mA level drastically improves THD performance.

I also revisit the tube vs. solid state rectification test. In the end, I conclude that increasing the negative feedback, biasing at 50mA, and using a tube rectifier and a quality set of output tubes results in the best THD performance of the VTA ST-70 amplifier.

Detailed listening tests will follow in a future video.

Disclaimer - Some claim that modern production EL34 cannot handle elevated bias levels and that running your ST-70 at 50mA regardless of the driver circuit could reduce tube life.

Resources & Links:
David Gillespie ST-70 Base Line Testing Article:
https://www.audioregenesis.com/documents/ST-70%20Base%20Line%20Testing.pdf

Dynaco Forum Bias Discussion:
https://dynacotubeaudio.forumotion.com/t2431p15-bias-voltage-for-st70-with-vta-driver-board

00:00 Introduction
00:42 Original ST-70 EL34 Bias Level
05:14 Bob Latino 40mA Recommendation
06:38 50mA Bias w/ 7.5K Feedback
08:03 50mA Bias w/ 3.3K Feedback
09:05 40mA Bias w/ 7.3K Feedback
10:02 40mA Bias w/ 3.3K Feedback
10:28 THD at Various Frequencies & Power Levels
12:30 Original vs "Improved" VTA
13:33 Solid State Rectifier Bias Calculation
17:14 THD w/ Solid State Rectifier
19:32 Solid State vs Tube Rectifier
21:08 Outro

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