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_
three_easy_payments's avatar
three_easy_payments
2,097 posts
 
I bet @ramtubes could provide stellar advice here
salectric
751 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.
gs5556
1,234 posts
 
I agree with salectric... positive feedback. Make sure that 4-ohm OPT lead is not grounded or the output will be in phase with the grid of the 6EU7.
bdp24's avatar
bdp24
9,792 posts
 
Roger Modjeski (ramtubes) may be able to give you some advice.
atmasphere's avatar
atmasphere
11,806 posts
 
 
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.
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