So 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:
7 Tube Stereo Output Amplifier
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)
YouTube
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:
7 Tube Stereo Output Amplifier
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)
YouTube
Sounds like positive feedback instead of negative feedback. Take the two wires that go from the power tube plates (6bq5 pin 7) to the output transformer and swap them over.
2nd on Oscillation. Try above or remove R9 3.9K ohms to see if the oscillation goes away. If it goes away, it can be caused by one of the following.
1. The transformer primary is reversed
2. The secondary (speaker common) was not grounded
3. Too much negative feedback
1. The transformer primary is reversed
2. The secondary (speaker common) was not grounded
3. Too much negative feedback
Is there insane noise at power on or do the tubes have to warm up first? If they do, it's probably positive feedback as mentioned above.
If not, there might be a current leaking from the AC to the circuitry. Double check your PT connections if that's the case.
Koda
If not, there might be a current leaking from the AC to the circuitry. Double check your PT connections if that's the case.
Koda
Last edited:
Thank you so much! I did this and it worked. It plays music now. Although when not plugged into a source; there's quite a bit of static noise. Is that normal?
I tried the first option. Thank you!
@koda it was positive feedback. Thank you for your response.
No.
That might simply be a noisy tube (probably not the output tubes). Try the input tube first.
- Status
- Not open for further replies.