Please help!

[ilya__]

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:

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)

 

[s-petersen]

It is possible you have the output transformer leads reversed on the output tubes is it both channels?

 

[grindfix]

Is it "howling" type of noise?
I suspect you may have grounded wrong side of the output transformer secondary windings and your feedback is Positive vs. Negative. I'd try disconnecting feedback, ground inputs and see what you get.
Other more experienced guys here may offer more help.
Good luck

 

[grindfix]

Scott beat me to it!

 

[ilya__]

Is it "howling" type of noise?
I suspect you may have grounded wrong side of the output transformer secondary windings and your feedback is Positive vs. Negative. I'd try disconnecting feedback, ground inputs and see what you get.
Other more experienced guys here may offer more help.
Good luck

Thank you. I have posted a video of how it sounds in my post. It sound more like a raging fighter plane actually, almost scary. I will try the modifications that you suggested.

 

[s-petersen]

If it only makes noise out of one channel, only that channel is backwards, it can also be caused by switching leads on the phase inverter to the output tubes

 

[ilya__]

If it only makes noise out of one channel, only that channel is backwards, it can also be caused by switching leads on the phase inverter to the output tubes

But Ive read the noise due to the phase inverter is a small one, right? The noise this makes is that of an extremely loud fighter plane. I'm sorry if i sound clueless, this is my first time.

 

[s-petersen]

If the leads from the phase inverter to the output tubes are reversed, it is the same as reversing the leads on the output transformers, it will oscillate badly

 

[grindfix]

I still think it’s easy to prove if feedback is reversed by simply disconnecting it from 4 Ohm tap and see if noise is gone.

 

[s-petersen]

I agree with that, certainly. Something I forgot about was crossed feedback, where 1 channel's feedback id connected to the other channel.

 

[ilya__]

So I just disconnect the 4ohm wire from my output trans. to my valve and switch it back on to see if theres no noise?

 

[UncleBingo]

He suggested also shorting the amplifier at the signal input which is typical for taking noise measurements. It sounds like regenerative feedback to me (the opposite of negative feedback)

 

[UncleBingo]

Your feedback loop should be coming off of the secondary side of the output transformer (one of the unused taps). removing the tap wire as suggested (not the speaker wires) should kill the howling. If it does stop the howling reverse the polarity on the two main secondary wires that connect to that speaker and re connect the negative feedback to whichever one of the unused secondary taps you were using before..

 

[UncleBingo]

If it's easier you can switch the leads on the primary side of the transformer and leave the secondary side alone. I usually go for the low voltage option whenever possible : )

 

[cademan]

Just reverse the plate wires on one channel only. And.........you can post schematics here.

 

[grindfix]

On very top of your schematic there is a feedback lead with 3.9k resistor in it.
Disconnect it from 4ohm tap and see if noise is gone.

 

[century tek]

Prolly reverse the plate connections on one channel. In my experience, this seems to be the main problem with kit or scratch builds.

I think the easiest thing to do would be to reverse the plate leads on one channel first. This will most likely cure that screech. If it doesn't, then do the other mentioned thing with the feedback resistor.

 

[grindfix]

Post closeup pictures that show connections between tubes and transformers. How do you know secondary transformer taps impedance?
Are leads labeled?

 

[ilya__]

If it's easier you can switch the leads on the primary side of the transformer and leave the secondary side alone. I usually go for the low voltage option whenever possible : )

Yes. I reversed the leads on the primary transformer by interchanging the wires at the plates and it worked. Thank you.

 

[ilya__]

Prolly reverse the plate connections on one channel. In my experience, this seems to be the main problem with kit or scratch builds.

I think the easiest thing to do would be to reverse the plate leads on one channel first. This will most likely cure that screech. If it doesn't, then do the other mentioned thing with the feedback resistor.

I did it for both and now it works as a normal amplifier. Thank you for your response!