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ST-70 Tube Amplifier DIY Kit

Lack of speaker ground shouldn't affect output tube current flow, but it will screw with feedback. Also could possibly cause oscillation which can show up as odd current flow in the output stage.
 
The underside photo is the one a member on diyaudio saw without the speaker ground not grounded to the amplifier. I have since ground the ground, if that makes sense. Cured the problem I was having with both amplifiers.

The harmonic profile for both builds is exactly what you want to see. Predominant 2nd harmonic with a touch of third. Exactly what Jean Hiraga said best sounding in an amplifier many years ago.
 
Ken Boyd said:
Looks great, I put a want to buy ad out as I would really like the Octal board but if one doesn't come around, I will probably end up with the one your used. I don't want to attempt to try and order again off tubes4hifi as never received the one I ordered and had to request a refund.
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Tubes4Hifi recommends a different PS transformer to be used on their St-70 kit if one uses the octal driver board. The heaters pull a lot more current. If I were you I would not consider it anymore. Check the data sheets yourself.
 
gadget73 said:
Lack of speaker ground shouldn't affect output tube current flow, but it will screw with feedback. Also could possibly cause oscillation which can show up as odd current flow in the output stage.
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On the toroidal build with the VTA driver board anytime I tried to bias above 30ma the bias would crash. This is when the speaker ground was not grounded. The EI build with the dynaco type drive board would bias at 40ma but I still had hum on my horns, not bad, I could not hear it on my test speakers. Also the distortion numbers were not good on either. Now both amplifiers on Arta have THD+ noise numbers at approximately .1% 1 watt. Too lazy to get up with precise numbers. About the lower limit of my test equipment and shop noise.
 
Hm, that really makes no sense unless it was some weird oscillation that was internally rectified to a very negative voltage at the grid. Either way, glad you figured it out and its working well now
 
gadget73 said:
Hm, that really makes no sense unless it was some weird oscillation that was internally rectified to a very negative voltage at the grid. Either way, glad you figured it out and its working well now
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I agree, made no sense to me either. Had me scratching my head trying multiple things for days. I had to have help on the diyaudio forum before I found the problem with my build. Of the dozen of so tube builds I have built I do not remember grounding the negative terminal of the speaker out. My guess is it has to do with the GFB network. I have been playing with adding a touch of feedback to my SE amplifiers but have not tied the negative speaker terminal to ground. I should have followed the schematic more carefully. It was right in front of my eyes.
 
Feedback from the output secondary requires that one of the output connections be connected to audio common. I won't work correctly otherwise. Think of it as another input to the amp. If it doesn't have a reference to audio common it is incomplete.
 
Yeah, no complete path without a ground reference off the speaker secondary. Plate to plate has the cathodes already connected to ground.
 
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