Vibration noise from toroid transformer. Is it possible to get rid of it?


My homemade amplifier (D.Walton 300B XLS MKIII schematic) is working normally with one exception. When I turn power on the amplifier  for the first 30-45 seconds appears vibration like silent but bored noise and this leads me crazy.  Others suggest not paying attention to this but I can't.

Maybe not quite well, I covered two main toroidal transformers under one cover and sealed it with epoxy.

My attempts to find the source of the noise were as follows:

I grounded inputs and verified voltages – everything – OK.

1. Then I removed all the tubes, turned power on – from the main transformers cover - dead silence - everything is OK. No load - no noise.

2. OK - I inserted GZ37 only, turned power on –  after about 10 - 15 sec appeared growing vibrating noise from the main transformer, which after 30 seconds became poorly heard.

3. Then I  disconnected one of toroidal transformers from the main. What was interesting I found high voltages in completely from the main disconnected second transformer, only not standard ones, but about 300 – 0 - 500 V.  When connected again both transformrs, voltages also have become standart 450 - 0 – 450 V on both transformers.

Maybe this electromagnetic interaction between two closely fitting toroidal transformers under one cover is the cause of noise? But I can’t do anything because these transformers are already deadly buried in epoxy. Replacing them with new ones and making anew different layout would cost me over 400  and almost all of the amplifier to be rebuilt. Is it possible to change something not so painfully? Maybe the vibrational noise reason is different?

Kindly Antanas


antanask
Sounds like you have too high a capacitive load after the rectifier. 
I agree with no romance! Unlike SS diodes a tube rectifier can only handle a limited amount of capacitance. What is your first capacitor after the GZ37?
They all vibrate to a certain degree. The crazy thing is the way manufacturers couple them to the chassis with bolts. 😳
Hi,
Thank you for responce. First capacitor is 4 uF.

This is a link to the schematic of my PSU:

https://ibb.co/nvVTSG

I also had hum problem during the first 35-45 sec after turning up of ampl but when I rearrange the heating of 300B XLS to DC  hum is gone

Kindly Antanas


Hi, I'm sorry to buy such a toroidal transformer. I was looking for a similar, but originally in Amplimo potted power toroidal trasformer - I did not find ... so I myself immersed two toroidal transformers into one cover and covered with epoxy ... I did not tighten them neither with each other nor to the chassis.. I thought that they would not vibrate in hardened epoxy
It's a link of the image of my homemade amp ;)

https://ibb.co/fiRYcG

Kindly Antanas
but when I rearrange the heating of 300B XLS to DC  hum is gone
Then the 300s are drawing too much current when cold and resistance is low.
Nice looking SET amp, antanas!