Hoffman Amplifiers Tube Amplifier Forum
Other Stuff => Video Clips => Topic started by: pompeiisneaks on August 08, 2016, 10:04:46 pm
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I've got the video here:
https://www.youtube.com/watch?v=__5VqD-fsv0&feature=youtu.be (https://www.youtube.com/watch?v=__5VqD-fsv0&feature=youtu.be)
and I've got the schematic I have to date I'll attach, please do let me know of any obviously 'off' issues with it. I have it working but I'm getting really low output in the PI/power side, the EF86 was odd in that I set it up with a 100k anode resistor, and a 390k for grid two, but the voltage on the anode was at like 80V and the G2 was 110V so I added a higher resistor, I think it was either 680k or 1M and that seemed to sort out the EF86, but now the output side is really quiet. I'll keep tinkering.
~Phil
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I see a few issues...
The EF86 screen needs a cap to ground.
The filaments will not light up according to your schematic. Point X must be on the left side of R17. Probably just a drawing error?
Now the big one. What you think is a phase inverter is not. The signals on the plates of each triode are in phase, therefore the signals going to the grids of the output tubes are also in phase. The two in phase signals are cancelling each other in the OT, resulting in a weak output. So, build a proper phase inverter, either LTP or cathodyne, and it will be much louder.
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I figured out the PI, I have that cathode that's shunting to ground C6. I've removed it and that's doe it. It's imbalanced. yeah that's a typo on the heaters, I've not even put in the LED yet, just doing a prototype. I'll fix that part. That cap on the Screen for the EF86 may be why I'm getting erratic behavior where it seems to work for a while, then not, and has a lot of weird noise. I'll add that. Thanks.
The PI I was going for is where the 1M resistor drops the signal significantly but makes it a negative feedback to the grid of the other half, but the cathode follows the inverse signal through to the anode of V3a. I may still have that wrong, but removing C6 made things quite a bit better. If I just add that grid cap on the EF86 I may even be closer, but I'm done for tonight. I'll hit it again tomorrow.
Thanks for the help!
~Phil
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Video 2, I mention the changes I have due to yours and other suggestions ;)
https://www.youtube.com/watch?v=ORJ_gGKLeGw&feature=youtu.be (https://www.youtube.com/watch?v=ORJ_gGKLeGw&feature=youtu.be)
~Phil
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You removed the 220K grid return resistors from the output tubes! Those resistors are crucial for the tube to work properly. Without them the tube cannot bias itself. Put them back.
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I can, but when they were in, the tubes output was so low it was almost inaudible with a guitar or a sine wave input from my phone. maybe something else is amiss then? I see the input from the EF86 is really hot (Input is about 50mV and I think output was like 50V? I am going to put the work on hold as I'm now building my AC30/4 but once done with that, I'm likely to put together a turret board and actually build the thing, the prototyping in general, when I did have it working, sounded really nice. But I think my shoddy setup made it much harder. I forgot biasing is the loop between that grid resistor and the cathode...
~Phil
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Video 3:
https://www.youtube.com/watch?v=OO0PHbyDP54 (https://www.youtube.com/watch?v=OO0PHbyDP54)
~Phil
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Video 4: https://www.youtube.com/watch?v=iqQPnDZU_MU&feature=youtu.be (https://www.youtube.com/watch?v=iqQPnDZU_MU&feature=youtu.be)
~Phil