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Original Message

TFA-204 300B Operating Points

Posted by jdrouin on March 7, 2017 at 12:34:03:

So, I'd like to breadboard a 6SN7 --> 300B SET circuit using Magnequest TFA-204 OPTs. It's a fairly well documented combination, but let me be up front that I'm trying to regain the sound of a pair of George Wright mono blocks I heard years ago, that have stuck in my memory ever since. This might be a sensitive topic given that some of you knew him well, so please let me know if it's inappropriate and I'll delete the thread.
Given that the TFA-204 is spec'd for 3K and 60mA, the operating points for the 300B would need to be pretty conservative. The usual idle current at 70% OPT maximum current capacity would be 42mA, falling in the range of the following from the WE 300B datasheet:
300V plate, 50mA, -63V grid into 3K for about 6w output
250V plate, 40mA, -52V grid into 3K for about 4w output
I've also seen some 300B SET builders (i.e. for the Fi Primer 300B) who liked:
310V plate, 42mA, -60V grid into 3K
The circuit I want to breadboard was a one-off that George was playing with when he sold them to my friend ca. 2000-2001: a WPA 3.5/Mono 8 (same topology) using a 300B and the TFA-204. I figured I'd start with a revers-engineered schematic posted by a WPA 3.5 owner at diyaudio (attached, accuracy uncorroborated), adjusting the voltages for 300B operation.
Do the above parameters seem viable for the attached circuit? Should I go closer to 350V/60mA -- even though it's already at max current?
Mike, if you're reading this, do you happen to remember the amp in question and its operating points?
If anyone knows of a more accurate schematic, or of how George's thinking would have suggested the op points, I'd really appreciate it.
Many thanks,
Jeff