2. amps-preamps
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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
ilya_
roberjerman
4,319 posts
 
It would be a lot easier if you got a photo of that Magnavox amp and copied the parts layout and wiring scheme! Trying to build any amp (tube or transistor) from a schematic is not a guarantee of success! See my post above! It is always best to directly copy an actual working amp!
roberjerman
4,319 posts
 
The heater wiring carrying the 6.3 volts AC should be twisted pairs kept close to the chassis! 
roberjerman
4,319 posts
 
Try swapping the + and - leads of each speaker. The feedback loop using R9 and R23 may be positive (instead of negative) because you may have miss-wired the two input connectors!
three_easy_payments's avatar
three_easy_payments
2,097 posts
 
I bet @ramtubes could provide stellar advice here
salectric
753 posts
 
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.
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