• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

Ebay 300B SE amp information

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Though I have a low level of hum which annoys me and haven't been able to resolve this doesn't take away from the sweet sound.

AC filaments? Balancing pots? It is hard, at least for me, to totally eliminate hum with a true triode with 5 or more watts using 103db speakers but with DC filaments and balancing pots I reduced it to an acceptable level. I do believe my build would be quite with 90db speakers. It takes some effort though. I used Arta to set my balancing pots which helped a lot.

I would like to incorporate this into my circuit but it might be difficult given it’s a PCB. there may be a way to tap into the circuit for someone who knows what they are doing (ie not me). I would probably need to rebuild the the amp using point to point wiring. refer to thread on SE 6sn7/300b on diyaudioproject.com site. DIY Audio Projects Forum • 300B SET Design Project
btw could you explain ‘Arta’? I would like to follow up.
 
The 300B is self-biased that you don't need to worry. Depending on the B+ voltage, say 400Vdc, the idle current shall be 80mA approximately. The OPT should be 2.5 to 3.5Kohm primary impedance.

Thanks. everything seems rather fine except for a low level of hum. there’s enough power with my 89db@1watt two way speakers. i can hear some bass down to 40 hz but can’t hear much from the tweeters above 10khz tones ( my old ears most likely). i would like to check the frequency response on a scope though.
opts are mentioned in the circuit above. i use the gsxe 15-3.5k. there not large but seem quite good enough.
 
In a future build (this was my first) I might go for larger opts if needed but I am really surprised how good the overall sound is with the 15 watt. I was listening to Led Zeppelin 2 and John Paul jones bass guitar was quite prominent in the sound which I hadn't noticed before with my ss amp. It may not be possible to re-create the organ notes from the big pipes but my speakers roll off at about 50 Hz anyway.
 
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