Parts layout is very important in any amplifier design! Keeping the power supply/transformer away from the input stage, wire length and routing ... Grounding issues? Did you use star grounding (tieing all grounds to a single point on the chassis)? Tieing all power supply grounds together and connecting them to the chassis near the transformer is one solution. So is using a bus bar running the length of the chassis and connected at one end as a common ground is another method.
Please help! Tube amp issue
I am extremely new to building tube amplifiers. I've never done this
before, but this concept intrigued me alot, therefore I decided to build
one. I followed a schematic of a 20W pc tube amplifer. To my knowledge,
and to checking my work multiple times, I have followed the schematic
religiously. I think it's from an old Magnavox amplifier.
When the amp switches on, there is an insane amount of noise, even when there is no audio input given. The only modification I have made is that I have changed is the main power tranformer's primary voltage due to where I live. (120x2)
I'm not allowed to post the schematic's diagram, but I may link the page:
http://www.wkinsler.com/radios/7tubeamp1.html
If anyone could please dissect the schematic to see if there could be a potential problem with the design itself, that would be fantastic.
I have also takena a video of my amp groaning the noise away to glory: (One of the pre amps do not work, but then i disconnected one side of the amp, and the noise still existed)
https://www.youtube.com/watch?v=Myj-aIommmA
When the amp switches on, there is an insane amount of noise, even when there is no audio input given. The only modification I have made is that I have changed is the main power tranformer's primary voltage due to where I live. (120x2)
I'm not allowed to post the schematic's diagram, but I may link the page:
http://www.wkinsler.com/radios/7tubeamp1.html
If anyone could please dissect the schematic to see if there could be a potential problem with the design itself, that would be fantastic.
I have also takena a video of my amp groaning the noise away to glory: (One of the pre amps do not work, but then i disconnected one side of the amp, and the noise still existed)
https://www.youtube.com/watch?v=Myj-aIommmA
12 responses Add your response
Your problem may be the same as what I encountered a few years ago with a DIY KT-77 amp. You may have reversed the output tube connections to your output transformer. Try rewiring the output transformer primary leads so that the leads previously connected to the “top” tube are now connected to the “bottom” tube and vice-versa. When the connections to the output tubes are reversed, the amp’s polarity is reversed and the feedback becomes feedforward. That would account for the noise on your video. Of course, I am assuming you have carefully traced the wiring to make sure the amp is wired correctly. |
The feedback loop using R9 and R23 may be positive (instead of negative) because you may have miss-wired the two input connectors!roberjerman and salectric got it. This is how an amplifier like this behaves when the output transformer is incorrectly phased. The best place to fix this is on the primary side of the transformer. |
Your problem may be the same as what I encountered a few years ago with a DIY KT-77 amp. You may have reversed the output tube connections to your output transformer. Try rewiring the output transformer primary leads so that the leads previously connected to the “top” tube are now connected to the “bottom” tube and vice-versa. When the connections to the output tubes are reversed, the amp’s polarity is reversed and the feedback becomes feedforward. That would account for the noise on your video. this has helped me a lot |
@jetter, OP solved the problem by swapping the wires of output transformer on primary side as suggested. https://www.diyaudio.com/forums/tubes-valves/330414-please-help-tube-amp-issue.html |